The aim of this diploma thesis is to investigate microstructure and thermal stability of ultra fine grained magnesium alloys. The thesis first summarises methods using plastic deformation in order to achieve ultra fine grained structure that are used to process metals. Then experimental methods employed in the experimental part including microhardness testing, scanning electron microscopy and positron annihilation spectroscopy are described. Brief summary of previous research on MgZnY alloys strengthened by quasicrystalline phases and Mg22Gd alloys is given. Finally, results of experimental investigation of MgZnY alloys with various Zn/Y ratios and Mg22Gd alloy are discussed. These results suggest that presence of phases in MgZnY alloys depend on Zn/Y ratio, hardness of these alloys depends on Zn content and that rapid cooling of MgZnY alloys annealed at 500 řC lead to significant increase in volume fraction of quasicrystalline icosahedral phase. In the second section of the experimental part thermal behaviour of Mg22Gd alloy is investigated. Furthermore, formation of GdH2 particles in Mg22Gd is examined and attributed to reaction of hydrogen decomposed from water vapour with gadolinium in areas rich in gadolinium. Finally, significant hardening of Mg22Gd alloy processed by high pressure torsion has been...

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第17956号 / 工博第3804号 / 新制||工||1582(附属図書館) / 30786 / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 辻 伸泰, 教授 松原 英一郎, 教授 安田 秀幸 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

Nanotechnology offers significant opportunities in providing solutions to existing engineering problems as well as breakthroughs in new fields of science and technology. In order to fully realize benefits from such initiatives, nanomanufacturing methods must be developed to integrate enabling constructs into commercial mainstream. Even though significant advances have been made, widespread industrialization in many areas remains limited. Manufacturing methods, therefore, must continually be developed to bridge gaps between nanoscience discovery and commercialization. A promising technology for integration of top-down nanomanufacturing yet to receive full industrialization is equal channel angular pressing, a process transforming metallic materials into nanostructured or ultraine grained materials with significantly improved performance characteristics. To bridge the gap between process potential and actual manufacturing output, a prototype top-down nanomanufacturing system identified as indexing equal channel angular pressing (IX-ECAP) was developed. The unit was designed to capitalize on opportunities of transforming spent or scrap engineering elements into key engineering commodities. A manufacturing system was constructed to impose severe plastic deformation via simple shear in an equal channel angular pressing die on 1100 and 4043 aluminum welding rods. 1/4 fraction factorial split-plot experiments assessed significance of five predictors on the response, microhardness, for the 4043 alloy. Predictor variables included temperature, number of passes, pressing speed, back pressure, and vibration. Main effects were studied employing a resolution III design. Multiple linear regression was used for model development. Initial studies were performed using continuous processing followed by contingency designs involving discrete variable length work pieces. IX-ECAP offered a viable solution in severe plastic deformation processing. Discrete variable length work piece pressing proved very successful. With three passes through the system, 4043 processed material experienced an 88.88% increase in microhardness, 203.4% increase in converted yield strength, and a 98.5% reduction in theoretical final grain size to 103 nanometers using the Hall-Petch relation. The process factor, number of passes, was statistically significant at the 95% confidence level; whereas, temperature was significant at the 90% confidence level. Limitations of system components precluded completion of studies involving continuous pressing. Proposed system redesigns, however, will ensure mainstream commercialization of continuous length work piece processing.

Equal-channel angular pressing (ECAP) is one of the severe plastic deformation (SPD) to produce ultra-fine grain (UFG) material, and its principle and microstructural developments. The majority of papers on SPD materials have been devoted to the face centered cubic (FCC) structure materials such as Al, Cu and Ni. The UFG of high alloy ECAP processing has been difficult up to now, but we were successful in this study. Fe-20%Cr steel with extremely low C and N has different slip behavior from the FCC. The mechanical properties and corrosion resistance were investigated in term microstructural evolution during ECAP processing. / 博士(工学) / Doctor of Philosophy in Engineering / 同志社大学 / Doshisha University

