Martensitic transformation path of NiTi

We present the structural evolution mechanism during the NiTi martensitic transformation and show the origins of this behavior in electronic and phononic anomalies. By employing highly precise all-electron density-functional theory calculations, we establish a barrierless transformation path for equiatomic NiTi consisting of a basal shear composed of bilayer 〈100〉 {011} stacking faults to the B2 phase followed by another basal shear which causes a relaxation of the structure's monoclinic angle and results in the B1 9′ phase. This path is traced to evolving Fermi-surface nesting regions, which drive the structural transformation between the austenitic and martensitic phases.