Stress-strain curves of sputtered thin films of Ti-Ni

Ti-Ni thin films with three different compositions (Ti-48at.%Ni, Ti-50at.%Ni and Ti-51at.%Ni) were prepared by sputtering. The sputter-deposited films were annealed at 773 K for 1 h to achieve crystallization. The stress-strain curves for the annealed films were measured over a wide temperature range (143-473 K) using a tensile tester. All the films showed yielding followed by a plateau region in the stress-strain curves. The strain in the plateau region partly or completely recovered on unloading at high test temperatures (superelasticity), whereas, at low test temperatures, the strain remained after unloading, but partly or completely recovered after subsequent heating (shape memory effect). The Ti-50at.%Ni thin films did not show perfect recovery in terms of either the superelasticity or shape memory effect. The Ti-48at.%Ni thin films showed a perfect shape memory effect, but not a perfect superelasticity. In contrast, the Ti-51at.%Ni thin films showed both a perfect shape memory effect and a perfect superelasticity.