Determination of the equilibrium transformation temperature ( T0) and analysis of the non-chemical energy terms in a CuAlNi single crystalline shape memory alloy

Abstract

A method for the determination of the equilibrium transformation temperature ( T 0) in CuAlNi single crystalline alloys, by traditional uniform heating and cooling of the specimen under constant uniaxial applied stress, σ, is presented and the T 0( σ) functions are constructed. Above a certain stress level the phase transformations, even in a multiple interface mode, can be driven in such a way that the thermal hysteresis loops have a rectangular part, from which T 0 can be determined via the well-known relation: T 0 = ( M s + A f)/2, where M s and A f are the martensite start and austenite finish temperatures, respectively. At low stress values the heating up branch of the hysteresis is different; it starts with a vertical part showing that the beginning of the austenite formation is free of the release of the elastic energy (this takes place only in the second part of this branch). Here the T 0 temperature can be determined as the arithmetic mean of M s and the austenite start temperature, A s. Using the experimentally determined stress dependence of the transformation strain, the T 0 vs. σ function was also constructed from the Clausius–Clapeyron relation and this curve fitted very well to the points obtained from the above relationships at high and low stress levels, respectively. The stress dependence of the non-chemical energy contributions to the phase transformation are also determined. It is shown that integrals of the differential values (the derivatives of the energy contributions by the transformed fraction) give self-consistent results with the (integral) quantities directly measured in differential scanning calorimetry (DSC) experiments or obtained from the area of the hysteresis loops.