Abstract
We have performed first-principles calculations for the crystal structures of binary shape memory alloys NiTi, PdTi, and PtTi along with the transformation from austenite to martensite. A detailed analysis of the transition pathway shows that both the cubic B2 and the orthorhombic B19 evolve to a twinned structure followed by a transformation to the monoclinic B19'. In contrast to the low-energy body-centered orthorhombic structure for NiTi, the existence of this twinned state is essential for storing the shape memory in smart alloys.