Dissertations / Theses on the topic 'Microstructural and mechanical characterizations'

Aluminum-alumina nanocomposites were synthesized using mechanical alloying of blended component powders of pure constituents. This study was performed on various powder mixtures with aluminum as the matrix and alumina as the reinforcement with volume fractions of 20, 30, and 50 % and Al[subscript 2]O[subscript 3] particle sizes of 50 nm, 150 nm, and 5 [mu]m. X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques were used for the crystal structure and microstructural characterization of the powders at different stages of milling. Al?O? powders with 50 nm and 150 nm particle size were predominantly of [gamma]-type, while Al[subscript 2]O[subscript 3] of 5 [mu]m size was of [alpha]-type. The main goal was to achieve uniform distribution of the Al?O? ceramic particles in the Al matrix, which was achieved on milling for 24 h in a SPEX mill or 100 h in a Fritsch Pulverisette planetary ball mill. The powders were consolidated in two stages: pre-compaction at room temperature followed by vacuum hot pressing (VHP) or hot isostatic pressing (HIP) techniques to a fully dense condition. The effect of reinforcement particle size and volume fraction on the stress-strain response, elastic modulus and yield strength of the composites was investigated. Nanoindentation and compression tests were performed to characterize the composite material. Yield strength of 515 MPa, compressive strength of 685 MPa and elastic modulus of 36 GPa were obtained from compression tests. Nanoindentation results gave the yield strength of 336 MPa, maximum shear stress of 194 MPa and an elastic modulus of 42 GPa. The low elastic modulus values obtained from the above tests might be because of localized yielding possibly due to residual stresses.
M.S.
Department of Mechanical, Materials and Aerospace Engineering;
Engineering and Computer Science
Materials Science and Engineering

PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO
CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO
Compósitos reforçados por fibras lignocelulósicas e, também, materiais estruturais de origem vegetal, tal como o bambu, vêm sendo cada vez mais empregados em diversos segmentos industriais, tendo em vista a crescente necessidade da sociedade de usar materiais provenientes de recursos naturais renováveis. Assim sendo, este trabalho visa analisar resíduos da agroindústria da produção sustentável de palmito e de coco, que são os caules das palmeiras Bactris gasipaes (pupunha) e Cocos nucifera (coqueiro). O objetivo em caracterizá-los é fundamentar uma via alternativa de obtenção de madeira, considerando os seguintes aspectos: a disponibilidade, a preservação do meio ambiente, o bom desempenho do material e o baixo custo. Neste trabalho foi feita a caracterização microestrutural da pupunha por microscopia eletrônica de varredura e microscopia óptica digital. O comportamento térmico e termo-mecânico da pupunha foi avaliado por termogravimetria e por análise termo-dinâmico mecânica. Foi avaliado ainda o comportamento mecânico em flexão, compressão e cisalhamento na linha de cola de corpos de prova usinados do estipe da pupunha, bem como se avaliou por difração de raios-X a estrutura cristalina e o grau de cristalinidade do material. Além disso, foi feita a caracterização da resistência à abrasão e avaliado o efeito do envelhecimento por absorção de água e por radiação UV nas propriedades à flexão da pupunha. Também foram avaliadas a resistência à abrasão e as propriedades mecânicas à flexão e à compressão do caule do coqueiro.
Composites reinforced by lignocellulosic fibers and structural cellulosic materials, such as bamboo, have being increasingly used in many industrial fields, owing to the growing society need to use materials from renewable resources. Therefore, this study aims to analyze two agro-wastes of the cococnut and heart of palm sustainable production, which are the trunks of these palms (Bactris gasipaes and Cocos nucifera). The main objective is to establish the foundation for an alternative way of obtaining wood, considering the following aspects: availability, environmental conservation, good performance and low material cost. In this work microestrutural characterization of pupunha trunk was done by scanning electron microscopy and digital optical microscopy. The thermal and the thermo mechanical behaviors were evaluated by thermogravimetric analysis and by dynamic thermo mechanics analysis. Also rated was the material mechanical behavior in bending, compressing and shearing in the glue line. The material crystal structure and the degree of crystallinity was tested by X-ray diffraction. The resistance to abrasion was checked and the effect of aging due to water absorption and UV radiation in the bending properties of pupunha. Finally we verified the abrasion resistance and mechanical properties of bending and compressing the coconut palm.

The use of aluminum alloys for commercial and military applications has increased substantially due to the alloysâ low areal density, toughness, and processability. It has recently been shown that an aluminum alloy, Al 2139, with copper, magnesium, and silver can be significantly toughened and strengthened by combinations of θâ and Ω precipitates and dispersed manganese particles. What has not been quantified are how these precipitates and dispersed particles affect behavior and what the material mechanisms and microstructural characteristics are that control the behavior of Al 2139 for strain-rates that span the quasi-static to high rates of strain. Hence, in this investigation, detailed transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), scanning electron microscopy (SEM), orientation imaging microscopy (OIM), and optical microscopy (OM) were used to delineate the different physical scales that range from the nano for the precipitates and dislocations to the micron for the dispersed particles, grain orientations and texture, grain-sizes, slip-bands, and grain-boundary orientations. The deformed specimens were from an Al 2139 plate that was impacted by 4340 steel fragmentation stimulating projectiles (FSPs) at impact velocities ranging from 813 to 1043 m/s. The majority of the projectiles were defeated by the Al 2139 plate, which is another indication of the alloyâs potential for damage mitigation and projectile defeat and resistance. Based on this detailed microstructural characterization, mechanisms for projectile defeat and full penetration are proposed. Deformation and damage modes include petalling on the impact face, shear cracking through the middle section of the plate due to projectile penetration, and discing due to bending stresses at a spall plane near the back of the plate. Shear cracking appears to be GB related, and the discing is dependent on the rolling direction. The extent of these modes for cross-sections where the target was penetrated was greater than that in regions where the projectile was defeated. For projectile defeat, large and elongated grains and precipitate deformation due to dislocation interaction can lead to highly ductile performance, which resists discing failure and plate penetration. Large grains significantly reduce the fraction of GBs, which then reduces the amount of GB cracking due to intense shear accumulation and spall. The elongation of the grains due to rolling also increased the dislocation densities, and subsequently the ductility of the grains, which reduced tensile failure due to the bending in the discing regions. High angle GBâs can also limit heterogeneous θâ precipitation at the GBâs, which would reduce intergranular fracture. Precipitation of Ω also increases the spall strength and decreases localized shear through its multiple cutting interactions with dislocations at the matrix interface. Dispersed particles also increase the strength of the alloy in high strain-rate applications by resisting localized shear. The results of this study are a first step in developing a tailored methodology that can be used to optimize microstructural characteristics and behavior of aluminum alloys for optimal strength and toughness.

Friction stir welding (FSWing) is a solid state, thermo-mechanical process that utilizes a non-consumable rotating weld tool to consolidate a weld joint. In the FSW process, the weld tool is responsible for generating both the heat required to soften the material and the forces necessary to deform and consolidate the former weld seam. Thus, weld tool geometry, material selection, and process parameters are important to the quality of the weld. To study the effects of the weld tool geometry on the resulting welds, a previous study was conducted using varying degree taper, microwave-sintered tungsten carbide (WC) weld tools to FSW Ti-6Al-4V. Fully consolidated welds were down selected for this study to evaluate the resulting mechanical properties and to document the microstructure. X-ray diffraction (XRD) was used to compare the parent material texture with that in the weld nugget. The purpose of this study is to quantify the temperatures obtained during FSWing by interpreting the resulting microstructure. This information is useful in process optimization as well as weld tool material selection.

PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO
COORDENAÇÃO DE APERFEIÇOAMENTO DO PESSOAL DE ENSINO SUPERIOR
PROGRAMA DE SUPORTE À PÓS-GRADUAÇÃO DE INSTS. DE ENSINO
Foram utilizados dois sistemas de aço API 5L X80, Nb-Cr e Nb-Cr-Mo, para obter as diferentes regiões da ZTA pertencentes a uma soldagem multipasse. Estas regiões são denominadas de: região de grãos grosseiros inalterados (RGGI), região de grãos refinados reaquecidos supercriticamente (RGRRS), região de grãos grosseiros reaquecidos intercriticamente (RGGRI), região de grãos grosseiros reaquecidos subcriticamente (RGGRS). Estas regiões foram obtidas para dois aportes de calor (1,2 e 2,5 kJ/mm) e a RGGRI por ser considerada a região onde poderiam ser formadas zonas frágeis localizadas (ZFL) foram utilizados também aportes de calor de 3,0 e 4,0 kJ/mm. Cada uma das regiões obtidas pela simulação física foi submetida a ensaios mecânicos de impacto Charpy e dureza, assim como a análises metalográficos por microscopia ótica (MO) e microscopia eletrônica de varredura (MEV). Foi possível observar que as microestruturas pertencentes a uma ZTA simulada obtidas com o equipamento (GleebleR3800) se mostram compatíveis com aquelas pertencentes a uma soldagem real. Este resultado comprova que as velocidades de resfriamento obtidas pela simulação foram similares àquelas da soldagem real. A adição de Mo ao sistema Nb-Cr-Mo não promoveu mudanças significativas tanto a nível microestrutural, observado por MO e MEV, como em termos de propriedades mecânicas.
Two API 5L steels grade X80 of the systems Nb-Cr and Nb-Cr-Mo, were submitted to physical simulation in order to obtain different regions of the HAZ similar to those of a multipass welding, the coarse grained heat affected zone (CGHAZ), supercritically coarse grained heat affected zone (SCCGHAZ), intercritically coarse grained heat affected zone (ICCGHAZ), subcritically coarse grained heat affected zone (SCGHAZ). The welding simulation was carried out on a Gleeble R 3800 considering two thermal cycles and different heat inputs 1.2, 2.5, 3.0 and 4,0 kJ/mm, typical of a girth weld. All HAZ zones were simulated only for 1.2 and 2.5kJ/mm. Since the ICCGHAZ is the probable weak link where a local brittle zone (LBZ) can occur, this region was simulated for all heat inputs studied. All simulated regions were subjected to traditional mechanical tests such as impact Charpy-V at -40 and -60C and microhardness Hv1kg. Metallographic analysis by optical microscopy (OM) and scanning electron microscopy (SEM) and fractography were also performed. The microstructures obtained for the different regions of the HAZ, by simulation were close to those of a real welding, however, the cooling rates obtained by simulation were slower than that obtained in a real welding. The mechanical properties and microstructure of the different regions of the HAZ for the systems NbCr and NbCrMo indicate that the microstructural and mechanical behavior of the intercritical region (ICCGHAZ) was considered to be similar to a local brittle zone (LBZ) for all conditions studied.

Super 13% Cr steels are used for offshore applications and have to meet strict tensile strength, yield strength, toughness, elongation, and hardness specifications. The mechanical properties of these steels are strongly dependent on the proportions of retained austenite and martensite, and consequently small changes in heat treatment parameters result in major changes in properties. In this work, a detailed investigation of the effect of heat treatment parameters on microstructural features, hardness and tensile properties of the commercial supermartensitic 13Cr-5Ni-2Mo steels in the asreceived and re-heat treated state were undertaken. The re-heat treatment was performed by re-austenitising the samples at 950 °C for 2 h, air-cooling, and then tempering (single and double temper) in the range of 550-700 °C. The effects of tempering temperature, time, and their combination P (P = (273+T (°C) x (40 + login t (h)) x 1000'1), the Holloman-Jaffe parameter, on retained austenite volume fraction, hardness and tensile properties were investigated. Retained austenite content increased with P to a peak value at P-36.9 and then decreased due to the formation of fresh martensite. However, the second temper increased retained austenite due to the re-transformation of fresh martensite to austenite. This resulted in refined grain size and a high dislocation density. An increase in P gave a decrease in the C and Ni content in austenite on tempering while the austenite grain size increased. The combination of these two effects led to a decrease in the stability of the retained austenite with P, as shown by the increased M. For the first temper at P > 36.9, the austenite present after the second temper was more stable than after the first temper as a result of re-distribution of C and Ni from the martensite to the austenite. 0.2% proof strength and hardness were inversely related to retained austenite content with P. Both hardness and 0.2% proof strength decreased linearly with increase in retained austenite content, but elongation showed the reverse trend. However, the slope of the relationships depended on whether fresh martensite was present or not. The results have shown that retained austenite with volume fraction between 16-30 vol% and size < 208 nm gives the optimum combination of strength, ductility and hardness.

Environmental issues and regulations are pushing the automotive industry to produce more efficient and environmentally friendly vehicles. To address these issues, reducing the weight of the vehicles by use of aluminum alloys is increasing. AA6022-T43 is a new sheet aluminum alloy designed specifically for automobile enclosure panels. Because this alloy is new, little data exists on its mechanical properties in the open literature. AA6022-T43 is received by the automotive industry in 1-mm thick sheet stock and subsequently stamped into the desired component. The design and manufacturing processes of the component are guided by the materials mechanical behavior. This study characterizes the mechanical and microstructural properties of uniaxially strained AA6022-T43.

