Dissertations / Theses on the topic 'Alloy grain structure'

<p>Cold chamber high pressure die casting, (HPDC), is an important commercial process for the production of complex near net shape aluminium and magnesium alloy castings. The work presented in the thesis was aimed at investigating the microstructure formation in this type of casting. The solidification characteristics related to the process and the alloys control the formation of grains and defects. This again has a significant impact on the mechanical properties of the castings.</p><p>The investigations were carried out mainly using the AM60 magnesium alloy and the A356 aluminium alloy. Two different casting arrangements were used: the cold chamber HPDC and the gravity die casting methods, which allowed for different flow and solidification conditions. The microstructures in the castings were investigated using optical microscopy, image analysis, scanning electron microscopy, electron back scatter diffraction measurements and electron probe microanalysis.</p><p>In the HPDC experiments, the shot sleeve solidification conditions were investigated primarily by changing the melt superheat on pouring. This significantly affected the microstructures in the castings. The fraction of externally solidified crystals (ESCs) was consistently found to be largest near the gate in both the AM60 and the A356 die castings. This was attributed to the inherent shot sleeve solidification conditions and the flow set up by the plunger movement. When the superheat was increased, a lower fraction of ESCs was found in the castings. Furthermore, a high superheat gave ESCs with branched dendritic/elongated trunk morphology whilst a low superheat generated coarser and more globular ESCs, both in the AM60 and the A356 castings. The ESCs typically segregated towards the central region of the cross sections at further distances from the gate in the die castings.</p><p>When a thin layer of thermal insulating coating was applied on the shot sleeve wall in the production of AM60 die castings, it nearly removed all ESCs in the castings. Using an A356 alloy, (and no shot sleeve coating), with no Ti in solution gave a significantly lower fraction of ESCs, whereas AlTi5B1 grain refiner additions induced an increase in the fraction of ESCs and a significantly finer grain size in the castings. The formation of globular ESCs was enhanced when AlTi5B1 grain refiner was added to the A356 alloy.</p><p>In controlled laboratory gravity die casting experiments, typical HPDC microstructures were created by pouring semi-solid metal into a steel die: The ESCs were found to segregate/migrate to the central region during flow, until a maximum packing, (fraction of ESCs of ~35-40%), was reached. The extent of segregation is determined by the fraction of ESCs, and the die temperature affects the position of the ESCs. The segregation of ESCs was explained to occur during flow as a result of lift forces.</p><p>The formation of banded defects has also been studied: the position of the bands was affected by the die temperature and the fraction of ESCs. Based on the nature of the bands and their occurrence, a new theory on the formation of defect bands was proposed: During flow the solid distribution from the die wall consists of three regions: 1) a solid fraction gradient at the wall; 2) a low solid fraction region which carries (3) a network of ESCs. A critical fraction solid exists where the deformation rate exceeds the interdendritic flow rate. When the induced stress exceeds the network strength, deformation can occur by slip, followed by liquid flow. The liquid flow is caused by solidification shrinkage, hydrostatic pressure on the interior ESC network, and gaps forming which draw in liquid.</p>

Hot tearing is a common and severe defect encountered in alloy castings and perhaps the pivotal issue defining an alloy's castability. Once it occurs, the casting has to be repaired or scraped, resulting in significant loss. Over the years many theories and models have been proposed and accordingly many tests have been developed. Unfortunately many of the tests that have been proposed are qualitative in nature; meanwhile, many of the prediction models are not satisfactory as they lack quantitative information, data and knowledge base. The need exists for a reliable and robust quantitative test to evaluate/characterize hot tearing in cast alloys. This work focused on developing an advanced test method and using it to study hot tearing in cast aluminum alloys. The objectives were to: 1) develop a reliable experimental methodology/setup to quantitatively measure and characterize hot tearing; and 2) quantify the mechanistic contributions of the process variables and investigate their effects on hot tearing tendency. The team at MPI in USA and CANMET-MTL in Canada has collaborated and developed such a testing setup. It consists mainly of a constrained rod mold and the load/displacement and temperature measuring system, which gives quantitative, simultaneous measurements of the real-time contraction force/displacement and temperature during solidification of casting. The data provide information about hot tearing formation and solidification characteristics, from which their quantitative relations are derived. Quantitative information such as tensile coherency, incipient crack refilling, crack initiation and propagation can be obtained. The method proves to be repeatable and reliable and has been used for studying the effects of various parameters (mold temperature, pouring temperature and grain refinement) on hot tearing of different cast aluminum alloys. In scientific sense this method can be used to study and reveal the nature of the hot tearing, for industry practice it provides a tool for production control. Moreover, the quantitative data and fundamental knowledge gained in this thesis can be used for validating and improving the existing hot tearing models.

