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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18987号 / 工博第4029号 / 新制||工||1620(附属図書館) / 31938 / 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 白井 泰治, 教授 松原 英一郎 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

Austenitic stainless steels are commonly used in nuclear reactors and have been considered as potential structural materials in fusion reactors due to their excellent corrosion resistance, good creep and fatigue resistance at elevated temperatures, but their relatively low yield strength and poor radiation tolerance hinder their applications in high dose radiation environments. High angle grain boundaries have long been postulated as sinks for radiation-induced defects, such as bubbles, voids, and dislocation loops. Here we provide experimental evidence that high angle grain boundaries can effectively remove radiation-induced defects. The equal channel angular pressing (ECAP) technique was used to produce ultrafine grained Fe-Cr-Ni alloy. Mechanical properties of the alloy were studied at elevated temperature by tensile tests and in situ neutron scattering measurements. Enhanced dynamic recovery process at elevated temperature due to dislocation climb lowers the strain hardening rate and ductility of ultrafine grained Fe-Cr-Ni alloy. Thermal stability of the ultrafine grained Fe-Cr-Ni alloy was examined by ex situ annealing and in situ heating within a transmission electron microscope. Abnormal grain growth at 827 K (600°C) is attributed to deformation-induced martensite, located at the triple junctions of grains. Helium ion irradiation studies on Fe-Cr-Ni alloy show that the density of He bubbles, dislocation loops, as well as irradiation hardening are reduced by grain refinement. In addition, we provide direct evidence, via in situ Kr ion irradiation within a transmission electron microscope, that high angle grain boundaries in nanocrystalline Ni can effectively absorb irradiation-induced dislocation loops and segments. The density and size of dislocation loops in irradiated nanocrystalline Ni were merely half of those in irradiated coarse grained Ni. The results imply that irradiation tolerance in bulk metals can be effectively enhanced by microstructure refinement.

Nanocrystalline metals exhibit a high strength/hardness but generally poor ductility during deformation regardless of their crystal structure which is often called the strength-ductility trade-off relationship and generally appears in most ultrafine-grained metals. The ultrafine-grained (UFG) high manganese austenitic twinning-induced plasticity (TWIP) steels have been found to overcome the strength-ductility trade-off but their underlying mechanism of discontinuous yielding behavior has not been well understood. In this study, our systematic TEM characterization suggests that the plastic deformation mechanisms in the early stage of deformation, around the macroscopic yield point, show an obvious association with grain size and nucleation of deformation twin was promoted rather than suppressed in UFG. More specifically, the main mechanism shifts from the conventional slip in grain interior to twinning nucleated from grain boundaries with decreasing the grain size down to less than 1 m. We also provide insights into the atomistic process of deformation twin nucleation at 3{111} twin boundaries, the dominant type of grain boundary in the UFG-TWIP steel of interest. In response to the external tensile stresses, the structure of coherent 3{111} twin boundary changes from atomistically smooth to partly defective by the grain boundary migration mechanism thus the "kink-like" defective step can act as a nucleation site for deformation twin, which deformation process is different from the one induced by dislocation pile-ups in coarse-grained counterparts and explain why UFG TWIP steel can retain the moderate ductility. In addition to the effect of grain size on deformation twin nucleation, grain boundary character was also taken into account. In coarse-grained TWIP steel, we experimentally reveal that deformation twin nucleation occurs at an annealing twin () boundary in a high-Mn austenitic steel when dislocation pile-up at boundary produced a local stress exceeding the twining stress, while no obvious local stress concentration was required at relatively high-energy grain boundaries such as or  A periodic contrast reversal associated with a sequential stacking faults emission from boundary was observed by in-situ transmission electron microscopy (TEM) deformation experiments, proving the successive layer-by-layer stacking fault emission was the deformation twin nucleation mechanism. The correlation between grain boundary character and deformation behavior was discussed both in low- and high-sigma value grain boundaries. On the other hand, localized strain concentration causes the nucleation of deformation twins at grain boundaries regardless of the grain boundary misorientation character in UFG TWIP steel. The invisibility of stacking fault (zero contrast) was also observed to be emitted at 3{111} boundaries in the coarse-grained TWIP steel, which deformation twin nucleation mechanism is found to be identical to UFG Fe-31Mn-3Si-3Al TWIP steel. / Doctor of Philosophy / High manganese (Mn) twin-induced