Effects of film composition and annealing on residual stress evolution for shape memory TiNi film

TiNi films with different Ni/Ti ratios were prepared by co-sputtering of a Ti50Ni50 (at.%) target with a separated Ti target at a temperature of 723 K. The stress values in the deposited films changed significantly with Ti contents and post-annealing temperatures due to the differences in phase transformation behaviors and intrinsic stress. For the film with Ti content of 51.3%, a two-step transformation was observed among martensite, R-phase and austenite, and residual stress was quite low at room temperature. After post-annealing the above film at 923 K, only one-stage transformation was observed. For the films with Ti contents of 47.3 and 53%, residual stress was quite high due to the high intrinsic stress and partial relaxation of stress caused by the R-phase transformation. When the films with Ti contents of 47.3 and 53% were annealed at 923 K, residual stress in films decreased significantly, because post-annealing could probably modify the film structure, reduce the intrinsic stress, increase the transformation temperatures, and cause martensite transformation above room temperature. For all the three types of films annealed at 1023 K for 1 h, the high thermal stress in the thin films could cause non-recoverable deformation during phase transformation, damage the shape memory effect, and result in the peeling-off of film from Si substrate.