Structure and Phase Transformations in Thermoelastic Ni(1−x)TiCux) Thin Films Prepared by D.C. Magnetron Sputtering.

Abstract

AbstractThe intermetallic compound NiTi and its copper containing variants, Ni(1−x)TiCu(x), are capableof stress induced displacive tansformations which give rise to superelasticity and shape-memory effects. Thin films of these alloys, which may find use as sensors, microactuators, or as protective surface microalloys, are of increasing interest. In this study, thin films in the Ni(1−x)TiCUx) system, ranging in thickness from 250 nm to 5 μm, were prepared with a triode-type D.C. magnetron sputtering apparatus using a ternary alloy target. Both homogeneous films and periodic multilayer structures (with interspersed Ti-rich layers) were fabricated. of particular interest were shifts in composition with respect to the target, and the structural and phase transformation characteristics of the sputtered films. These phenomena were examined using transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), and energy dispersive X-ray microanalysis. Transformation temperatures and enthalpies were determined by differential scanning calorimetry and confirmed by 4-point D.C. resistivity measurements. The as-sputtered films were amorphousbut crystallized during annealing at 923K. Electron diffraction and XPS data suggested the presence of oxygen. Single layer film compositions were shifted to slightly lower Ti fractions which depressed the onset of the transformation sequence by ˜35K and degraded phase stability during annealing. It was possible, however, to produce periodic multilayer films which showed evidence of thermoelastic phase transformations during DSC, resistivity, and in-situ TEM experiments. These films were stable with respect to vacuum annealing at 923 K for 6 h.