Thesis (Ph. D.) -- University of Maryland, College Park, 2006.
Thesis research directed by: Dept. of Mechanical Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

The twin objectives of the investigation were (i) to synthesize Al/Al2O3 metal matrix composites (MMCs) with uniform distribution of the Al2O3 reinforcement in the Al matrix and (ii) to evaluate the effect of volume fraction and size of the reinforcement on the mechanical behavior of MMCs. This was achieved by successful synthesis of Al-Al2O3 MMCs with volume fractions of 5, 10, 20, 30, and 50%, and particle sizes of 50 nm, 150 nm, and 5 µm of Al2O3 synthesized from blended component powders by a high-energy milling technique. A uniform distribution of the Al2O3 reinforcement in the Al matrix was successfully obtained after milling the powders for a period of 20 h with a ball-to-powder weight ratio of 10:1 in a SPEX mill. The uniform distribution of Al2O3 in the Al matrix was confirmed by characterizing these nanocomposite powders by scanning electron microscopy and X-ray mapping. The energy dispersive spectroscopy and X-ray diffraction techniques were employed to determine the composition and phase analysis, respectively. The milled powders were then consolidated for subsequent mechanical characterization by (i) magnetic pulse compaction (MPC) (ii) hot-isostatic pressing (HIP), (iii) vaccum hot pressing (VHP), and (iv) a combination of vaccum hot pressing and hot-isostatic pressing (VHP+HIP). However, successful consolidation of the powders to near-full density was achieved only through VHP+HIP for the 5 and 10 vol. % Al2O3 samples with 50 nm and 150 nm particle sizes. The fully dense samples were then subjected to mechanical characterization by compression testing and nanoindentation techniques. The strength and elastic modulus values obtained from compression testing showed an increase with increasing volume fraction and decreasing particle size of the reinforcement. The nanoindentation results were, however, contradictory, and the presence of residual stresses in the samples was attributed as the cause for the deviation in values.
M.S.M.S.E.
Department of Mechanical, Materials and Aerospace Engineering;
Engineering and Computer Science
Materials Science and Engineering

Les travaux exposés dans cet ouvrage portent sur l'élaboration, la caractérisation microstructurale et les propriétés mécaniques de solutions solides nanolamellaires de phases dites MAX. Les phases MAX représentent une classe exceptionnellement étendue de céramiques. Elles répondent à une formule générale du type Mn+1AXn (n=1, 2 ou 3) où M est un métal de transition, A est un métal des groupes IIIA ou IVA, et X est un métalloïde (C ou N). Nous avons dans un premier temps réalisé l'optimisation de la synthèse, par métallurgie des poudres, de Ti3AlC2 pur. Une nouvelle phase, Ti3SnC2, ayant été découverte au laboratoire en 2007, les travaux se sont alors focalisés sur la synthèse de solutions solides du type Ti3AlxSn(1-x)C2 par pressage isostatique à chaud. Nous nous sommes, par la suite, attachés à la caractérisation microstructurale de ces solutions solides en étudiant notamment les variations du paramètre de maille, du taux de distorsion des octaèdres [Ti6C] et des prismes trigonaux [Ti6AlxSn(1-x)]. Enfin, nous avons déterminé la dureté intrinsèque et le module d'élasticité des différentes solutions solides en fonction du taux de substitution en utilisant la nanoindentation. Par ailleurs, des essais de compression, uniaxiale et sous confinement de gaz, ont été réalisés à température ambiante, afin d'étudier et de comparer les mécanismes de déformation de Ti3AlC2 et de la solution solide Ti3Al0. 8Sn0. 2C2. Les relations entre modifications microstructurales et propriétés mécaniques sont discutées. Nous montrons notamment que Ti3AlC2 et Ti3Al0. 8Sn0. 2C2 peuvent être considérés comme des matériaux "Kinking Non-linear Elastic"
The work described in this thesis concerns the elaboration, the microstructural characterization and the mechanical properties of nanolaminated MAX phases solid solutions. The MAX phases represent a large class of ceramics. They are a family of ternary nitrides and carbides, with the general formula Mn+1AXn (n=1, 2 or 3), where M is an early transition metal, A is a metal of the groups IIIA or IVA, and X is either carbon or nitrogen. We performed at first the optimization of the synthesis, by powder metallurgy, of highly pure Ti3AlC2. Since a new MAX phase, Ti3SnC2, has been discovered in the laboratory in 2007, the study has been further focused on the synthesis of Ti3AlxSn(1-x)C2 solid solutions by hot isostatic pressing. In a second step, the microstructural characterization of these solid solutions has been carried out, by studying, in particular, the variation of the cell parameters, the distortion rates of [Ti6C] octahedrons and [Ti6AlxSn(1-x)] trigonal prisms. Finally, we have determined the intrinsic hardness and the elastic modulus of the various solid solutions as a function of the Al content by using the nanoindentation. Besides, uniaxial and gas confining compression tests were realized at room temperature, to study and compare the deformation mechanisms of Ti3AlC2 and Ti3Al0. 8Sn0. 2C2. The relationship between microstructural modifications and mechanical properties are discussed. We show in particular that Ti3AlC2 and Ti3Al0. 8Sn0. 2C2 can be considered as "Kinking Non-linear Elastic" materials

This research work focuses on a comparative study on weld bead geometries of three different welding techniques: Cold Metal Transfer (CMT), Metal Inert Gas Pulse Synergic (MIG P) and MIG Manual Standard (MIG M). Bead-on-plate tests were performed using ER4043 (AlSi5%) as a filler material on the 3.18 mm thick plates of AA6061-T6. Current (80 A, 100 A and 120 A) and welding speed (7.5, 10.5 and 13.5 mm/sec) were used as input process parameters while shielding gas flow rate and contact tip to workpiece distance (CTWD) were maintained constant as 15 l/min and 10mm respectively. The weld beads processed by all the three techniques are compared by analysing the weld bead geometry. Microstructural characterization is carried out using optical microscopy and Field Emission Scanning Electron Microscope (FESEM). CMT has high dilution and penetration with low heat input. Compared to MIG P and MIG M, CMT shows a drastic reduction in residual stresses. Multi-response mathematical model is established for prediction of weld bead geometry in CMT, MIG P and MIG M welding of AA6061-T6 using ER4043 (AlSi5%) as a filler material. Central composite face-centered design (CCFCD) under response surface methodology (RSM) is employed to develop the design matrix for conducting the experiments. The developed model is employed in finding the optimal process parameters for good weld bead aesthetics. Current (I) and welding speed (S) are opted as input process parameters for response output such as penetration, dilution and heat input. This model is proficient to forecast the main effects and interactive effects of two factor of the opted welding process parameters. Results show that higher current values with low welding speed results in deeper penetration, high amount of dilution with higher heat input and vice versa. With lower heat input, CMT has high dilution and penetration with respect to MIG P and MIG M welding. The optimal process parameters are 92.518A and 7.50mm/sec for CMT, 109.418A and 10.873mm/sec for MIG P, 110.847A and 11.527mm/sec for MIG M with 61.11%, 68.80% and 72.6% desirability, respectively. Predicted output values generated from regression model equation obtained from welding process parameters are very close and sometimes overlaid on actual output that obviously demonstrates the suitability of the second order regression equations. A vi good amount of penetration and dilution with low heat input is required for better joint efficiency. The requirements projected by many industries for stronger, lighter, more efficient and cost-effective combined alloys in the welding of two dissimilar materials or dissimilar thickness. The current industry trend is the coalescence of various aluminium alloys of varying thicknesses. CMT welding process was used for joining of AA6061-T6 and AA6082-T6 using ER4043 filler wire and inspected the effect of different process parameters on mechanical properties of welded butt joints. Current (I), welding speed or travel speed (TS) and gas flow rate (Q) are the input welding process parameters that are to be optimized. Different heat input is studied w.r.t welding speed, current and gas flow rate. Heat inputs ranging from 100+, 200+ and 300+ J/mm is achieved at constant welding speed of 9, 7 and 5 mm/sec respectively at variable currents and flow rates. Bead geometry variables such as penetration (P), reinforcement (R) and contact angle (CA) are distinguished at different heat inputs. Mechanical properties such as tensile test and microhardness for different heat input were investigated. Microstructural characterization of base metal (BM), fusion line (FL) and weld metal (WM) is carried out. High-Resolution X-ray Diffraction (HR-XRD) technique based on cosα method is used for residual stress measurements at different heat inputs. Tensile fractured surfaces were examined by FESEM and energy-dispersive X-ray spectroscopy (EDX). Butt joints of various different process parameters were fabricated with the help of full factorial CCFCD under RSM to optimize the tensile properties, microhardness and residual stresses. Grey relation analysis (GRA) with Principal component analysis (PCA) is incorporated with CCFCD for finding out the optimal process parameter by considering multi-response parameters simultaneously. ANOVA was executed to interpret the impact of process parameters on the mechanical properties of the weldments. Results showed that the most dominant process parameter was found to be the welding speed. The optimal process parameter obtained via GRA-PCA technique is I3-TS1-Q1 (I - 100 A, TS - 5 mm/sec and Q - 14 L/min having heat input 352 J/mm) which produces 226 MPa of ultimate tensile strength, 12.6 % of elongation, 68.7 HV of microhardness and -152.3 MPa of compressive residual vii stress. Desirability of optimality level obtained through CCFCD was 65.99 % and significantly improved to 97.07 % through GRA-PCA. Nowadays, to enhance the structural efficiency, ultrasonic vibrations are combined with other manufacturing processes such as welding. It gives considerable advantages in terms of improved mechanical properties, adequate surface strength, improved material flow and uniform grain growth etc. Ultrasonic assisted cold metal transfer (U-CMT) welding is performed to fabricate the joints and improvements in mechanical properties and microstructural modifications are studied. Non-destructive technique (NDT) such as radiography technique (RT) is used to test weld consistency. Results revealed improved weld bead geometry with the aid of ultrasonic vibrations for the same welding parameters. The tensile strength and micro-hardness are enhanced. Samples with ultrasonic vibration experiences grain refining as compared to without vibration samples. As compared with CMT, U-CMT joints are rich in Al-Si eutectic structure. Al-Si structure is in globular form with reduced porosity level.

Mixtures used for the production of porcelain tiles consist of inorganic raw materials such as illitic and kaolinitic clays, tempering materials such as sands, and feldspars as fluxing agents. Illitic and kaolinitic clays are used as plasticizers in the mixture, while feldspars are fluxing agents which create an abundant melted phase with a proper viscosity during the firing process of the tiles that results in a compact vitreous structure of the final product. Sands represent the skeleton of the porcelain tile and at the same time limit the shrinkage during firing. During the preparation of this thesis, two innovative mixtures have been formulated, in which the introduction of local industrial and urban post-consumer wastes substituting clays and feldspars lead to advantages regarding energy consumption and improved characteristics of the final product. The samples obtained in each phase of the laboratory production process were tested to verify if the innovative mixtures are suitable for an industrial technological transfer. The results obtained show a stable behavior of the mixtures in terms of shrinkage during the firing process, while the optimal firing temperature has been reduced significantly leading to a considerable energy saving during this phase of the production.

This study focuses on joining pearlitic ductile cast iron with low carbon steel by welding and investigation of this joint in microstructural and mechanical viewpoints. For this purpose E355 steel and GJS600-3 cast iron were joined using metal active gas (MAG) welding process by G3Si1 filler wire. The joining process is shaped mainly by the problems related to the low weldability of cast. Preheating was applied to prevent formation of cooling cracks and effects of post weld heat treatments (PWHT) were surveyed. Micro examination and micro hardness tests were applied to characterize the general microstructure. Grain size measurements were done for E355. Hardness profiles, tension and impact toughness properties were designated via mechanical tests. Fatigue behavior was surveyed and general fracture characteristics were determined via scanning electron microscopy (SEM) examinations. According to study it was concluded that successful weld joint could be formed between the materials by suitable preheating. Formation of ledeburitic white cast iron and martensite in heat affected zone of cast piece was witnessed. It was possible to lower high hardness values of these phases gradually by increasing post weld heat treatment temperatures. The weld joint behaved superior to the base metals under tension and fatigue tests. In tension tests failure occurred at E355 base metal whereas fatigue loading resulted failure at GJS600-3. A great impact toughness variation was determined between two pieces. It is seen that heat treatments had minor effect on mechanical test results except hardness. The study was concluded that a useful fusion weld joint between these materials can be built.

Les travaux exposés dans cet ouvrage décrivent la synthèse, la caractérisation microstructurale et les propriétés physiques de solutions solides nanolamellaires des phases MAX. Les phases Mn+1AXn (M : métal de transition, A : un métal des groupes IlIA ou IV A, et X: carbone ou azote) constituent une famille de nitrures et de carbures ternaires (n = 1 à 3), qui possèdent les meilleures propriétés des métaux et les meilleures propriétés des céramiques.Lors d'une première étape, nous nous concentrons sur la synthèse de solutions solides pures et denses de Ti2AICxNy par compression isostatique à chaud. Les variations des paramètres de maille sont étudiée et discutée an fonction du taux de substitution (carbone-azote) et du taux de lacune (sur le site X). Lors d'une seconde étape, nous étudions les propriétés mécaniques et les propriétés de transport électronique des solutions solides Ti2AICxNy et des phases Ti2AICx et Ti2AINy. La technique de nanoindentation pour déterminer la dureté et le module élastique en fonction du taux de substitution et de lacune. Nous démontrons que la substitution conduit à une amélioration des propriétés mécaniques tandis que l'introduction de lacune conduit à une détérioration de ces propriétés. La résistivité électrique augmente lorsque des lacunes et/ou un effet de substitution sont introduits. Dans le cas de la substitution, nous démontrons que le désordre introduit est faible et que seule la diminution du temps de relaxation explique l'augmentation de la résistivité (interaction électron-phonons). Dans le cas de l'introduction de lacunes, nous montrons que ces dernières conduisent à une modification du temps de relaxation et probablement à une modification de la densité de porteurs.Enfin, l'anisotropie des propriétés de transport électronique a été mise en évidence par des mesures de résistivité réalisée avec le courant électrique circulant dans le plan de base et avec le courant électrique circulant selon l'axe c. Nous démontrons les propriétés de transport dans le plan de base peuvent être comprises en utilisant un modèle à une bande et un mécanisme de conduction assuré par des électrons ayant le comportement de trous
The work discussed in this thesis concerns the synthesis, the microstructural characterization and the physical properties of nanolaminated MAX phase's solid solution. The Mn+1AXn phases (M: transition metal, A: IlIA or IV A group element, and X: either carbon or nitrogen) are a class of ternary nitrides and carbides (n=l to 3), which possess sorne of the best properties ofmetal and sorne of the best properties of ceramics.In a first step, we focus on the synthesis of highly pure and dense Ti2AICxNy solid solutions by hot isostatic pressing. The influence of the substitution of C atoms by N atoms and the influence of vacancy content on the solid solution lattice parameters is discussed. In a second step, we investigate the mechanical and transport properties of Ti2AICxNy solid solutions and oftheir related Ti2AICx and Ti2AINy end-members. Hardness and elastic modulus has been studied using nanoindentation tests. It is demonstrated that sol id solution effect leads to a hardening effect whereas the presence vacancy leads to a softening effect. The electrical resistivity is shown to increase with vacancy content and substitution rate. Such an effect is discussed in terms of disorder and relaxation time variation. Finally, the anisotropic transport properties of MAX phases is studied and discussed. The anisotropy of transport properties has been evidenced by direct measurement of the resistivity along the basal plane and along the c-axis. It is demonstrated that transport property in the basal plane can be understood in the framework of a single band model with hole-like states as charge carrier

