Dissertationen zum Thema „Multi-Element alloys“

Bulk metallic glasses and multi-principal element alloys represent relatively new classes of multi-component engineering materials designed for satisfying multiple functionalities simultaneously. Correlating the microstructure with mechanical behavior (at the microstructural length-scales) in these materials is key to understanding their performance. In this study, the structure evolution and nano-mechanical behavior of these two classes of materials was investigated with the objective of fundamental scientific understanding of their properties. The structure evolution, high temperature nano-mechanical behavior, and creep of two Zr-based alloys was studied: Zr41.2Ti13.8Cu12.5Ni10.0Be22 (Vitreloy1) and Zr52.5Ti5Cu17.9Ni14.6All0 (Vitreloy105). Devitrification was found to proceed via the formation of a metastable icosahedral phase with five-fold symmetry. The deformation mechanism changes from inhomogeneous or serrated flow to homogenous flow near 0.9Tg, where Tg is the glass transition temperature. The creep activation energy for Vitreloy1 and Vitreloy105 were 144 kJ/mol and 125 kJ/mol, respectively in the range of room temperature to 0.75Tg. The apparent activation energy increased drastically to 192 kJ/mol for Vitreloy1 and 215 kJ/mol for Vitreloy105 in the range of 0.9Tg to Tg, indicating a change in creep mechanism. Structure evolution in catalytic amorphous alloys, Pt57.5Cu14.7Ni5.3P22.5 and Pd43Cu27Ni10P20, was studied using 3D atom probe tomography and elemental segregation between different phases and the interface characteristics were identified. The structure evolution of three multi-principal element alloys were investigated namely CoCrNi, CoCrFeMnNi, and Al0.1CoCrFeNi. All three alloys formed a single-phase FCC structure in as-cast, cold worked and recrystallized state. No secondary phases precipitated after prolonged heat treatment or mechanical working. The multi-principal element alloys showed less strain gradient plasticity compared to pure metals like Ni during nano-indentation. This was attributed to the highly distorted lattice which resulted in lesser density of geometrically necessary dislocations (GNDs). Dislocation nucleation was studied by low load indentation along with the evaluation of activation volume and activation energy. This was done using a statistical approach of analyzing the "pop-in" load marking incipient plasticity. The strain rate sensitivity of nanocrystalline Al0.1CoCrFeNi alloy was determined by in situ compression of nano-pillars in a Pico-indenter. The nanocrystalline alloy demonstrated a yield strength of ~ 2.4 GPa, ten times greater than its coarse grained counterpart. The nanocrystalline alloy exhibited high strain rate sensitivity index of 0.043 and activation volume of 5b3 suggesting grain boundary dislocation nucleation.

High entropy alloys (HEA) are a relatively new group of alloys first introduced in 2004. They usually contain 5 to 6 different principle elements. Each of these elements comprise 5-35 at. % of the chemical composition of the alloy. There is a growing interest in the research community about the development of these alloys as well as their engineering applications. Some HEAs have interesting properties that have made them well suited for higher temperature applications, particularly refractory uses, while some have been shown to maintain their mechanical properties even at cryogenic temperatures. Initially, the HEA research was focused on developing alloys with equiatomic compositions as it was believed that the single phase HEA would only form at such composition ratios. However, further research have found multiple HEAs with non-equiatomic chemical compositions. A major question that needs to be answered at this point is how to identify these non-equiatomic single phase alloy systems. Unlike the conventional alloys, the HEAs do not have a base element as a solvent, which complicates the identification of new alloy systems via conventional development techniques. To find a potential HEA, alloy development techniques of both exploratory and computational natures are being conducted within the community. Even though multiple HEAs have been successfully identified and fabricated by these techniques, in most cases they require extensive experimental data and are relatively time consuming and expensive. This study proposes a thin film combinatorial approach as a more efficient experimental method in developing new HEA alloy systems. In order to study HEA systems with different crystal structures, nominal HEA compositions were selected, including: CoFeMnNiCu in order to achieve face centered cubic (FCC) HEA, OsRuWMoRe to obtain hexagonal closed packed (HCP) and VNbMoTaW in an attempt to form a body centered cubic (BCC) crystal structure. Thin film samples were fabricated by simultaneous magnetron sputtering of the elements onto silicon wafer substrates. The arrangement of the sputtering targets yielded a chemical composition gradient in the films which ultimately resulted in the formation of various phases. Some of these phases exhibited the desired single-phase HEA, albeit with non-equiatomic chemical compositions. Bulk samples of the identified HEA compositions were prepared by arc melting mixtures of the metals. Microstructure of both thin film samples and bulk samples were characterized via scanning electron microscopy (SEM), focused ion beam (FIB) and energy dispersive x-ray spectroscopy (EDX). The crystal structures of the samples were studied by X-ray diffraction (XRD) and electron backscattered diffraction (EBSD) technique. Applying nano-indentation technique, the mechanical properties of some of the samples were screened over the composition gradient as well. By applying this combinatorial thin film approach, single-phase FCC, HCP and BCC HEAs were detected and successfully produced in bulk form. Additionally, screening the properties of the compositionally gradient thin films, as well as their chemical composition and crystal structure, provided a thorough understanding of the phase space. This experimental approach proved to be more efficient in identifying new alloy systems than conventional exploratory development methods.