Analyse de l'evolution de la macrostruture et de la microstruture des alliages de fonderie aluminium-silicium as7g en fonction de la composition, des conditions de solidification et des traitements thermiques. L'etude de la macrostruture s'est faite a partir de la reconstruction des zones eutectiques en utilisant des transformations morphologiques et en respectant les conditions de la connaissance locale simple. Quantification de la macrostructure a partir des analyses granulometriques et stereologiques. L'analyse de la microstructure est realisee a partir d'une etude globale de la forme des particules, de silicium. Mise au point et etude critique des procedures d'analyse automatique d'images

Additive Manufacturing (AM) is an innovative manufacturing process which offers near-net shape fabrication of complex components, directly from CAD models, without dies or substantial machining, resulting in a reduction in lead-time, waste, and cost. For example, the buy-to-fly ratio for a titanium component machined from forged billet is typically 10-20:1 compared to 5-7:1 when manufactured by AM. However, the production rates for most AM processes are relatively slow and AM is consequently largely of interest to the aerospace, automotive and biomedical industries. In addition, the solidification conditions in AM with the Ti alloy commonly lead to undesirable coarse columnar primary β grain structures in components. The present research is focused on developing a fundamental understanding of the influence of the processing conditions on microstructure and texture evolution and their resulting effect on the mechanical properties during additive manufacturing with a Ti6Al4V alloy, using three different techniques, namely; 1) Selective laser melting (SLM) process, 2) Electron beam selective melting (EBSM) process and, 3) Wire arc additive manufacturing (WAAM) process. The most important finding in this work was that all the AM processes produced columnar β-grain structures which grow by epitaxial re-growth up through each melted layer. By thermal modelling using TS4D (Thermal Simulation in 4 Dimensions), it has been shown that the melt pool size increased and the cooling rate decreased from SLM to EBSM and to the WAAM process. The prior β grain size also increased with melt pool size from a finer size in the SLM to a moderate size in EBSM and to huge grains in WAAM that can be seen by eye. However, despite the large difference in power density between the processes, they all had similar G/R (thermal gradient/growth rate) ratios, which were predicted to lie in the columnar growth region in the solidification diagram. The EBSM process showed a pronounced local heterogeneity in the microstructure in local transition areas, when there was a change in geometry; for e.g. change in wall thickness, thin to thick capping section, cross-over’s, V-transitions, etc. By reconstruction of the high temperature β microstructure, it has been shown that all the AM platforms showed primary columnar β grains with a <001>β.