L'objectif de cette thèse était de contribuer au développement et à la compréhension des phénomènes physiques pilotant les évolutions microstructurales et les propriétés mécaniques des aciers Dual-Phase. Dans un souci d'utilisation sur les lignes de production industrielle, le développement d'outils numériques à champ moyen a été retenu, utilisables pour une large gamme de compositions chimiques et de cycle thermique. Une calibration des modèles sur des aciers ternaires provenant de coulées de laboratoire a été effectuée avant leur validation sur deux nuances industrielles. Un modèle de prédiction de la cinétique de recristallisation (MiReX) a été développé à partir de la composition chimique, du taux de réduction par laminage à froid et de leur état de précipitation prévu en utilisant un couplage avec un logiciel de prédiction de la cinétique de précipitation (Preciso). Un modèle pour la transformation de phase entre la ferrite et l'austénite, basé sur la minimisation de l'énergie du système global (GEM), a été proposé pour les traitements isothermes et anisothermes. Il reproduit la cinétique de la formation de l'austénite sur un cycle thermique industriel comprenant une rampe de chauffage et un maintien pour les systèmes quaternaires en tenant compte de la dissolution de la cémentite. Un modèle permettant de prédire la température du début de la transformation martensitique a également été développé pour les aciers bi-phasés, en tenant compte de la taille des grains austénitiques et de l'enrichissement en carbone et en manganèse à l'interface. Enfin, une nouvelle loi d'interaction basée sur les observations lors d'un essai de traction ex-situ a permis d'améliorer une prédiction modèle des propriétés mécaniques des aciers DP. La considération de l'étape de revenu de la martensite sur les propriétés des aciers DP a été proposée après l'analyse des mécanismes métallurgiques impliqués dans le revenu par des mesures de pouvoir thermoélectrique et de dureté
The aim of this thesis was to contribute to the development and understanding of the physical phenomena driving the microstructural evolutions and the mechanical properties of Dual-Phase steels. In a concern of use on industrial production lines, the development of physics-based mean-field numerical tools was retained, usable for a wide range of chemical composition and thermal cycle parameters. A calibration of the models on ternary steels from laboratory castings was carried out before their validation on two industrial grades. A model for predicting recrystallization kinetics (MiReX) was developed based on the chemical composition, the reduction ratio by cold rolling and their predicted precipitation state using a coupling with a software for predicting precipitation kinetics (Preciso). A model for the phase transformation between ferrite and austenite, based on the minimization of the global system energy (GEM), has been proposed for isothermal and anisothermal treatments. It reproduces the kinetics of austenite formation on an industrial thermal cycle including a heating ramp and holding for quaternary systems taking into account the dissolution of the cementite. A model for predicting the temperature of the beginning of martensitic transformation has also been developed for two-phase steels, taking into account austenitic grain size and carbon and manganese enrichment at the interface. Finally, a new interaction law based on observations during an ex-situ tensile test has allowed the improvement of a prediction model of the mechanical properties of DP steels. A consideration of the tempering step of martensite on the mechanical properties of DP steels has been proposed after the analysis of metallurgical mechanisms involved in tempering bythermoelectric power and hardness measurements

L’utilisation des composites à matrice céramiques (CMC), et particulièrement les matériaux SiC/SiC, à la place des alliages métalliques dans les moteurs aéronautiques peut permettre de réduire leur consommation. Ils possèdent en effet des propriétés physiques et mécaniques très intéressantes à haute température : faible densité, résistance élevée aux chocs thermiques et rupture non-fragile. Dans ces matériaux, une fine couche est insérée entre les fibres et la matrice : l’interphase. Le nitrure de bore pyrolytique est le matériau d’interphase de choix pour les applications visées. La bibliographie souligne bien la nécessité et la difficulté de « contrôler » l’intensité des liaisons interfaciales fibres/matrice (F/M) grâce à l’interphase. Mais l’influence exacte de la cristallinité et de l’épaisseur des interphases de type BN sur son contrôle, et par conséquent sur le comportement mécanique final du CMC industriel est encore mal connue.Une première problématique abordée dans cette thèse est l’échelle du CMC de laboratoire à utiliser. En effet jusqu’ici, les matériaux modèles les plus couramment employés sont les mini- et les micro- composites 1D. Ils peuvent être élaborés facilement et rapidement par dépôt chimique en phase vapeur mais ne rendent pas compte de phénomènes inévitablement présents au sein du composite industriel. C’est pourquoi l’utilisation de nouveaux matériaux modèles 2D comme les « monostrates » comprenant un pli de tissu, l’interphase BN et une matrice de SiC dont la porosité peut être comblée par du Si comme dans le cas des CMC industriels est plus pertinente et est proposée. Cependant, de par la faible épaisseur des éprouvettes, les protocoles de caractérisation et de tests mécaniques ont dû être revus. Il s’agit notamment de caractériser la liaison F/M par deux paramètres : la contrainte de cisaillement de la liaison interfaciale (τi) et le module de cisaillement du matériau (G12).Pour la partie mécanique, des essais de traction monotone et cyclée dans l’axe des fibres du renfort (exploités à l’aide de modèles micromécaniques), des essais de cisaillement Iosipescu, ainsi que des essais de push out ont été mis au point et exploités. Des analyses de micro caractérisation par microscopie électronique (MEB, FIB-MEB, MET) ont été réalisées avant et après essais mécaniques avec des interphases de différentes configurations afin de relier les différences de microstructures et d’épaisseurs aux chemins de fissuration matricielle et aux comportements mécaniques macroscopiques des composites. La liaison F/M la plus forte est notamment obtenue quand le degré de cristallisation et l’anisotropie structurale du BN sont peu élevés, pourvu que l’épaisseur de l’interphase soit suffisante
The use of ceramic matrix composites (CMC), and particularly SiC/SiC materials, in place of metal alloys in aircraft engines has the potential to reduce their fuel consumption. They have very interesting physical and mechanical properties at high temperatures: low density, high resistance to thermal shock and non- brittle failure. In these materials, a thin layer is inserted between the fibres and the matrix: the interphase. Pyrolytic boron nitride is the interphase material of choice to achieve the desired applications. The bibliography clearly highlights the need and difficulty of "controlling" the intensity of fibre/matrix interfacial bonds (F/M) thanks to the interphase. But the exact influence of the crystallinity and thickness of BN-type interphases on its control, and consequently on the final mechanical behaviour of the industrial CMC, is still insufficiently known.A first issue addressed in this thesis is the scale of the CMC to be used in the laboratory. Indeed, so far, the most commonly used model materials are 1D mini and micro composites. They can be easily and quickly prepared by chemical vapour deposition but do not account for phenomena inevitably present in the industrial composite. Therefore, the use of new 2D model materials such as "monostrates" comprising a single-ply woven, the BN interphase and a SiC matrix in which the porosity can be filled with Si as in the case of industrial CMCs is more relevant and is proposed. However, due to the thinness of the specimens, the characterization and mechanical testing protocols had to be reviewed. This involves characterizing the F/M bond by two parameters: the shear stress of the interfacial bond (τi) and the shear modulus of the material (G12).For the mechanical part, monotonic and cycled tensile tests in the fibre axis of the reinforcement (operated using micromechanical models), Iosipescu shear tests, as well as push-out tests were developed and used. Micro characterization analyses by electron microscopy (SEM, FIB-SEM, TEM) were performed before and after mechanical tests with interphases of different configurations in order to link the differences in microstructures and thicknesses to the matrix cracking paths and macroscopic mechanical behaviours of the composites. The strongest F/M bond is obtained notably when the degree of crystallization and structural anisotropy of the BN are low, provided that the interphase is thick enough

La synthèse par métallurgie des poudres de nickel à grains ultrafins (UFG) a été effectuée, et l’effet de l’affinement de la microstructure sur le comportement mécanique et les propriétés physiques a été étudié. La possibilité de coupler le broyage et le frittage flash est étudiée avec des résultats prometteurs. Des échantillons de haute densité avec des tailles de grains d = 0.65 – 4 µm, caractérisés par une fraction élevée des joints de grains Σ3 et un faible niveau de contrainte ont été synthétisés. Les propriétés mécaniques des échantillons UFG montrent une bonne combinaison ductilité-résistance mécanique, avec un impact mineur des porosités présentes. L’étude de l’influence de la taille de grain dans le régime UFG sur les propriétés mécaniques montre une limite d’élasticité supérieure à celle attendue et une capacité d’écrouissage plus faible. Ces observations sont cohérentes avec la microstructure déformée à rupture, étudiée par diffraction d’électrons rétrodiffusés et microscopie électronique en transmission. Une haute diffusivité, mesurée par des expériences de traceurs radioactifs, montrent des profils de pénétration très différents liés aux structures de porosités diverses présents dans les échantillons. Ces différentes structures sont aussi responsables de la densification rétrograde observée, uniquement pour les échantillons frittés à partir de poudres broyées
The present manuscript concerns the synthesis of ultrafine grained (UFG) Ni by powder metallurgy, and the study of the influence of UFG microstructures on the mechanical behavior and physical properties. The possibilities of coupling ball milling and Spark Plasma Sintering are presented showing promising results. Highly dense homogeneous specimens are obtained, with average grain sizes d = 0.65 - 4 µm, and microstructures highlighted by a high fraction of Σ3 grain boundaries dependent on grain size. The mechanical properties in tensile testing for UFG samples are evaluated showing a good combination of strength and ductility, with little impact from porosities, the major drawback of powder metallurgy. The influence of grain size in the UFG regime on the mechanical properties is investigated, showing strength values that deviate from the expected behavior for grain refinement. Likewise, a reduced strain hardening capacity is depicted which correlates to the microstructural observations performed on the deformed state. High diffusivity measured by means of radiotracer experiments is observed in the sintered samples, displaying different penetration profiles that relate to diverse porosity structures. Such structures are also responsible for retrograde sintering observed exclusively in samples processed from BM powders

Natural Gas with more than 80% methane has a liquefaction temperature around -165 ° C. Temperature at which the gas reduces its volume by a factor of 600/1. This operating temperature makes the use of ferritic materials unfeasible because they have a brittle fracture mode below a critical temperature, called transition temperature. For the construction of large containers, the most commonly used material is Steel A-553-T1 which has a nominal content of 9% nickel and whose crystalline structure is formed by a martensite matrix with some austenite reacted. This microstructure is achieved through double heat treatment; Tempering and tempering. To weld this steel, for this application, it is not possible to use materials of 9% Ni feed, in view of the impossibility of performing the necessary thermal treatments to achieve homogeneity of properties. On the other hand, the austenitic welding consumables present a ductile behavior with a high energy absorbed even to -196ºC and within these NiCrMo family nickel base alloys have a high mechanical strength, and a coefficient of thermal expansion close to the steel 9% Ni. Within this family of nickel base alloys, the Hastalloy C-276 alloy has been used, which increases its mechanical strength by solid solution, the main alloys being chromium and molybdenum both about 15% and with 2.5% of Tungsten and 5% iron. Although this is an alloy that is essentially single-phase Gamma, the last liquid is usually transformed into carbides or TCP phases as the pass Mu and P. These three phases have a very close composition so that their identification through the EDX is not possible . Currently, most of the tanks being built have a storage capacity between 150,000 and 200,000, so the sheet thickness of 1 was ferrule between 27 to 50mm, implying that the welds are multi-pass, Requiring between 16 and 30 passes to fill the joints of this first ferrule. Normally the vertical joints are welded with manual or semi-automatic processes while the horizontal joints are welded with automatic submerged arc process. According to the atmospheric tank design standards for cryogenic storage, the thickness of the sheets is determined by the Maximum Admissible Efforts that are calculated from the mechanical strength of the weakest structural element, the base metal or the weld joint. In the case of welding, the mechanical strength of the weld is determined from the cylindrical specimen tensile test obtained from the deposited metal, from the homologation coupon of the welding process. During the homologation of the manual or semi-automatic procedures, the results obtained in the tests of longitudinal and transverse traction are equivalent. In the case of automatic welding In addition to the low values obtained from the cylindrical tensions of the weld metal of the horizontal joints with respect to the transverse tensions, it is very often observed that an important difference in the resistance presented by the different tensile tests of the same welded specimen , These differences being much greater than the observed difference between two experimental conditions. Prior to this experimental work, 6 other tests and a few procedural approvals were carried out, in which sheets of 12, 21, 26.5 and 27 mm of thickness were used, with 2.4 mm and 1.6 mm threads always of the classification AWS A5.14 ER-NiCrMo-4, corresponding to the Hastalloy C-276 alloy, with different fluxes, stiffening levels, cylindrical probe diameter etc. This PhD work was carried out on the seventh test carried out in the summer of 2008 at Lincoln Electric Cleveland, where 4 fluxes, 2 wire diameters, alternating and continuous current and two voltage levels were tested, with an experimental design 23 with each flux. As all DOE tests were performed, 8 specimens were welded per flux, and a total of 32 specimens were welded. The purpose of this test was to select the best flux wire pair, and determine the optimum parameters to maximize the mechanical strength of the weld metal. The base material used in this experiment were A553 T1 steel sheets, with 9% Ni and annealed and tempered with a thickness of 21mm. The joint design of these specimens is asymmetrical and unbalanced "X" with a 1mm bead and a 2mm spacing. Following the actual joint design of the production plates. In order to prevent the melting bath from being picked up, a flux backing was placed. The tests performed on each specimen were as follows: Cylindrical traction welding metal: 4 per test pieces Charpy V Notch at -196 ° C Macro General Chemical Analysis, performed on the side faces of the macros. Chemical analysis on tensile specimens. Microhardness tests Vickers and Knoob. The wire-flux pair selected in these tests has been used for the welding of eight tanks: three in Spain: two in Gijón 2011-2013 and one in Bilbao 2014-2015; A tank in Chile, 2011-2013 and four other tanks in China, 2011-2013. With this pair, good results have been obtained in the approvals of welding procedures of these projects, both in the transversal tensions and in the cylindrical tensions, fulfilling the requisites of resistance necessary in each project. During the production, a welding metal with very few inclusions of slag has been deposited, presenting good degreasing and degassing. The objective of this research is to determine the factors that produce the variability of results in the tensile tests, correlating the structural and micro structural factors with the mechanical properties of the deposited metal, in order to maximize its mechanical resistance.
Hasta mediados del siglo XX, el gas era considerado como un residuo de la explotación petrolera con importantes barreras tecnológicas y económicas para su procesado y comercialización por lo que gran parte de este era quemado en los países de producción. Desde finales del siglo XX, el aumento de la demanda de energía sumado con los altos niveles de contaminación producido por la quema de petróleo y carbón hicieron que se desarrollen las tecnologías y normas para el transporte seguro y rentable de los gases derivados del petróleo. Desde entonces, El gas natural ha tenido una penetración muy importante en la cadena de consumo debido a su alto poder energético y a la baja cantidad, comparada con el petróleo y carbón, de residuos, sólidos y gaseosos, que han hecho que este se perciba como un combustible limpio. El transporte de este producto se realiza en estado líquido, por medio de 2 tecnologías, presurización o por enfriamiento, LPG y LNG. La primera requiere de plantas de presurizado y gasoductos. Las distancias económicamente rentables para la conducción por gasoducto rondan la docena de miles de kilómetros, requiriendo de plantas de re presurización a lo largo del gasoducto. Cuando la distancia entre los productores y los consumidores que muy grande la licuación por enfriamiento a presión atmosférica es la opción más económica y segura. En este caso en, el gas obtenido del pozo se conduce hasta la planta de licuefacción donde se realiza la separación de los distintos componentes sólidos, líquidos y gaseosos, por procesos de filtración y licuación diferencial. El Gas natural producto de este proceso se almacena temporalmente en un tanque de LNG mientras en cargado en el barco que lo transportará a destino. Una vez en destino el barco descarga a un tanque de LNG, de donde se suministra a la planta de regasificación. De esta el gas es canalizado a alta presión por los gasoductos de distribución o a las plantas de generación eléctrica. El Gas Natural, con más de un 80% de metano tiene una temperatura de licuefacción alrededor de los -165ºC. Temperatura a la cual el gas reduce su volumen por un factor de 600/1. Esta temperatura de operación hace inviable el uso de materiales ferríticos, debido a que estos presentan un modo de fractura frágil por debajo de una temperatura crítica, llamada de transición. Para la construcción de grandes contenedores el material más usado en es Acero A-553-T1 que tiene un contenido nominal de 9% de níquel y cuya estructura cristalina está formada por una matriz de martensita con algo de austenita revenida. Esta microestructura se consigue a través del doble tratamiento térmico; de temple y revenido. Para soldar este acero, para esta aplicación, no se pueden usar materiales de aportes similares al 9%Ni en vista de la imposibilidad de realizar los tratamientos térmicos necesarios para conseguir la homogeneidad de propiedades. Por otro lado, los consumibles de soldadura austeníticos presentan un comportamiento dúctil con una alta energía absorbida incluso a -196ºC y dentro de estos las aleaciones base níquel de la familia NiCrMo presentan una alta resistencia mecánica, y un coeficiente de expansión térmica cercano a del acero 9%Ni. Dentro de esta familia de aleaciones base níquel se ha usado la aleación Hastalloy C-276, la cual incrementa su resistencia mecánica por solución sólida, siendo los principales aleantes el cromo y molibdeno ambos alrededor de 15%, y con un 2,5% de tungsteno y un 5% de hierro. Aun que ésta es una aleación que en esencia es monofásica Gamma, el último liquido suele transformarse en carburos o fases TCP como la pase Mu y P. Estas tres fases tienen una composición muy cercana por lo que su identificación a través del EDX no es posible. Actualmente, la mayoría de los tanques que se están construyendo tienen una capacidad de almacenamiento entre los 150.000 y 200.000, por lo que el espesor de chapa de la 1 era virola de entre 27 a 50mm, lo que implica que las soldaduras son multi pasada, necesitándose entre 16 y 30 pasadas para rellenar las juntas de esta primera virola. Normalmente las juntas verticales se sueldan con procesos manuales o semi-automáticos mientras que las horizontales se sueldan con proceso automático de arco sumergido. Según las normas de diseño de tanques atmosféricos para almacenamiento criogénico, el espesor de las chapas está determinado por los Esfuerzos Máximos Admisibles que se calculan a partir de la resistencia mecánica del elemento estructural más débil, el metal base o la junta de soldadura. En el caso de la soldadura, la resistencia mecánica de ésta se determina a partir del ensayo de tracción con probeta cilíndricas obtenidas del metal depositado, a partir del cupón de homologación del procedimiento de soldadura. Durante la homologación de los procedimientos manuales o semi automáticos los resultados obtenidos en los ensayos de tracción longitudinal y transversal son equivalentes. En el caso de la soldadura automática de las juntas horizontales, los resultados obtenidos de las tracciones transversales siempre han sido muy superiores a los resultados de las tracciones cilíndricas. Además de los bajos valores que se obtienen de las tracciones cilíndricas del metal de soldadura de las juntas horizontales con respecto a las tracciones transversales, con mucha frecuencia se observa que una importante diferencia en la resistencia presentada los diferentes ensayos de tracción de una misma probeta soldada, siendo estas diferencias mucho mayores que la diferencia observada entre dos condiciones experimentales. Situación que dificulta la correcta interpretación de los resultados de los diferentes ensayos o pruebas realizadas. Antes de este trabajo experimental se realizaron otros 6 ensayos y unas cuantas homologaciones de procedimientos, en las cuales se usaron chapas de 12, 21, 26,5 y 27mm de espesor, con hilos de 2,4mm y 1,6mm siempre de la clasificación AWS A5.14 ER-NiCrMo-4, correspondiente a la aleación Hastalloy C-276, con diferentes fluxes, niveles de rigidización, diámetro de probeta cilíndrica etc. Este trabajo de doctorado se he realizado sobre el séptimo ensayo realizado en verano de 2008 en las instalaciones de Lincoln Electric Cleveland, en que se probaron 4 fluxes, 2 diámetros de hilo, corriente alterna y continua y dos niveles de voltaje, desarrollándose un diseño experimental 23 con cada flux. Como se realizaron todas las pruebas correspondientes al DOE, se soldaron 8 probetas por flux, y en total 32 probetas. El objetivo de este ensayo era seleccionar el mejor par alambre fundente, y determinar los parámetros óptimos para maximizar la resistencia mecánica del metal de soldadura. El material base usado en este experimento fueron chapas de acero A553 T1, con 9%Ni y templadas y revenidas con un espesor de 21mm. El diseño de junta de estas probetas es en “X” asimétrica y desbalanceada con un talón de 1mm y una separación de 2mm. Siguiendo el diseño de junta real de las chapas de producción. Con el fin de evitar que el baño de fusión se descuelgue se colocó un respaldo de flux. Los ensayos realizados a cada probeta han sido los siguientes: Tracciones Cilíndricas de metal de soldadura: 4 por probetas Charpy V Notch a -196ºC Macro Análisis Químico General, realizado sobre las caras laterales de las macros. Análisis Químico en las probetas de tracción. Ensayos de microdureza Vickers y Knoob. El par alambre-fundente seleccionado en estas pruebas ha sido usado para el soldeo de ocho tanques: tres en España: dos en Gijón 2011-2013 y uno en Bilbao 2014-2015; un tanque en Chile, 2011-2013 y otros cuatro tanques en China, 2011-2013. Con este par se han conseguido buenos resultados en las homologaciones de procedimientos de soldadura de estos proyectos, tanto en las tracciones transversales como en las tracciones cilíndricas, cumpliendo con los requisitos de resistencia necesarios en cada proyecto. Durante la producción se ha depositado un metal de soldadura con muy pocas inclusiones de escoria, presentando buen desescoriado y desgasificado. El objetivo de este trabajo de investigación es determinar los factores que producen la variabilidad de resultados el los ensayos de tracción, correlacionando los factores estructurales y micro estructurales con las propiedades mecánicas del metal depositado, con el fin de maximizar su resistencia mecánica.

