Design of TiNi alloy two-way shape memory coil extension spring

This work presents a study of the development of the two-way shape memory effect (TWSME) by themomechanical training and its degradation due to working cycles in a shape memory alloys (SMAs) spring. A TWSME extension spring that could extend upon heating and contract upon cooling was obtained by constrained annealing and thermomechanical training. A stable TWSME recovery rate can reach 60–70% after training for 200 cycles. The investigation showed that constrained annealing temperature and the thermomechanical training procedure had a great effect on the TWSME. After 1000 working cycles, the TWSME recovery rate still maintained at 45% stably. The effect of the thermomechanical training on transformation characters was also investigated by differential scanning calorimetry (DSC). The inverse martensitic transformation temperatures increased and martensitic transformation temperatures decreased with increasing thermomechanical training cycles, which was attributed to the dislocations introduced into the TiNi shape memory alloys.