Energetics of (114) twinning in B2 NiTi under coupled shear and shuffle

The (1 1 4) twin growth in the austenitic regime in NiTi (B2 lattice) is characterized precisely at the atomistic scale. This twinning mode is consistent with experimental observations but has not been well understood despite its importance. Combined shears, shuffles and interface shifts operate in a complex way to generate the (1 1 4) twin. Pure shear on the (1 1 4) plane results in a pseudo-twin, and interchange shuffles are necessary to convert this to a reflective twin and render a lower energy state. Additional atomic shifts at the twin-matrix interfaces result in sharp boundaries, further lowering the energy barriers. We established the energy barrier to be 148 mJ m(-2) during (1 1 4)[2 2 (1) over bar] twin boundary growth of four layers at a time. The entire potential energy surface and the mean energy path during twinning can be derived from our simulations, providing the required insight into the atomistic processes involved. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.