In the present study, the applicability of the hybrid plasma arc welding, in which a keyhole is responsible of deep penetration and a filler wire electrode supplies a high deposition rate, was examined. The microstructural evolutions in grade 2205 duplex stainless steel plates joined by keyhole and melt-in techniques were investigated. The specimens obtained from welded plates having thickness of 8 mm were examined via optical and scanning electron microscopy. Metallographic investigations were supported by X-ray diffraction and energy dispersed spectra analyses by characterizing the phases formed after welding. Impact toughness properties, hardness profiles, and crack propagation behavior of welding zones were quantitatively and qualitatively compared for mechanical characterization. Fracture characteristics were determined via scanning electron microscopy examinations. It was observed that single-pass HPA weldment seemed to be free of secondary austenite precipitation in acicular form, which is inevitable in multi-pass conventional arc welding methods. Besides &delta
-ferrite was successfully kept under 70%, which is presented as a limit to not to deteriorate the mechanical properties of DSS. High linear welding speed and high power density supplied by HPAW presented narrower weld metal and heat affected zone with not only lower hardness but also higher impact toughness energies. Synergic effect of the keyhole formed by a plasma arc and the metal transfer supplied by gas metal arc gave reasonable dilution in the weld metal. Furthermore, fatigue crack growth tests revealed that crack propagation rates in HPAW joints were comparable to GMAW joints.

Among the commercial aluminum alloys, aluminum 319 (Al-7wt%Si-4wt%Cu) type alloys are popularly used in automobile engine parts. These alloys have good casting characteristics and excellent mechanical properties resulting from a suitable heat treatment. To get a high strength in the 319 type alloys, grain refining, reducing the porosity, solid solution hardening, and precipitation hardening are preferred. All experimental variables such as solidification condition, composition, and heat treatment are influence on the precipitation behavior; however, precipitation hardening is the most significant because excess alloying elements from supersaturated solid solution form fine particles which act as obstacles to dislocation movement. The challenges of the 319 type alloys arise due to small size of precipitate and complex aging response caused by multi components. It is important to determine the chemical composition, crystal structure, and orientation relationship as well as precipitate morphology in order to understand the precipitation behavior and strengthening mechanism. In this study, the mechanical properties and microstructure were investigated using transmission electron microscopy and three dimensional atom probe tomography. The Mn and Mg effects on the microstructure and mechanical properties are discussed with crystallographic study on the iron intermetallic phases. The microstructural evolution and nucleation study on the precipitates in the low-Si 319 type aluminum alloys are also presented with sample preparation and analysis condition of TEM and 3DAP tomography.

Cette étude porte sur la caractérisation des mécanismes d'adhésion et d'endommagement de produits colaminés à froid, afin de pouvoir proposer des procédés optimisés. Celle-ci s’inscrit dans le cadre de la participation au projet MeGaN (pour Module Electronique GaN) qui porte le développement de nouvelles technologies de modules de puissances à base de composants à grand gap « GaN », compatibles avec des applications hautes températures et hautes tensions. Notre travail porte essentiellement sur l’intégration d’un substrat innovant (i-TBC), un composite architecturé cuivre invar doté de ponts thermiques pour un bon compromis dialatation thermique/conductivité thermique pour accueillir les composants électroniques développés dans le cadre de ce projet. Ainsi, une première partie de ce travail est axée sur la caractérisation microstructurale du substrat i-TBC durant les étapes de son élaboration, l’objectif est de comprendre l’impact du procédé de colaminage sur la formation de l’adhésion des interfaces de cuivre dans les ponts thermiques. On a ainsi mis en évidence que la recristallisation de grains et la microstructure continue à travers l'interface Cu-Cu était garante d'une bonne adhérence de celle-ci. Dans la seconde partie, nous nous sommes focalisés sur la caractérisation de la tenue mécanique du substrat i-TBCdans des conditions de cycles thermiques passifs. Pour ce faire, des essais de fatigue thermique et de choc thermique nous permettent de déterminer la sensibilité de la tenue mécanique des interfaces à la fois à l’amplitude et à la vitesse de variation de température. La conclusion de cette étudeest que les paramètres de colaminage doivent permettent un compromis entreadhérence du pont Cu-Cu et des interfaces Cu-Invar pour augmenter significativement la durée de vie du composant. Enfin, nous avons procédé à l'analyse thermo-mécanique des propriétés intrinsèques du substrat seul et de l'assemblage électronique complet. Les propriétés intrinsèques ont été établies en termes de comportement mécanique du composite Cu-Invar et d'endommagement des interfaces sous la forme de propagation d'une fissure à l'interface Cu-Cu
This work is part of ‘MeGaN” project which focuses on the development of new power module technologies based on "GaN" wide gap components, compatible with high temperature and high voltages applications. In This study, a new substrate an innovative thermal bridge composite (i-TBC) has been developed, obtained by roll bonding of two copper sheets separated by perforated invar. The i-TBC is an “architectured” composite material that combines good thermal conductivity associated to copper and limited CTE due to the presence of invar. A particularity of the i-TBC consists of the formation of copper bonding area through the invar perforations during the cold rolling called thermal bridges. These thermal bridges, ensure good thermal conductivity of the i-TBC. Thus, a first part of this work focuses on the microstructural characterization of the i-TBC substrate during the stages of its elaboration, the objective is to understand the impact of the elaboration steps on the adhesion formationof the copper interfaces. in thermal bridges. It was thus demonstrated that the cold welding obtained along the interface Cu-Cu was a guarantee of good adhesion. In the second part, we focused on the characterization of the mechanical strength of the i-TBC substrate under passive thermal cycling conditions. To do this, tests of thermal fatigue and thermal shock allow us todetermine the sensitivity of the mechanical resistance of the interfaces to both the amplitude and the speed of temperature variation. The conclusion of this study is that the parameters of cold rolling must allow a compromise between adhesion of the Cu-Cu bridge and Cu-Invar interfaces to significantly increase the lifetime of the substrate. Finally, a finite element analysis (FEA) wasperformed. firstly, the thermal modeling validated the thermal performance of the i-TBC substrate in an electronic assembly.Then, the intrinsic properties were established in terms of mechanical behavior of the Cu-Invar composite and deterioration of the interfaces in the form of propagation ofa crack at the Cu-Cu interface

La fabrication de cavités SRF à hautes performances est essentielle pour augmenter l’énergie de collision dans de nouveaux accélérateurs de particules. L’utilisation de procédés de fabrication à haute vitesse, comme l’électro-hydro formage, peut être bénéfique, mais requiert une compréhension détaillée des propriétés mécaniques des matériaux déformés à haute vitesse et de l’impact sur leurs microstructures. Les objectifs de cette thèse sont d’étudier les propriétés mécaniques de monocristaux de niobium et de tôles polycristallines de niobium à haute pureté et de cuivre OFE déformés à des taux de déformation d’environ 10−4 s−1 à 103 s−1. Les résultats de cette étude sont séparés en deux parties selon le matériau étudié. En partie I, la caractérisation de monocristaux de niobium se concentre sur les propriétés mécaniques en traction et en compression à des taux de déformation d’environ 10−4 s−1 à 103 s−1 et sur la microstructure (MEB, EBSD, MET et nanoindentation) d’éprouvettes déformées. Les effets de l’orientation des cristaux, la vitesse de déformation et de la direction de chargement sur les propriétés mécaniques, la rotation des cristaux et la structure de dislocations sont présentés. En partie II, la formabilité de tôles polycristallines de niobium et de cuivre OFE sont présentées à l’aide de courbes limites de formage (CLF) obtenues à un taux de déformation quasi-statique. Les CLFs de ces matériaux offrent des données importantes pour les techniques de formage conventionnel, tels que l’emboutissage et le repoussage. Cette seconde partie présente aussi les propriétés mécaniques d’éprouvettes de cuivre OFE recuites et de tôles de niobium polycristallines soudées par faisceau d’électron et déformées en traction et en compression à des taux de déformation de 10−3 à 103 s−1
Manufacturing of superconducting radiofrequency (SRF) cavities with high performances is paramount to increase the collision energy in new particle accelerators. The use of high-speed sheet forming techniques, such as electro-hydraulic forming, can be beneficial, but requires a detailed understanding of the mechanical properties of the materials being deformed and the consequence on their microstructure. This thesis focuses on the characterization of high-purity niobium single crystals, polycrystalline niobium sheets, and polycrystalline OFE copper sheets. The results from this study are separated in two parts. In Part I, the characterization of niobium single crystals focused on the mechanical properties in tension and compression at strain rates of 10-4 to 103 s-1 and on the microstructure (analyzed using SEM, EBSD, TEM, and nanoindentation) of the deformed specimens. The effect of crystal orientation, strain rate, and loading direction on the mechanical properties, the crystal rotation, and the dislocation substructures are presented. In Part II, the forming limit diagram (FLD) of polycrystalline niobium sheets and OFE copper sheets were measured at a quasi-static strain rate. The FLDs of those materials should provide important data for manufacturers using conventional techniques, such as deep-drawing and spinning. The second part also presents the mechanical properties of electron beam (EB) welded polycrystalline niobium sheets and OFE copper sheets deformed in tension and compression at strain rates of 10-3 to 103 s-1

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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)
Titânio e suas ligas são utilizados em implantes biomédicos devido a suas excelentes propriedades de volume, como resistência mecânica. Porém, tais propriedades estão diretamente ligadas à microestrutura, que por sua vez depende do processamento. O objetivo deste trabalho foi avaliar a microestrutura e a evolução da recristalização da liga Ti30Ta deformada a frio, após tratamento térmico. Os lingotes da liga Ti30Ta foram obtidos em um forno de fusão a arco voltaico. A liga foi tratada em um forno tubular sob vácuo a 950ºC por 24 horas para garantir a homogeneidade química e então foi forjada a frio em barras de 10 mm de diâmetro. As amostras foram solubilizadas a 900ºC por 2 horas e temperadas em água. Discos de 3 mm de espessura foram usados para análise. Para a avaliação da recristalização, foi realizado um tratamento térmico a 750ºC, 800ºC e 850ºC por 12 horas seguido de têmpera em água. A morfologia, composição e estrutura foram investigadas usando microscopia óptica, microscopia eletrônica de varredura, microscopia eletrônica de transmissão e difração de raios X. As análises de microscopia e difração de raios X indicaram a presença apenas da fase martensítica α'' na amostra solubilizada e a presença das fases α'' e α' nas amostras recristalizadas. Os ensaios mecânicos mostraram que houve uma melhoria nas propriedades mecânicas após a recristalização, sendo a temperatura de 750° que teve maior influência, porém houve também um incremento no módulo de elasticidade, o que é indesejado.
Titanium and its alloys have been used in biomedical devices due their excellent bulk properties, such as mechanical strength. However, these properties are very sensitive of microstructures which depend of the processing. The purpose of this study was to evaluate the microstructure and evolution of recrystallization of the could-swaged Ti30Ta alloy after heat treatment. Ingots of Ti30Ta alloy were obtained in arc melting furnace. Alloys were treated at 950° C for 24 hours for chemical homogenization in vacuum tubular furnace and cold worked by swaging into rods with 10 mm in diameter. Samples where then solution treated at 900° C for 2 hours and water quenched. Discs with 10 mm in diameter and 3 mm in thickness were used for analysis. For recrystallization evaluation, heat treatment was carried out at 750º C, 800º C and 850º C for 12 hours followed by water quenching. The morphology, composition and structure were investigated using optical microscopy, scanning electron microscopy, transmission electron microscopy and XRD (X-rays Diffraction Analysis). The microscopy and XRD analysis indicate the presence of the martensitic phase α'' for the solution treated sample and the presence of both α'' and α' phases for the recrystallized samples. Mechanical tests shows an increase in the mechanical strength to all the samples after recrystallization, with the temperature of 750° being the most impacting, however, it led an unwanted increase to the elastic modulus.

