Martensitic transformations in sputter-deposited shape memory Ti-Ni films

Abstract

Composition and heat-treatment dependences of the martensitic transformations in sputter-deposited shape memory Ti-Ni films were investigated by differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). Films were deposited on quartz substrates by sputtering using a target of an equiatomic TiNi alloy. The substrate temperature was kept below 423K during sputtering. The compositions of the films were determined by electron probe micro-analysis using a calibration line prepared from bulk samples of well-established compositions. The as-deposited films were found to be amorphous and to crystallize on heating above 750K. The amorphous films were heat-treated at various temperatures between 673 and 1123K and the transformation behavior of the heat-treated films was examined by DSC and TEM. Obtained results are as follows. (1) During crystallization heat treatment, no precipitation occurs in near-equiatomic Ti-Ni films, whereas Ti2Ni particles precipitate in the Ti-rich films and Ti3Ni4 particles do in the Ni-rich films. (2) The B2→R transformation temperature becomes maximum (335K) at the equiatomic composition and decreases with increasing Ni-content with a rate of 96K/at.%Ni for 50.0-50.5at.%Ni alloys which becomes a single B2 phase when crystallized by holding at 773K. It is constant (335K) in the Ti-rich side range. It also is constant in the Ni-rich range (287K with the films crystallized by holding at 773K). (3) The B2-B19' transformation temperature becomes maximum at the equiatomic composition and decreases with increasing Ni-contcnt. It is constant in the Ti-rich range.