Processing and Microstructures of Ti-Ta High Temperature Shape Memory Alloys

Ti-Ta shape memory alloys (SMAs) show a martensitic transformation at temperatures close to 500K. Most importantly, they are very ductile, and in contrast to many other high high-temperature SMAs (HT-SMAs), they show an excellent workability. However, the preparation of Ti-Ta SMAs is challenging because the melting point of Ta is close to the evaporation point of Ti. Furthermore, the solidification of molten Ti-Ta is associated with decomposition processes. In the present work we show, how homogeneous Ti-Ta alloys can be prepared through arc melting, heat treatments and thermomechanical processing. The resulting materials were characterized through differential scanning calorimetry, optical, scanning and transmission electron microscopy. Ti-Ta ingots have to be remelted several times to ensure a macroscopic homogeneity. Homogenization annealing at 1100°C for 25 hours is required to sufficiently reduce microscopic concentration gradients resulting from solidification. Ti-Ta alloys can be easily deformed through cold rolling. Deformation levels of up to 90% can be achieved, even without any pre-heating. In the present work, we also characterize martensite twin structures and omega-phase precipitates which form during thermal cycling and aging.