Sputter alloying of Ni,Ti and Hf for fabrication of high temperature shape memory thin films

Abstract

In the present paper, the fabrication and characterisation of typical high temperature Ni(Ti+Hf) alloyed thin films produced by simultaneous sputter deposition from separate elemental Ni, Ti and Hf targets are presented. Film composition, determined by energy dispersive X-ray spectroscopy, was controlled by adjusting the ratio of powers applied to each target. Films deposited at room temperature had an amorphous structure and subsequent annealing at 550°C was carried out in a high vacuum environment, based on crystallisation temperature evaluation by differential scanning calorimetry (DSC). High temperature martensitic transformation, confirmed by DSC and variable temperature X-ray diffraction (XRD), was achieved by deposition of (Ti+Hf) rich Ni–Ti–Hf films. Any slight change of composition towards Ni rich reduced the transformation temperature. Atomic force microscopy and XRD illustrated that the films had a fine grain structure (∼100 nm). One way shape memory effect was observed at ∼200°C in a film with composition of 15·6 at.-%Hf.