On mechanical properties of NiTi SMA wires prestrained by cold rolling

This study investigates mechanical properties and recovery stress of NiTi shape memory alloy (SMA) wires prestrained by different degrees of cold rolling. Using differential scanning calorimetry (DSC), the phase transformation temperature in cold rolled wires are compared with as received one. Monotonic and hysteretic tensile tests are conducted to determine the stress-strain curves of each cold rolled wire. Thickness recovery ratio, recovery and residual stress of constrained wires are measured during temperature variations. It is found that cold rolling can reduce detwinning process in tensile behavior and finally eliminate it with more cold work. Moreover, the optimal degree of cold rolling is obtained in terms of maximum induced recovery stress, safety of storage and minimum morphological changes and costs. Based on the experimental data, the degree of cold rolling can be categorized into three ranges of slight, moderate and extensive.