Rozprawy doktorskie na temat „Mn TWIP/TRIP Steels”

Abstract Austenitic high-Mn (15–30 wt.%) based twinning-induced plasticity (TWIP) steels provide great potential in applications for structural components in the automotive industry, owing to their excellent tensile strength-ductility property combination. In certain cases, these steels might also substitute austenitic Cr-Ni stainless steels. The aim of this present work is to investigate the high-temperature flow resistance, recrystallisation and the evolution of microstructure of high-Mn steels by compression testing on a Gleeble simulator. The influence of Al alloying (0–8 wt.%) in the hot rolling temperature range (800°C–1100°C) is studied in particular, but also some observations are made regarding the influence of Cr alloying. Microstructures are examined in optical and electron microscopes. The results are compared with corresponding properties of carbon and austenitic stainless steels. In addition, the mechanical properties are studied briefly, using tension tests over the temperature range from -80°C to 200°C. Finally, a preliminary study is conducted on the corrosion behaviour of TWIP steels in two media, using the potentiodynamic polarization technique. The results show that the flow stress level of high-Mn TWIP steels is considerably higher than that of low-carbon steels and depends on the Al concentration up to 6 wt.%, while the structure is fully austenitic at hot rolling temperatures. At higher Al contents, the flow stress level is reduced, due to the presence of ferrite. The static recrystallisation kinetics is slower compared to that of carbon steels, but it is faster than is typical of Nb-microalloyed or austenitic stainless steels. The high Mn content is one reason for high flow stress as well as for slow softening. Al plays a minor role only; but in the case of austenitic-ferritic structure, the softening of the ferrite phase occurs very rapidly, contributing to overall faster softening. The high Mn content also retards considerably the onset of dynamic recrystallisation, but the influence of Al is minor. Similarly, the contribution of Cr to the hot deformation resistance and static and dynamic recrystallisation, is insignificant. The grain size effectively becomes refined by the dynamic and static recrystallisation processes. The tensile testing of TWIP steels revealed that the Al alloying and temperature have drastic effects on the yield strength, tensile strength and elongation. The higher Al raises the yield strength because of the solid solution strengthening. However, Al tends to increase the stacking fault energy that affects strongly the deformation mechanism. In small concentrations, Al suppresses martensite formation and enhances deformation twinning, leading to high tensile strength and good ductility. However, with an increasing temperature, SFE increases, and consequently, the density of deformation twins decreases and mechanical properties are impaired. Corrosion testing indicated that Al alloying improves the corrosion resistance of high-Mn TWIP steels. The addition of Cr is a further benefit for the passivation of these steels. The passive film that formed on 8wt.% Al-6wt.%Cr steel was found to be even more stable than that on Type 304 steel in 5–50% HNO3 solutions. A prolonged pre-treatment of the steel in the anodic passive regime created a thick, protective and stable passive film that enhanced the corrosion resistance also in 3.5% NaCl solution.

Ce travail est centré sur la modélisation de l’évolution de microstructures induite par le procédé SMAT pour des aciers TWIP/TRIP. Les caractéristiques de base d’un acier TWIP/TRIP sont mises en évidence par MEB, DRX et nanoindentation. La nanoindentation est appliquée sur différentes couches pour l’étude des propriétés mécaniques du matériau à gradient de microstructure. Un modèle basé sur la densité de dislocations tenant compte de l’influence du maclage et de la transformation de phase martensitique dans un acier TWIP/TRIP est proposé pour étudier l’influence des paramètres induits par SMAT. Ensuite, la densité de dislocations dans l’austénite et celle dans la martensite ainsi que les fractions volumiques de macles et de martensite au cours d’un impact sont évaluées numériquement avec un modèle d’éléments finis. Par ailleurs, afin d’étudier l’évolution de l’endommagement pendant le processus d’impact, l’endommagement a été introduit dans le modèle de la densité de dislocations. Enfin, un modèle visco-élastoplastique basé sur la densité de dislocations considérant l’effet de la taille de grain, le maclage et le taux de déformation a été proposé afin de mieux comprendre les effets du SMAT sur les propriétés mécaniques d’un acier TWIP/TRIP avec gradient de microstructure. Ainsi, pour étudier la réponse globale du matériau SMATé, la loi de mélange est utilisée en considérant le gradient du matériau comme une structure composée de différentes couches avec des microstructures et des propriétés mécaniques différentes
