Thin-film shape-memory materials for high-temperature applications

Many applications of shape-memory alloys (SMAs) are likely to require development of alloys having much higher martensite transformation temperatures than are currently available. This article reviews recent reports on a few promising systems, with emphasis on sputtered films in the NiTiX group. NiTiX is a name for the ternary ordered substitutional solutions of Hf, Zr, Au, Pd, or Pt, in the β-NiTi lattice. These have been the most extensively studied compounds and the only high-temperature SMAs yet fabricated as thin films. For films, the exaggerated kinetics of physical vapor deposition and the generous surfaces of the planar form confer economic and technical advantages, especially in cases involving costly additions or troublesome solidification issues. The outlook for technically useful SMAs operating at up to 240°C is quite good. Operation at temperatures above 500°C may be possible, but issues of thermal stability will pose significant challenges.