Deformation mechanism of NiTi shape memory alloy subjected to severe plastic deformation at low temperature

Based on local canning compression, nickel-titanium shape memory alloy (NiTi SMA) with B2 austenite structure at room temperature was subjected to severe plastic deformation (SPD) at −150 °C which is much lower than martensite finish temperature (Mf). Transmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM) were used for investigating microstructural evolution of the NiTi samples subjected to the different deformation degrees. In the process of large plastic deformation, the NiTi sample mainly undergoes martensitic reorientation, dislocation slip, formation of martensite-like plates, occurrence of submicrocrystalline grains, nanocrystallization and amorphization. It can be concluded that SPD is able to lead to nanocrystallization and amorphization of NiTi SMA at low temperature. The formation of the dislocation cells in the deformed NiTi sample plays an important role in nanocrystallization of NiTi SMA. A high density of vacancy and dislocation defects induced in the nanocrystalline grains lay the foundation for amorphization of NiTi SMA. In the case of the smaller plastic strain, the deformed B19′ martensite can return to the B2 austenite at room temperature by means of thermal driving force. In the case of larger plastic strain, the survival of the B19′ martensite phase at room temperature is attributed to the mechanical stabilization of the martensite phase and the increase of reverse transformation temperature.