Effect of heat treatment on shape memory behavior of Ti-rich Ti-Ni thin films

Thin films of Ti-rich Ti-Ni were prepared by sputtering. The sputter-deposited films were annealed at three different temperatures of 773, 873 and 973 K for three different times of 1, 10 and 100 h in order to crystallize them. After the heat treatment, the shape memory behavior was examined with a thermomechanical tester. The shape memory behavior was found to be very sensitive to the annealing conditions, being different from that in bulk specimens. The martensitic and reverse martensitic transformation temperatures increased with increasing annealing temperature and time. The film annealed at 973 K for 100 h showed a single-stage shape change associated with the martensitic transformation, while the other annealed films showed a two-stage shape change associated with both the martensitic and R-phase transformations. It was found from transmission electron microscopy that the difference in the shape memory behavior between thin films and bulk specimens comes from a difference in the distribution of Ti2Ni particles. That is, line Ti2Ni particles distribute inside the TiNi grain in the annealed films except the one annealed at 973 K for 100 h. On the other hand, the structure of the film annealed at 973 K for 100 h is similar to that normally observed in bulk specimens, where Ti2Ni particles distribute along the gain boundaries.