Shape memory behavior of Ti–Ni–Cu thin films

Three kinds of Ti-rich Ti–Ni–Cu thin films (Ti51.9Ni41.6Cu6.5, Ti51.6Ni36.8Cu11.6, Ti51.5Ni33.1Cu15.4) and three kinds of (Ni, Cu)-rich Ti–Ni–Cu thin films (Ti48.9Ni44.9Cu6.2, Ti48.5Ni40.0Cu11.5, Ti48.6Ni35.9Cu15.5), numbers indicate at.%, were prepared by sputtering and annealed at 773, 873 and 973 K for 1 h. X-ray diffraction patterns showed that the Ti-rich thin films contain Ti2Ni and Ti2Cu phases, while the (Ni, Cu)-rich thin films contain a TiNiCu phase. The B2 → B19′ martensitic transformation occurred below 10 at.% Cu, while the B2 → B19 martensitic transformation occurred above 10 at.% Cu. The transformation temperature increased with increasing Cu content and annealing temperature. The temperature hystereses were small compared with those of Ti–Ni binary alloy thin films. The maximum recoverable strain decreases with increasing annealing temperature. It also decreases with increasing Cu content when the Cu content is more than 10 at.%. The critical stress for inducing plastic deformation increases with decreasing annealing temperature and increasing Cu content.