Les alliages à éléments principaux multiples (MPEA) constituent une nouvelle catégorie d'alliages intéressante pour le stockage de l'hydrogène. Contrairement à un alliage conventionnel dans lequel 1 ou 2 éléments sont ajoutés en petite quantité à un élément à forte concentration, ici au moins 4 éléments sont mélangés dans des proportions quasi égales. Selon les compositions, l'augmentation de l'entropie de mélange peut permettre la formation d'une solution solide monophasée (de structure cubique ou hexagonale principalement). La pression d'équilibre est généralement inférieure à 1 bar, ce qui signifie que l'hydrure présente une grande stabilité thermodynamique. Cette faible valeur de la pression d'équilibre ne convient pas aux applications de stockage, car la réaction de déshydruration nécessite une quantité d'énergie conséquente pour se produire. Dans le but d'améliorer le premier plateau de pression d'équilibre, de nouvelles compositions sont conçues sur la base de la classification de type AB, avec A un élément formant un hydrure stable et B un élément formant un hydrure instable. Cette thèse porte sur la synthèse, les études microstructurales et structurales et des propriétés de sorption d'alliages à quatre éléments, principalement des éléments de transition
Multiple principal element alloys (MPEAs) are an interesting new class of alloys for hydrogen storage. Unlike a conventional alloy in which 1 or 2 elements are added in small quantities to a high-concentration element, here at least 4 elements are mixed in almost equal proportions. Depending on the composition, the increase in mixing entropy can lead to the formation of a single-phase solid solution (mainly cubic or hexagonal in structure). Equilibrium pressure is generally less than 1 bar, which means that the hydride is thermodynamically stable. This low equilibrium pressure is not suitable for storage applications, as the dehydridation reaction requires a significant amount of energy to occur. In order to improve the first equilibrium pressure plateau, new compositions are designed on the basis of the AB type classification, with A a stable hydride-forming element and B an unstable hydride-forming element. This thesis deals with the synthesis, microstructural and structural studies and sorption properties of four-element alloys, mainly transition elements

