Transformation start stress in non-textured shape memory alloys

Yield (transformation start stress) surfaces of shape memory alloys (SMAs) are investigated using a modified micro-mechanical model. It is based on the assumption that the driving energy for transformation is approximately a constant. By using the lattice structure and correspondence of materials in transformation, the yield surfaces of four SMAs, namely NiTi, NiAl, CuAlNi and CuZnGa, are investigated. Their yield surfaces in the π plane are presented by a formula for maximum scheme and polynomial functions for average scheme, respectively. We also investigate the effect of different Young's moduli between austenite and martensite. We study the yield stress versus temperature relationship. It appears that the V-shape of the yield surface against temperature relationship occurs only in a cooling process, which agrees with observations reported in the literature.