Crystallization process and shape memory properties of Ti-Ni-Zr thin films

The crystallization process of as-deposited Ti-Ni-(10.8-29.5)Zr amorphous thin films was investigated. The Ti-Ni-Zr as-deposited films with a low Zr content exhibited a single exothermic peak due to the crystallization of (Ti,Zr)Ni with a B2 structure. In contrast, a two-step crystallization process was observed in the Ti-Ni-Zr thin films with a high Zr content. Shape memory behavior of Ti-Ni-Zr thin films heat treated at 873-1073 K was investigated by thermal cycling tests under various stresses. The martensitic transformation start temperature increased with increasing Zr content until reaching the maximum value, then decreased with further increasing Zr content. The inverse dependence of transformation temperature on Zr content in the thin films with a high Zr content is due to the formation of a NiZr phase during the crystallization heat treatment. The formation of the NiZr phase increased the critical stress for slip but decreased the recovery strain.