九州大学 研究者情報
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植木 翔平(うえき しようへい) データ更新日:2024.01.29

助教 /  工学研究院 機械工学部門 材料力学講座


原著論文
1. Shohei Ueki, Yoji Mine, Yu-Lung Chiu, Paul Bowen, Kazuki Takashima, Effects of crystallographic orientation and lamellar configuration on fatigue crack propagation in single-colony structures of Ti–6Al–4V alloy: Alternating shear crack growth vs. damage accumulation crack propagation, Materials Science and Engineering A, https://doi.org/10.1016/j.msea.2023.145885, 890, 145885, 2024.01, In this study, the fatigue crack propagation mechanisms in lamellar colonies of Ti–6Al–4V alloy were examined using miniature compact-tension specimens with a single-colony structure at the crack tip. Fatigue tests were performed on colonies with different α-phase crystallographic orientations and lamellar configurations with respect to the notch plane and direction, followed by post-fatigue metallographic analysis. Crack propagation occurs by three main mechanisms: 1) alternating shear due to in-plane prismatic slip, 2) damage accumulation via dislocation–dislocation interaction due to out-of-plane slip, and 3) interlamellar decohesion due to damage accumulation via lamellar interphase boundary–dislocation interaction. The slope of da/dN vs. ΔK is higher for alternating shear crack growth than for damage accumulation crack propagation via dislocation–dislocation interaction. This is due to an incubation period before substantive crack extension in the latter case, which is dominated by degree of strain accommodation associated with the activated slip systems. When the lamellar interphase boundaries are nearly perpendicular to the loading axis, the crack growth rates drastically increase by interlamellar decohesion. This is attributed to the reduced incubation period due to slip incompatibility at the interphase boundary where numerous crack nuclei were formed during damage accumulation process..
2. Shohei Ueki, Kaoru Koga, Yoji Mine, Kazuki Takashima, Crystallographic characterisation of hydrogen-induced twin boundary separation in type 304 stainless steel using micro-tensile testing, ISIJ International, 10.2355/isijinternational.ISIJINT-2018-608, 59, 5, 927-934, 2019.05, Micro-tensile behaviour and the corresponding microstructural evolution under hydrogen pre-charging conditions were examined on single-crystalline and twinned bi-crystalline specimens with the same [111] loading axis to elucidate the hydrogen-induced twin boundary separation in type 304 stainless steel. A hydrogen pre-charge increased the flow stress during tensile testing but decreased the elongation-to-failure in both single-crystalline and twinned specimens. Although the hydrogen-charged single-crystalline specimen exhibited a quasi-cleavage, the presence of a twin boundary induced a premature failure at the twin boundary interface. Flat-facetted features due to the twin boundary separation had linear steps in the three directions, which corresponded to the intersections between the twin plane and the other {111} close-packed planes of austenite. Matching halves of the fracture surface along the three directions perpendicular to the linear steps, i.e. on the (111) twin plane, revealed two sets of concavity–flat surface and a peak-and-valley correspondence. In addition, electron backscatter diffraction analysis of the substructures below the fracture surfaces revealed that martensite variants developed mainly with their habit planes parallel to the most favourably shear-stressed plane in each crystal, and they grew towards the concavities on the fracture surfaces. These findings suggest that the hydrogen-induced twin boundary separation is triggered by cracks generated by the high hydrogen concentration at the twin boundary due to deformation-induced martensitic transformation, and this is followed by coalescence of cracks through hydrogen-enhanced alternating shear on the slip planes situated symmetrically with respect to the twin boundary..
3. Aya MATSUSHITA, Shohei UEKI, Yoji MINE, Kazuki TAKASHIMA, Comparative study of microstructure-sensitive fatigue crack propagation in coarse- And fine-grained microstructures between stable and metastable austenitic stainless steels using miniature specimen, ISIJ International, 10.2355/ISIJINTERNATIONAL.ISIJINT-2020-659, 61, 5, 1688-1697, 2021.05, Microstructure-sensitive fatigue crack propagation was studied on coarse- and fine-grained stainless steels with different austenite stabilities using miniature compact-tension specimens. For coarse-grained 310S stable austenitic steel, the crack growth rate was increased by shear-localised bands formed ahead of the crack tip. For fine-grained 310S with an average grain size of ∼0.25 μm, the crack-tip plastic strain was concentrated on the grains favourable to dislocation multiplication, rather than being dependent on the distance from the crack surface, which led to discontinuous crack propagation. Consequently, the fatigue crack growth rate was lower in the fine-grained 310S steel than in the coarse-grained one. In 304 metastable austenitic steel, the fatigue crack propagated within the martensite that formed ahead of the crack tip, and the crack growth rate was lower than that in the 310S steel. The grain refinement of 304 steel to a ∼0.99 μm average grain size enhanced the crack growth resistance. Electron back-scatter diffraction analysis of the fracture surface revealed microstructural fragmentation due to single-variant transformation for each grain in the fine-grained 304 steel. These findings indicate that the microstructural evolution ahead of the crack tip dominates the rate of mechanically short fatigue crack propagation in austenitic stainless steels..
