Sputter-deposited Ni-rich NiTi thin films: Mechanical behavior and composition sensitivity

In this study, we investigated the mechanical behavior of sputter-deposited Ni-rich NiTi thin films with various chemical compositions. Freestanding thin films were successfully produced by employing magnetron sputtering and microfabrication techniques. Notably, films containing Ni within the range of 52.6–57.6 at. % exhibited distinct stress-induced martensitic transformations. Among these, 53.3 at. % and 54.2 at. % Ni–Ti films demonstrated a remarkable combination of strength exceeding 1.5 GPa and elastic strain approaching 5%. Conversely, films with the highest Ni content displayed nearly linear elastic behavior, showcasing an exceptional tensile strength of 2.2 GPa. As the element composition deviated from the equiatomic ratio, the grain size of the deposited films underwent a transition from several micrometers to nano-scale. In addition, the critical stresses showed lower sensitivity to chemical composition compared to bulk alloys, indicating that superelastic behavior persists over a wider composition range in Ni-rich NiTi thin films. By comparing the NiTi matrix and precipitate morphology between bulk alloys and thin films, we discussed the variations in the mechanical behavior of thin films.