Tetsuya Matsunaga

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.

Single crystal Cu–Al–Mn shape memory alloy fabricated by abnormal grain growth is a strong candidate for actuators of heat switches that can adjust the heat flow to control the temperature inside a spacecraft appropriately. In this study, we proposed a suitable thermomechanical treatment of Cu–16.9Al–11.4Mn (mol%) alloy to stabilize its single–crystal orientation within the desired orientation region, i.e. RD//<510>, through forming texture before single crystallization heat treatment. Strong texture of the desired orientation was formed after rolling and annealing at 740°C 1 h of needle–like α+β duplex microstructure derived from Bainite microstructure. The texture became stronger when the above treatment was repeated twice. After forming a texture, single crystallization was achieved by oscillating the temperature between 740°C and 500°C five times and final annealing of 900°C 1 h. Finally, single crystals of the desired orientation were obtained with a probability of 80%. Repeated tensile tests on the obtained single crystals showed a good superelastic properties with superelastic strain of 7%.

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.

To reveal grain-size dependency and mechanism of creep of 1050 aluminum at low temperatures, creep tests were performed for the samples with grain sizes (d) of 1.0&ndash;47 &micro;m at 233&ndash;473K. Dislocation creep rate-controlled by non-diffusion process was observed at low temperatures, i.e., T<400K and 280K for coarse (CG) and ultrafine grained (UFG) specimens, respectively. The former showed creep at more than 0.2% proof stress, whereas the latter did it at less than that stress. Creep behavior of UFG aluminum was similar to ambient-temperature creep of hexagonal close-packed metals because apparent activation energy was about 30 kJ/mol. Although grain-size exponent was small, i.e. p=0&ndash;0.3, in CG and UFG regions, transient region was observed at d=1.7&ndash;10 &micro;m and creep rate decreased of about one order. At high temperatures, CG and UFG aluminum showed conventional dislocation creep rate-controlled by dislocation-core diffusion.