plasticity (TWIP) steel is a new type of steels which exhibit pronounced strain hardening rate so that offering an extraordinary potential to adjust the strength-ductility relationship. This key advantage will help implement the current development of lightweighting components in automobile industry due to a considerable reduction of material use and an improved press formability. Such outstanding ductility can be contributed by the pronounced strain hardening rate during every such deformation processes, which is highly associated with several different controlling parameters, i.e., SFE, grain orientation, grain size, and grain boundary characters. In this study, we take particular attention to the effect of grain size and grain boundary characters on deformation twinning behavior besides well-known parameters such as SFE and grain orientation. The effect of grain size on deformation twinning behavior was found to be deeply associated with the yielding behavior in TWIP steel, i.e., a discontinuous yielding behavior with a unique yield drop was observed in ultrafine-grained TWIP while a continuous yielding behavior was observed in coarse-grained counterpart. Our TEM characterization indicates that the microstructural features of grains >10 m are different from the microstructural features in grains < 1 m. In over-10 m grains, normal dislocation slips and the formation of in-grain stacking faults are the main deformed microstructure. However, in the under-1 m grains, the in-grain dislocation slip is inhibited, but the deformation twinning is promoted at grain boundaries. This deformation transition from in-grain slip to twinning at grain boundary appears to be responsible for the discontinuous yielding behavior observed in stress-strain curve. The effect of grain boundary character on deformation twinning was examined in both coarse- and ultrafine-grained TWIP steels. In coarse-grained TWIP steel, we found that deformation twinning behavior varies as the function of boundary structure, i.e., different atomic configuration. Coherent twin boundary can act as a nucleation site for deformation twin as a localized strain concentration was introduced by dislocation pile-ups. On the other hand, incoherent boundaries can act as a deformation twin nucleation site by a boundary relaxation mechanism, i.e., grain-boundary dislocations can dissociate into partial dislocations to both side of boundary to accommodate the misfit between grains. In UFG TWIP steel, we found that the coherent twin boundary can act as a deformation twin nucleation site without presence of dislocation pile-ups. Alternatively, twin boundary becomes defective with a "kink-like" step by boundary migration. As a result, this defective step would progressively accumulate localized strain field thus stimulate the nucleation of deformation twin. Such study provides a novel insight into the UFG TWIP steel and a roadmap toward controlling TWIP effect.

A micromanufatura via usinagem apresenta algumas dificuldades, principalmente aquelas relacionadas à formação do cavaco, pois a espessura de corte passa a ter a dimensão do tamanho de grão do material da peça e da microgeometria da aresta de corte. Em operações de microcorte, a microestrutura do material é um fator importante no controle da geração da superfície da peça, mecanismo de formação de cavaco, etc. Este trabalho de pesquisa avaliou o efeito do tamanho ultrafino dos grãos do material da peça sobre os fenômenos inerentes ao corte no microfresamento. As variáveis de usinagem investigadas foram avanço por dente (fz), velocidade de corte (vc), diâmetro da microfresa (d&#934) e raio de aresta de corte (re), visando avaliar o mecanismo de formação do cavaco, acabamento da peça e integridade superficial. Os materiais utilizados nos experimentos foram um aço bifásico (ferrita-perlita) com tamanho de grão ferrítico de 11 µm e outro de microestrutura homogênea de grãos ultrafinos com 0,7 µm, ambos com mesma composição química e baixo-carbono. Dois grupos de ensaios foram propostos: (1) macro e microfresamento e (2) microfresamento de canais. O tipo de usinagem foi o de fresamento de topo, sem emprego de fluido de corte. Os ensaios de usinagem foram executados em centros de usinagem CNC. As ferramentas de corte foram de metal duro com recobrimentos, diâmetro 16 mm na escala macro de usinagem, 200 e 800 µm na escala micro. A adequação da microestrutura do material da peça à redução da escala de usinagem, através do mecanismo de refino de grão, gerou alguns aspectos favoráveis à microusinagem, como melhor acabamento (Ssk&#8776;0 e Sku&#8776;3), formação de cavaco contínuo e menor formação de rebarbas com a redução da espessura de corte (fz&#8804re), possibilitando aplicações em microfabricação por corte com ferramenta de geometria definida utilizando aços baixo carbono, antes limitadas à estruturas na construção civil e peças obtidas por conformação mecânica. / Micro manufacturing by means of machining presents difficulties, mainly those related to chip formation, since chip thickness become as small as normal material grain size, as well as the cutting edge radius. At such micro cutting operations material microstructure ascends as a very important issue in terms of machining output, i.e. surface roughness, subsurface damages, cutting