Continuous cyclic deformations of a Ni/W film studied by synchrotron X-ray diffraction

In this paper, a method to study cyclic deformations is used in a polycrystalline Ni thin film 600 nm thick with a W cap-layer of 20 nm. The Ni/W bilayer film was deposited onto compliant polyimide substrate allowing the film to be subjected to a relatively large tensile applied stress range during loading-unloading of the film/substrate composite. Film deformations were in situ measured both by synchrotron X-ray diffraction and digital image correlation during continuous cyclic uniaxial tensile test. This work focuses on the characterization of the deformation mechanisms during the first cycles of loading-unloading tensile test. It is found that up to a strain of 0.5% corresponding to an applied stress of 1.1 GPa the elastic strain of the Ni film follows precisely the strain applied to the substrate, but for larger applied stress the elastic strain of the metallic film, though completely reversible, is smaller than the substrate true strain. The results open out onto characteristic strain-strain hysteresis curves, indicative of the very first step of fatigue processes in the Ni/W thin film.