Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

The yielding behaviour and deformed microstructure of lath martensite blocks in ultra-low carbon steel were examined using micro-sized tensile specimens with different habit-plane orientations to elucidate anisotropic slip behaviour. The resolved shear stress at 0.2 % proof stress for slips parallel to the habit plane was lower than that for slips across the habit plane. Post-yielding electron backscatter diffraction analysis demonstrated that slip transfer across sub-block and lath boundaries occurred readily in the in-habit-plane direction, and with difficulty in the out-of-habit-plane direction, owing to the interaction between incident and low-angle grain (sub-block and lath) boundary dislocations after the onset of plastic deformation. These findings suggest that the anisotropic and localised slip behaviour in martensite blocks is due to differences in the interaction between activated dislocations and low-angle grain boundaries, depending on the habit-plane orientation.

DOI： 10.1016/j.scriptamat.2024.116389

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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.

DOI： https://doi.org/10.1016/j.msea.2023.145885

Repository Public URL： https://hdl.handle.net/2324/7164812

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.msea.2019.138830

Repository Public URL： https://hdl.handle.net/2324/7164811

Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

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.

DOI： 10.2355/isijinternational.ISIJINT-2022-088

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Repository Public URL： https://hdl.handle.net/2324/7161265

Authorship：Lead author   Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Iron and Steel Institute of Japan  

Crystallographic Characterisation of Hydrogen-induced Twin Boundary Separation in Type 304 Stainless Steel Using Micro-tensile Testing
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 <110> 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. <112> 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.

DOI： 10.2355/tetsutohagane.tetsu-2021-086

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Other Link： https://www.jstage.jst.go.jp/article/tetsutohagane/108/1/108_TETSU-2021-086/_pdf

Repository Public URL： https://hdl.handle.net/2324/7161104

Event date： 2019.9

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