Processing and characterization of NiTi-based shape memory alloy thin films

Abstract

In the present work, the properties of two NiTi-based shape memory alloy thin films, namely Ti50Ni25Cu25 ribbon and Ni-Ti-Hf thin films, and the influencing factors have been extensively studied. Major attention has been paid to rapid thermal annealing of the initially amorphous materials and their resulted properties, including crystallization behavior, microstructure evolution, transformation characteristics and constraint shape memory effect. As a result, the processing-microstructure-property relationship has been established, which provides guidelines on optimization of the properties. The crystallization behavior of NiTi-based thin films is characterized by a single-stage transformation. The addition of Cu reduces the crystallization temperature and activation energy of the initially amorphous alloy; whereas, the addition of Hf has a contrary effect. With the help of rapid thermal annealing, the initially amorphous Ti50Ni25Cu25 ribbon can be fully crystallized by annealing at 400 ºC for 30 s, which is significantly lower than the crystallization temperature under conventional thermal annealing. This is attributed to the assistance of the extra energy available from the higher internal stress field associated with free volume in amorphous state.