Static and Dynamic Mechanical Behavior of Martensitic NiTi Shape Memory Alloys

The austenitic phase of NiTi shape memory alloys (SMAs) shows higher stiffness (40-90 GPa for austenite, 20-70 GPa for martensite) and more recoverable strain (up to 8%) as compared with the martensitic phase of NiTi SMAs. Since the temperature range between the martensite finish temperature (M_f) and the austenite finish temperature (A_f) is often  small leading to phase transformation with only a small temperature change and the important functionality of martensitic SMA devices involve the phase transformation from martensite-to-austenite for recover, it is important to examine the mechanical behavior of both phases in martensitic NiTi SMAs. The deformation mechanism is very different between these two phases; pseudoelastic effect occurs in austenitic state and the shape memory effect occurs in the martensitic state. In the present research, mechanical behavior of martensitic NiTi SMAs under both quasi-static (0.001 s^-1) and dynamic (300 s^-1, 600 s^-1, 1000 s^-1) compression and the corresponding microstructure and deformation mechanisms such as deformation twinning and phase transformation were investigated both at room temperature in the martensitic state and above A_f in the austenitic state.