Influence of applied stress on the transformation behaviour and martensite evolution of a Ti–Ni–Cu shape memory alloy

The influence of applied stress on the transformation behaviour and martensite evolution of a Ti50Ni41Cu9 (at.%) shape memory alloy was investigated by in situ synchrotron diffraction. The initial microstructure contained both the B19 and B19′ martensites as well as Ti2(Ni,Cu) precipitates. Uniaxial loading at room temperature resulted in a nonlinear behaviour at stresses above 70 MPa, which was related to a 1) B19 → B19′ transformation, and 2) mechanical detwinning of the B19′, creating a strong alignment of the (020)B19′ to the tensile axis. Thermally driven transformation cycles were conducted at applied stresses of 50, 100 and 150 MPa, and in each case the transformation sequence was observed to be two step B19′ ⇄ B19 ⇄ B2. A significant elongation of the material occurred during cooling, the majority of which was associated with the B2 → B19 transformation, where the (020)B19 was found to align with the tensile axis. Following the transformation cycle, a similar preferential alignment of the B19′ was observed at each of the applied stress level. Therefore, is it suggested that the favourable martensite configuration can be achieved by either mechanical detwinning during loading above the yield stress, or via variant selection during transformation under load.