京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第18266号 / 工博第3858号 / 新制||工||1592(附属図書館) / 31124 / 京都大学大学院工学研究科機械理工学専攻 / (主査)教授 福永 俊晴, 教授 白井 泰治, 准教授 徐 蛟 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第18986号 / 工博第4028号 / 新制||工||1620(附属図書館) / 31937 / 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 田中 功, 教授 乾 晴行 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第19709号 / 工博第4164号 / 新制||工||1642(附属図書館) / 32745 / 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 乾 晴行, 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

No description available.

<p>     </p> <p>Martensitic transformation underlies the phenomena of super-elasticity within shape memory alloys and the production of advanced steels. Experimentation has demonstrated that defects and microstructural changes strongly influence this process. With simulations granting up to an atomic-level understanding of the impact that grain boundaries and precipitates have upon the solid-to-solid phase transformation. Yet the role that dislocations partake in the martensitic transformation and its microstructures remains unclear or disputed. </p> <p><br></p> <p>Therefore, we utilize large-scale molecular dynamics (MD) simulations to study the forward and reverse transformation of martensitic material modeled after Ni63Al37 shape via thermal cycling loading. The simulations indicate that dislocations retain martensite well above the martensite start temperature and behave as nucleation sites for the martensite. We found that a reduction in dislocation density with cycle correlated with a decrement in the Ms and As transition temperatures, in agreement with the experiment. It was found that competing martensite variants could develop stable domains as dislocation density reduced sufficiently which resulted in multi-domain structures. Furthermore, the critical nuclei size of the martensite variant was able to be extracted from our results. </p>

No description available.

<p> Case hardening of steels is extensively used throughout general engineering to produce components with a hardened layer whilst retaining a tough core. This is usually accomplished using different sources of energy, e.g. flame and induction being the most common. In recent years, a new case hardening technology, named 'Grind-Hardening' has surfaced. In this method, the heat dissipated during grinding is utilized to induce martensitic phase transformation in the surface layer of a component. Therefore it is possible to incorporate grinding and surface hardening into a single operation to develop a cost-effective production method. The grinding process then becomes an integrated heat treatment process.</p> <p> In the present study on 'grind hardening', a numerical thermal model has been developed to compute the temperature distribution beneath the ground surface to predict the extent of surface hardening and the case depth. Grinding experiments were conducted in order to examine the influence of various process variables such as wheel depth of cut, feed speed, and wheel preparation. AISI 52100 and 1045 steels were used in this study to evaluate the behavior of plain and alloy steels during grind hardening. Effective case depth was measured using a Vickers hardness tester and was found to be over 0.5 mm for a target hardness of 513 Hv. Microstructure was analyzed using optical and scanning electron microscopes. The microstructure was observed to have fine martensitic laths which give rise to remarkable high hardness.</p> / Thesis / Master of Applied Science (MASc)

京都大学 / 新制・課程博士 / 博士(工学) / 甲第23196号 / 工博第4840号 / 新制||工||1756(附属図書館) / 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 田中 功, 教授 乾 晴行 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

No / Martensitic transformations are of considerable technological importance, a particularly promising application being the possibility of using martensitic materials, possibly proteins, as tiny machines. For organic crystals, however, a molecular level understanding of such transformations is lacking. We have studied a martensitic-type transformation in crystals of the amino acid DL-norleucine using molecular dynamics simulation. The crystal structures of DL-norleucine comprise stacks of bilayers (formed as a result of strong hydrogen bonding) that translate relative to each other on transformation. The simulations reveal that the transformation occurs by concerted molecular displacements involving entire bilayers rather than on a molecule-by-molecule basis. These observations can be rationalized on the basis that at sufficiently high excess temperatures, the free energy barriers to concerted molecular displacements can be overcome by the available thermal energy. Furthermore, in displacive transformations, the molecular displacements can occur by the propagation of a displacement wave (akin to a kink in a carpet), which requires the molecules to overcome only a local barrier. Concerted molecular displacements are therefore considered to be a significant feature of all displacive transformations. This finding is expected to be of value toward developing strategies for controlling or modulating martensitic-type transformations.

The mechanical behavior of structural materials used in nuclear applications is significantly degraded as a result of irradiation, typically characterized by an increase in yield stress, localization of inelastic deformation along narrow dislocation channels, and considerably reduced strains to failure. Further, creep rates are accelerated under irradiation. These changes in mechanical properties can be traced back to the irradiated microstructure which shows the formation of a large number of material defects, e.g., point defect clusters, dislocation loops, and complex dislocation networks. Interaction of dislocations with the irradiation-induced defects governs the mechanical behavior of irradiated metals. However, the mechanical properties are seldom systematically correlated to the underlying irradiated microstructure. Further, the current state of modeling of deformation behavior is mostly phenomenological and typically does not incorporate the effects of microstructure or defect densities. The present research develops a continuum constitutive crystal plasticity framework to model the mechanical behavior and deformed microstructure of bcc ferritic/martensitic steels exposed to irradiation. Physically-based constitutive models for various plasticity-induced dislocation migration processes such as climb and cross-slip are developed. We have also developed models for the interaction of dislocations with the irradiation-induced defects. A rate theory based approach is used to model the evolution of point defects generated due to irradiation, and coupled to the mechanical behavior. A void nucleation and growth based damage framework is also developed to model failure initiation in these irradiated materials. The framework is used to simulate the following major features of inelastic deformation in bcc ferritic/martensitic steels: irradiation hardening, flow localization due to dislocation channel formation, failure initiation at the interfaces of these dislocation channels and grain boundaries, irradiation creep deformation, and temperature-dependent non-Schmid yield behavior. Model results are compared to available experimental data. This framework represents the state-of-the-art in constitutive modeling of the deformation behavior of irradiated materials.