L’objectif principal de cette thèse est de développer des techniques de modélisation et de simulation multi-échelles avancées et efficaces pour les matériaux architecturés et composites à base d’Alliages à Mémoire de Forme (AMF). À cette fin, un modèle générique 3D multi-échelles pour les AMF architecturés est implémenté dans ABAQUS, où un modèle thermodynamique, proposé par Chemisky et al. [1], est adopté pour décrire le comportement constitutif local de l’AMF, et la méthode des éléments finis multi-échelles (EF2) pour réaliser l’interaction en temps réel entre le niveau microscopique et le niveau macroscopique. L’instabilité élastique des fibres au niveau microscopique est également étudiée efficacement dans ce cadre en introduisant la Méthode Asymptotique Numérique (MAN) et la Technique des Coefficients de Fourier à Variation Lente (TCFVL). Pour améliorer l’efficacité du calcul de l’approche simultanée à plusieurs échelles, dans laquelle d’énormes problèmes microscopiques sont résolus en ligne pour mettre à jour les contraintes macroscopiques, des méthodes de calcul multi-échelles basées sur les données sont proposées pour les structures composites. En découplant les échelles corrélées dans le cadre FE2, les problèmes microscopiques sont résolus hors ligne, tandis que le coût du calcul macroscopique en ligne est considérablement réduit. De plus, en formulant le schéma data-driven en contrainte et déformation généralisées, le calcul par la technique Structural-Genome-Driven est développé pour les structures composites à parois minces
The main aim of this thesis is to develop advanced and efficient multiscale modeling and simulation techniques for Shape Memory Alloys (SMAs) composite and architected materials. Towards this end, a 3D generic multiscale model for architected SMAs is implemented in ABAQUS, where a thermodynamic model, proposed by Chemisky et al. [1], is adopted to describe the local constitutive behavior of the SMA, and the multiscale finite element method (FE2) to realize the real-time interaction between the microscopic and macroscopic levels. Microscopic fiber instability is also efficiently investigated in this framework by introducing the Asymptotic Numerical Method (ANM) and the Technique of Slowly Variable Fourier Coefficients (TSVFC). To improve the computational efficiency of the concurrent mulitscale approach, in which tremendous microscopic problems are solved online to update macroscopic stress, data-driven multiscale computing methods are proposed for composite structures. Decoupling the correlated scales in concurrent FE2 framework, microscopic problems are solved offline, while the online macroscopic computational cost is significantly reduced. Further, by formulating the data-driven scheme in generalized stress and strain, Structural-Genome-Driven computing is developed for thin-walled composite structures

