Shape memory behavior of Ti-rich Ti-Ni thin films formed by sputtering

Abstract
The shape memory behavior of Ti-rich Ti-Ni thin films (Ti-45.2, 47.0, 47.9 at. %Ni) annealed at 773, 823, 873K for 1h was investigated. Transmission electron microscopy revealed that Ti-45.2at. %Ni thin films contain randomly oriented Ti₂Ni particles, while the other two films contain Ti₂Ni precipitates with the same orientation as that of the TiNi matrix. In addition to the Ti₂Ni precipitates, GP zones were also observed in a Ti-47.9at. %Ni thin film annealed at 773K for 1h. Every specimen showed a two-stage transformation in a low stress range. The martensitic transformation temperature was found to decrease with increasing Ti content and decreasing annealing temperature. However, thin films containing GP zones showed significantly low transformation temperatures in spite of a low Ti content. The residual strain was found to increase with increasing Ti content and decreasing annealing temperature. The transformation strain was found to increase with decreasing Ti content and annealing temperature. In particular, the thin films containing GP zones showed a large transformation strain. The transformation temperatures of Ti-rich Ti-Ni thin films were generally higher than those of Ni-rich Ti-Ni thin films.