Detwinning process and its anisotropy in shape memory alloys

摘要

Detwinning in crystalline solids is a unique deformation mechanism partially responsible for the shape memory effect in addition to phase transformation. Owing to an insignificant dislocation process during detwinning leading to inelastic deformation, the residual strain can be recovered through a reverse transformation. The maximum shape recovery strain is intrinsically related to the lattice geometry and twinning mode. While the magnitude of shape recovery strain is related to a competition of detwinning versus dislocation generation responsible for the macroscopically observed martensite deformation. The detwinning magnitude is directional, and in the polycrystalline materials, it is related to the textures. Without textures, the detwinning process in polycrystalline solid is isotropic. With textures, the detwinning process is enhanced for certain directions and reduced for other directions and so do the shape recovery strain. The anisotropy in detwinning process allows the possibility of maximizing the potential of the polycrystalline shape memory alloys. This paper presents recent results on the anisotropy of detwinning as a function of loading mode and texture orientation. The anisotropy in detwinning process is also responsible for the direction-dependence of the shape recovery strain. The fundamental reason responsible for this detwinning anisotropy is associated with the combination of twinning types, texture orientation and loading direction, which can be further treated mathematically based on a physical model.