forces, etc. This research evaluated the effect of the intervention on the metallurgical microstructure of the material on the cutting phenomena inherent in micromachining. The variables investigated were the feed per tooth (ft), cutting speed (vc), micro end-mill diameter (d&#934) and cutting edge radius (re). The materials used in the experiments were a steel two-phase (ferrite-pearlite) with ferritic grain size of 11 µm and similar one with homogeneous microstructure and ultrafine grains (0.7 µm), both low carbon. The mechanism of chip formation, surface finish and surface integrity were investigated and correlated with the studied variables. Two groups of machining experiments were proposed: (1) macro and micro end-milling and (2) microchannels. Overall, the type of machining was the end milling, without using cutting fluid. The machining tests were carried on a CNC machining center. The cutting tools are coated, diameter 16 mm in macro scale of machining, 200 and 800 &#956m in micro scale. the adequacy of the microstructure of the workpiece material to the reduce the scale of machining generated some favorable aspects to micromachining, such as better finishing (Ssk&#8776;0 e Sku&#8776;3), continuous chip formation and lesser burr formation by reducing the cutting thickness (fz&#8804re), enabling micromanufacturing applications for low carbon steels, once limited to structures in the civil construction and pieces obtained by mechanical forming.

A micromanufatura via usinagem apresenta algumas dificuldades, principalmente aquelas relacionadas à formação do cavaco, pois a espessura de corte passa a ter a dimensão do tamanho de grão do material da peça e da microgeometria da aresta de corte. Em operações de microcorte, a microestrutura do material é um fator importante no controle da geração da superfície da peça, mecanismo de formação de cavaco, etc. Este trabalho de pesquisa avaliou o efeito do tamanho ultrafino dos grãos do material da peça sobre os fenômenos inerentes ao corte no microfresamento. As variáveis de usinagem investigadas foram avanço por dente (fz), velocidade de corte (vc), diâmetro da microfresa (d&#934) e raio de aresta de corte (re), visando avaliar o mecanismo de formação do cavaco, acabamento da peça e integridade superficial. Os materiais utilizados nos experimentos foram um aço bifásico (ferrita-perlita) com tamanho de grão ferrítico de 11 µm e outro de microestrutura homogênea de grãos ultrafinos com 0,7 µm, ambos com mesma composição química e baixo-carbono. Dois grupos de ensaios foram propostos: (1) macro e microfresamento e (2) microfresamento de canais. O tipo de usinagem foi o de fresamento de topo, sem emprego de fluido de corte. Os ensaios de usinagem foram executados em centros de usinagem CNC. As ferramentas de corte foram de metal duro com recobrimentos, diâmetro 16 mm na escala macro de usinagem, 200 e 800 µm na escala micro. A adequação da microestrutura do material da peça à redução da escala de usinagem, através do mecanismo de refino de grão, gerou alguns aspectos favoráveis à microusinagem, como melhor acabamento (Ssk&#8776;0 e Sku&#8776;3), formação de cavaco contínuo e menor formação de rebarbas com a redução da espessura de corte (fz&#8804re), possibilitando aplicações em microfabricação por corte com ferramenta de geometria definida utilizando aços baixo carbono, antes limitadas à estruturas na construção civil e peças obtidas por conformação mecânica. / Micro manufacturing by means of machining presents difficulties, mainly those related to chip formation, since chip thickness become as small as normal material grain size, as well as the cutting edge radius. At such micro cutting operations material microstructure ascends as a very important issue in terms of machining output, i.e. surface roughness, subsurface damages, cutting forces, etc. This research evaluated the effect of the intervention on the metallurgical microstructure of the material on the cutting phenomena inherent in micromachining. The variables investigated were the feed per tooth (ft), cutting speed (vc), micro end-mill diameter (d&#934) and cutting edge radius (re). The materials used in the experiments were a steel two-phase (ferrite-pearlite) with ferritic grain size of 11 µm and similar one with homogeneous microstructure and ultrafine grains (0.7 µm), both low carbon. The mechanism of chip formation, surface finish and surface integrity were investigated and correlated with the studied variables. Two groups of machining experiments were proposed: (1) macro and micro end-milling and (2) microchannels. Overall, the type of machining was the end milling, without using cutting fluid. The machining tests were carried on a CNC machining center. The cutting tools are coated, diameter 16 mm in macro scale of machining, 200 and 800 &#956m in micro scale. the adequacy of the microstructure of the workpiece material to the reduce the scale of machining generated some favorable aspects to micromachining, such as better finishing (Ssk&#8776;0 e Sku&#8776;3), continuous chip formation and lesser burr formation by reducing the cutting thickness (fz&#8804re), enabling micromanufacturing applications for low carbon steels, once limited to structures in the civil construction and pieces obtained by mechanical forming.

