Influence of applied stress on shape memory characteristics of Ni50Ti45Cu5 (at.%) alloy subjected to thermomechanical cycling

Shape memory alloys have made rapid progress in many domains, primarily biomedical (endovascular stents, orthodontic archwires), and engineering (smart actuators, robotics, hydraulic couplings). The selection of a shape memory alloy for the indented application is based on its characteristic phase transformation temperatures. These characteristic temperatures are influenced by myriad parameters, such as composition, microstructure of the alloy, defect density, etc. When an shape memory alloy under an external load is subjected to cyclic operations to perform useful work, for example, actuators, these characteristic temperatures are modified. This study, therefore, aims to understand the influence of external loading on the shape memory characteristics of a Ni50Ti45Cu5 (at.%) alloy. A wire of 1.43 mm diameter and length of 100 mm was subjected to heating and cooling between its phase transformation temperatures in a cyclic manner under constant stress (of up to 60 MPa). The maximum recovery strain, actuation/retraction rate, and the stress influence coefficient were determined and compared with those of the other Ni-Ti and Cu-based shape memory alloys. The results show that raising the load level causes an increase in the transition temperatures, especially the Ms (martensite start temperature) rather than the other phase transformation temperatures (martensite finish (Mf), austenite start (As), austenite finish (Af)). It also significantly affects the recovery strain and the rate of retraction during forward transformation and the symmetry of operation.