Built on the recent successes in understanding the crystallographic mechanism for short fatigue crack (SFC) growth across a grain boundary (GB) and developing an experimental method to quantify the GB resistance against short crack growth, a microstructure-based model was developed in this study to simulate the growth behaviors of SFCs in 3-D, by taking into account both the driving force and resistance along at each point along the crack front in an alloy. It was found that the GB resistance was a Weibull function of the minimum twist angle of crack deflection at the boundary in AA2024-T3 Al alloys. In the digital microstructure used in the model, the resistance at each GB that the short crack interacted with could be calculated, as long as the orientations of grains and the crack were known. In the model, an influence function accounting for the overlapping effect of the resistance from the neighboring grain boundaries was proposed, allowing for calculation of the total resistance distribution along the crack front. In order to overcome the time consuming problem for the existing equations to derive the distribution of stress intensity factor along the crack front under cyclic loading, an analytical equation was proposed to quantify the stress intensity factor distribution along an irregular shape planar crack. By introducing two shape-dependent factors, the fractured area and the perimeter of the crack front, the newly proposed equation could readily and accurately derive the stress intensity factor distribution along the crack front that had large curvatures and singularities. Finally, a microscopic-scale Paris’ equation was proposed that took into account both the driving force, i.e., stress intensity factor range, and the total resistance to calculate the growth rate at each point along crack front. The model developed in this work was able to incorporate microstructure, such as grain size and shape, and texture into simulation of SFC growth in 3-D. It was capable of simulating all the anomalous growth behaviors of SFCs, such as the marked scatters in growth rate measurement, retardation and arrest at grain boundaries, and crack plane deflection at grain boundaries, etc. The model was used to simulate the growth behaviors of SFCs initiated from prefractured constituent particles in order to interpret the multi-site fatigue crack initiation observed in AA2024-T351 Al alloys. Three types of SFCs were observed initiating from these particles, namely, type-I non-propagating cracks; type-II cracks which were arrested soon after propagating into the matrix; and type-III propagating cracks. To quantitatively study the 3-D effects of particle geometry and micro-texture on the growth behaviors of micro-cracks in these particles, rectangular micro-notches with different dimensions were fabricated using focused ion beam in the selected grains on the T-S planes in AA2024-T351 Al alloys, to mimic the pre-fractured particles in these alloys. Knowing the notch dimensions or particle shape, grain orientation and GB geometry, the simulated crack growth behaviors were consistent with the experimental observations, and the model was able to verify that the three types of cracks evolved from these particles were mainly associated with the thickness and width of the pre-fractured particles, though the particle geometry and grain orientation could also affect the behaviors of fatigue crack initiation at the particles. When the widths of the particles were less than 15 μm, like in most high strength Al alloys, the simulated results confirmed that the crack type was only associated with the particle thickness, consistent with the experimental results in AA2024-T351 alloys with a strong rolling texture. The lives for the SFCs to reach 0.5 mm in length were quantified with the model in the AA2024 alloy, revealing that there was a bimodal distribution in the life spectrum calculated, with the longer life peak being related to larger twist angles of crack deflection at the first GB the cracks encountered and the shorter life peak being associated with small twist angles (< 5°) at the first GB. The model further demonstrated the influence of grain structure on SFC growth by considering two different grain structures with the same initial short crack, namely, a layered grain structure with only the primary GBs perpendicular to the surface and the layered grains with both primary and secondary GBs. Depending on their positions and geometry, the secondary GBs could still exert a strong retarding effect on SFC growth on surface. The model was validated by matching to the growth rate measured on surface of a SFC in an AA8090 Al-Li alloy. Good consistency was achieved between the simulated and experimentally measured growth rates when both the primary and secondary GBs were considered in the model. The model developed in this study exhibits its potential applications to optimizing the microstructure and texture in alloys to enhance their fatigue resistance against fatigue crack growth, and to satisfactory life prediction of engineering alloys.

Thermoelectric (TE) ZrNiSn samples with a half-Heusler atomic structure were synthesized by mechanical alloying (MA) and consolidation by either Spark Plasma Sintering (SPS) or hot pressing (HP). X-Ray diffraction patterns of as milled powders and consolidated samples were compared and analyzed for phase purity. Thermal conductivity, electrical conductivity and Seebeck coefficient are measured as a function of temperature in the range 300 K to 800 K and compared with measurements reported for high temperature solid state reaction synthesis of this compound. HP samples, compared to SPS samples, demonstrate increased grain growth due to longer heating times. Reduced grain size achieved by MA and SPS causes increased phonon scattering due to the increased number of grain boundaries, which lowers the thermal conductivity without doping the base system with addition phonon scattering centers. Mechanical characterization of the samples by microindentation and depth sensing indentation for hardness and elastic modulus will be discussed.