4. Yuki Tampa, Kosuke Takagi, Shohei Ueki, Motoki Ohta, Yoji Mine, Kazuki Takashima, Comparative Study of Shear Fracture between Fe-based Amorphous and Ultrafine-grained Alloys Using Micro-tensile Testing, ISIJ International, 10.2355/isijinternational.ISIJINT-2022-088, 62, 8, 1741-1749, 2022.08, Micro-tensile tests were employed to clarify the post-plastic-instability behavior in the shear fractures of specimens with the dimensions of 18×25×50 μm3 made from iron-based amorphous (AM) and ultrafine-grained (UFG) alloys. The AM specimen yielded by localized shear bands with an inclination angle of ~52° with respect to the loading axis, followed by sliding off almost throughout the entire specimen thickness. Micro-tensile and micro-shearing tests revealed that the Mohr failure envelope of the AM specimens could be described by a quadratic equation rather than a linear equation. Therefore, the sliding-off process is assisted by the applied normal stress, which suggests that it is caused by free-volume coalescence. For the UFG specimen, yielding set in by shear band formation with an inclination angle of ~45° with respect to the loading axis, following the Tresca criterion. Necking after shear band diffusion formed a triaxial stress state, which resulted in a final shear fracture plane via void coalescence in the UFG specimens. Voids formed along the intersection of the primary shear bands with secondary shear bands during the necking process. This indicates that the deviation of the shear fracture plane in the UFG specimen was determined by the strain development process. A comparison of the post-plastic-instability behavior between the AM and UFG specimens suggests that the external control of triaxial stress conditions is key to improving the formability of AM specimens..
5. Shohei Ueki, Yoji Mine, Kazuki Takashima, Crystallographic study of hydrogen-induced twin boundary separation in type 304 stainless steel under cyclic loading, Corrosion Science, 10.1016/j.corsci.2017.10.013, 129, 205-213, 2017.12, This study focused on the martensitic transformation during fatigue crack growth in twinned crystals using small compact-tension specimens to elucidate the hydrogen-induced twin boundary separation in type 304 stainless steel. In the uncharged specimen, martensite variants were formed with their habit planes parallel to the most highly shear-stressed slip plane. The crack extended in martensite that was earlier formed ahead of the crack tip and deflected from the twin boundary. Hydrogen-induced twin boundary separation occurred predominantly at the interface between martensite and austenite in a medium stress intensity range. As the stress intensity range increased, martensite variants symmetrically arranged with respect to the twin plane dominated, suggesting that a slip-off crack growth mechanism was facilitated by hydrogen..
6. Shohei Ueki, Takuya Matsumura, Yoji Mine, Shigekazu Morito, Kazuki Takashima, Microstructural fatigue crack growth in single-packet structures of ultra-low carbon steel lath martensite, Scripta Materialia, 10.1016/j.scriptamat.2019.08.004, 173, 80-85, 2019.12, Fatigue crack growth in single-packet structures with different habit-plane orientations was examined using miniature compact-tension specimens of ultra-low carbon steel to elucidate the elementary process of microstructural crack growth in lath martensite. The growth rates of fatigue cracks were higher in propagating along block boundaries than in running across blocks, while both cracks had a rippled fracture surface. Post-fatigue-test electron microscopy results suggest that the crack prefers to grow along the block boundaries with high slip symmetry rather than take a roundabout route through the cell boundaries formed owing to the interaction of activated dislocations with pre-existing dislocations inside blocks..
7. Shohei Ueki, Yoji Mine, Kazuki Takashima, Microstructure-sensitive fatigue crack growth in lath martensite of low carbon steel, Materials Science and Engineering A, 10.1016/j.msea.2019.138830, 773, 2020.01, The elementary process of microstructural fatigue crack growth in single-packet structures with different habit-plane orientations to the loading axis was examined using miniature compact-tension specimens to comprehend the intrinsic resistance to fatigue crack growth in lath martensite of low carbon steel. When the angle between the loading axis and the normal to the habit plane (φ) was within ~45°, the cracks propagated nearly parallel to the habit plane. In packets with their habit planes nearly perpendicular (φ > ~70°) to the notch direction, the crack grew across the laths because of damage accumulation through the activation of the out-of-plane slips, which increased the fatigue crack growth resistance. Post-fatigue-test electron microscopy revealed that the microstructural inhomogeneity of the low carbon steel enhanced the strain localisation in coarse laths, which probably led to the premature fatigue crack growth. When the crack grew perpendicular to the habit plane, the intrinsic resistance to fatigue crack growth was higher in the fine-lath region than in the coarse-lath region..
