Energetics of twinning in martensitic NiTi

This work elucidates the details of twinning in martensite as the major deformation mode in NiTi undergoing thermoelastic phase transformation. We report on generalized planar fault energy and generalized stacking fault energy barriers in NiTi shape memory alloys in the monoclinic martensite state (B19′). The energy barrier for (0 0 1) twin migration is 7.6 mJ m ^-2, which is smaller than that for twinning in soft face-centered cubic metals. The conjugate (1 0 0) twinning corresponds to a higher energy barrier of 41 mJ m^-2. Both modes of twinning have been observed experimentally, the (0 0 1) at lower stresses, and the (1 0 0) mode at higher stresses. The unstable stacking fault energy for slip is also calculated, and the results show that twin migration is favored over slip in the B19′ state. We suggest that the results provide a quantitative methodology for the development of new shape memory alloys in which twinning can occur at stress levels far below that corresponding to plastic deformation.