AA6061 Al alloys modified with addition of Mn, Cr and Cu were homogenized at temperatures between 350 ºC and 550 ºC after casting. STEM experiments revealed that the formation of α-Al(MnFeCr)Si dispersoids during homogenization were strongly affected by various factors such as heating rate, concentration of Mn, low temperature pre-nucleation treatment and homogenization temperature. Through analysis of the STEM results using an image software Image-Pro, the size distributions and number densities of the dispersoids formed during different annealing treatments were quantitatively measured. It was revealed that increasing the heating rate or homogenization temperature led to a reduction of the number density and an increase in size of the dispersoids. The number density of dispersoids could be markedly increased through a low temperature pre-nucleation treatment. A higher Mn level resulted in the larger number density, equivalent size and length/width ratio of the dispersoids in the alloy. Upsetting tests on two of these Mn and Cr-containing AA6061 (Al-Mg-Si-Cu) Al alloys with distinctive Mn contents were carried out at a speed of 15 mm s-1 under upsetting temperature of 450 ºC after casting and subsequent homogenization heat treatment using a 300-Tone hydraulic press. STEM experiments revealed that the finely distributed α-Al(MnFeCr)Si dispersoids formed during homogenization showed a strong pinning effect on dislocations and grain boundaries, which could effectively inhibit recovery and recrystallization during hot deformation in the two alloys. The fractions of recrystallization after hot deformation and following solution heat treatment were measured in the two alloys with EBSD. It was found that the recrystallization fractions of the two alloys were less than 30%. This implied that the finely distributed α-dispersoids were rather stable against coarsening and they stabilized the microstructure by inhibiting recovery and recrystallization by pinning dislocations during deformation and annealing at elevated temperatures. By increasing the content of Mn, the effect of retardation on recrystallization were further enhanced due to the formation of higher number density of the dispersoids. STEM and 3-D atom probe tomography experiments revealed that α-Al(MnFeCr)Si dispersoids were formed upon dissolution of lathe-shaped Q-AlMgSiCu phase during homogenization of the modified AA6061 Al alloy. It was, for the first time, observed that Mn segregated at the Q-phase/matrix interfaces in Mn-rich regions in the early stage of homogenization, triggering the transformation of Q-phase into strings of Mn-rich dispersoids afterwards. Meanwhile, in Mn-depleted regions the Q-phase remained unchanged without segregation of Mn at the Q-phase/matrix interfaces. Upon completion of α-phase transformation, the atomic ratio of Mn and Si was found to be 1:1 in the α-phase. The strengthening mechanisms in the alloy were also quantitatively interpreted, based on the measurements of chemical compositions, dispersoids density and size, alloy hardness and resistivity as a function of the annealing temperature. This study clarified the previous confusion about the formation mechanism of α-dispersoids in 6xxx series Al alloys. Four-point bend fatigue tests on two modified AA6061 Al alloys with different Si contents (0.80 and 1.24 wt%, respectively) were carried out at room temperature, f = 20 Hz, R = 0.1, and in ambient air. The stress-number of cycles to failure (S-N) curves of the two alloys were characterized. The alloys were solution heat treated, quenched in water, and peak aged. Optical microscopy and scanning electron microscopy were employed to capture a detailed view of the fatigue crack initiation behaviors of the alloys. Fatigue limits of the two alloys with the Si contents of 0.80 and 1.24 wt% were measured to be approximately 224 and 283.5 MPa, respectively. The number of cracks found on surface was very small (1~3) and barely increased with the applied stress, when the applied stress was below the yield strength. However, it was increased sharply with increase of the applied stress to approximately the ultimate tensile strength. Fatigue crack initiation was predominantly associated with the micro-pores in the alloys. SEM examination of the fracture surfaces of the fatigued samples showed that the crack initiation pores were always aspheric in shape with the larger dimension in depth from the sample surface. These tunnel-shaped pores might be formed along grain boundaries during solidification or due to overheating of the Si-containing particles during homogenization. A quantitative model, which took into account the 3-D effects of pores on the local stress/strain fields in surface, was applied to quantification of the fatigue crack population in a modified AA6061 Al alloy under cyclic loading. The pores used in the model were spherical in shape, for simplicity, with the same size of 7 μm in diameter. The total volume fraction of the pores in the model were same as the area fraction of the pores measured experimentally in the alloy. The stress and strain fields around each pore near the randomly selected surface in a reconstructed digital pore structure of the alloy were quantified as a function of pore position in depth from the surface using a 3-D finite element model under different stress levels. A micro-scale Manson-Coffin equation was used to estimate the fatigue crack incubation life at each of the pores in the surface and subsurface. The population of fatigue cracks initiated at an applied cyclic loading could be subsequently quantified. The simulated results were consistent with those experimentally measured, when the applied maximum cyclic stress was below the yield strength, but the model could not capture the sudden increase in crack population at UTS, as observed in the alloy. This discrepancy in crack population was likely to be due to the use of the spherical pores in the model, as these simplified pores could not show the effects of pore shape and their orientations on crack initiation at the pores near surface. Although it is presently very time-consuming to calculate the crack population as a function of pore size and shape in the alloy with the current model, it would still be desirable to incorporate the effects of shape and orientation of the tunnel-shaped pores into the model, in the future, in order to simulate the fatigue crack initiation more accurately in the alloy.

La premiere partie de cette etude a ete consacree a la conception de deux nouveaux alliages de brasure de l'alumine en visant des caracteristiques optimales pour l'utilisateur. L'essentiel du travail a consiste en une etude des proprietes de mouillage, realisee par la methode de la goutte posee sous vide pousse, avec les deux alliages cupdti et nifecrsiti sur substrats mono et polycristallins d'alumine. Les interfaces et les produits de reaction ont ete caracterises par rugosimetrie, microscopie electronique a balayage et micro-analyse, et les resultats discutes a l'aide d'arguments thermodynamiques. La deuxieme partie a ete consacree a des aspects fondamentaux de la realisation par thermocompression d'une liaison entre monocristaux d'alumine et de nickel. Le travail a montre que l'on pouvait transposer a un couple metal-ceramique un modele de cinetique de soudage developpe pour des couples metalliques. Un calcul de reseau de coincidence a permis de confirmer que l'une des orientations cristallographiques relatives frequemment mentionnee dans la litterature, etait tres certainement energetiquement favorable. Avec cette orientation relative, nous avons pu effectivement synthetiser une serie d'echantillons bicristallins par thermocompression sous vide secondaire. La caracterisation de l'interface a ete faite par microscopie optique, microscopie electronique a balayage et en transmission.

