Abstract
Ti-rich Ti-Ni thin films of Ti-45.2, 46.1, 47.0, 47.9, 48.5 at%Ni were prepared by sputtering. The sputter-deposited thin films were annealed at 773, 823 and 873 K for 1 h. Transmission electron microscopy revealed that Ti-45.2 at%Ni thin films contain randomly oriented Ti2Ni particles, while the other films contain Ti2Ni precipitates with the same orientation as that of TiNi matrix. In addition to these Ti2Ni precipitates, GP zones were also observed in Ti-47.9 and 48.5 at%Ni thin films annealed at 773 K for 1 h. The shape memory behavior of the annealed Ti-Ni thin films was investigated with a thermomechanical tester. It was found that martensitic transformation temperature decreases with increasing Ti content and decreasing annealing temperature. However, thin films containing GP zones show low transformation temperatures despite of low Ti content. On the other hand, R-phase transformation temperature was not found to be sensitive to film composition, except for thin films with GP zones, which show low R-phase transformation temperatures as well as low martensitic transformation temperatures. Critical stress for slip deformation was found to increase with increasing Ti content and decreasing annealing temperature. GP zones were helpful in increasing the critical stress of Ti-47.9 and 48.5 at%Ni thin films. Finally, recoverable strain was found to increase with decreasing Ti content and annealing temperature. Especially, the thin films containing GP zones showed large recoverable strains.