Microstructural and mechanical characterizations of heat treatable Al-Mg-Si-Cu based wrought aluminum alloys have been studied. The aim of this work was to produce fine grained, high strength alloy by adjusting processing conditions: deformation, solutionizing and aging. First, primary characterization was carried out via SEM-EDS analyses and tensile tests. Then an extensive experimental study has been carried out on two sets of samples. The first set has been studied to determine the ideal conditions for solutionizing and aging processes by analyzing the variation of hardness with different solutionizing and aging time and temperature. The second set, have first been mechanically deformed by swaging at four different deformations and four different temperatures, then heat treated. The hardness measurements have been carried out before and after solutionizing and also after aging. Finally, recrystallization behavior has been investigated by measuring grain size before and after solutionizing treatment using image analyzer software. The initial characterizations showed that Mg2Si and complex iron, manganese bearing intermetallics were the primary particles observed in the &
#945
-Al matrix. Nearly 140HB hardness could be obtained with solutionizing at 530°
C and aging at 175°
C for 8 hours which was determined as the optimum treatment for obtaining peak hardness. When shaping (deformation) was concerned
strength loss was the overall outcome of any hot or cold deformation before solutionizing
which was most probably due to the destruction of the initial microstructure. Improvement in the percent elongation was the promising aspect of this application. Strength loss was increased for samples deformed at higher temperatures and higher reductions.

O presente trabalho visou desenvolver ligas à base de Níquel-Cromo para prótese dentária, que confiram as melhores características metalúrgicas e químicas para posterior produção e aplicação. Para isto foram adquiridas amostras de ligas comerciais de 8 fabricantes. Os materiais foram caracterizados em termos de composição química, propriedades mecânicas, caracterização microestrutural, temperatura de fusão, coeficiente de expansão térmica, corrosão e biocompatibilidade. Para isto, utilizou-se as fluorescência de raios X (XRF-WDS), ICP-OES, ensaios de tração, ensaios de dureza, ensaios de corrosão, avaliação de citotoxicidade, microscopia eletrônica de varredura (MEV), microanálise eletrônica (EDS), difratometria de Raios X (DRX), análise térmica diferencial (DTA), dilatometria e avaliação de fluidez. A maioria dos resultados composicionais não condiz com as composições fornecidas pelos fabricantes. Os resultados mostraram claramente a presença de um eutético típico formado pelas fases Niss e NiBe nas ligas contendo Be, que pode ser considerado uma \"impressão digital\" da presença deste elemento nestas ligas. É possível verificar a formação de intermetálicos na região interdendritica para as ligas isentas de Be, sendo identificado um siliceto (Nb6Ni16Si7) em uma das ligas, que possui temperatura de inicio de fusão inferior ao eutético Niss e NiBe das ligas com Be. De uma forma geral, os resultados dos ensaios de dureza mostraram valores superiores de dureza para as ligas com Be. Foi escolhida uma das ligas comerciais para a reprodução em escala laboratorial (LAB) e industrial (IND). Estas ligas produzidas apresentaram as mesmas características microestruturais, químicas e físicas da liga comercial. Nos ensaios de fluidez, a liga LAB apresentou o mesmo comportamento da liga comercial. A liga IND apresentou limite de escoamento e módulo de elasticidade de ?e0,02%= 810MPa e E= 200 GPa, respectivamente. Os ensaios de corrosão mostraram comportamento passivo tanto para a liga comercial quanto para a liga IND em saliva artificial e enxaguante bucal. O ensaio de citotoxicidade da liga IND indicou comportamento não citotóxico em controle negativo (Chapa de Ti). Os resultados obtidos com a liga IND indicam a possibilidade de fabricação desta liga Ni-Cr para próteses dentárias em escala industrial.
This study aimed to develop nickel-chromium base alloys for dental prostheses with the best characteristics for subsequent production and application. Samples were acquired from eight manufacturers of commercial alloys. The materials were characterized in terms of chemical composition, mechanical properties, microstructure, melting temperature, coefficient of thermal expansion, corrosion and biocompatibility. For this, we have used X-ray fluorescence (XRF-WDS), ICP-OES, tensile tests, hardness testing, corrosion testing, evaluation of cytotoxicity, scanning electron microscopy (SEM), electron probe microanalysis (EDS) , X-ray diffraction (XRD), differential thermal analysis (DTA), dilatometry and evaluation of fluidity. Most of the chemical compositional determined in this study were not consistent with the compositions provided by the manufacturers. The results clearly showed the presence of a typical eutectic microstructure formed by Niss and NiBe in alloys containing Be, which can be considered a \"fingerprint\" of the presence of this element in these alloys. It was possible to verify the formation of intermetallics in the interdendritic region of the Be-free alloys. In one of the alloys this intermetallic was identified as (Nb6Ni16Si7), this alloy presenting a initial melting temperature below the eutectic melting of the Niss+NiB eutectic. Overall, the results of hardness tests showed superior hardness for alloys with Be. One of the commercial alloys was chosen for reproduction in laboratory (LAB) and industrial (IND) scales. The alloys produced showed the same microstructural characteristics, chemical and physical characteristics of the commercial alloy. In the fluidity tests, the LAB alloy showed the same behavior of the commercial alloy. The IND alloy presented yield strength and modulus of elasticity ?e0, 02% = 810MPa, E = 200 MPa, respectively. The corrosion tests showed passive behavior for both commercial and IND alloy in artificial saliva and mouthwash solutions. The cytotoxicity assay indicated that the IND alloy is not cytotoxic in the negative control (Ti plate). The results obtained with the IND alloy indicate the possibility of fabrication this alloy for dental prosthesis on an industrial scale.

Resumo: Os principais objetivos deste trabalho foram desenvolver estruturas bainíticas e multifásicas através de diversas rotas de tratamentos térmicos, visando as melhores combinações de propriedades mecânicas, fornecendo subsídios científicos/tecnológicos para as indústrias brasileiras. Em certos componentes de veículos aeroespaciais tem sido usado aço de baixa liga e ultra-alta resistência temperados e revenidos com elevada resistência devida a estrutura martensítica mas com baixa tenacidade. Uma melhoria na tenacidade é conseguida com redução controlada de resistência através do revenimento. O novo conceito, para aços avançados que combinam alta resistência com boa tenacidade, está simbolizado pelas microestruturas bainíticas e multifásicas. Neste projeto foi feito um estudo do efeito das microestruturas nas propriedades mecânicas de um aço AISI 4340. Foram analisadas diversas microestruturas, desde aquelas inteiramente bainíticas até microestruturas multifásicas com teores variados de ferrita, bainita, martensita e austenita retida. Os resultados foram comparados com aqueles obtidos por têmpera por resfriamento continuo e com as diversas rotas de transformação isotérmica. As combinações de propriedades mecânicas estão relacionadas com as frações volumétricas das fases e a bainita melhora significativamente a ductilidade do aço, mantendo a resistência elevada e melhorando a combinação resistência/ductilidade. O aço possui baixo coeficiente de encruamento e é possível conseguir resistências entre 1000 MPa e 1400 MPa com alongamento entre 13% e 25%, combinação esta superior aquelas encontradas para o mesmo aço quando temperado e revenido em óleo.
Abstract: The main goals of this study were to develop bainitic and multiphasic structures through several routes of heat treatment, in order to reach the better combination of mechanical properties, providing scientific/technological subsidies to Brazilian industries. In some of aerospatial vehicles components have been used quenched and tempered ultra-high-strength low-alloy steel where the martensitic structure is responsible for the high-strength and low toughness levels. Toughness improvements can be achieved by strength reduction control during tempering. The new concept for advanced steels, that combine high-strength and good toughness, is correlated with the bainitic and multiphasic microstructures. In this work the effect of microstructures on the mechanical properties of AISI 4340 steel. Has been analysed several microstructures, from those totally bainitic until multiphasics microstructures with various ferrite, bainite, martensite and retained austenite content. The results were compared with those obtained by quenching through continuous cooling transformation and several routes of isothermal transformation. The combinations of mechanical properties are related with volume fraction of present phases and the bainite improved significantly the toughness steel., keeping the high strength and improving the strength/toughness combination. This steel has low coefficient of hardness and is possible to achieve strengths between 1000 MPa e 1400 MPa with percentual elongation between 13% e 25%, this combination is better than that found to the same steel when quenched and tempered in oil.
Orientador: Tomaz Manabu Hashimoto
Coorientador: Antonio Jorge Abdalla
Banca: Marcelo dos Santos Pereira
Banca: Jorge Otubo
Mestre

The penchant towards development of high performance materials for light weighting engineering systems through various thermomechanical processing routes has been soaring vigorously. Friction stir processing (FSP) - a relatively new thermomechanical processing route had shown an excellent promise towards microstructural modification in many Al and Mg alloy systems. Nevertheless, the expansion of this process to high temperature materials like titanium alloys and steels is restricted by the limited availability of tool materials. Despite it challenges, the current thesis sets a tone for the usage of FSP to tailor the mechanical properties in titanium alloys and steels. FSP was carried out on three near beta titanium alloys, namely Ti6246, Ti185 and Tiβc with increasing β stability index, using various tool rotation rates and at a constant tool traverse speed. Microstructure and mechanical property relationship was studied using experimental techniques such as SEM, TEM, mini tensile testing and synchrotron x-ray diffraction. Two step aging on Ti6246 had resulted in an UTS of 2.2GPa and a specific strength around 500 MPa m3/mg, which is about 40% greater than any commercially available metallic material. Similarly, FSP on an ultra-high strength steel―Eglin steel had resulted in a strength greater than 2GPa with a ductility close to 10% at around 4mm from the top surface of stir zone (SZ). Experimental techniques such as microhardness, mini-tensile testing and SEM were used to correlate the microstructure and properties observed inside SZ and HAZ's of the processed region. A 3D temperature modeling was used to predict the peak temperature and cooling rates during FSP. The exceptional strength ductility combinations inside the SZ is believed to be because of mixed microstructure comprised of various volume fractions of phases such as martensite, bainite and retained austenite.

O emprego de compósitos na construção civil, como os de matriz cimentícia e pasta reforçada com fibras, tem se disseminado consideravelmente nos últimos anos. Uma grande variedade de fibras sintéticas, como o polipropileno, tem sido utilizada com sucesso para reforçar compósitos cimentícios. No entanto, o interesse mundial na utilização de produtos com menor impacto ambiental estimula a busca por materiais para substituir fibras sintéticas. As fibras vegetais, biodegradáveis, pode ser ótima alternativa devido à abundância, ao baixo custo, ao menor consumo de energia para sua produção e às propriedades mecânicas apropriadas. Fibra de curauá, planta nativa do estado do Amazonas, com plantações em escala comercial, é usada na fabricação de cordas, cestos ou como reforço em matrizes orgânicas. Suas propriedades mecânicas são semelhantes às de polipropileno. A tecnologia de extrusão é viável nas indústrias de fibrocimento, pois produz compósitos com matriz de alta densidade e ótimo empacotamento, baixa permeabilidade e boa adesão fibra matriz. No entanto, o processo de extrusão bem-sucedido de produtos cimentícios depende principalmente das propriedades reológicas do cimento fresco reforçado com fibras. As fibras vegetais podem promover o sequestro de água e interferir fortemente no escoamento, na coesão e no fluxo de pasta de cimento fresco. A incorporação de fibras vegetais influencia os materiais à base de cimento no estado fresco e afeta propriedades no estado endurecido. Neste contexto, o objetivo da pesquisa é avaliar a influência das fibras de curauá e de polipropileno em propriedades reológicas e em propriedades mecânicas da pasta de cimento fresco. Foram preparadas formulações sem fibras, como referência, e com 1 e 2% de reforço em massa, fibras com comprimento de 6 e 10 mm. Utilizaram-se duas técnicas reológicas: Squeeze flow e reômetro extrusor para analisar o fluxo de pastas cimentícias. Por meio de dados experimentais, como força/deslocamento, e de análise numérica da pressão do reômetro extrusor, foram determinados: tensão inicial de cisalhamento (σ0), limite de cisalhamento (τ0), tensão de escoamento (α) e tensão de cisalhamento (β). As propriedades mecânicas foram determinadas em máquina de ensaio MTS. Módulo de ruptura (MOR), tenacidade à fratura (TFT) e energia de fratura (EF) foram calculados. Os resultados reológicos indicam que a pasta cimentícia reforçada com fibras de curauá apresentou maior força, menor deslocamento e aumento da pressão de extrusão em fibras de curauá em relação às pastas cimentícias reforçadas com fibras de polipropileno. O comprimento das fibras influenciou mais o fluxo da mistura do que o teor de fibra. Compósitos cimentícios reforçados com fibra de polipropileno apresentaram melhores resultados mecânicos de MOR, TFT e EF em relação aos compósitos reforçados com curauá. Após os 200 ciclos de envelhecimento, os resultados mecânicos dos compósitos reforçados com as fibras de curauá diminuíram devido a mineralização das fibras. Os resultados de nanoindentação, como dureza e módulo elástico, aumentaram após os 200 ciclos. As metodologias aplicadas para avaliar o comportamento reológico e mecânico do fibrocimento durante a extrusão facilitará a futura transferência dessa tecnologia ao setor produtivo, com produtos potencialmente de melhor qualidade.
The use of composites in construction, as matrix and paste cement reinforced with fibers, has spread considerably in recent years. A wide variety of synthetic fibers such as polypropylene have been successfully used to reinforce cementitious composites. However, worldwide interest in the use of products with lower environmental impact stimulates the search for materials to replace synthetic fiber. Vegetable fiber, biodegradable, can be a great alternative because of the abundance, low cost, the lowest energy consumption for its production, appropriate mechanical properties. Curauá fiber, native plant from Amazon, with crops on a commercial scale, is used in the manufacture of ropes, baskets or as reinforcement in organic matrix. Its mechanical properties are similar to those of polypropylene. Extrusion technology is feasible in the fibercement industry, because it produces composites with high density matrix and great packaging, low permeability and good adhesion fiber matrix. However, successful extrusion process of cementitious products mainly depends on the rheological properties of fresh cement reinforced with fibers. The vegetable fibers can promote water kidnapping and strongly interfere in the flow, cohesion and fresh cement slurry flow. The incorporation of vegetable fibers influences the based cementitious materials in the fresh state and affects properties in the hardened state. In this context, the objective of the research is to evaluate the influence of curauá and polypropylene fiber in rheological and mechanical properties of fresh cement paste. Formulations without fiber, used as reference, 1 and 2% content by weight of reinforcement, fibers with a length of 6 to 10 mm were prepared. Two rheological techniques were used: Squeeze flow and extruder rheometer to analyze the flow of cement pastes. Through experimental data, as strength/displacement and numerical analysis of the pressure extruder rheometer were determined: yield stress corresponding to zero velocity (σ0), initial shear stress (τ0), effect of the velocity on yield stress (α) and effect of velocity in the shear stress (β). The mechanical properties were determined in MTS testing machine. Modulus of rupture (MOR), fracture toughness (TFT) and fracture energy (EF) were calculated. The rheological results indicate that the cement paste reinforced with curauá fiber showed higher strength, smaller displacement and increased extrusion pressure with curauá fibers compared to cementitious paste reinforced with polypropylene fibers. The length of the fibers influence the flow of the mixture more than the fiber content. Composites reinforced with polypropylene fibers presented higher values of MOR, TFT and EF compared to composites reinforced with curauá fiber. After 200 ageing cycles, the mechanical results of composites reinforced with curauá fibers decreased due to mineralization of the fibers. The nanoindentation results, as hardness and elastic modulus, increased after 200 cycles. The methodologies used to assess the rheological and mechanical behavior of fibercement during extrusion facilitate future transfer of this technology to the productive sector, with potentially higher quality products.

