Crystal orientation effects on the onset of plasticity in single-crystal NiTi

The plastic deformation of several orientations of a monocrystalline NiTi shape memory alloy was investigated via nanoindentation. The onset of plasticity, indicated by a pop-in, was used to determine the activation volume for each orientation. Unlike other reports, we found the activation volume to be dependent on orientation; in particular, the (1 0 0) orientation had an activation volume double that of the (1 1 0) and (1 1 1) orientations. This is attributable to different recoverable transformation strain behaviors. These observations illuminate the deformation mechanisms that may exist in materials that undergo phase transformations.