This work focuses on modeling the microstructure evolution induced by SMAT in TWIP and/or TRIP steels. The features of the generated gradient microstructure of a 304L TWIP/TRIP steel are characterized by SEM, XRD, and nanoindentation. Nanoindentation is applied on different layers for the investigation of the mechanical properties of the gradient microstructure. Based on the experimental results, a dislocation density model considering the influence of twinning and martensitic transformation of TWIP/TRIP steel is proposed to investigate the effect of SMAT controlling parameters. Then the dislocation density of the austenite and that of the martensite as well as the volume fraction of twinning and martensitic transformation during impact loading is numerically evaluated using a full finite element model. Afterwards, to study the evolution of the damage during the SMAT process, the damage was introduced in the dislocation density model. Finally, a dislocation density based visco-elastoplastic model considering the effect of grain size, dislocation density, twin, and strain rate was proposed to further understand the effect of impact loadings on the mechanical properties of TWIP/TRIP steel with gradient microstructure based on the results of nanoindentation tests. Then to study the overall elastoplastic response of the SMATed material, the rule of mixtures is used by considering the gradient material as a gradient structure consisting of different layers with distinct microstructures and mechanical properties

Thermomechanical processing, which is a combination of deformation and heat treatment, is an optimum method to control the microstructural evolution and, accordingly, to generate the desired mechanical properties of materials. TRIP (Transformation-Induced-Plasticity) behavior is a powerful mechanism with which to improve mechanical properties. The basis of TRIP behavior is the retention of austenite with optimum characteristics (volume fraction, stability, size, morphology and composition) at room temperature. The transformation of retained austenite to martensite during deformation can lead to TRIP-enhanced properties. This work deals with the effects of thermomechanical processing parameters on the microstructural characteristics of TRIP steels, primarily from the point of view of the retained austenite condition in Si-Mn and Si-Mn-Nb bearing TRIP steels. Initially, a new test technique based on continuous cooling compression (CCC) testing was developed to find the critical temperatures of thermomechanical processing (TMP). A major finding from the CCC test is the ability to determine the $Ae sb3$ (equilibrium austenite-to-ferrite transformation) temperature. In the CCC test, the $Ae sb3$ appeared to be associated with an increase in the rate of increase in flow stress with decreasing temperature. In order to further evaluate this result, neutron diffractometry at high temperatures was used to monitor any crystallographic changes associated with the metastable region. The results revealed an increase in the rate of contraction of the austenite lattice as the temperature decreases through the metastable state (below $Ae sb3),$ compared with that observed as the temperature decreases through the stable austenite region. Having defined the critical TMP temperatures, the effects of processing parameters on the state of the retained austenite were examined by changing the thermomechanical processing conditions. These effects were more fundamentally considered by tak

O desenvolvimento e utilização de novos materiais, mais leves e com propriedades mecânicas superiores aos atuais, se mostram extremamente importantes devido à redução de peso e consequentemente redução na emissão de gases poluentes que poderiam gerar. As ligas de Fe-Mn-C com elevados teores de Mn (20-30%) representam um desenvolvimento muito recente de aços austeníticos, que, através dos seus mecanismos diferenciados de deformação reúnem elevada resistência mecânica com grande ductilidade. Essa nova classe de materiais estruturais possibilita uma efetiva redução de custos na produção através do reduzido tempo de processamento (sem a necessidade de tratamentos térmicos especiais e de processamentos termomecânicos controlados). A soldagem é, atualmente, o mais importante processo de união de metais usado no setor industrial. Dentro da variada gama de processos de soldagem existentes, a soldagem por fricção e mistura mecânica (SFMM, em inglês: Friction Stir Welding - FSW) se destaca por ser um processo de união no estado sólido que apresenta uma série de vantagens sobre as tecnologias convencionais de soldagem por fusão. Do ponto de vista metalúrgico, uma das suas principais vantagens se manifesta justamente na junção de materiais dissimilares, visto que o grau de mistura de composições e as transformações de fases entre materiais incompatíveis podem ser minimizados. Outra vantagem é que há um refino de grão no cordão de solda comparado com a microestrutura fundida que se forma nos processos convencionais. Este trabalho teve como objetivo produzir em escala laboratorial os aços de alta liga ao manganês com efeito TRIP, avaliar o impacto da velocidade de rotação da ferramenta na soldagem por fricção e mistura mecânica e avaliar a microestrutura e propriedades mecânicas das juntas soldadas. A microestrutura das juntas soldadas caracterizou-se pela presença apenas da zona de mistura e do metal base, além da formação de \'anéis de cebola\' na zona de mistura, esta não mostrou sinais de transformação martensítica induzida por deformação e sofreu recristalização dinâmica para todas as velocidades de rotação investigadas com a formação de grãos refinados e com morfologia equiaxial. Os corpos de tração fraturaram todos nos metais de base, mostrando que as propriedades mecânicas da zona de mistura foram superiores à do metal base e que a variação de aporte térmico alcançada com a velocidade de rotação da ferramenta não comprometeu a qualidade das juntas soldadas.