Les excellentes proprietes mecaniques des superalliages leurs sont conferees par la precipitation de la phase gamma prime issue des divers traitements thermiques. L'etape de trempe qui succede a l'elaboration par metallurgie des poudres des disques de turboreacteur en superalliage a base de nickel, conditionne fortement l'etat mecanique et microstructural final du materiau. L'objectif de ce travail est de developper et de valider un modele qui decrit la cinetique de precipitation de la phase gamma prime afin d'optimiser la microstructure resultant de cette trempe. Une serie de demarches experimentales a permis d'analyser et d'identifier le comportement du materiau au cours de ce traitement thermique. L'influence de la vitesse de refroidissement sur la microstructure finale et sur l'allure de la fraction volumique transformee est etudiee experimentalement par dilatometrie de trempe et observation electronique a transmission. Ces observations montrent que la coalescence s'effectue suivant la loi de lsw et que l'on retrouve la sequence de transformation morphologique sphere cube octocube dendrite observee dans le cas des transformations anisothermes. La dilatometrie permet egalement de mettre en evidence le domaine de precipitation de la phase gamma prime. De la meme facon, l'influence de la temperature de mise en solution sur la microstructure et sur la fraction volumique f#v est analysee. De plus, les enregistrements dilatometriques permettent d'apprecier la dissolution de la phase gamma prime en cours de chauffage a partir de microstructures variees. Enfin, la microstructure du n18 en cours de refroidissement est suivie par hyper-trempe a environ 10#5 k/s dans un four concu specialement pour cette etude. Une famille tres fine de phase gamma prime est mise en evidence par met et diffusion de neutrons aux petits angles. L'ensemble des resultats obtenus permettent de valider le modele base sur la theorie classique de nucleation et les lois de coalescence de lsw et permet de rendre compte de la capacite du calcul a decrire la precipitation intragranulaire (secondaire et tertiaire) au cours de traitements thermiques divers

Les Alliages à Mémoire de Forme Magnétiques (AMFM) sont des matériaux actifs qui présentent des comportements inhabituels par rapport aux matériaux " classiques ". Ils peuvent par exemple présenter de larges déformations réversibles sous l'action d'un champ magnétique ou sous une action mécanique. Ce sont des candidats potentiels pour des applications dans des domaines de pointe (automobile, aéronautique, spatial, etc.). Les AMFM présentent par ailleurs un avantage indéniable par rapport aux matériaux à mémoire de forme " thermique " en raison de leur réponse dynamique à haute fréquence. Il est bien connu que ces comportements sont dus à un couplage magnéto-mécanique et à un phénomène physique lié à l'orientation des variantes de martensite. L'objectif de cette thèse est d'analyser les comportements magnéto-mécaniques des AMFM. Pour ce faire, nous étudions expérimentalement et théoriquement, la réorientation martensitique dans les AMFM. Tout d'abord, une analyse énergétique en 2D/3D est proposée et intégrée dans des diagrammes d'état pour une étude systématique de la réorientation martensitique dans les AMFM sous chargements tridimensionnels quelconques. Ainsi, des critères de large déformation réversible sous des chargements cycliques sont obtenus. L'analyse énergétique montre que les AMFM, sollicités sous chargement multiaxiaux présentent plus d'avantages que ceux sollicités en 1D ; en particulier, on montre que l'état multiaxial permet d'augmenter (d'améliorer) la contrainte fonctionnelle, ce qui augmente le champ d'application des ces matériaux. Ensuite, afin de valider les prédictions de l'analyse énergétique, des expériences bi-axiales ont été effectuées sur des éprouvettes en AMFM. Les résultats révèlent que la dissipation intrinsèque et la déformation de transformation dues à la réorientation martensitique sont constantes dans tous les états de contraintes. De plus, les résultats ont permis de valider nos prédictions théoriques quant à l'augmentation de la contrainte fonctionnelle. Enfin, afin de prédire les comportements magnéto-mécaniques des AMFM sous des chargements multiaxiaux, un modèle tridimensionnel est développé dans le cadre de la thermodynamique des processus irréversibles avec liaison interne. Toutes les variantes de martensite ont été considérées et l'effet de température a également été pris en compte. Les simulations numériques montrent un très bon accord (rejoignent/confirment les résultats) avec les résultats expérimentaux existant dans la littérature. Le modèle a ensuite été programmé dans un code de calcul par éléments finis afin d'étudier les comportements non linéaires de flexion des poutres en AMFM. L'effet géométrique et l'effet d'anisotropie du matériau ont été systématiquement pris en compte.