La combinaison de deux mécanismes permettant d’accroitre la résistance mécanique (précipitation et joints de grains) a été explorée dans cette thèse afin d’augmenter les propriétés d’alliages d’aluminium à durcissement structural de la série 7###. Les techniques d’élaboration par déformation plastique intense permettent d’obtenir des alliages nano-structurés comportant une grande densité de joints de grains qui permet une augmentation conséquente de la limite d’élasticité selon la loi de Hall-Petch. Cependant, la grande densité de défauts (dislocations, lacunes, joints de grains…) et les contraintes internes générées par cette déformation donnent lieu à des nanostructures hors équilibre intrinsèquement instables lors d’un traitement de précipitation. Cela se traduit d’une part par une croissance rapide des grains et d’autre part par des changements dans les mécanismes de précipitation (précipitation hétérogène, cinétique accélérée). Nous avons étudié les nanostructures obtenues par déformation plastique intense via HPT et HPS (High pressure torsion/sliding) sur un alliage modèle, Al-2%Fe, et un alliage commercial AA7449 enrichi en fer. La stratégie choisie était de stabiliser la structure à grains ultra-fins par des nanoparticules d’intermétalliques riches en fer (épinglage de Zener) pour permettre une précipitation homogène de phases durcissantes et de combiner ainsi les 2 mécanismes pour accroître la limite élastique. Dans ce contexte, nous nous sommes plus particulièrement intéressés à : 1) l’influence des solutés sur les mécanismes physiques de recristallisation dynamique conduisant à la nanostructuration ; 2) les mécanismes spécifiques impliqués lors de la co-déformation de phases aux comportements mécaniques très différents ; 3) les transformations de phase pouvant conduire soit à la formation d’une solution solide sursaturée ou bien au contraire à la décomposition d’une solution solide par précipitation induite par déformation ; 4) aux relations entre les nanostructures ainsi générées, leur stabilité thermique et leur résistance mécanique. L’observation des microstructures et la compréhension des mécanismes induits par la déformation ainsi que les relations avec le comportement mécanique ont été entrepris grâce à l’utilisation de nombreuses techniques : la microscopie électronique à balayage, en transmission, ASTAR (cartographie d’orientation au MET), ainsi que la sonde atomique tomographique. L’étude de la précipitation a été réalisée par DSC (calorimétrie différentielle à balayage), SAXS (diffusion des rayons X aux petits angles) et MET in-situ. Finalement, la relation avec le comportement mécanique a été établie sur la base d’essais de traction et de mesures de microdureté<br>The combination of two mechanisms to increase mechanical strength, namely precipitation and grain size reduction, has been explored in this thesis in the aim of increasing the properties of age hardenable aluminum alloy from the 7### series.Manufacturing by severe plastic deformation makes it possible to obtain nanostructured alloys with high density of grain boundaries, which allows increasing the yield strength according to the Hall-Petch law. However, the high density of defects (dislocations, vacancies, grain boundaries ...) and the internal stresses generated by this deformation results in inherently unstable nanostructures when precipitation heat treatment is performed. These nanostructures experience rapid grain growth and drastic changes in precipitation mechanisms (heterogeneous precipitation, accelerated kinetics).In this work we have studied nanostructures obtained by severe plastic deformation using HPT and HPS (High pressure torsion / sliding) on a model alloy, Al-2% Fe and a commercial alloy AA7449 enriched with iron. The strategy was to stabilize the ultra-fine grain structure by intermetallic iron-rich nanoparticles (Zener pinning) to allow homogeneous precipitation hardening and thus combine the two mechanisms to increase the yield strength. In this context, we have particularly investigated: 1) the influence of solutes on the physical mechanisms leading to dynamic recrystallization nanostructuring; 2) specific mechanisms involved in co-deforming phases with very different mechanical behaviors; 3) the phase transformations that may lead either to the formation of a supersaturated solid solution or, on the contrary, to the decomposition of a solid solution by deformation-induced precipitation; 4) the relationship between the nanostructures thus generated, their thermal stability and related mechanical properties.The observation of the microstructures and understanding of the mechanisms induced by the deformation and relations with the mechanical behavior has been undertaken with many techniques: scanning and transmission electron microscopy (SEM/TEM), ASTAR (orientation mapping by TEM), and atom probe tomography. The study of precipitation was carried out by DSC (differential scanning calorimetry), SAXS (small angle X-ray scattering) and in-situ TEM. Finally, the relationship with the mechanical behavior has been established on the basis of tensile tests and micro-hardness measurements