The development and application of new light materials with superior mechanical properties is extremely important to weight reduction in vehicles and consequently reduction of greenhouse gases emission. The Fe-Mn-C steels with high Mn (20-30%) are a recent development of austenitic steels, which, due to their different mechanisms of deformation, possesses high strength and high ductility as well. In addition, this new type of structural steel allows an effective reduction of manufacturing costs due to its reduced processing time (it does not require special heat treatments and controlled thermo mechanical processing). Welding has been one of the most important processes for joining metals. Among the available welding processes, friction stir welding (FSW) is notable for being a solid state process with great advantages over the conventional welding methods. In the mettalurgical point of view, welding dissimilar materials is a significant advantage of FSW over the other process. The main reason is the reduction of mixture of material and phase transformations between the incompatible materials in the weld. Moreover, grain refinement is another advantage from the process. The present study aimed to produce laboratorial scale high Mn steels with TRIP effect, investigate the impact of tool speed ont the microstructure and mechanical properties of friction stir welded joints. The microstructure of the welded joints exhibited only the stirred zone (SZ) and the base material (BM), besides the presence of ´onion rings´ within the stirred zone. The SZ exhibited no signs of martensite suggesting that dynamic recrystallization have occurred for all the speed tested. Moreover, the grains in the SZ had equiaxial morphology and were significantly refined. The fracture of the tensile specimens occurred in the base material, bringing to light that the welding process was beneficial to the mechanical properties. Furthermore, the variation of heat input achieved with the speed did not compromise the quality of welded joints.

Estudou-se a cinética das transformações de fase em resfriamento contínuo e em tratamentos isotérmicos de cinco ligas de aços TRIP microligados com Nb, contendo teores variáveis de Mn e Si, através de ensaios dilatométricos, de caracterização morfológica dos produtos de transformação e de cálculos termodinâmicos e simulações numéricas usando os programas Thermocalc ® e Dictra®. Foram determinados os diagramas RC para a transformação da austenita, e foi estudada a influência da precipitação de ferrita pró-eutetóide e de bainita na fração volumétrica de austenita retida. Através dos diagramas de resfriamento contínuo foi possível delimitar a extensão do campo intercrítico dos cinco aços analisados, com determinação da janela de resfriamento e seus intervalos de temperaturas. Isso permitiu projetar os ciclos de resfriamento controlado a serem aplicados durante o processamento termomecânico dos Aços TRIP-D, TRIP-E e TRIP-H. Os cálculos pelo modelo numérico de redistribuição de carbono e de elementos substitucionais na interface ferrita/austenita, bem como as medidas de microanálise química por WDS e EDS permitiram verificar que a taxa de crescimento da ferrita pró-eutetóide é controlada pela difusão do carbono na austenita. Para tempos curtos de tratamento, o modelo de crescimento que melhor se ajusta é o do equilíbrio local com partição negligível de soluto. Verificou-se através de tratamentos isotérmicos no campo bainítico, que o silício atrasa a precipitação de carbonetos durante a reação bainítica, o que justifica o aumento da estabilidade da austenita retida no aço de maior Si (TRIP-H), quando comparado com o aço de menor Si (TRIP-E). Baseado nos resultados dos estudos das transformações de fase por resfriamento contínuo foram selecionadas as ligas TRIP-D, TRIP-E e TRIP-H, para simular dois esquemas de laminação controlada por meio de ensaios de torção a quente. Nesses ensaios foram variados o grau de deformação e a temperatura de acabamento, de modo a estudar os efeitos dos parâmetros de deformação mecânica na fração transformada dos diferentes constituintes microestruturais, e em particular na fração volumétrica de austenita retida. O primeiro ensaio refere-se à laminação controlada por recristalização estática (LCRE) e o segundo à laminação convencional (LCC), com temperatura de acabamento de 1030°C e 850°C, respectivamente. O resfriamento consistiu em dois tratamentos isotérmicos consecutivos: o primeiro no campo intercrítico (austenita + ferrita), e o segundo no campo bainítico. O aumento do grau de deformação na simulação por torção a quente da laminação controlada por recristalização estática, levou a um aumento da porcentagem de austenita retida obtida durante o resfriamento controlado (de 9 a 14,0 %). O acúmulo de energia de deformação abaixo da TNR na simulação do processo de laminação controlada convencional provocou uma diminuição da fração volumétrica de austenita retida bem como da concentração de carbono contido nela. Os perfis de Mn e C obtidos a partir de análises químicas com EDS e WDS em amostras do aço TRIP-E, deformadas com deformação total de 2,1 e deformação total de 2,8, mostram a contribuição do refinamento de grão para a difusão destes elementos na frente da interface ferrita/austenita, durante a precipitação de ferrita pró-eutetóide.