碩士
國立清華大學
材料科學工程學系
102
This study investigates the superconductivity of as-cast and as-heat-treated ternary to 6-element multi-component alloys (hereafter abbreviated as the alloys) that are made of non-equal molar Nb-Zr-Ti by addition of minor Hf, V, Ta and Ge. The alloys are principally single random BCC Nb-rich solid solutions. Difference in formation enthalpies between elements, especially for pairs with Ta and Ge, and the driving forces by heat treatments induce a Zr-bearing precipitation from the Nb-rich BCC phase to form a Zr(Ge)-rich phase. This results in a change of Nb/Zr ratio in the Nb-rich BCC phase, and thus affects the superconductivity of the alloys. The critical temperature of the alloys, Tc, ranges from 8 K to 11 K. The room-temperature resistivity of the as-cast alloys varies from 21 μΩ–cm to 35 μΩ–cm. Compared with the electrical resistivity of other multi-component ones, the alloys have a lower electrical resistivity. The residual resistivity ratio RRR value is from 1.2 to 1.3, which mentions that the resistivity is principally controlled by the impurity atoms in the alloys. If one simply emphasizes the e/a ratio from the content of Nb and Zr in the alloys, the Tc of the alloys approximately follows the Matthias empirical rule. However, factors affecting Tc, besides the e/a, include lattice distortion due to multiple element addition, and the characteristics of individual elements in the alloys. The alloys are typically type II superconductors. The upper critical magnetic field Hc2 is estimated to be in the range of 5 T to 9 T. At 2 K &; 5 T, the critical current density Jc has the value of approximately 105 A/cm2. This property has something to do with the precipitates and has yet nothing to do with the lattice distortion of the alloys.

碩士
國立清華大學
材料科學工程學系
92
A good thermoelectric material requires high Seebeck coefficient (S) and electrical conductivity (σ) but low thermal conductivity (κ), which are not common found in traditional metallic alloys. As compound with traditional metallic alloy, high entropy alloy of multi constituent element is a newly developed metallurgical concept. A variety of different forms of the multi-element alloys have been demonstrated to possess superior mechanical and chemical properties. However, the electrical and thermoelectric properties of the multi-element alloys have not been thoroughly explored yet. In this study, multi-element alloys were prepared by arc-melting and thermal annealing. Their thermoelectric properties were characterized after appropriate sample preparation. We find two multi-element alloy systems based on half-Heusler structure：ZrTiSnSiNi2 and ZrTiSnGeNi2. The thermoelectric figure of merits (ZT , Z= S2/(ρ×κ)) of these multi-element alloys were equal to be 0.02 and 0.045, respectively, at room temperature. These multi-element alloys showed 3 times increase in power factor (S2/ρ) in the middle temperature range (250℃ to 300℃). By adding Nb element into the alloy forming Zr0.9Ti0.9Nb0.2SnSiNi2, the thermoelectric figure of merit was found to be 0.078 at room temperature. The multi- element alloys were identified to be a composite material containing semiconductor-like phase and metallic-like phase by X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectrometer (EDS).

碩士
國立清華大學
材料科學工程學系
94
A effect of Sb doping (x=0~0.5) on the thermoelectric properties of annealed Ti0.5Zr0.5NiSn1-xSbx multi-element alloys is studied based on Ti0.5Zr0.5NiSn quaternary alloy with half-Heusler structure. The result shows both the absolute value of Seebeck coefficient and electrical resistivity decrease as the quantity of Sb doping increases at room temperature. The change of resistivity is marked for the alloy with slight Sb doping. The maximum power factor(S2/λ) is 2.83×10-3W/m-K2 for the annealed Ti0.5Zr0.5NiSn1-xSbx (x=0.005) alloy. It is three times larger than that of Ti0.5Zr0.5NiSn which is 0.72×10-3W/m-K2. The maximum ZT value is about 0.16 for the Ti0.5Zr0.5NiSn1-xSbx (x=0.005) alloy at room temperature which is four times larger than that of Ti0.5Zr0.5NiSn. Therefore, the lightly Sb doping would dramatically improve the thermoelectric properties of the Ti0.5Zr0.5NiSn alloy system. Besides, the results of thermoelectric properties measurement at high temperature reveal that the absolute value of Seebeck coefficient increases as the temperature rises. The resistivity of Sb doped alloys increases with rising temperature, indicating that the Sb doping would promote the metallic transport properties. Consequently, a slight Sb doping (below 5at% substitution of Sn atoms)would enhance the thermoelectric properties of Ti0.5Zr0.5NiSn1-xSbx alloy system.