博士<br>國立臺灣大學<br>材料科學與工程學研究所<br>101<br>The Ag-8Au-3Pd wire can achieve enrichment of annealing twins by improvement of drawing and annealing processes. It remains almost the same grain size during aging at 600℃ for 180 minutes, thus possesses high thermal stability. Besides, the twinned-grain percentage of this wire reachs 65%, more than conventional Ag-8Au-3Pd wire, gold wire and copper wire. After high temperature storage, this wire still has higher tensile strength, yield stress, elongation than conventional Ag-8Au-3Pd wire, keeping high electrical conductivity meantime. This innovative annealing-twinned Ag-8Au-3Pd wire with diameter of 17.5 μm can sustain 0.3A for 61 hours, which is much higher than conventional Ag-8Au-3Pd wire’s 25hours, reaching more than twice the life. This result can be attributed to the annealing twins’ strengthening effect, the stepwise structure and reconstruction during curren stressing. The reconstruction is produced by primary slip and perpendicular secondary slip, the two directional slips result atom pile up and generate hillock. Theis annealing-twinned Ag-8Au-3Pd wire possesses higher tensile strength and elongation than conventional Ag-8Au-3Pd wire with different current stressing time. Since these superior properties, this annealing-twinned Ag-8Au-3Pd wire has potential to replace the existing gold and conventional Ag-8Au-3Pd bonding wire. The conducitivity and lifetime during current stressing can be increasd by reducing the addition of Au or Pd. The AgPd wire having much higher lifetime than annealing-twinned Ag-8Au-3Pd because silver possess high electrical and thermal conductivity, lower the Joule heating and temperature during current stressing, retarding the electromigration. Although pure copper has higher electrical and thermal conductivity than silver alloy, the most anti-corrosive CuPd wire oxidate during current stressing at room temperature, leading to a higher local current density and breaking at early stage, having a lowest lifetime. However, reducing the addition of Au or Pd also decreases the thermal stability of wire, leading to a larger grain size after high temperatre storage. However, the silver bonding wires’ grain sizes are still smaller than wire diameter after aging at 600℃ for 120 minutes, except pure silver wire. Owing to high resistance to electromigration and better thermal stability, the Ag low Pd wire also has potential to replace existing bonding wires.

碩士<br>國立成功大學<br>機械工程學系<br>107<br>In this work, the mechanical properties and fracture behaviors of the structures with different grain sizes and Gradient-Nano-Grain (GNG) of Cu50Ag50 alloy were individually investigated under the uniaxial tensile stress. Molecular dynamics (MD) simulation with Finnis-Sinclair (FS) potential was adopted in numerical simulation. Grain sizes of 16.97 nm, 12 nm and 2.68 nm were selected in the samples with the mean grain sizes and GNG structure. Common neighbor analysis (CNA) and radial distribution function (RDF) method were employed for structure identification. Pre-cracked Cu50Ag50 alloy models under uniaxial tensile were also considered to investigate the crack growth and propagation. Simulation results indicate that the maximum strength and the inverse Hall-Petch (H-P) relationship can be observed at the grain size of 12 nm, and the ultimate tensile strength (UTS) is as high as 1.84 GPa. The UTS decreases with the decrease of the mean grain sizes, while the ductility increases and the plastic deformation become more uniform. Moreover, for GNG structure, stress induced phase transformation can be found during uniaxial tensile test. The initial crystalline structure transforms into a high proportion of amorphous structure, and the grain boundaries between the initial grains disappear. The effect of GNG structure on mechanical properties is not significant. Moreover, it was found that the crack tip exhibits blunting due to plastic deformation after the action of uniaxial tensile stress. Therefore, the Cu50Ag50 alloy shows good resistance of crack propagation than other brittle materials.

This research work presents the first comprehensive study on nanocrystalline materials produced in bulk quantities using a novel continuous electrodeposition process. A series of nanocrystalline Ni and Ni-Fe alloy continuous foils were produced and an intensive investigation into their structure and various properties was carried out. High-resolution transmission electron microscopy (HR-TEM) revealed the presence of local strain at high and low angle, and twin boundaries. The cause for these local strains was explained based on the interpretation of non-equilibrium grain boundary structures that result when conditions of compatibility are not satisfied. HR-TEM also revealed the presence of twin faults of the growth type, or &ldquo;growth faults&rdquo;, which increased in density with the addition of Fe. This observation was found to be consistent with a corresponding increase in the growth fault probabilities determined quantitatively using X-ray diffraction (XRD) pattern analysis. Hardness and Young&rsquo;s modulus were measured by nanoindentation. Hardness followed the regular Hall-Petch behaviour down to a grain size of 20 nm after which an inverse trend was observed. Young&rsquo;s modulus was slightly reduced at grain sizes less than 20 nm and found to be affected by texture. Microstrain based on XRD line broadening was measured for these materials and found to increase primarily with a decrease in grain size or an increase in intercrystal defect density (i.e., grain boundaries and triple junctions). This microstrain is associated with the local strains observed at grain boundaries in the HR-TEM image analysis. A contribution to microstrain from the presence of growth faults in the nanocrystalline Ni-Fe alloys was also noted. The macrostresses for these materials were determined from strain measurements using a two-dimensional XRD technique. At grain sizes less than 20 nm, there was a sharp increase in compressive macrostresses which was also owed to the corresponding increase in intercrystal defects or interfaces in the solid.

