Martensitic Transformations and Shape Memory Effect in Ti-Ni Sputter-Deposited Thin Films

Films deposited on quartz substrates by sputtering in an argon atmosphere using a sputtering target of a equiatomic TiNi alloy were investigated by differential scanning calorimetry and electron microscopy. The substrate temperature was kept below 423K during sputtering. Transmission electron microscope observation showed that the as-deposited films are amorphous and they crystallize by heating above 750K.The amorphous films were heat treated at various temperatures between 573 and 993K for crystallization. The composition of the films were determined by electron probe micro analysis using a calibration line prepared from bulk samples of well-established compositions. For it was found that Ti content decreases by sputtering, film composition was controlled by placing Ti pieces on the target and changing their sizes. In this way it was successful to obtain films of various composition. By the crystallization heat treatment, no precipitation occurred in the 50.0at%Ti-Ni film, whereas T i 2Ni precipitated in the 53.2at%Ti-Ni and 51.6at% Ti-Ni foils, and Ti3Ni4 precipitated in the 48.6at%Ti-Ni film. The R-phase transformation occurred reversibly on cooling and heating these films in the electron microscope. It was confirmed by constant load thermal cycling tests that the films show a very good shape memory.