Submitted by Aelson Maciera (aelsoncm@terra.com.br) on 2017-08-16T19:31:06Z No. of bitstreams: 1 DissDAGP.pdf: 24957220 bytes, checksum: d8c9cd6b22c7e40f1a1f5c7c88deb93a (MD5) / Approved for entry into archive by Ronildo Prado (bco.producao.intelectual@gmail.com) on 2018-01-30T16:50:04Z (GMT) No. of bitstreams: 1 DissDAGP.pdf: 24957220 bytes, checksum: d8c9cd6b22c7e40f1a1f5c7c88deb93a (MD5) / Approved for entry into archive by Ronildo Prado (bco.producao.intelectual@gmail.com) on 2018-01-30T16:50:13Z (GMT) No. of bitstreams: 1 DissDAGP.pdf: 24957220 bytes, checksum: d8c9cd6b22c7e40f1a1f5c7c88deb93a (MD5) / Made available in DSpace on 2018-01-30T16:57:24Z (GMT). No. of bitstreams: 1 DissDAGP.pdf: 24957220 bytes, checksum: d8c9cd6b22c7e40f1a1f5c7c88deb93a (MD5) Previous issue date: 2017-03-03 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Biomedical devices currently in use (prostheses, implants) have satisfactory performance in many cases. However, sometimes the body reacts to the device insertion and may lead to its rapid replacement. Some of these disadvantages can be solved by the use of titanium and its alloys, due to their excellent combination of corrosion resistance, wear resistance and biocompatibility compared to other competing biomaterials. This work presents the possibility of obtaining near β titanium alloy with ultrafine grains produced by severe plastic deformation. For this, the Ti-13Nb-13Zr alloy was processed by high-pressure torsion processing method. Samples were processed with different loads and number of turns. The samples were evaluated by Vickers microhardness. As-received and deformed samples were analyzed through X-Ray diffraction. The microstructures were observed by optical microscopy and scanning electron microscope and the microtexture and phase mappings of the material evaluated through the ASTAR equipment in the transmission electron microscope. After characterization, it was observed that there is a refinement of the microstructure and increase of the microhardness of the Ti-13Nb-13Zr alloy deformed by HPT. Due to the superior microhardness of the deformed material and the results of phase transformations indicate a potential application as nanostructured biomaterial. / Os dispositivos biomédicos utilizados atualmente (próteses, implantes) possuem desempenho satisfatório em muitos casos. No entanto, às vezes, o corpo reage à inserção destes dispositivos exigindo a sua rápida substituição. Algumas destas desvantagens podem ser resolvidas pelo uso de titânio e suas ligas, devido à sua excelente combinação de resistência à corrosão, resistência ao desgaste e biocompatibilidade em comparação com outros biomateriais concorrentes. Este trabalho apresenta a possibilidade de obtenção de liga de titânio quase β com grãos ultrafinos produzidos por deformação plástica severa para três diferentes condições iniciais de microestrutura. Para isso, a liga Ti-13Nb-13Zr foi processada pelo método de processamento de torção sob alta pressão (High-Pressure Torsion - HPT). As amostras foram processadas com diferentes cargas e número de voltas e avaliadas por meio de microdureza Vickers. Amostras como recebidas e deformadas foram analisadas através de difração de raios X. As microestruturas foram observadas por meio de microscopia óptica e eletrônica de varredura. A microtextura e mapeamentos de fase do material foram avaliados através do equipamento ASTAR no microscópio eletrônico de transmissão. Após a caracterização, foi observado que existe um refinamento da microestrutura e aumento da microdureza da liga Ti-13Nb-13Zr deformada por HPT. A microdureza superior do material deformado e os resultados de transformações de fase apontam para uma potencial aplicação como biomaterial nanoestruturado.