碩士<br>國立中興大學<br>材料工程學系所<br>94<br>In a conventional hot-rolled 5038 al-Alloy thick plate, the crystalline structure at the central part in the thickness direction comprises primarily slender grains. However, the grain structure is always equiaxed near the surface of the rolling plate. In this experiment, the shape of the slab before hot rolling was changed to a trapezoid. A main goal is to increase the amount of plastic strain, increasing the dislocation density in the central part of the plate hot-rolled from the trapezoidal aluminum slab. The experimental results indicated that, according to TEM observations, the center of the plate of hot-rolled trapezoidal slab had a higher dislocation density than the center of the rectangular slab. Subsequently, heat treatment caused the treated grains to become equiaxed. Therefore, an equiaxed grain structure that was uniform in the thickness direction of a hot-rolled thick plate could be obtained because the hot rolling of the trapezoidal slab caused profound lateral strain, in addition to extensive deformation in the rolling direction. The excess deformation resulted in a high dislocation density in the central region of the as-hot rolled plate, increasing the strain energy that was stored for recrystallization.

碩士<br>國立成功大學<br>材料科學及工程學系碩博士班<br>94<br>Fully-annealed 5083 Al casting alloy (5083C) has an equiaxed grain structure. Because of its high specific tensile strength, great corrosion resistance, and uniform expansion properties, it has been used to manufacture the cases of heating furnaces in semiconductor industry in recent years. Sometimes, 5083C Al thin sheets are also used to produce some components of semiconductor equipments. Friction stir welding (FSW) is a solid-state joining process providing better mechanical properties than fusion welding process. Friction stir processing (FSP) is an emerging technique based on the principle of FSW. In this research, it was investigated the tensile properties for the stir zone of Al alloy 5083C to judge the FSW technique is suitable for Al alloy 5083C. The results of this research also could help us to know if the FSP technique can modify the working properties of Al alloy 5083C thin sheets under severe bending strain. The results of tensile test show that FSP technique increase the uniform elongation from 10% to 18%, and total elongation from 11% to 22%. In addition, the strain hardening exponent ‘n’ is found to increase with the FSP technique. It means that FSP technique is helpful to improve the resistance to instable deformation. According to the previous studies we conjecture that the strain hardening exponent should increase as a result of grain and intermetallic compounds (Al6(Mn,Fe),Al6Mn) refinement and homogenization. Besides, it can be inferred the relationship between cracks and intermetallic compounds (IMC) by observations of the microstructures near fracture surface. It is always found that the IMC Al6(Mn,Fe) near the fracture surface was broken in all tensile specimens. So, we conjecture that Al6(Mn,Fe) is the beginning of the cracks. Because of the large cracks in 5083C during tensile test, the specimens is broken rapidly after necking occurring. It is found that the cracks near fracture surface in the FS-processed tensile specimens are smaller and homogenously distributed. So, the FS-processed tensile specimens deformed remarkably after necking. FSP technique enhances the ductility of 5083C. Microstructures of Al alloy 5083C could be obviously refined and homogenized by FSP technique, however the strength is not increased. There might be some key factors that we did not concern to affect the strength. Dynamic strain aging or serrated yielding has been wildly investigated in Al alloy 5083. But the effect of FSP technique to dynamic strain aging was not concerned in previous studies. In this study, critical strain and stress drop are used to analyze dynamic strain aging. The results show that critical strain decreases and stress drop increases after FSP.