Quantitative diffusion analysis in multicomponent metallic systems has been a formidable task historically and despite decades of research, most of the diffusivity estimations were limited to interdiffusion and some intrinsic diffusion coefficients in binary systems and interdiffusion coefficients in a few ternary systems until recently. The experimental complications associated with the need to intersect (n-1) serpentine diffusion paths in the n dimensional space for determining the 〖(n-1)〗^2 interdiffusion coefficients lead to various approaches like average diffusivity, square root diffusivity estimations that approximate a representative value of the diffusivity across a composition range from a single experiment. However, these values are not material constants and do not provide any information about the atomic interactions. This lack of diffusivity data in multicomponent systems has hampered the development of mobility databases essential for various simulations and physico-chemical studies of materials. This work resolves the issues with quantitative multicomponent diffusion analysis via several newly proposed methods that solves the issue of intersecting diffusion paths through the application of special constrained diffusion paths. The equations necessary to apply these methods are derived and their application is discussed mathematically and applied experimentally to the model alloy system, the NiCoFeCr equiatomic multiprincipal element alloy to compare with available radiotracer data measured for this system. The work first employs the pseudo-binary diffusion couple approach that develops a rectilinear diffusion path in the multicomponent space to the NiCoFeCr system to estimate the tracer coefficients from the intrinsic coefficients at the marker plane. The mathematical formulations derived for the same justify its namesake and the obtained tracer coefficients can be used to back calculate the intrinsic and interdiffusion coefficients. The pseudo-ternary method improves on the shortcomings of the pseudo-binary diffusion couple method and enables the estimation of tracer coefficients of three components by crossing two constrained diffusion paths in a 2d plane in addition to the main and cross interdiffusion coefficients. The body diagonal method originally proposed for determination of interdiffusion coefficients is modified here to determine the tracer coefficients of all components using only two diffusion profiles thus reducing the errors associated with crossing (n-1).paths per the original approach. This work then explores the possibilities of crossing dissimilar constrained diffusion paths by crossing pseudo ternaries of different types. Strategically crossing a rectilinear pseudo-binary diffusion path with a serpentine conventional (body diagonal) diffusion path overcomes all the previous drawbacks of pseudo binary, pseudo ternary and body diagonal methods to determine the full set of diffusivities at any desired composition and generalizes the constrained diffusion path approach to any order multicomponent system. The obtained tracer coefficients show a good match with the diffusivities measured in radio tracer experiments. Finally, based on the ideas from the constrained diffusion path experiments in the NiCoFeCr system, a constrained path approach is devised to measure the diffusivities in an Al based NiCoFeCr multiprincipal element alloy system which was not possible earlier due to unavailability of radio isotopes and the complexities of interdiffusion experiments in higher order systems. The obtained tracer diffusivities, show an excellent match with the trends extrapolated from lower order systems. Calculated intrinsic and interdiffusion coefficients demonstrate the importance of vacancy wind effect as well as the issues with using diffusivities having different dependent components to make predictions on diffusion trends among different elements.

