Development of composition-insensitive SMA thin films with high recovery strength and large recovery strain

Abstract
After reviewing the microstructures formed in Ti-rich Ti-Ni based thin films and the associated shape memory properties, it is shown that thin films of composition-insensitive shape memory alloys (SMA) can be developed by utilizing the formation of coherent bct thin plate precipitates. Ti-rich sputter-deposited thin films of Ti-Ni-Cu alloys are presented as such an example, which exhibit excellent shape memory properties of 500 MPa recovery stress with about 5 % recovery strain for the Ti-composition range of 51-54 at. %. Necessary conditions for producing such thin films are (1) Ti-rich content, i.e., over 50 at. %, (2) the amorphous initial state, (3) heat treatment at low temperatures near the crystallization temperature and (4) ternary alloy system. It is suggested that the same approach will be possible for the other ternary alloys such as Ti-Ni-Pd, Ti-Ni-Hf, etc..