La synthèse par métallurgie des poudres de nickel à grains ultrafins (UFG) a été effectuée, et l’effet de l’affinement de la microstructure sur le comportement mécanique et les propriétés physiques a été étudié. La possibilité de coupler le broyage et le frittage flash est étudiée avec des résultats prometteurs. Des échantillons de haute densité avec des tailles de grains d = 0.65 – 4 µm, caractérisés par une fraction élevée des joints de grains Σ3 et un faible niveau de contrainte ont été synthétisés. Les propriétés mécaniques des échantillons UFG montrent une bonne combinaison ductilité-résistance mécanique, avec un impact mineur des porosités présentes. L’étude de l’influence de la taille de grain dans le régime UFG sur les propriétés mécaniques montre une limite d’élasticité supérieure à celle attendue et une capacité d’écrouissage plus faible. Ces observations sont cohérentes avec la microstructure déformée à rupture, étudiée par diffraction d’électrons rétrodiffusés et microscopie électronique en transmission. Une haute diffusivité, mesurée par des expériences de traceurs radioactifs, montrent des profils de pénétration très différents liés aux structures de porosités diverses présents dans les échantillons. Ces différentes structures sont aussi responsables de la densification rétrograde observée, uniquement pour les échantillons frittés à partir de poudres broyées. / The present manuscript concerns the synthesis of ultrafine grained (UFG) Ni by powder metallurgy, and the study of the influence of UFG microstructures on the mechanical behavior and physical properties. The possibilities of coupling ball milling and Spark Plasma Sintering are presented showing promising results. Highly dense homogeneous specimens are obtained, with average grain sizes d = 0.65 - 4 µm, and microstructures highlighted by a high fraction of Σ3 grain boundaries dependent on grain size. The mechanical properties in tensile testing for UFG samples are evaluated showing a good combination of strength and ductility, with little impact from porosities, the major drawback of powder metallurgy. The influence of grain size in the UFG regime on the mechanical properties is investigated, showing strength values that deviate from the expected behavior for grain refinement. Likewise, a reduced strain hardening capacity is depicted which correlates to the microstructural observations performed on the deformed state. High diffusivity measured by means of radiotracer experiments is observed in the sintered samples, displaying different penetration profiles that relate to diverse porosity structures. Such structures are also responsible for retrograde sintering observed exclusively in samples processed from BM powders.

This work deals with fatigue properties and stability of microstructure of ultrafine-grained (UFG) copper prepared by severe plastic deformation by means of equal channel angular pressing (ECAP) method. The effect of different fatigue loading regimes and thermal exposition on microstructural changes was investigated and the fatigue lifetime curves were experimentally determined. The research attention was focussed on localization of cyclic plastic deformation and fatigue crack initiation in UFG structure. Experimental results indicate that after stress-controlled fatigue loading (both symmetrical and asymmetrical) the microstructure remains ultrafine; no grain coarsening was observed. Contrary to this, strain-controlled fatigue loading results in formation of bimodal structure. Grain coarsening was observed also after thermal exposition at 250 °C for 30 minutes. Annealing at lower temperatures does not result in grain coarsening or development of bimodal structure. Fatigue loading results in development of surface relief in form of cyclic slip markings. Their density, distribution and shape differ for particular fatigue loading regimes. Differences in crack initiation mechanism in low- and high-cycle fatigue region were found. Nevertheless, the characteristic feature for all loading regimes was stability of UFG microstructure in the region of cyclic slip bands and fatigue cracks.