Orientador: Ana Paula Rosifini Claro
Resumo: Dentre os materiais utilizados em aplicações biomédicas, o titânio e suas ligas apresentaram destaque devido a suas excelentes propriedades de volume, como por exemplo, resistência mecânica. No entanto, essas propriedades estão diretamente ligadas à microestrutura, que estão diretamente relacionadas com o seu processamento. O objetivo deste trabalho foi avaliar a influência da deformação plástica severa por Equal Channel Angular Presssing (ECAP) na microestrutura e propriedades mecânicas da liga experimental Ti25Ta25Nb3Sn. Os lingotes da liga Ti25Ta25Nb3Sn foram processados em forno de fusão a arco voltaico com atmosfera controlada e submetidos a tratamento térmico de homogeneização para posterior forjamento a frio. Para o processamento por ECAP os lingotes foram previamente tratados por solubilização. Uma matriz com canal de 90º foi utilizada no processamento por ECAP, seguindo a rota Bc, processando o material com 2 e 4 passes. A morfologia, composição e estrutura foram investigadas usando microscopia eletrônica de varredura, microscopia eletrônica de transmissão e análise por EBSD. As análises de microscopia e EBSD mostraram uma estrutura com uma mistura de grãos refinados e grãos grosseiros após 2 passes e uma homogeneidade maior de grãos refinados após 4 passes. A difração de Raios X indicou a formação da fase α'' e β após 2 passes do processamento por ECAP e após 4 passes apenas a fase β estava presente na estrutura.
Doutor

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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)
Universidade Estadual Paulista (UNESP)
Os principais objetivos deste trabalho foram desenvolver estruturas bainíticas e multifásicas através de diversas rotas de tratamentos térmicos, visando as melhores combinações de propriedades mecânicas, fornecendo subsídios científicos/tecnológicos para as indústrias brasileiras. Em certos componentes de veículos aeroespaciais tem sido usado aço de baixa liga e ultra-alta resistência temperados e revenidos com elevada resistência devida a estrutura martensítica mas com baixa tenacidade. Uma melhoria na tenacidade é conseguida com redução controlada de resistência através do revenimento. O novo conceito, para aços avançados que combinam alta resistência com boa tenacidade, está simbolizado pelas microestruturas bainíticas e multifásicas. Neste projeto foi feito um estudo do efeito das microestruturas nas propriedades mecânicas de um aço AISI 4340. Foram analisadas diversas microestruturas, desde aquelas inteiramente bainíticas até microestruturas multifásicas com teores variados de ferrita, bainita, martensita e austenita retida. Os resultados foram comparados com aqueles obtidos por têmpera por resfriamento continuo e com as diversas rotas de transformação isotérmica. As combinações de propriedades mecânicas estão relacionadas com as frações volumétricas das fases e a bainita melhora significativamente a ductilidade do aço, mantendo a resistência elevada e melhorando a combinação resistência/ductilidade. O aço possui baixo coeficiente de encruamento e é possível conseguir resistências entre 1000 MPa e 1400 MPa com alongamento entre 13% e 25%, combinação esta superior aquelas encontradas para o mesmo aço quando temperado e revenido em óleo.
The main goals of this study were to develop bainitic and multiphasic structures through several routes of heat treatment, in order to reach the better combination of mechanical properties, providing scientific/technological subsidies to Brazilian industries. In some of aerospatial vehicles components have been used quenched and tempered ultra-high-strength low-alloy steel where the martensitic structure is responsible for the high-strength and low toughness levels. Toughness improvements can be achieved by strength reduction control during tempering. The new concept for advanced steels, that combine high-strength and good toughness, is correlated with the bainitic and multiphasic microstructures. In this work the effect of microstructures on the mechanical properties of AISI 4340 steel. Has been analysed several microstructures, from those totally bainitic until multiphasics microstructures with various ferrite, bainite, martensite and retained austenite content. The results were compared with those obtained by quenching through continuous cooling transformation and several routes of isothermal transformation. The combinations of mechanical properties are related with volume fraction of present phases and the bainite improved significantly the toughness steel., keeping the high strength and improving the strength/toughness combination. This steel has low coefficient of hardness and is possible to achieve strengths between 1000 MPa e 1400 MPa with percentual elongation between 13% e 25%, this combination is better than that found to the same steel when quenched and tempered in oil.

La microstructure des matériaux constitue un outil essentiel pour optimiser les propriétés mécaniques des structures et ainsi améliorer leurs performances. Les modèles de Cauchy ne sont pas toujours adaptés à la description de la réponse dynamique de certains matériaux microstructurés montrant des comportements mécaniques exotiques. Les théories de milieux continus généralisés peuvent être de bonnes candidates pour modéliser ces matériaux d’une façon plus précise et plus réaliste, aussi bien en statique qu’en dynamique, puisqu’elles peuvent décrire, même d’une façon simplifiée, la manifestation macroscopique de la présence d’une microstructure. Ce manuscrit est organisé comme suit : - Dans le chapitre 1 nous introduisons les aspects généraux de la mécanique des renforts fibreux.- Dans le chapitre 2 nous rappelons certains concepts fondamentaux concernant la mécanique des milieux continus classiques. De plus, nous introduisons les théories de deuxième gradient à l’aide du Principe des Travaux Virtuels.- Dans le chapitre 3 nous nous proposons de présenter une première modélisation des renforts fibreux de composites en mettant en place des modèles numériques discrets. Cette modélisation discrète permet de rendre compte de certains effets de la microstructure des renforts fibreux sur leur comportement macroscopique global. En particulier, il sera montré que la flexion locale des mèches à l’échelle mesoscopique a un effet non-négligeable sur le comportement macroscopique global de ces matériaux. Dans un deuxième moment nous introduisons une modélisation continue de deuxième gradient pour la description des mêmes matériaux et nous montrons que les termes d’ordre supérieur permettent une description satisfaisante des effets de flexion locale sur-cités.- Dans le chapitre 4 on particularise le cadre général de la mécanique des milieux continus introduit dans le chapitre 2 au cas particulier des milieux continus 2D. On mettra un accent fort sur l’interprétation géométrique des mesures de déformation de deuxième gradient qui seront directement reliées aux courbures dans le plan de certaines lignes matérielles. Ces lignes matérielles seront ensuite interprétées dans les chapitres suivantes comme décrivant les mèches des renforts fibreux de composites qu’on se propose d’étudier.- Dans le chapitre 5 nous introduisons une hypothèse cinématique forte sur les déformations admissibles, en supposant que les mèches du renfort considéré sont inextensibles. Cette hypothèse nous permettra de construire un modèle simplifié de premier gradient pour le comportement des renforts de composites 2D qui est encore représentatif de leur comportement mécanique. Une méthode numérique permettant de montrer certaines solutions concernant le cas du bias extension test est codée en Mathematica et les résultats obtenus sont discutés
Dered materials in the simplest and more effective way. However, there are some cases in which the considered materials are heterogeneous even at relatively large scales and, as a consequence, the effect of microstructure on the overall mechanical behavior of the medium cannot be neglected. In such situations, Cauchy continuum theory may not be useful to fully describe the mechanical behavior of considered materials. It is in fact well known that such continuum theory is not able to catch significant phenomena related to concentrations of stress and strain and to specific deformation patterns in which high gradients of deformation occur and which are, in turn, connected to particular phenomena which take place at lower scales. Generalized continuum theories may be good candidates to model such micro-structured materials in a more appropriate way since they are able to account for the description of the macroscopic manifestation of the presence of microstructure in a rather simplified way. The present manuscript is organized as follows: In ch.1 a general description of fibrous composite reinforcements is given. In ch.2 some fundamental issues concerning classical continuum mechanical models are recalled. In ch.3 we start analyzing some discrete and continuum models for the description of the mechanical behavior of 2D woven composites. At this stage of the manuscript, we want to show how some discrete numerical simulations allowed us to unveil some very special deformation modes related to the effect of the local bending of fibers on the overall macroscopic deformation of fibrous composite reinforcements. Such discrete simulations showed rather clearly that microscopic bending of the fibers cannot be neglected when considering the deformation of fibrous composite reinforcements. For this reason, we subsequently introduced a continuum model which is able to account for such microstructure-related effects by means of second gradient terms appearing in the strain energy density. In ch.4 we reduce the general continuum mechanical framework introduced in ch.2 to the particular case of 2D continua. In ch.5 we introduce a strong kinematical hypothesis on the admissible deformations, assuming that the yarns composing the woven reinforcements are inextensible

Os aços inoxidáveis têm um amplo campo de aplicação, por ter alta resistência mecânica e à corrosão quando trabalha em temperaturas elevadas. Uma aplicação recorrente é em reatores nucleares, podendo ser utilizado no vaso de pressão e nas estruturas de contenção do reator. O aço inoxidável austenítico foi muito utilizado no núcleo de reatores para revestimento do combustível nuclear. No entanto, foi substituído por uma liga de zircônio denominada zircaloy, em consequência da menor absorção de nêutrons térmicos desta liga. Após o acidente de Three Miles Island o aço inoxidável voltou a ser usado para esta aplicação. Para atenuar a corrosão intergranular, muito característica em aços inoxidáveis austeníticos, utiliza-se elementos estabilizantes como o nióbio. O aço inoxidável AISI 348 é estabilizado com nióbio. Neste trabalho, estudou a soldagem de tubos de AISI 348 e tampões de mesmo material soldados pelo processo GTAW (Gas Tungsten Arc Welding) sob diversas condições, procurando-se obter penetração de soldagem de 110 % da espessura do tubo e reforço do cordão de solda inferior a 0,15 mm. As amostras soldadas foram submetidas à caracterização microestrutural com microscopia ótica e microscopia eletrônica, utilizando também a técnica EDS (Espectroscopia de Energia Dispersiva). Foram realizados ensaios mecânicos de tração, fadiga, microdureza Vickers e arrebentamento, como também verificado a susceptibilidade à corrosão intergranular. O cordão de solda passou por ensaios não destrutivos de inspeção visual, dimensional, líquido penetrante, raios X e ensaio de estanqueidade por fuga de gás hélio. A microdureza não apresentou diferenças nas regiões da solda, não sendo possível identificar claramente a zona afetada pelo calor. O arrebentamento ocorreu a uma distância acima de 30 mm do cordão de solda, sendo o resultado considerado aprovado. No ensaio de tração, a ruptura ocorreu no cordão de solda e no metal de base tubo, o local da ruptura dependeu do afastamento lateral do eletrodo em relação à junta soldada. O ensaio de fadiga com corrente de 40 A obteve número de ciclos de 2,14 x 105. Este valor é 50% maior no tempo de vida comparado com amostra de 30 A. No ensaio de corrosão intergranular, as amostras que foram submetidas não apresentaram sensitização no contorno de grão. A análise por EDS identificou áreas com carbonetos oriundos do processo de fabricação do tubo.
Stainless steels have a wide range of applications, since it has high mechanical and corrosion resistance when working at elevated temperatures. A recurring application is in nuclear reactors, can be used in the pressure vessel and the reactor containment structures. The austenitic stainless steel has been widely used in the core reactors for nuclear fuel cladding. However, it was replaced with a zirconium alloy called zircaloy, due to lower thermal neutron absorption of this alloy. After the accident at Three Mile Island stainless steel came to be used for this application. In order to mitigate intergranular corrosion, very characteristic in austenitic stainless steel, is used stabilizing elements such as niobium. The stainless steel AISI 348 is stabilized with niobium. In this work, welding AISI 348 tubes and caps of the same material welded by GTAW process (Gas Tungsten Arc Welding) under various conditions, seeking to obtain welding penetration of 110% of the tube thickness and reinforcement of the weld bead less than 0.15 mm. The welded samples were subjected to microstructural characterization with optical and electron microscopy, also using the technique EDS (Energy Dispersive Spectroscopy). Mechanical tests of traction were performed, fatigue, Vickers hardness and burst, but also verified the susceptibility to intergranular corrosion. The weld bead has undergone non-destructive testing visual inspection, dimensional, liquid penetrant, X-rays and leakage test by helium gas leak. The microhardness showed no differences in weld regions, it is not possible to clearly identify the heat affected zone. The burst occurred at a distance of more than 30 mm of the weld bead. In the tensile test, the rupture occurred in the weld bead and base metal tube, the location of rupture depended on the lateral spacing of the electrode in relation to the welded joint. The fatigue test with 40 A current obtained number of cycles of 2.14 x 105. This value is 50% higher in the life time compared with sample 30 A. In the intergranular corrosion test, samples were subjected showed no sensitization in the grain boundary. Analysis by SDS identified areas carbides derived from the tube manufacturing process.

