Microscopic Theory of the Shape Memory Effect in TiNi

Abstract

An ab initio study of the Bain strain and orthorhombic distortions in the TiM compounds ( $M\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\mathrm{Ni}$, Cu, Rh, Pd, and Ag) shows that the $B2$ structure of the shape memory alloy TiNi, unlike that of the other TiM compounds, is stable against these distortions, and that TiNi has some flat total energy minima. A thermodynamic model, built on these findings, displays some features of the shape memory effect. An electronic structure analysis reveals the importance of the Ni metallic bonding to the $B2$ phase stability and to the detwinning mechanism.