Effect of Heat Treatment on Shape Memory Behavior of Ti-Ni Thin Films

Thin films of Ni-rich Ti-Ni and Ti-rich Ti-Ni were prepared by sputtering. The films were annealed at 773 and 973K for various times from 3.6 to 360ks in order to crystallize them. After the heat treatments, the shape memory characteristics of the films were investigated by measuring the strain during cooling and heating under various constant stresses between 40 and 520MPa. The shape memory behavior of the films was found to be very sensitive to the annealing conditions. The Ni-rich films annealed at 773K showed a two-stage shape change, while the ones annealed at 973K showed a single-stage shape change. The martensitic and reverse martensitic transformation temperatures increased with decreasing annealing temperature and increasing annealing time. On the other hand, the annealed Ti-rich films showed a two-stage shape change except the film annealed at 973K for 100h. In this film, the R-phase transformation which caused the two-stage shape change was not detected. The martensitic and reverse martensitic transformation temperatures increased with increasing annealing temperature and annealing time.