A systematic analysis of transformation stress anisotropy in shape memory alloys

Abstract

Prior experimental observations reveal that the transformation stress and strain of single-crystal shape memory alloys (SMAs) are strongly anisotropic and moderately asymmetric. Models capable of evaluating the anisotropy of transformation stress are now available for unconstrained and constrained transformation of single crystals. In the present investigation, anisotropy and asymmetry indices are systematically evaluated for a set of seven SMAs, for both the unconstrained and the constrained transformation. Within the group Cu-Al-Ni, Cu-Zn-Al, Cu-Zn-Ga, Cu-Zn and NiAl, the anisotropy decreases with increasing misorientation of the habit plane and shear direction from the ideal {011} {011} shear system orientation. On the basis of this observation, it is hypothesized that low anisotropy is an essential feature of technologically useful SMAs. The anisotropy indices for several synthetic SMAs are examined to evaluate the changes needed to decrease the anisotropy of SMAs such as Cu-Zn and NiAl.