Shape memory behaviour of Ti51.5Ni(48.5−x)Cux (x = 23.4–37.3) thin films with submicron grain sizes

The shape memory behaviours of Ti51.4Ni25.2Cu23.4, Ti51.3Ni21.1Cu27.6, Ti51.2Ni15.7Cu33.1 and Ti51.4Ni11.3Cu37.3 thin films annealed at 773, 873 and 973 K for 1 h were investigated. The Ti51.3Ni21.1Cu27.6 film annealed at 773 K, the Ti51.2Ni15.7Cu33.1 film annealed at 873 K, and the Ti51.4Ni11.3Cu37.3 films annealed at 873 and 973 K showed a perfect shape memory effect at a stress as high as 1 GPa. This improvement in shape memory behaviour was attributed to their fine grain sizes less than 500 nm. Whereas the Ti51.2Ni15.7Cu33.1 and Ti51.4Ni11.3Cu37.3 films annealed at 873 K or higher showed a martensitic transformation start temperature above room temperature, these films annealed at 773 K were in the parent phase at room temperature owing to their very fine grain sizes. The effects of Cu content and annealing temperature on the shape memory behaviour of the Ti51.5Ni48.5−xCux (x > 27) films with submicron grain sizes were discussed in comparison with those of the Ti51.5Ni48.5−xCux (x < 24) films on the basis of their microstructures.