The phase transformation kinetics of five Nb microalloyed Si-Mn TRIP steels was studied under continuous cooling and isothermal treatments, using dilatometric techniques, morphologic characterization, Thermocalc computational thermodynamics and Dictra numerical simulation. WDS and EDS X-ray microanalysis and Dictra numerical modeling of C, Mn and Si distribution during transformation showed that the reaction is carbon diffusion controlled and growth occurs under local equilibrium with negligible partition. CCT diagrams for austenite transformation were determined and the effect of the amount of proeutectoid ferrite and bainite precipitation on the volume fraction of retained austenite was also estimated. The CCT diagrams allowed determining the boundaries of the critical zone and the processing window to obtain bainite plus austenite microstructures. Based on this information cooling cycles were selected to perform thermomechanical treatments. Three TRIP steels were selected to simulate, in a hot torsion testing machine, two different controlled rolling sequences: Recrystallization Controlled Rolling and Conventional Controlled Rolling. The influence of the degree of deformation and the finishing temperature on the amount of retained austenite was studied. After rolling the cooling cycle comprised two isothermal treatments, one in the austenite + ferrite field and the other in the bainitic field. Increasing the strain during simulation of Recrystallization Controlled Rolling led to an increase in the volume fraction of retained austenite to the range 9 to 14 %. The energy stored during simulation bellow TNR of the Conventional Controlled Rolling led to a decrease in the volume fraction and in the carbon content of retained austenite. The Mn and C contents measured by EDS and WDS analysis of TRIP-E steel, showed that grain refinement due to recrystallization contributes to diffusion of these elements in front of the ferrite/austenite interface during precipitation.

A crescente escassez dos recursos energéticos renováveis, bem como o contínuo aumento dos seus custos tem requerido nas últimas décadas uma redução drástica no consumo de energia utilizada para o transporte de cargas e passageiros. A indústria siderúrgica pode contribuir decisivamente neste contexto, disponibilizando no mercado aços de maior resistência mecânica, os quais podem ser utilizados em estruturas mais esbeltas. Os aços com elevados teores de Mn (15-30%) representam um desenvolvimento muito recente de ligas ferrosas puramente austeníticas, que reúnem resistência mecânica elevada e grande ductilidade. Além disso, trata-se de ligas de baixo custo devido à eliminação dos elevados teores de Ni necessários para a estabilização da austenita e ao reduzido tempo de processamento, que dispensa tratamentos térmicos e processamentos termomecânicos controlados. Por outro lado, a redução de peso estrutural no setor automobilístico requer não somente a pesquisa de novos aços, mas também a utilização de componentes híbridos, resultantes, entre outros, da união dos aços austeníticos alto Mn com aços comerciais estruturais de alta resistência e baixa liga (ARBL). Nesta dissertação, estudou-se, portanto, a soldabilidade pelo processo de fricção e mistura mecânica (SFMM) de aço austenítico alto Mn com efeito TRIP (plasticidade induzida por transformação martensítica) com aço ARBL processado termomecanicamente tipo XABO500 (ThyssenKrupp Steel, limite de escoamento > 460 MPa). As placas de aço TRIP foram fabricadas na EESC-USP com composição Fe-22.5% Mn-0.4% C através de fundição sob atmosfera protetora de argônio, tratamento térmico de homogenização e laminação a quente a 1150°C. As juntas dissimilares TRIP-ARBL foram produzidas com chapas de 3.5 mm de espessura. Os ensaios de soldagem SFMM foram conduzidos com ferramenta de compósito PCBN-WRe. O aporte térmico de soldagem foi variado através do uso de três velocidades de rotação da ferramenta: 300, 400 e 500 rpm, e o avanço foi de 100 mm/min. Dois deslocamentos (offsets) da ferramenta foram investigados: +1.0 e +2.0 mm em direção ao aço TRIP. Os resultados revelaram um acabamento superficial satisfatório das juntas soldadas com 300 e 400 rpm. A penetração de soldagem aumentou com a velocidade de rotação da ferramenta e com um maior deslocamento da ferramenta em direção ao aço TRIP devido ao crescimento do aporte térmico. A SFMM produziu em ambos os lados das juntas dissimilares uma microestrutura caracterizada apenas por zona de mistura (ZM) e zona termicamente