Friction Stir Processing (FSP) is a promising thermomechanical technique that is used to modify the microstructure of metals locally, and thereby locally improve mechanical properties of the material. FSP uses a simple and inexpensive tool, and has been shown to eliminate pores and also reduce the sizes of intermetallics in aluminum alloys. This is of great interest for research on solidification, production and performance of aluminum alloy castings because FSP can enhance the structural quality of aluminum casting significantly by minimizing the effect of those structural defects. In the literature, there is evidence that the effectiveness of FSP can change with tool wear of the tool used. Therefore, a study was first conducted to determine the effect of FSP time on the tool life and wear in 6061-T6 extrusions. Results showed the presence of two distinct phases in the tool life and wear. Metallographic analyses confirmed that wear in Phase I was due to fracture of the threads of the tool and Phase II was due to regular wear, mostly without fracture. Moreover, built-up layers of aluminum were observed between threads. The microhardness profile was found to be different from those reported in the literature for 6061-T6, with Vickers hardness increasing continuously from the the stir zone to the base material. To investigate the degree of effectiveness of FSP in improving the structural quality of cast A356 alloys, ingots with different quality (high and low) were friction strir processed with single and multiple passes. Analysis of tensile test results and work hardening characteristics showed that for the high quality ingot, a single pass was sufficient to eliminate the structural defects. Subsequent FSP passes had no effect on the work hardening characteristics. In contrast, tensile results and work hardening characteristics improved with every pass for the low quality ingot, indicating that the effectiveness of FSP was dependent on the initial quality of the metal. The evolution of microstructure, specifically the size and spacing of Silicon (Si) eutectic particles, was investigated after friction stir processing of high quality A356 castings with single and multiple passes. Si particles were found to coarsen with each pass, which was in contrast with previous findings in the literature. The nearest neighbor distance of Si particles also increased with each FSP pass, indicating that microstructure became progressively more homogeneous after each pass. In the literature, the improvement observed after FSP of Al-Si cast alloys was attributed to the refinement of Si particles. Tensile data from high quality A356 ingot showed that there was no correlation between the size of Si particles and ductility. To the author’s knowledge, this is the first time that the absence of a correlation between Si particle size and ductility has been found.

Les aciers ODS (Oxide Dispersion Strengthened) ferrito-martensitiques, font partie des matériaux candidats à la fabrication des pièces pour le gainage de combustible des réacteurs nucléaires de 4ième génération. L’assemblage type « bouchon-gaine » se fait par le procédé de soudage par résistance (SPR) en bout ; procédé de soudage en phase solide, connu pour avoir un impact limité sur la dispersion des nano-oxydes dans la zone soudée par rapport aux procédés de soudage par fusion. Un des objectifs de ces travaux est d’évaluer et de comprendre les effets du SPR sur la résistance mécanique finale de l’assemblage « bouchon-gaine » en acier 11Cr-ODS. Une approche couplant caractérisations microstructurales et mécaniques avec des simulations numériques (du procédé de SPR et des essais mécaniques) est adoptée. L’originalité de cette approche réside aussi dans le développement de deux géométries spécifiques d’éprouvettes de traction, permettant de localiser les sollicitations dans la zone soudée. En effet, les sollicitations thermomécaniques sévères induites par le SPR génèrent des hétérogénéités microstructurales au matériau avec des conséquences directes sur la résistance mécanique. Des microstructures complexes en termes de taille de grain, de texture locale, de phases en présence (ferrite, martensite, ferrite résiduelle) et de type de grain (recristallisé ou déformé) sont obtenues. Les essais mécaniques réalisés ont mis en évidence que la résistance mécanique de l’assemblage soudé peut être principalement associée à la zone interne du plan de joint, formant un angle d’environ 45° par rapport à l’axe de la gaine. Cette zone subit une déformation plastique significative, présente les valeurs de dureté les plus élevées et affiche un affinement plus prononcé de la microstructure. Un second objectif est l’évaluation des effets d’un traitement thermique post-soudage sur les propriétés microstructurales de la soudure et sur la résistance mécanique de l’assemblage soudé. Son effet est significatif s’il est réalisé au-dessus de la température de transformation de phase Ac3, tandis qu’il est limité, s’il est effectué en dessous d’Ac3. Lors d’essais de traction à température ambiante, la zone de rupture est déplacée de la zone soudée vers le métal à l’état de réception lorsque l’assemblage a subi un traitement thermique post-soudage adéquat
Oxide Dispersion-Strengthened (ODS) ferrito-martensitic alloys are among the candidate materials for the manufacture of fuel cladding parts for 4th generation nuclear reactors. The « plug-clad » assembly is carried out by the Pressure Resistance Welding (PRW) process; a solid phase welding process known to have a limited impact on the dispersion of nano-oxides in the welded zone compared with fusion welding processes. One of the aims of this work is to assess and understand the effects of PRW on the final mechanical strength of the 11Cr-ODS steel plug-clad assembly. An approach coupling microstructural and mechanical characterizations with numerical simulations (PRW process and mechanical tests) is adopted. The originality of this approach also lies in the development of two specific geometries for tensile samples, enabling the localization of stresses in the welded zone. Indeed, the severe thermomechanical loadings imposed on the material during the PWR process generate microstructural heterogeneities in the material with direct consequences on its mechanical resistance. Complex microstructures in terms of grain size, local texture, phases (ferrite, martensite, residual ferrite) and grain type (recrystallized or deformed) are obtained. The mechanical tests indicate that the mechanical resistance of the welded assembly is primarily associated with the internal zone of the joint plane, forming an angle of approximately 45° with respect to the axis of the clad. This area is submitted to significant plastic deformation, presents the highest hardness values, and exhibits a more pronounced refinement of the microstructure. A second objective is the evaluation of the effects of a post-welding heat treatment on the microstructural properties of the weld and on the mechanical strength of the welded assembly. Its effect is significant if it is carried out above the phase transformation temperature, Ac3, while it is limited if below Ac3. During tensile tests at room temperature, the fracture zone is moved from the welded area to the as-received metal when the assembly has undergone adequate heat treatments

Este trabalho versa sobre a caracterização de uma junta soldada em cheio (bead-onplate) de liga AA356 reforçada com partículas de SiC, soldada por um Laser de fibra de alta potência. A soldagem foi realizada em duas amostras com diferentes condições térmicas, tratadas termicamente T6 (solubilização e envelhecimento) antes da soldagem (amostra A) e após a soldagem (amostra B). Nas amostras A e B foram realizadas as análises de materialográfia via Microscopia Óptica de luz reflexiva (M.O) e Microscopia Eletrônica de Varredura (MEV), também foram realizadas análises microquímica por Energia Dispersiva de Raios-X (EDX), ensaio mecânico de microdureza e difração de Raios-X. Os corpos de prova submetidos ao ensaio de tenacidade em Flexão três pontos tinham condições térmicas iguais a da amostra A, pois essa condição térmica é a condição que normalmente o Compósito de Matriz Metálica (CMM) exibe em serviço. A inspeção materialográfica por M.O identificou a interferência do tratamento térmico T6 realizado na amostra A, fazendo que fosse possível identificar a baixa proporção das partículas de Si na matriz, conforme análise se aproximava da zona fundida (ZF), e a presença de uma estrutura metaestável com a presença dendritas na ZF, aos quais foram diretamente correlacionados com os resultados da microdureza. A amostra B exibiu uma microestrutura bem homogênea em relação à amostra A, em termos de dispersão das partículas de Si e presença de dendritas na ZF. A fratográfia por MEV em modo de imageamento por elétrons secundários, permitiu analisar as superfícies de fratura do compósito em estudo (AA356+SiC), fraturado após o ensaio de tenacidade em Flexão três pontos, identificando a presença de muitos dimples que formavam uma estrutura alveolar, conhecida como uma estrutura típica de um regime dúctil. Por meio desta técnica, também foi possível detectar os principais mecanismos de tenacificação nos CMM, tais como: trincamento, descolamento ou destacamento das partículas de SiC, e o crescimento e coalescência de dimples na estrutura da matriz, os quais foram identificados e correlacionados com o desempenho mecânico dos corpos de prova analisados. A microanálise química por EDS permitiu o mapeamento dos elementos químicos presentes nas regiões do Metal Base (MB) e na Zona Termicamente Afetada (ZTA) do CMM. Por meio desta técnica, foi possível identificar a presença das partículas de Si e SiC, os elementos químicos presentes nas regiões dendriticas, os elementos fragilizantes presentes na microestrutura do CMM, tais: como Fe, Cr e Mn, e a presença de carbeto de Aluminio-Silício (Al4SiC4) presentes nas ZF, em forma de agulhas. A microanálise química foi realizada tanto nas regiões das juntas soldadas quanto nas superfícies de fraturas provenientes do ensaio de tenacidade em Flexão três pontos.
This work focuses on the characterization of a joint welded bead-on-plate of SiC particulate-reinforced A356-alloy welded by high power fiber laser. The welding was achieved in two samples with different conditions, both with T6 applied, before (sample A) and after (sample B) the welding process respectively. Samples A and B were performed materialographic analysis by Optic Microscopy of light reflected (O.M) and scanning electron microscopy (SEM), were also performed chemical microanalysis by energy dispersive X-ray , mechanical testing microhardness and X-ray diffraction. Specimens submitted to the three point bending toughness test present a thermal condition similar to sample A, because this thermal condition is the condition that normally the Metal Matrix Composite (MMC) exhibits in service. The inspection metallographic by (O.M) identified heat treatment T6 interference in the sample A, and though this is it was possible to identify low proportion of Si particles in the matrix, while the analysis was approaching fused zone, and the presence of a metastable structure with formation of dendrites in the fused zone, were which promptly correlated with results of microhardness. The sample B exhibited more homogenous a microstructure in terms of dispersion of Si particles. SEM fractography in secondary electron imaging mode allowed to analyze fracture surface of MMC, identifying the presence of more microvoids creating an alveolar structure typical of an ductile regime. Through this technique, it also was possible to detect main toughening mechanisms for MMC, such as, cracking, debonding and growth and coalescence dimples in the structure the of matrix which were identified and correlated with performance of specimens analyzed. EDS micro-chemical analysis allowed to map chemistry elements present in various regions of CMM, such as, Base Metal (BM), heat affected zone (HAZ) and Fused Zone (FZ). Through this technique, it was possible to identify and quantify the presence of Si and SiC particles, the elements present in the dendrites and presence of embrittlement elements in the microsctructure of MMC, such as, Fe,Cr and Mn and presence of needle-shapped Aluminium-Silicon carbides (Al4SiC4) in the FZ of sample A. Chemical microanalyses were performed both in regions of welded joints and in surface of fracture from the three points bending toughness test.

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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)
Os aços Maraging são aços de ultra-alta-resistência, tem composição à base de ferro ligado com cerca de 18% de níquel, com limite de esoamento entre 1.400 a 2.500 MPa. Possuem propriedades superiores, tais como a alta ductilidade, boa soldabilidade, tratamento térmico simples sem deformações, boa usinabilidade, alta resistência e resistência ao desgaste. A sua boa tenacidade permiti que suporte tensões repetidas de fadiga por maior tempo, comparado com outros aços de alta resistência. Para o presente trabalho, a partir de diversos testes de solda a laser, aplicação de tratamentos térmicos e tratamento superficial de nitretação a plasma, foram estudadas as propriedades mecânicas através de ensaios de tração, fadiga, impacto, dureza e rugosidade. Foram também analisadas, com auxílio do Microscópio Óptico, as características microstuturais da zona de fusão (ZF), zona térmica afetada (ZTA) pelo calor da solda e camada nitretada. Com o auxílio de um Microscópio Eletrônico de Varredura (MEV) analisou-se o mecanismo da fratura e foram realizadas análises por E.D.S. (Energia Dispersiva de Raios X). Foram realizados vários teste com tempos e temperaturas para se definir a melhor condição de envelhecimento, obteve-se a temperatura de 480°C por 3 horas como satisfatoria. A solda a laser mostrou-se eficaz para a união das chapas, com perda de resistência inferior a 10%, quando comparada com a resistência do aço maraging sem solda. As análises da superfície fraturada mostram que a ruptura dos corpos de prova soldados ocorrereu na região da zona fundida, e possui natureza dúctil, com a formação de dimples, para todas as condições de tratamento térmico, o processo de soldagem e nitretação a plasma não alteraram o tipo de fratura típica dos aços maraging. A vida em fadiga não foi prejudicada pela nitretação, porém, foi observado uma redução da vida em fadiga do material soldado comparado com o material sem solda. Os resultados indicam, que o tratamento térmico de envelhecimento elevou as propriedades mecânicas do aço maraging tanto do material soldado como do nitretetado a plasma, com valores de resistência de aproximadamente 2.000 MPa e alongamento em torno de 8%.
Maraging Steels are ultra-high-strength, that have their composition based on iron alloyed with approximately 18% Nickel, with yield strength between 1.400 to 2.400 MPa. They have superior properties, such as high ductility, good weldability, simple heat treatment without deformation, good machinability, high strength and wear resistance. Their hardness and resistance allow them to supports repeated stress of longer fatigue in comparison with other high strength steels. The maraging steel has a significant cost advantage due to the good workability, predictable and uniform retraction during heat treatment, making this steel be promising. For this work, from various laser welding tests, heat treatment and surface treatment of plasma nitriding, The mechanical properties were studied by means of testing: traction; fatigue; impact; hardness; and roughness. We also analyze, with the aid of optical microscope, the microstructural characteristics of the fusion zone (FZ), the heat affected zone (HAZ) due the welding and by nitrided layer. With the aid of SEM (Scanning Electron Microscope) and EDS (Energy Dispersive Scanning), we analyzed the fractured mechanisms. Several tests were carried out with times and temperatures to determine the best aging condition and the chosen temperature was 480°C for 3 hours. The application of laser welding seems to be effective for joining the plates with loss in the yield strength or tensile strength less than 10%, compared with the strength of maraging steel without welding. The analyses of the fractured surface showed that the rupture of the welded samples occurred in the fused zone region and has ductile nature, with formation of dimples for all heat treatment conditions. The welding and plasma nitriding process have not modified the type of typical fracture of the maraging steel. The life in fatigue was not affected by nitrided, however, they observed a reduction in fatigue life of welded material, compared with no soldering material. The results showed that aging heat treatments increased the mechanical properties of maraging steel both welded and plasma nitrided material, whit strength values about 2.000 MPa and elongation nearly 8%.

