Microscopic theory of the shape memory effect in TiNi

An ab initio study of the Bain strain and orthorhombic distortions in the TiM compounds (M = 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.