The concern of elasticity in stress-induced martensitic transformation in NiTi

It has been reported in the literature that moduli of elasticity of the austenite and martensite of near-equiatomic NiTi differ often by a factor of 3. It is expected that a phase transformation between the two phases may be induced by the application of a stress, according to thermodynamic principles. This is due to the fact that the difference in elastic energy caused by the change in modulus of elasticity serves as a driving force for the transformation, similar to the effect of the lattice distortion of the martensite on the transformation. A thermodynamic equation expressing this effect is derived. It is expected, based on the understanding of this equation, that the relationship between the critical stress and the temperature for a thermoelastic martensitic transformation is non-linear if the transformation involves a large change in modulus of elasticity. Therefore, this equation may be used for either of two purposes: to clarify the reliability of the experimentally determined moduli of elasticity of the two phases or to verify the Clausius-Clapeyron relation between the critical stress and the temperature for the transformation. (C) 1999 Elsevier Science S.A. All rights reserved.