V-shape in Young's modulus versus strain relationship in shape memory alloys upon mechanical loading

The evolution of Young's modulus in NiTi polycrystalline wires of different initial phases in uniaxial tension was investigated. A V-shape with the bottom at round the hardening start point was found in all experiments. This is a phenomenon that has not yet been reported in the literature. Similar phenomenon was also observed in an austenite CuAlNi single crystal under uniaxial compression although appearing within a much narrower strain range. Optical observation and Wyko interferometer study of the CuAlNi sample surface revealed that the occurrence of V-shape is accompanied by the abrupt reorientation of martensite variant(s). It is concluded that same mechanism should be responsible for the similar phenomenon in polycrystals.