afetada (ZTA), não sendo observada a formação de zonas termomecanicamente afetadas (ZTMA). Na ZM do aço ARBL, a SFMM produziu uma microestrutura polifásica, contendo misturas de ferrita acicular, bainita e martensita. O lado TRIP da ZM não exibiu sinais de transformação martensítica induzida por deformação e sofreu recristalização dinâmica com a formação de uma austenita refinada em comparação com o metal de base. A junta produzida com menor aporte térmico (300 RPM e Offset +1) apresentou os maiores picos de dureza na ZM do aço TRIP devido à maior taxa de resfriamento e, consequentemente, a microestrutura mais fina. Apesar dos maiores picos de dureza, a junta produzida com 300 RPM e Offset +1 apresentou o melhor desempenho no ensaio de tração, atingindo o maior percentual de alongamento a fratura e rompendo no metal de base ARBL. Isso se deve provavelmente à formação de ferrita acicular mais fina na ZM do aço ARBL com microestrutura entrelaçada e de maior tenacidade, se comparado com o metal de base ARBL.
The increasing scarcity of renewable energy resources and their continuously rising costs have required in the last decades a drastic reduction in the energy consumption for the transportation of goods and passengers. The steel industry can decisively contribute in this context by providing the market with steel grades of increased mechanical strength, which can be incorporated into light-weight structures. Steels with high Mn contents (15-30%) represent a recent development of austenitic ferrous alloys that combine elevated mechanical strength with high ductility. In addition, those steel grades correspond to low cost alloys due to the replacement of the high Ni contents necessary to stabilize the austenite as well as the reduced manufacturing time that does not involve subsequent heat treatments or controlled thermo-mechanical processing. On the other hand, the reduction of structural weight in the automotive sector does not only require the research on novel steels, but also the use of hybrid components that result among others from joining austenitic high-Mn steels to commercial structural high-strength low-alloyed (HSLA) steel grades. In this work, we studied therefore the friction stir weldability of an austenitic high-Mn steel with TRIP (transformation induced plasticity) effect to the thermomechanically processed HSLA XABO500 steel grade (ThyssenKrupp Steel, yield strength > 460 MPa). High-Mn TRIP steel plates were produced at the EESC-USP with the chemical composition of Fe-22.5% Mn-0.4% C by casting under protective argon atmosphere, followed by homogenization treatment and hot rolling at 1150°C. The dissimilar TRIP-HSLA joints were produced using 3.5 mm thick plates. The friction stir welding (FSW) experiments were carried out with a tool made of a PCBN-WRe composite. The heat input was varied by using three tool rotational speeds: 300, 400 and 500 rpm. The welding speed was set to 100 mm/min. Two different tool offsets were investigated: +1.0 and +2.0 mm towards the high-Mn TRIP steel. The results revealed that a satisfactory surface finishing is achieved for the butt-joints produced with 300 and 400 rpm. The welding penetration increased for higher tool rotational speeds and larger tool offsets towards the TRIP steel because of an increased heat input. FSW produced at both sides of the dissimilar joints a microstructure characterized by only stir zone (SZ) and heat-affected zone (HAZ). Thermo-mechanical affected zones (TMAZ) could not be observed. In the SZ of the HSLA steel, FSW produced a multiphase microstructure that contains a mixture of acicular ferrite, bainite and martensite. The TRIP side of the SZ did not exhibit traces of strain induced martensitic transformation and underwent dynamic recrystallization with the formation of a fine-grained austenite in comparison to the base material. The butt-joint produced with the lowest heat input (300 RPM and Offset +1) developed the highest hardness peaks in the SZ of the TRIP steel because of the increased cooling rate and, consequently, the more refined microstructure. In spite of the hardest zones, the butt-joint produced with 300 RPM and offset +1 achieved the best performance in the tensile tests by reaching the largest elongation to fracture and having the failure in the HSLA base material. This is likely promoted by the formation of a more refined acicular ferrite in the SZ of the HSLA steel with interpenetrated microstructure and enhanced toughness in comparison to the HSLA base material.

