Twinning In B2 and B19' Phases of Ni-Ti Alloys

Twinning is an integral part of the martensitic microstructure of Ni-Ti shape memory alloys. Twinnning occurs as the substructure of martensite in Ni-Ti alloys, as the intervariant interface and as deformation twinning. Twinning is responsible for the reversibility of the microstructure and pseudoplasticity. In all, the {112} and the {114} twinning systems have been reported as the deformation twins of the B2 phase, the (001) compound twins, {011}, {111 } and {1 1-1} type-I and {011 } type-II twins as the substructure and intervariant interface, and, the {20-1} as the deformation twin of the B19’ phase. The aim of the present study is to determine the crystallographically possible twinning systems of the B2 and B19’ structure, and, to establish the ones that are energetically feasible to occur as deformation twins. Ab-initio calculations were carried out within the pseudo-potential description of the atoms to determine the energy landscapes of the twinning systems. The results show that the {114} and {20-1}twins cannot form by deformation twinning of B2 and B19’, respectively. On the other hand, the (001) compound twin has the lowest activation energy of all possible deformation twinning systems. This paper will discuss the implications of these results with respect to pseudoplastic deformation and reversibility of martensitic microstructure.