8. Shohei Ueki, Ryuji Oura, Yoji Mine, Kazuki Takashima, Micro-mechanical characterisation of hydrogen embrittlement in nano-twinned metastable austenitic stainless steel, International Journal of Hydrogen Energy, 10.1016/j.ijhydene.2020.07.120, 45, 51, 27950-27957, 2020.10, Micro-tensile tests were performed on single variants of nano-twinned 304 stainless steel with hard and soft orientations to determine the effect of nano-twins on hydrogen embrittlement in the metastable austenitic steel. Nano-twins were introduced into the 304 steel by deformation above the martensite-start temperature. The introduction of nano-twins increased the yield stress to 647 MPa for the hard-oriented crystals and 540 MPa for the soft-oriented crystals. For the soft-oriented crystals, the hydrogen charge induced quasi-cleavage although it did not change the uniform strain. For the hard-oriented crystals, the introduction of nano-twins retarded hydrogen-induced twin boundary separation. The relationship between the reduction of area and ultimate tensile strength revealed that the degree of strengthening and mitigation of hydrogen-induced ductility loss depended on the orientation of the nano-twins. These suggest that controlling the orientation and distribution of nano-twins allows for simultaneous strengthening and mitigation of hydrogen-induced ductility loss in metastable austenitic steel..
9. Shohei Ueki, Yoji Mine, Kazuki Takashima, Excellent mechanical properties of taenite in meteoric iron., Scientific reports, 10.1038/s41598-021-83792-y, 11, 1, 4750-4750, 2021.02, Meteoric iron is the metal that humans first obtained and used in the earliest stage of metal culture. Advances in metallographic analysis techniques have revealed that meteoric iron largely comprises kamacite, taenite, and cohenite, which correspond to ferrite, austenite, and cementite in artificial steel, respectively. Although the mechanical properties of meteoric irons were measured previously to understand their origin and history, the genuine mechanical properties of meteoric iron remain unknown because of its complex microstructure and the pre-existing cracks in cohenite. Using micro-tensile tests to analyse the single-crystalline constituents of the Canyon Diablo meteorite, herein, we show that the taenite matrix exhibits excellent balance between yield strength and ductility superior to that of the kamacite matrix. We found that taenite is rich in nitrogen despite containing a large amount of nickel, which decreases the nitrogen solubility, suggesting that solid-solution strengthening via nitrogen is highly effective for the Fe-Ni system. Our findings not only provide insights for developing advanced high-strength steel but also help understand the mysterious relationship between nitrogen and nickel contents in steel. Like ancient peoples believed that meteoric iron was a gift from the heavens, the findings herein imply that this thought continues even now..
10. 植木 翔平, 古賀 薫, 峯 洋二, 高島 和希, マイクロ引張試験を用いた304型ステンレス鋼の水素誘起双晶境界分離の結晶学的評価, 鉄と鋼, 10.2355/tetsutohagane.tetsu-2021-086, 108, 1, 97-106, 2022.01.
11. Shohei Ueki, Yoji Mine, Xinyu Lu, Yu Lung Chiu, Paul Bowen, Kazuki Takashima, Effect of geometric lath orientation on fatigue crack propagation via out-of-plane dislocation glide in martensitic steel, SCRIPTA MATERIALIA, 10.1016/j.scriptamat.2021.114045, 203, 2021.10, Strain accumulation ahead of the fatigue crack tip in the martensite lath of a medium-carbon steel was examined using a three-dimensional electron backscatter diffraction (3D-EBSD) technique. The objective of this study is to explain the crack propagation mechanism due to the activation of out-of-plane slips with their Burgers vectors having no component of the crack growth direction, which exhibits high resistance to fatigue crack growth. The 3D-EBSD analysis revealed little misorientation in the crystal, concurrent with the fatigue crack propagation in the coarse laths oriented favourably for dislocation glide in their longitudinal directions. This suggests that these laths contributed to strain accommodation. In contrast, strain preferentially accumulated in the coarse laths oriented unfavorably for the longitudinal slip, promoting crack propagation. These indicate that the geometrical anisotropy and distribution of marten site laths dominate the fatigue crack propagation resistance in martensitic carbon steel. (c) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved..

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