碩士<br>國立成功大學<br>機械工程學系<br>102<br>In this study, 7075-T6 aluminum alloy was examined under different grain directions, strain rates and temperatures using split-Hopkinson pressure bar in order to investigate its dynamic deformation behaviors and microstructure characteristics. The cylindrical specimens were prepared from longitudinal and transverse direction, respectively. Impact tests were performed under different strain rates of 103 s-1, 2×103 s-1, 3×103 s-1 and 5×103 s-1 at room temperature for transverse specimens and at both room temperature and high temperature of 350ºC for longitudinal specimens. The results reveal that the mechanical properties and microstructures of the current alloy are greatly affected by directional grain structures, temperature and strain rates. It is found that the flow stresses and strain rate sensitivity increase with increasing strain rate. However, the activation volume decreases as the strain rate is increased. The flow behavior in high temperature conditions exhibits a lower work hardening rate, and a higher strain rate sensitivity. This result indicates a pronounced work hardening effect is appeared in high temperature as high strain rate loading is imposed. Finally, the Zerilli-Armstrong model is shown to provide an adequate description of the stress-strain response of 7075-T6 specimens under the considered grain direction, strain rate and temperature. Furthermore, according to the microscopic results, the relationship between the dislocation density, dislocation cells and grain size can be expressed using the modified Hall-Petch equation.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1986.<br>Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 174-179).

碩士<br>國立臺灣大學<br>材料科學與工程學研究所<br>106<br>In this study, recrystallization behavior and grain growth characteristics of homogenized and 80% cold-rolled FeCoNiCrMn、FeCoNiCrPd、FeCoNiCr、FeCoNiMn、CoNiCr and CoNiMn multi-component equiatomic medium / high entropy alloys(MEAs/HEAs) heated at different annealing time and temperature are investigated. The values of δ、∆H_mix and VEC in these six alloys all indicate the formation of single-phase FCC structure. The hardness of recrystallized alloys decreases as the grain size increases, and obeys the Hall-Patch equation, Hv=H0+KHd-1/2. The ternary CoNiCr alloy has the highest KH value, whereas the quinary FeCoNiCrPd alloy has the lowest one. This phenomenum is caused by the largest stacking fault energy and smallest shear modulus exhibited in FeCoNiCrPd alloy. After 80% cold rolling, the surface hardness of the ternary CoNiCr alloy is hardest, indicating that solid solution hardening in MEAs/HEAs is not determined by the number of soluted elements but by the kind of the element. Furthermore, all six alloys have the grain growth exponents 1/n higher than 2 with FeCoNiCrPd alloy having the highest activation energy of grain growth, say 831.9kJ/mol which is much higher than that in the conventional alloys. This characteristic suggests that the effects of the solute drag and the sluggish diffusion control the grain boundary motion. Moreover, the self-diffusion activation energy of the element exhibited in the alloy also affects the MEAs/HEAs activation energy of grain growth.

碩士<br>國立臺灣大學<br>化學工程學研究所<br>103<br>The evolution of grain structures, especially the types of grain boundaries (GBs), during directional solidification is crucial to the electrical properties of multicrystalline silicon used for solar cells. To study this, the electric molten zone crystallization (EMZC) of silicon wafers at different drift speeds from 2 to 6 mm/min was considered. It was found that <111> orientation was dominant at the lower drift velocity, while <112> orientation at the higher drift velocity. Most of the non-∑GBs tended to aligned with the thermal gradient, but some tilted toward the unfavorable grains having higher interfacial energies. On the other hand, the tilted ∑3 GBs tended to decrease during grain competition, except at the higher speed, where the twin nucleation became frequent. The competition of grains separated by ∑GBs could be viewed as the interactions of GBs that two coherent ∑3n GBs turned into one ∑3n GB following certain relations as reported before. On the other hand, when ∑ GBs met non-∑ GBs, non-∑ GBs remained which explained the decrease of ∑ GBs at the lower speed. Nakajima et al. has recently proposed multicrystalline SiGe with microscopic compositional distribution as a novel annual photovoltaic material. We also studyed the evolution of grain structures of multicrystalline SiGe at different Ge concentration from 0 to 12.45 at. % was considered. It was found that <111> orientation still was dominant at high Ge concentration. Nakajima et al. has recently proposed multicrystalline SiGe with microscopic compositional distribution as a novel annual photovoltaic material. We also studyed the evolution of grain structures of multicrystalline SiGe at different Ge concentration from 0 to 12.45 at. % was considered. It was found that <111> orientation still was dominant at high Ge concentration.