Microstructure and shape memory behavior of annealed Ti–36.8 at.% Ni–11.6 at.% Cu thin film

Amorphous films of Ti–36.8 at.% Ni–11.6 at.% Cu, formed by sputtering, were annealed at 773, 873 and 973 K for 1 h and their structure and shape memory behavior were investigated. Transmission electron microscopy and electron diffraction showed that the structure of such a film evolves in the following sequence: (1) Guinier–Preston zones form at 773 K, (2) Ti2Cu plate-shaped precipitates and Ti2Ni spherical precipitates appear at 873 K and (3) Ti2Ni spherical precipitates form at 973 K. The strain–temperature curves under various constant stresses of the annealed films were measured. They showed a two-step deformation associated with the two-step martensitic transformation of B2 → B19 → B19′. The transformation temperature increased with increasing annealing temperature. The temperature hystereses of the films were largely reduced compared with those of the binary alloy thin films. The residual strain decreased with decreasing annealing temperature. This improvement in shape memory effect was ascribed to the presence of fine precipitates such as Guinier–Preston zones and Ti2Cu precipitates.