Martensitic transformation criteria in Cu–Al–Be shape memory alloy—In situ analysis

Polycrystalline Cu–11.5 wt.% Al–0.5 wt.% Be shape memory alloy specimens were subjected to both X-ray diffraction stress analysis and optical microscopy during stress-induced martensitic transformation at room temperature. During in situ uniaxial tensile test, the stress state of several grains with different orientations was correlated with martensitic variant development. Variant selection was verified with Schmid factor calculation, and a critical resolved shear stress of 70 MPa for martensitic transformation was identified. As the transformation developed, it was observed that the austenitic stress state in the grains depends mainly of martensitic volume fraction, number and arrangement of variants. The stress in the tensile direction decreases when the martensitic fraction increases.