Microstructure and shape-memory behavior of annealed Ti_51.5Ni_33.1Cu_15.4 thin films

Abstract

The structure and shape-memory behavior of Ti_51.5Ni_33.1Cu_15.4 amorphous films, prepared by sputtering and annealed at 773, 873 and 973 K for 1 h, have been investigated. The microstructure of the thin films evolved in the following sequence as a function of the annealing temperature: (1) plate precipitates of Guinier–Preston (GP) zones at 773 K, (2) spherical precipitates of a Ti₂Ni phase and plate precipitates of a Ti₂Cu phase at 873 K, (3) grain boundary precipitates of a Ti₂Ni phase and a Ti₂Cu phase at 973 K. With increasing annealing temperature, the transformation temperature increased. The temperature hysteresis was small compared to that of Ti_51.5Ni_48.5 thin films. As the annealing temperature increased, the transformation strain decreased and the residual strain increased. It was found that the substitution of Cu for Ni stabilizes coherent plate precipitates and, thus, improves the shape-memory effects of Ti_51.5Ni_33.1Cu_15.4 films annealed at 773 and 873 K compared with those of Ti_51.5Ni_48.5 films.