Microstructure and shape memory behavior of Ti55.5Ni44.5−xCux (x = 11.8–23.5) thin films

•Ti–Ni–Cu films with 55.5 at% Ti were formed by sputtering.•The films annealed at 773–973 K for 1 h had no precipitates within the B2 grains.•The shape memory behavior was sensitive to the B2 grain size.•The B2 grain size was controlled by the grain size of a second phase.•The effects of Cu content and annealing temperature were discussed.

The microstructure and shape memory behavior of Ti55.5Ni45.5−xCux (x = 11.8–23.5) thin films annealed at 773, 873, and 973 K for 1 h were investigated. None of the films except the Ti55.4Ni32.8Cu11.8 film annealed at 773 K for 1 h had any precipitates in the B2 grain interiors and their grain sizes were small (less than 1 μm). Increasing the annealing temperature caused grain growth and thus a decrease in the critical stress for slip and an increase in the martensitic transformation start temperature (Ms). The grain size was also controlled by the growth of a second phase. In the three-phase equilibrium region of Ti2Ni, Ti2Cu and TiNi, Ti2Cu grains grew faster than Ti2Ni grains, leading to a decrease in the critical stress for slip and an increase in the Ms temperature with increasing Cu content.