The details of twinning in martensite, as the major deformation mode of NiTi undergoing thermo-elastic phase transformation were studied. Generalized planar fault energy and generalized stacking-fault energy barriers in NiTi shape-memory alloy in the monoclinic martensite state (B19′) were reported. The energy barrier to (001) twin migration was 7.6mJ/m2, which was smaller than that for twinning in soft face-centered cubic metals. The conjugate (100) twinning corresponded to a higher barrier, of 41mJ/m2. Both modes of twinning were observed experimentally, (001) at lower stresses and (100) at higher stresses. The unstable stacking-fault energy for slip was calculated, and the results showed that twin migration was favored over slip in the B19′ state. It was suggested that the results provided a quantitative method for the development of new shape-memory alloys in which twinning could occur at stress levels far below that corresponding to plastic deformation.

Energetics of Twinning in Martensitic NiTi. T.Ezaz, H.Sehitoglu, H.J.Maier: Acta Materialia, 2011, 59[15], 5893-904