Crystallization and martensitic transformation behavior of NiTi shape memory alloy thin films

The microstructure evolution and shape memory properties of near-equiatomic Ni-Ti thin films were investigated. Ni-Ti thin films sputter-deposited at room temperature are usually amorphous in their as-deposited state. This observation provides an opportunity to control the microstructure by adjusting the crystallization conditions. The temperature dependence of the crystallite nucleation and growth rates is measured for amorphous Ni-Ti thin films sandwiched between two SiN x layers. Crystallites are shown to nucleate homogeneously in the film and to grow with an interface-controlled mechanism. The reaction between Ni-Ti and surrounding layers results in a small composition shift at these interfaces and suppresses heterogeneous nucleation at these interfaces. The crystal growth rate shows a film thickness dependence and is much slower in thinner films. We propose that hydrogen present in surrounding SiN x layers is responsible for this decrease of the crystal growth velocity. By manipulating nucleation and growth rates, unprecedented control over the microstructure of the films is possible. Martensitic transformation behavior of Ni-Ti thin films of submicron thicknesses was investig