Crystallization and martensitic transformation of sputter-deposited Ti-Ni thin films

Ti-Ni alloy films were deposited on quartz substrates by sputtering in argon atmosphere, using a sputtering target of an equiatomic TiNi alloy. The composition of the films was determined by electron probe micro-analysis using a calibration line prepared from bulk samples of well-established compositions. Since it was found that the Ti content in a deposited film was less than that of the target material, the film composition was controlled by placing Ti pieces on the target and changing their sizes. In this way films with various compositions have been obtained. TEM microscope observations showed that the as-deposited films are amorphous when the substrate temperature is kept below 423 K during sputtering, and that they are crystallized when the specimens are heated above 708 K. By heat treatment at temperatures above 708 K, Ti2Ni precipitated in Ti-46.8 at. percent Ni and Ti-48.4 at. percent Ni films, and Ti3Ni4 precipitated in the Ti-51.4 at. percent Ni film, but no precipitation occurred in the Ti-50.0 at. percent Ni film. The martensitic transformation in these films was investigated by differential scanning calorimetry and electron microscopy. The R-phase transformation occurred reversibly on cooling and heating the films in the electron microscope. It was confirmed by constant load thermal cycling tests that the films show very good shape memory. (Author)