Twinning Induced Plasticity (TWIP) and Transformation Induced Plasticity (TRIP) steels possess excellent combination of high strength and high ductility. Deformation twinning and strain induced martensitic phase transformation are the dominant deformation mechanisms in TWIP and TRIP steels, respectively. The TWIP and TRIP effects are strongly dependent on stacking fault energy (SFE), which essentially decides the operating deformation mechanisms. In TWIP steels, which are characterized by SFE in the range 18 - 45 mJ/m2, deformation takes place primarily by twinning, whereas in TRIP steels where the SFE below 18 mJ/m2, deformation is by martensitic transformation. TWIP steels contain very high amount of manganese (Mn) for the stabilization of austenitic phase and to keep the SFE within the prescribed range. In TRIP steels, the Mn content is rather low. The present thesis primarily deals with the evolution of the microstructure, texture and mechanical properties in TWIP/TRIP steels for a range of Mn content. Chapter 1 of the thesis presents the general introduction of the TWIP/TRIP steels and an extended review of published literature on these materials. The experimental procedures and the methodology of data analysis are presented in chapter 2. Chapter 3 deals with the effect of deformation on the evolution of microstructure and texture in a high Mn steel. The evolution of microstructure and texture has been examined for deformation up to very large strains. In chapter 4, two medium Mn steels with 18 and 12 wt. pct. Mn have been studied about evolution of microstructure and texture, hence the associated the deformation mechanisms have been explored. For both the materials deformation texture has been characterized by the evolution of Bs-type texture. Deformation twinning has been identified as main mechanisms in the early stages of deformation and shear banding at intermediate to large deformation. Thermal stability of the deformed microstructure has been examined and recrystallization mechanisms have been identified. Chapter 5 deals with the effect of Al addition on deformation mechanisms, texture evolution and mechanical properties in medium Mn TWIP steels. The effect of Al addition has been studied more comprehensively. It was found that different Al containing alloy led to different mechanical response in terms of yield strength and strain hardening, which has been attributed to different propensity of twinning. In chapter 6, the basic principles of alloy design, deformation mechanisms and mechanical properties have been investigated for low Mn steels (<10 wt. pct. Mn), exhibiting TWIP/TRIP effects. It has been found that in these materials, microstructural development takes place on elemental partitioning during inter-critical annealing. The phase fraction of austenite in the microstructure depends on the inter-critical annealing temperature, annealing time, chemical composition and rolling conditions before annealing. An elaborate study on possible alloying additions and their consequences have also been discussed. Overall conclusions pertaining to the investigations carried out in the entire thesis have been summarized in chapter 7 along with the suggestions for the future work.