碩士
國立臺灣大學
機械工程學研究所
93
The superior properties of aluminum-alloy attracted attentions from the light-trend industry recently. Although the principal manufacturing process of the aluminum-alloy hollow products has been extruded by using porthole dies, the seamless extrusion with a mandrel has considerable potential because of its competitive productivity and performance. So the seamless extrusion process of aluminum-alloy tubes at elevated temperatures was studied in the present study by the finite element analysis and experiments. The trend of processing load and temperature on single-hole die extrusion was studied at first, the finite element software DEFORM was employed to simulate the hollow tubes processes. The experiment of single-hole die extrusion was practiced to verify the analyzed results. The experimental results obtained in the present study show good agreement with simulations. It shows the finite element software DEFORM is suitable to process of seamless tube extrusion. In order to increase the productivity, multi-hole die with several mandrels is applied. But the defects, which are the bias of mandrel and the unbalanced thickness of production caused by material flow are critical issues. The influence of important process parameters such as the temperature, extrusion speed, production dimension, billet size, number and position of holes on die are analyzed by simulations. According to the results of analysis, it shows the serious effect could be caused by the number and position of holes on die. Besides, the material flow is better when the hole is near the focus of action area. The bias phenomenon of mandrel could be improved after die stress analysis by the software DEFORM. The results of this study can be reference resources for related academic research and can also be used to develop related products for industry production.

碩士
國立成功大學
機械工程學系碩博士班
94
The present study investigates the microstructure and strength of multi-element alloy by quenching from the liquid with the theory of molecular dynamics simulation. The applied potential parameter is the semi-empirical potential parameter of the tight-binding method. The fabrication parameter is analyzed by Honeycutt-Anderson analysis and the relation between the stress and strain of the tensile test, including the influences of the types and proportions of the added elements on the structure and strength of alloy. The factor which caused the good amorphous formation was also investigated in this study. More bigger variations between the added atoms are, more easier to obtain high proportion of amorphous structure. By adding same proportion of zirconium atoms, comparing with other added atoms, large number of amorphous structure will be obtained in the result. Nevertheless, due to that there was specified proportion of BCC structure in the alloy, amorphous structure can only reach the maximum proportion of 80%. The BCC structure mostly composed of the intermetallic compound and the crystalline structure of zirconium at high temperature. Because the size effect of amorphous alloy is slight, the strength of nanowire is equivalent to its bulk material. This showed that the strength of amorphous alloy is far greater than that of crystalline alloy and the strength of amorphous alloy is with respect to the type of added atoms. Improvement and inference of molecular dynamics simulation were mentioned at the final of this study, which provided the aspect of future investigation.

碩士
國立中正大學
物理所
96
In order to simplify the manufacturing process for making isotropic magnet and also improving the magnetic properties, we adopt the copper mold casting method to fabricate permanent magnetic Nd9.5Febal.Ti3-xMxB15(M=Nb and Zr) alloy rods (a diameter of 0.9 mm and 16 mm in length). The cosubstitution of sufficient Ti/Nb and Ti/Zr, respectively, not only led to the finer microstructure, but also improved the exchange coupling effect between the grains of the cast magnet significantly. The optimal magnetic properties of Br=6.6 kG, iHc=9.6 kOe and (BH)max=8.2 MGOe could be achieved in Nd9.5Febal.Ti2.5Zr0.5B15 alloy rod. On the other hand, to understand the effect of Zr substitution in NdFeTiB system, different manufacturing processes, direct casting and melt spinning, were employed in Nd9.5Febal.Ti3-xZrxB15(x=0 and 0.5) alloys. The results showed that the whole magnetic properties were improved not only in the magnets made by direct casting but also in those by melt spinning technique. It is presumed that the anisotropy field HA of 2:14:1 phase is increased by the entrance of Zr atoms in the lattice, resulting in the improvement of the iHc and (BH)max simultaneously. In RyFebal.Ti2.5Zr0.5B15(R=Nd and Pr; y=7-11.5) alloys, the iHc increased and σ12kOe decreased with the increase of the rare earth element content, arisen from the increase of the volume fraction of magnetically hard 2:14:1 phase. While in Nd9.5Febal.Ti2.5Zr0.5Bx(x=14-20), the iHc increased and Br decreased, due to the finer grain and the appearance of amorphous phase, with increasing B content in the magnets. Finally, the influences of various elements (C, Co and Cr) on magnetic properties of Nd9.5-zFebal.-yTi2.5Zr0.5B15-xM (M=Cx, Coy and Crz；x=0.5, y=2.5 and z=1) alloy rods has also been investigated. The coercivity was increased due to the finer grain with substituting the C, Co and Cr element. Besides, the curie temperature of Nd2Fe14B phase was increased by proper Co substitution, leading to the improvement of the thermal stability of the magnets.