Nanometer-scale-thick films of metals and semiconductor heterostructures are used increasingly in modern technologies, from microelectronics to various areas of nanofabrication. Processing of such ultrathin-film materials generates structural defects, including voids and cracks, and may induce structural transformations. Furthermore, the mechanical behavior of these small-volume structures is very different from that of bulk materials. Improvement of the reliability, functionality, and performance of nano-scale devices requires a fundamental understanding of the atomistic mechanisms that govern the thin-film response to mechanical loading in order to establish links between the films' structural evolution and their mechanical behavior. Toward this end, a significant part of this study is focused on the analysis of atomic-scale mechanisms of plastic deformation in freestanding, ultrathin films of face-centered cubic (fcc) copper (Cu) that are subjected to biaxial tensile strain. The analysis is based on large-scale molecular-dynamics simulations. Elementary mechanisms of dislocation nucleation are studied and several problems involving the structural evolution of the thin films due to the glide of and interactions between dislocations are addressed. These problems include void nucleation, martensitic transformation, and the role of stacking faults in facilitating dislocation depletion in ultrathin films and other small-volume structures of fcc metals. Void nucleation is analyzed as a mechanism of strain relaxation in Cu thin films. The glide of multiple dislocations causes shearing of atomic planes and leads to formation of surface pits, while vacancies are generated due to the glide motion of jogged dislocations. Coalescence of vacancy clusters with surface pits leads to formation of voids. In addition, the phase transformation of fcc Cu films to hexagonal-close packed (hcp) ones is studied. The resulting martensite phase nucleates at the film's free surface and grows into the bulk of the film due to dislocation glide. The role of surface orientation in the strain relaxation of these strained thin films under biaxial tension is discussed and the stability of the fcc crystalline phase is analyzed. Finally, the mechanical response during dynamic tensile straining of pre-treated fcc metallic thin films with varying propensities for formation of stacking faults is analyzed. Interactions between dislocations and stacking faults play a significant role in the cross-slip and eventual annihilation of dislocations in films of fcc metals with low-to-medium values of the stable-to-unstable stacking-fault energy ratio, γs/γu. Stacking-fault-mediated mechanisms of dislocation depletion in these ultrathin fcc metallic films are identified and analyzed. Additionally, a theoretical analysis for the kinetics of strain relaxation in Si 1-x Ge x (0 ≤ x ≤ 1) thin films grown epitaxially on Si(001) substrates is conducted. The analysis is based on a properly parameterized dislocation mean-field theoretical model that describes plastic-deformation dynamics due to threading dislocation propagation; the analysis addresses strain relaxation kinetics during both epitaxial growth and thermal annealing, including post-implantation annealing. The theoretical predictions for strain relaxation as a function of film thickness in Si 0.80 Ge 0.20 /Si(001) samples annealed after growth, either unimplanted or after He + implantation, are in excellent agreement with reported experimental measurements.

Titanium is a widely applied material in industries and characterized by highly anisotropic mechanical behaviour. To study the special property of titanium, many kinds of mechanical loading tests have been conducted. Moreover, researchers attempted to reproduce these experiments with numerical methods. This paper will present an overview about the deformation mechanisms and related representative studies of titanium. Among the numerical methods, Taylor type and self-consistent crystal plasticity models are two of the most common ones seen in literature. Simulation of some mechanical loading tests using visco-plastic self-consistent model was carried out and compared with the results given by Taylor type model. It has been found that self-consistent model prevails in the reproduction of stress-strain response and texture evolution. During the calculation of self-consistent model, there are totally 4 kinds of self-consistent schemes available for linearization process. The author investigated 4 groups of simulation works using different self-consistent schemes. But no evident distinction has been observed. The application of visco-plastic self-consistent model in commercial purity titanium is studied at the end. The simulation results successfully captured the general features of 9 mechanical loading tests. / Thesis / Master of Applied Science (MASc)

No description available.

No description available.

No description available.

