Microstructure and shape memory behaviour of annealed Ti_51.5Ni_(48.5-x)Cu_x (x = 6.5–20.9) thin films

Abstract

Ti-rich Ti–Ni–Cu amorphous films (Ti_51.9Ni_41.6Cu_6.5, Ti_51.6Ni_36.8Cu_11.6, Ti_51.5Ni_33.1Cu_15.4 and Ti_51.7Ni_27.4Cu_20.9), formed by sputtering, were annealed at 773, 873 and 973 K for 1 h and their structures and shape memory behaviours investigated. All the films annealed at 773 K for 1 h exhibited Guinier–Preston (GP) zones, but these precipitates were absent after annealing at 873 K or higher. Instead of GP zones, coherent plate precipitates of a Ti₂Cu phase were formed in films annealed at 873 K for 1 h, when the Cu content was between 11.6 to 20.9 at.%. The strain–temperature curves under constant stresses of Ti_51.6Ni_36.8Cu_11.6 and Ti_51.5Ni_33.1Cu_15.4 films showed a two-step deformation associated with the B2 ↔ B19 ↔ B19′ transformation, whereas Ti_51.9Ni_41.6Cu_6.5 and Ti_51.7Ni_27.4Cu_20.9 films showed a single-step deformation associated with the B2↔B19′ and B2↔B19 transformations, respectively. The two kinds of plate precipitates, GP zones and a Ti₂Cu phase were found to be effective to increase the critical stress for plastic strain.