Abstract
The superelasticity of Ti-48.3, 50.0 and 51.5 at% Ni thin films was evaluated by measuring the stress-strain curves on loading and unloading. It was found that perfect superelasticity is obtainable above the reverse R phase transformation finish temperature for both the Ti-48.3 at% Ni and Ti-51.5 at% Ni thin films, while the Ti-50.0 at% Ni thin film does not show perfect superelasticity. The Ti-50.0 at% Ni thin film was found to be susceptible to plastic deformation because of a single phase. This plastic strain hinders the perfect shape recovery after unloading. In contrast, in the Ti-48.3 at% Ni thin film, crystallization from the amorphous state produces fine distribution of coherent Guinier-Preston zones. These precipitates are considered to impede plastic deformation during stress-induced martensitic transformation, resulting in a perfect shape recovery after unloading, that is, a perfect superelasticity. On the other hand, in the Ti-51.5 at% Ni thin film subjected to solution and aging treatments, fine distribution of Ti3Ni4 was observed and this fine precipitate is considered to be responsible for perfect superelasticity by suppression of plastic deformation.