Os aços de alta resistência e baixa liga (ARBL) são amplamente empregados nas indústrias automotiva, petrolífera e naval por apresentarem boas propriedades mecânicas e boa soldabilidade. A seleção do processo de soldagem utilizado para a fabricação de componentes depende de vários fatores, entre eles a qualidade do cordão de solda e a capacidade de produção, ambas obtidas ao se utilizar a soldagem a laser, processo que vem sendo cada vez mais utilizado nas indústrias. As propriedades dos cordões de solda são afetadas pelo processo de soldagem e pelos parâmetros do processo, e o conhecimento do desempenho mecânico do cordão de solda é essencial para garantir a confiabilidade da união soldada. A caracterização mecânica de cordões de solda é complexa devido aos diversos fatores que podem afetá-lo, como as alterações metalúrgicas, diferentes propriedades mecânicas nas regiões do cordão de solda, presença de descontinuidades, tensão residual e complexo estado de tensão devido à geometria da junta. O presente trabalho obteve uniões entre chapas de aço ARBL pelo processo a laser em duas condições diferentes: com alto aporte térmico e baixo aporte térmico. A utilização de diferentes aportes térmicos gerou cordões de solda com microestruturas distintas, e avaliou-se, deste modo, o comportamento mecânico associando-o à microestrutura gerada. Para a avaliação da microestrutura utilizou-se da microscopia óptica e eletrônica de varredura, e para a caracterização mecânica empregou-se a aferição da dureza, ensaio de tração, ensaio de tenacidade à fratura, avaliada pelo CTOD, e ensaio de propagação da trinca por fadiga. Devido às alterações metalúrgicas causadas pelos diferentes aportes térmicos verificou-se diferente comportamento mecânico entre os cordões de solda em função da microestrutura obtida.
High-strength low-alloy steels (HSLA) are widely used in the automotive and oil industries due to their good mechanical properties and good weldability. The selection of the welding process used to manufacture components depends on several factors, including the quality of the weld bead and the production capacity, both obtained when using laser welding, a process, which is being increasingly used in the industries. The weld beads properties are affected by the welding process and by its parameters, and the knowledge of the weld bead mechanical performance is essential to ensure the reliability of the welded joint. The weld beads mechanical characterization is complex due to various factors that may affect it, such as metallurgical changes, different mechanical properties in the regions of the weld bead, presence of discontinuities, residual stress and complex state of tension due to the geometry of the joint. The present work obtained the joining of HSLA steel sheets by laser process in two different conditions, with high heat input and low heat input. The use of different heat inputs produced weld beads with different microstructures; therefore, the welded joints were evaluated according to their mechanical behavior, relating it with the microstructural generated. For the microstructure evaluation, optical microscopy and scanning electron microscopy were used, and for the mechanical characterization hardness determination, tensile test, fracture toughness test (evaluated by CTOD) and fatigue crack propagation were performed. Due to the metallurgical changes caused by the different heat inputs, a different mechanical behavior was observed between the weld beads as a function of the microstructure obtained.

COORDENAÇÃO DE APERFEIÇOAMENTO DO PESSOAL DE ENSINO SUPERIOR
Ainda hoje, no Brasil, o fibrocimento à base de matriz cimentícia reforçados com amianto permanecem como o principal compósito fibroso utilizado em larga escala de produção, apesar da constatação dos seus malefícios, como doenças respiratórias e câncer, causados por esta fibra durante o processo de extração, fabricação e instalação. Nos EUA, União Européia e Canadá a produção de materiais reforçados com amianto foi proibida ou sofreu sérias restrições ao seu uso. No Brasil há uma promessa do governo federal de se iniciar a diminuição da extração do amianto a partir de 2005 até sua completa extinção. Frente a inadequação do uso do amianto para produção de materiais de construção sem riscos à saúde da sociedade, surge a necessidade de se encontrar um substituto com propriedades físicas e mecânicas adequadas que seja ecológico e de custo adequado. Neste trabalho é analisado o uso de polpas celulósicas de bambu, com dois estágios de tratamento diferentes, quais sejam polpas antes do processo de refino e após este processo, para reforço de matrizes cimentícias, utilizando o processo de produção Hatschek em escala laboratorial. As polpas celulósicas foram utilizadas nas porcentagens de 4, 6, 8, 10, 12, 14 e 16% em relação a massa do cimento, sendo utilizada a matriz plena como referência. Foi estudado ainda a substituição parcial do cimento por um resíduo de fábrica de tijolos na proporção de 20, 30 e 40% em relação a massa do cimento e o uso de dois tipos de equipamentos de produção diferentes. O objetivo deste trabalho é determinar a porcentagem ideal de reforço para os dois tipos de polpas utilizadas, discutindo as propriedades físicas, mecânicas e microestruturais de cada um destes compósitos.
Still today, in Brazil, the fibre cement to the base amianthus is the main fibrous composite to the base of main cement with wide production scale, despite verification of its damages, as breathing diseases and cancer, caused by this fiber during the extraction process, production and installation. In the USA, European Union and Canada the production of materials to the base of amianthus are prohibited or they suffered serious restrictions. In Brazil, the federal government wile begin to extinguish the amianthus extraction in 2005 due to the problems used for amianthus writer allow, it is necessary to get a new material with appropriate physical and mechanical properties, that is ecofriendly and of appropriate cost. In this work the use of pulps bamboo cellulose`s is analyzed, with two types of treatment, which are pulps before the process of I refine and after this process, for reinforcement of main cimentícias, using the production process Hatschek in scale laboratories. The pulps cellulose`s was used in the percentages of 4, 6, 8, 10, 12, 14 and 16% in relation to mass of the cement, being used the full head office as reference. In addition, it was substitution of the cement by a residue of the it manufactures of bricks in the proportion of 20, 30 and 40% in relation to mass of the cement and two types of equipment of different production. The aim of this work is to determine the ideal percentage of reinforcement for the two used types of pulps used, discussing the physical properties, mechanics and microstructure of each one of these composites.

Orientador: Marcelo dos Santos Pereira
Resumo: O desenvolvimento dos aços avançados de alta resistência tem sido a principal estratégia por parte das siderúrgicas para atenderem às demandas das indústrias automobilísticas por um material que permita a redução de peso, diminuição do consumo de combustível e, simultaneamente, que possibilite uma melhora na resistência ao impacto e na segurança dos passageiros. Neste contexto, o aço bifásico apresenta grande potencial tecnológico de aplicação, principalmente em componentes estruturais, em virtude de suas propriedades mecânicas especiais, como alta resistência à tração, alta taxa de encruamento e muito boa ductilidade. No entanto, é necessário minimizar os problemas dimensionais provocados pelo efeito do retorno elástico (springback) durante a estampagem, assim como, as falhas durante o processamento. Este trabalho tem por objetivo caracterizar um aço bifásico da classe 1000 MPa (DP980), quanto à microestrutura e propriedades mecânicas. A caracterização microestrutural, por meio de técnicas de microscopia óptica e eletrônica de varredura, determinaram as fases presentes de maneira qualitativa e quantitativa, assim como suas morfologias, com o objetivo de correlacionar a microestrutura com as propriedades mecânicas. Evidenciou-se a presença de grãos alongados, acompanhando o sentido de laminação da chapa e uma fração volumétrica de martensita de 51,4%. As características em relação aos processos de estampagem foram estudadas por meio do ensaio de dobramento para determinar o r... (Resumo completo, clicar acesso eletrônico abaixo)
Abstract: The development of advanced high strength steels (AHSS) has been the main strategy for steelmakers to meet the demands of the automotive industry for a material that reduces weight and fuel consumption, provides shock resistance and vehicle safety. Within this context, dual-phase steel (DP) shows great technological potential, mainly in structural parts, due to its special mechanical properties, such as high tensile strength, work hardening, and ductility. However, it is necessary to minimize the dimensional problems caused by springback during stamping, along with the processing failures. This work aims to characterize a 1000 MPa (DP980) dual-phase steel regarding its microstructure and mechanical properties. The microstructural characterization, through optical microscopy and scanning electron microscopy, determined the present phases in a qualitative and quantitative manner, as well as their morphologies, in order to correlate microstructure with mechanical properties. The presence of elongated grains, following the direction of lamination of the sheet, as well as a volumetric fraction of martensite of 51.4% were observed. Features of the stamping processes were examined through a bending test to determine the springback in samples submitted to bending punches with radii of 5 mm and 0.3 mm. The samples tested with the 5 mm radius punch show a strong springback in function of the high strength of the steel. With the 0.3 mm radius punch, the springback decreases significantl... (Complete abstract click electronic access below)
Doutor

The recent advances in the Materials Engineering have led to the development of new materials with customized microstructure in which the properties of its constituents and their geometric distribution have a considerable effect on determination of the macroscopic properties of the substance. Direct inclusion of the material microstructure in the analysis on a macro level is challenging since spatial meshes created for the analysis should have enough resolution to be able to accurately capture the geometry of the microstructure. In most cases this leads to a huge finite element model which requires a substantial amount of computational resources. To circumvent this limitation a number of homogenization techniques were developed. By considering a small element of the material, referred to as Representative Volume Element (RVE), homogenization methods make it possible to include the effects of a material’s microstructure on the overall properties at the macro level. However, complexity of the microstructure geometry and the necessity of satisfying periodic boundary conditions introduce additional difficulties into the analysis procedure. In this dissertation we propose a hybrid homogenization method that combines Asymptotic homogenization with MeshFree Solution Structures Method (SSM). Our approach allows realistic inclusion of complex geometry of the microstructure that can be captured from micrographs or micro CT scans. In addition to unprecedented flexibility in handling complex geometries, this method also provides a completely automatic analysis procedure. Using meshfree solution structures simplifies meshing to creating a simple cartesian grid which only needs to contain the domain. This also eliminates manual modifications which usually needs to be performed on meshes created from image data. A computational platform is developed in C++ based on meshfree/asymptotic method. In this platform also a novel meshfree solution structure is designed to provide exact satisfaction of periodic boundary conditions for boundary value problems such as homogenization. Performance of the developed platform is tested over 2D and 3D domains against previously published data and/or conventional finite element methods. After getting satisfactory results, homogenized properties are used to compute localized stress and strain distributions over inhomogeneous structures. Furthermore, effects of geometric features of pores/inclusions on homogenized mechanical properties is investigated and it is demonstrated that the developed platform could provide an automated quantitative analysis tool for studying effects of different design parameters on homogenized properties.

Ce travail vise à améliorer la compréhension de la microstructure d’aciers ferritiques appelés aciers ODS. Ils sont fabriqués par métallurgie des poudres, ce qui inclut le cobroyage d’une poudre ferritique avec une fine poudre d’oxydes, suivi d'une consolidation à haute température. La consolidation permet de former un matériau dense renforcé par des particules nanométriques qui sont responsables des bonnes propriétés mécaniques à haute température. Cependant, les procédés conventionnels, notamment la Compaction Isostatique à Chaud, provoquent des microstructures hétérogènes qui étaient jusqu’à ce jour mal comprises. Ainsi, la technique rapide de consolidation assistée par courant électrique appelée "Spark Plasma Sintering" (SPS), a été testée afin d’étudier la microstructure. Pour la première fois, on montre que d’excellentes propriétés mécaniques peuvent être obtenues par SPS, comparables à celles des matériaux ODS obtenus classiquement par Compaction Isostatique à Chaud, mais avec un temps de procédé largement réduit. Cependant, la consolidation par SPS échoue quand il s’agit d’obtenir une micro-structure ferritique homogène. En effet, malgré la cinétique rapide de consolidation, on obtient des grains dits ultrafins (D < 500 nm) entourée de grains plus grossiers (D >10 μm). Une caractérisation microstructurale poussée a permis de comprendre l’évolution du matériau durant la consolidation. Un modèle d’évolution microstructurale a été proposé. Le calcul des pressions gouvernant la mobilité des interfaces souligne l’importance de la déformation plastique hétérogène issue du cobroyage des poudres. Par ailleurs, il est montré que la précipitation des particules d’oxydes ancre les joints de grains et stabilise la microstructure hétérogène, même à très haute température. On montre aussi qu’augmenter la teneur en renforts n’empêche pas la croissance anormale mais permet de contrôler la fraction et la taille de grains ultrafins, et donc les propriétés mécaniques des ODS. Parce que les particules jouent un rôle primordial dans la croissance des grains, une caractérisation fine de l’état de précipitation a été réalisée sur les matériaux consolidés par SPS. L’étude par Microscopie Electronique en Transmission, Diffusion des Neutrons et Sonde Atomique révèle une grande densité d’oxydes qui varient en taille et en composition chimique. Un modèle thermodynamique de type germination/croissance/coalescence a été développé pour simuler les cinétiques de précipitation des phases Y2O3 et Y2Ti2O7 durant les étapes de consolidation non isothermes. Tant les résultats expérimentaux que numériques démontrent la précipitation rapide des nano-particules qui sont ensuite extrêmement stables durant les recuits. Ce modèle permet de mieux comprendre la spécificité des microstructures et de la précipitation dans les ODS, de la formation rapide de particules nanométriques à la précipitation grossière d’oxydes de titane aux interfaces
This work aims to lighten the understanding of the behavior of a class of metallic materials called Oxide-Dispersion Strengthened (ODS) ferritic steels. ODS steels are produced by powder metallurgy with various steps including atomization, mechanical alloying and high-temperature consolidation. The consolidation involves the formation of nanoparticles in the steel and various evolutions of the microstructure of the material that are not fully understood. In this thesis, a novel consolidation technique assisted by electric field called "Spark Plasma Sintering" (SPS) or "Field-Assisted Sintering Technique" (FAST) was assessed. Excellent mechanical properties were obtained by SPS, comparable to those of conventional hot isostatic pressed (HIP) materials but with much shorter processing time. Also, a broad range of microstructures and thus of tensile strength and ductility were obtained by performing SPS on either milled or atomized powder at different temperatures. However, SPS consolidation failed to avoid heterogeneous microstructure composed of ultrafine-grained regions surrounded by micronic grains despite of the rapid consolidation kinetics. A multiscale characterization allowed to understand and model the evolution of this complex microstructure. An analytical evaluation of the contributing mechanisms can explain the appearance of the complex grain structure and its thermal stability during further heat treatments. Inhomogeneous distribution of plastic deformation in the powder is argued to be the major cause of heterogeneous recrystallization and further grain growth during hot consolidation. Even if increasing the solute content of yttrium, titanium and oxygen does not impede abnormal growth, it permits to control the fraction and the size of the retained ultrafine grains, which is a key-factor to tailor the mechanical properties. Since precipitation through grain boundary pinning plays a significant role on grain growth, a careful characterization of the precipitation state was performed on consolidated ODS steels. The experimental data obtained by transmission electron microscopy, small angle neutron scattering and atom-probe tomography evidenced the presence of dense and nanosized particles in SPS ODS steels, similarly to what is observed in conventional ODS steels. This is of great importance since it proves that the precipitation is very rapid and mainly occurs during the heating stage of the consolidation process. Using a thermodynamic model, the precipitation kinetics of Y2O3 and Y2Ti2O7 were successfully reproduced at various consolidation temperatures. Both experimental and numerical findings agree with the rapid precipitation of nanoparticles that are then extremely stable, even at high temperature. Consequently, this model can be an efficient tool to design ODS steelsby the optimization of the precipitation state