Effects of composition and annealing on shape memory behavior of Ti-rich Ti-Ni thin films formed by sputtering

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 the 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 these Ti-Ni thin films was investigated with a thermomechanical tester. The following results were obtained with respect to the martensitic transformation. (1) The transformation temperature decreases with increasing Ti content and decreasing annealing temperature. However, thin films containing GP zones show low transformation temperatures despite of the low Ti contents. (2) The critical stress for plastic deformation associated with the transformation increases with increasing Ti content and decreasing annealing temperature. GP zones seem to increase the critical stresses of Ti-47.9 and 48.5 at%Ni thin films annealed at 773 K for 1 h. (3) The recoverable strain increases with decreasing Ti content and decreasing annealing temperature. Especially, the thin films containing GP zones show large recoverable strains. On the other hand, the R-phase transformation showed the following features. (1) The transformation temperature is insensitive to film composition and heat treatment, being almost 335 K, though thin films containing GP zones show relatively low R-phase transformation temperatures. (2) No residual strain is detected for any specimen after a thermal cycle. These characteristics suggest that the shape memory effect due to the R-phase transformation is suitable for practical use.