Stress-induced martensitic phase transformations in polycrystalline CuZnAl shape memory alloys under different stress states

The effect of different uniaxial and triaxial stress states on the stress-induced martensitic transformation in CuZnAl was investigated. Under uniaxial loading, it was found that the compressive stress level required to macroscopically trigger the transformation was 34 pct larger than the required tensile stress. The triaxial tests produced effective stress-strain curves with critical transformation stress levels in between the tensile and compressive results. It was found that pure hydrostatic pressure was unable to experimentally trigger a stress-induced martensitic transformation due to the large pressures required. Traditional continuum-based transformation theories, with transformation criteria and Clausius-Clapeyron equations modified to depend on the volume change during transformation, could not properly predict stress-state effects in CuZnAl. Considering a combination of hydrostatic (volume change) effects and crystallographic effects (number of transforming variants), a micromechanical model is used to estimate the dependence of the critical macroscopic transformation stress on the stress state.