Abstract
The shape memory behaviour of (Ni,Cu)-rich Ti–Ni–Cu thin films (Ti48.9Ni44.9Cu6.2, Ti48.5Ni40Cu11.5, Ti48.6Ni35.9Cu15.5, Ti48.3Ni28.4Cu23.3, Ti48.3Ni23.9Cu27.8 and Ti48.5Ni18Cu33.5) annealed at 773, 873 and 973 K for 1 h was investigated. The films with 6.2, 11.5–15.5 and 23.3–33.5 at% Cu showed a single-stage deformation due to a B2 ↔ B19′ transformation, a two-stage deformation due to the B2 ↔ B19 ↔ B19′ transformation and a single-stage deformation due to the B2 ↔ B19′ transformation, respectively. The martensitic transformation start temperature (M s) increased with increasing Cu content and then levelled off for more than 15 at% Cu, indicating a high Ms temperature of 345 K. Temperature hystereses were almost 15 K for all films with more than 10 at% Cu. The critical stress for slip increased with increasing Cu content and increased significantly for the Ti48.5Ni18Cu33.5 film, whereas the maximum recoverable strain significantly decreased for the Ti48.5Ni18Cu33.5 film. With decreasing annealing temperature, the critical stress for slip increased, but the M s temperature decreased. It was found that films with 11.5 at% Cu or more, annealed at 873 K, showed a high martensitic transformation temperature and a high critical stress for slip.