Effect of stretching-bending deformation and aging treatment on phase transformation behavior and superelasticity of Ti-50.8 at.% Ni alloy

In order to improve the functional stability of Ti-50.8 at.% Ni alloys, a new plastic deformation method and device suitable for sheets or wires was developed in this work, called "stretching-bending deformation (SBD)". After SBD process, aging treatments were carried out at various temperatures (523-823 K) and various durations (0.5-12 h) to explore the effects of SBD process and aging treatment on the phase transformation behavior and superelasticity of NiTi alloys. The results indicate that the repeated stretching-bending process is an effective method to improve the functional stability of NiTi alloys, which introduces a certain amount of dislocations and refines the grain size from 9.1 mu m in as-annealed state to 3.2 mu m in the state after 15 SBD passes. After repeated SBD treatment, the martensitic transformation occurs in a wide temperature range, and the phase transformation peak becomes wider. Aging at 623 K for 0.5-1 h after 7 SBD passes helps to improve the functional stability. This is due to refined grains with an average grain size of 5.4 mu m and favorable dislocations introduced by SBD and nanoscaled Ni4Ti3 precipitates (<15 nm) induced by aging.