碩士
國立高雄大學
化學工程及材料工程學系碩士班
99
This study focused on the effect of Mn and Al alloys on the mechanical properties in cold-rolled TWIP steels, however, the effect of strain rate on mechanical properties was also investigated. In this study, steels were adopted as the test materials. According to thermodynamic calculation, the stacking fault energy of the 21Mn、24Mn and 24Mn-4Al were 21.9 mJ/m2, 29.2 mJ/m2 and 58.7 mJ/m2, respectively. It reveals that stacking fault energy increased with addition of manganese and aluminum. As the mechanical property, tensile test was carried out to investigate the TWIP steels tensile strength (T.S.), elongation (El.), and toughness (T.S.×El.) with various strain rates of 3.3×10-3s-1、3.3×10-2s-1、10-1s-1. The results showed that 21Mn possessed the maximum (T.S.) of 907.6MPa. and 24Mn possessed the maximum elongation (El.) of 78.9%. In addition, 24Mn-4Al possessed the maximum Y.S. of 352.9MPa. Summary the results of mechanical properties showed that 24Mn steel possessed the superior toughness of 69645.03MPa% (T.S.×El. value), while strain rate of 3.3×10-3s-1. 21Mn and 24Mn-4Al steels were 54819.04 MPa% and 43918.75 MPa%, respectively. From XRD results, 24Mn and 24Mn-4Al steels possessed stableγphase, even though after deformed. It was noticed the 21Mn specimen possessed γ(F.C.C.) structure before tensile test and obtained the phase transformation fromγ transform to εafter tensile test. The TEM results revealed ε-martensite and twin co-exist in 21Mn, it supported that deformation mechanism is TRIP and TWIP under tensile test. Only mechanical twin structure was observed in 24Mn, resulting in obtaining TWIP deformation mechanism. Moreover 24Mn-4Al possessed dislocation entanglement phenomenon and twin structure; as a result its deformation mechanism was classified to dislocation slip and partly TWIP mode. As the effect of strain rate on mechanical property with TWIP steels, the elongation (El.) of 21Mn was increased from 60.4% to 75.07% with strain rate increasing. In addition, the value of T.S.×El. was also increased to 68024.02 MPa%. However, 24Mn possessed the highest value of T.S.×El. reached to 69029 MPa%. But the strain rate was not affect the mechanical property of 24Mn-4Al significantly.

碩士
義守大學
材料科學與工程學系碩士班
98
The effects of Mn contents (19wt%、21wt%、24wt%) and Al contents (2wt%、4wt%) on the mechanical characteristics of twinning induced plasticity steels (TWIP steels) were investigated. Two major works on the TWIP steels with various Mn and Al contents were carried out in this study including: (1) the varied of stacking fault energy (SFE), high temperature ductility, and microstructure of TWIP steels with Al content; (2) the effect of hot rolling and cold rolling on microstructure and mechanical property of TWIP steels.The SFE of TWIP steels were calculated using thermodynamics. It is found that the SFE increases with inreasing Mn and Al contents. The SFE of TWIP steels studied are ranging from18 to 58 mJ/m2.Concerning the high temperature ductility, the reduction of area (RA) of all the steels decreased with decreasing deformation temperature. When the steels were deformed at 1100℃, the steel with 24wt% Mn content exhibited highest RA. The RA of TWIP steels with 24 wt% Mn content decreased with the addition of Al. The RA of the steel with 24wt%Mn-4wt%Al content was lower than 40% when it was deformed at 900℃, indicating a very poor hot workability.The mechanical performance of as-hot rolled TWIP steels are indicated by the product of tensile strength and total elongation (T.S.×El.). The steel with 21wt% Mn content exhibited best T.S.×El. product, whereas the steel with 24wt%Mn-4wt%Al content exhibited lowest product.Concerning the mechanical performance of TWIP steels subjected to SAT (SAT, 1000℃×20min.), both the steels with 21 and 24 wt% Mn contents exhibited the product over 70 000MPa%, whereas the steel with 24wt%Mn-4wt%Al content gain possessed lowest product. Supposed that the deformation mechanism is between twining induced plasticity and slip with the stacking fault energy of Al element about 60mJ/m2. Compare with C-Mn steels, TWIP steels possess the superior plasticity with Al content because of the slip mechanism.

This thesis performed within the Collaborative Research Center 799 describes a development of the metal-ceramic thermodynamic databases as applied to the design of the TRIP-Matrix-Composite materials. A wide range of theoretical and experimental investigations have been carried out in the relevant systems of Fe–Mg–Zr–O, Mg–Ti–Zr–O and Mg–Mn–Zr–O. Thermodynamic data were obtained using experimental methods of calorimetry and ab-initio calculations. Phase relations in the constituent binary and ternary systems have been studied using different types of static and dynamic methods. The obtained results allowed an assessment of thermodynamic parameters of the aforementioned systems using CALPHAD approach. The thermodynamic calculations have been performed to predict interfacial reactions within the composite material as well as to made recommendations for the design and further development of production processes for TRIP-Matrix-Composite materials.