Serious issues including solidification porosity, columnar grains, and large grain sizes are common during fusion-based metal additive manufacturing due to the inherent melting and solidification that occurs during printing. In recent years, a high-temperature, rapid plastic deformation technique called additive friction stir deposition (AFSD) has shown great promise in overcoming these issues. Because the deposited material stays in the solid state during printing, there are no melting and solidification events and the process can result in as-printed material that is fully-dense with equiaxed, fine grains. As AFSD is an emerging process, developing an understanding of the synergy between material deformation and the resultant microstructure evolution, especially the strain magnitude, its influence on dynamic microstructure evolution, and material flow details, is imperative for the full implementation of AFSD. Therefore, the purpose of this work is to investigate the severe plastic deformation in AFSD through complementary studies on the concurrent evolution of shape and microstructure during the deposition of dissimilar aluminum alloys. In this work, we systematically study (1) the entire deposition via dissimilar cladding along with (2) specific volumes within the deposited layer via embedded tracers printed at varied processing parameters. X-ray computed tomography and electron backscatter diffraction are employed to visualize the complex shape of the deposits and understand the microstructure progression. Investigation of dissimilar cladding of homogeneous AA2024 feed-rods onto an AA6061 substrate establishes a working understanding of the mechanisms related to material flow and microstructure evolution across the whole deposit (macroscopic shape evolution) as well as at the interface between the deposit and the substrate. Variations in tooling and rotation rate affect the interfacial features, average grain size, and depth of microstructural influence. The non-planar and asymmetric nature of AFSD on the macro-scale is revealed and a maximum boundary of deposited material is established which gives a frame of reference for the next material flow study within the deposition zone. An understanding of the mesoscopic morphological evolution and concurrent dynamic microstructure evolution of representative volumes within the deposition zone is determined by comparing depositions of hybrid feed-rods (AA6061 matrix containing an embedded tracer of AA2024). Samples were printed with and without an in-plane velocity to compare initial material feeding to steady-state deposition. Variations in initial tracer location and tool rotation rate/in-plane velocity pairs affect the final morphology, intensity of mixing, and microstructure of the deposited tracer material. The tracer material undergoes drastic mesoscopic shape evolution from millimeter-scale cylinders to long, curved micro-ribbons. There is simultaneous grain refinement in AA2024 via geometric dynamic recrystallization during initial material feeding, after which the grain size remains relatively constant at a steady-state size. The lower bound of strain is estimated based on extrusion, torsion, and shear-thinning factors. The step-by-step mesoscopic deformation and microstructure evolution is further elucidated by characterizing depositions of hybrid feed-rods with a series of embedded tracers. The AFSD tooling is stopped quickly at the end of the deposition with a quench applied to "freeze" the sample. X-ray computed tomography reveals multiple intermediate morphologies including the progression from a cylinder to a tight spiral, to a flattened spiral shape, and to a thin disc. EBSD mapping shows that a refined microstructure is formed soon after the material leaves to tool head with areas off the centerline reaching a fully recrystallized state more quickly. The findings from this work summarize the current understanding of the link between material deformation and microstructure evolution in AFSD. Hopefully these first fundamental studies on the co-evolution of material flow and grain structure during AFSD can inspire future work, especially in the area of heterogeneous multi-material printing. / Doctor of Philosophy / Additive friction stir deposition (AFSD) is a new metal 3D printing process that uses friction to heat up and deposit materials rather than using a laser to melt the material into place. This is beneficial since it avoids problems that come from melting and solidification (e.g., porosity, hot cracking, residual stresses, columnar grains). Since AFSD is such a new technology, an understanding of some of the fundamental processing science is needed in order to predict and control the performance of the resultant parts. This is because the processing of a material affects its structure (at multiple scales, for example macro-, micro-, atomic) which then affects the properties a material will exhibit which, finally, dictates the performance of the overall part. Therefore, the purpose of this work is to explore how the feed material is transformed and deposited into the final layer after printing and to link the original processing conditions to the resultant structure. To investigate the interface between the deposited layer and the substrate, we use a simple feed-rod of one aluminum alloy (AA2024) and deposit it onto a substrate of another aluminum alloy (AA6061). To look at just one small volume within the deposited layer, we use a hybrid feed-rod that is mostly AA6061 except for small cylinders of AA2024 that are placed either in the center or on the edge of the feed-rod so that we can track the AA2024. Printing these feed-rods under different processing conditions will help us understand the connection between processing and structure. Using a characterization technique called X-ray computed tomography we can visualize a 3D representation of the final position for the AA2024 material. In order to evaluate the structure on the micro-scale, a characterization technique called electron backscatter diffraction is used to show the individual grains of our metal. The main contributions of this work are as follows: 1) a lower bound of strain is estimated for AFSD, 2) various intermediate deformation steps are captured for the tracer cylinders including a progression from cylinder to multiple spiral shapes to a thin disc to long ribbons, 3) these deformation steps are linked to different microstructures, and 4) changing the tool geometry and other processing parameters significantly alters the range of shapes and microstructures developed in the deposited material. These findings bring us closer to a fully controllable system as well as sparking some interesting areas for future research because of the complex shapes we observed. These results could lead to the customization and optimization of 3D spirals, ribbons, etc. designed for a certain application.