Prediction of the detwinning anisotropy in textured NiTi shape memory alloy

Deformation of shape memory alloys via detwinning of thermally formed martensite twins is a unique microstructural process, which leads to a macroscopic stress plateau in the stress-strain curves. Since this type of deformation mechanism involves insignificantly the dislocation processes. the original shape of the shape memory alloys can be partially or even fully recovered upon subsequent heating to a critical temperature. The response of twinned domains under stresses plays a critical role in the anisotropy of both the mechanical (stress-strain curve) and the thermomechanical (stress-strain-temperature curve) behaviour of textured shape memory alloys. Recent results obtained by Liu et al. (1999. Acta mater., 47, 645) have shown that the relation between the shear direction of texturally distributed martensite twins and the loading direction plays a critical role in the anisotropy of both microscopic and macroscopic deformation processes. Based on this observation, the present research analyses the orientation dependence of the detwinning process from a crystallographic approach and using mechanics of heterogeneous materials. The obtained results are found to agree with the the experimental observations. Specifically, for a NiTi sheet with given textures, the predicted response of two types of martensite twin, namely (011) type II and (001) compound twins, as a function of loading direction agrees well with the major features of experimental observations.