松永 哲也

Twinnability in face-centered cubic metallic solids is discussed from the viewpoint of wavenumber (k) space. The {111} deformation twinning is one of the main plastic deformation modes enabling the rearrangement of a mirror-symmetry lattice structure across a twin boundary. The crystalline rearrangement around 112¯ in k-space is triggered by changes in the electron density distribution caused by the strain-induced modulation of the electronic band structure around this orientation. This modulation is defined by the twinnability, ε112¯, which reflects the difficulty of the redistribution of electron density around 112¯. This index clarifies that the deformation twinning is suppressed in aluminum (Al) due to its large ε112¯ value. This result rationalizes the disorder between platinum and Al observed in conventional twinnability assessments based on stacking fault energy.

Abstract The effect of alloying elements on the martensitic transformation behavior of Ti–Pd series high-entropy shape memory alloys was investigated. Five compositions incorporating Zr, V, Ni, and Pt were synthesized. DSC analysis revealed that the coherency of the martensitic structure formed by the added elements is crucial for maintaining a high transformation temperature. The findings provide insights into the compositional design of high-temperature shape memory alloys with enhanced thermal stability and functional performance.

Abstract The near-α Ti–6Al–4Zr–4Nb (wt pct) alloy is a recently developed alloy with potential for aerospace applications. This study evaluates the microstructure and mechanical properties of Ti–6Al–4Zr–4Nb produced by spark plasma sintering (SPS). The SPS samples sintered in the α + β regions exhibited equiaxed α with a neighboring thin β phase. Above the β-transus temperature, the α/β lamellar structure formed, allowing control of the grain size (100 ~ 200 μm). The compressive strength and the creep property of the SPS samples were compared with the LBPDed and the forged samples. The compressive strength of the SPS sample was lower than that of the LPBFed sample but similar to the forged sample. The SPS samples exhibited longer creep rupture life (2220 hours) than LPBFed samples (1730 hours), but shorter than the forged sample (4109 hours). The creep deformation mechanism of the lamellar structure in the SPS sample was dislocation creep.

Heat-resistant Ti-Al-Nb-Zr alloys, which don’t contain Sn, have been designed to obtain good oxidation resistance above 600 °C. In addition, to design Ti alloys with best balance of creep and fatigue properties, prior β grain size which affects fatigue properties and lamellar microstructure which affects creep properties were controlled by heat treatment. In the present study, the effect of microstructure on creep properties of one of the alloys, i.e., Ti-7.5Al-4Nb-4Zr alloy, with the bimodal (B), the lamellar structures in small prior β grains (L_S), and the lamellar in large prior β grains (L_L) were investigated at 600 °C. The creep deformation mechanism for each microstructure was a power-law creep. However, the creep life varied depending on the microstructures. The longest creep life was obtained in L_S with prior β grain size of 90 μm and interlamellar spacing of approximately 10 μm, while the shortest creep life was obtained in L_L with prior β grain size of 550 μm and fine interlamellar spacing of less than 2~3 μm. This suggests that creep life is more affected by interlamellar spacing than by prior β grain size.