Two-way shape memory effect and alternating current driving characteristics of a TiNi alloy spring

Two-way shape memory effect (TWSME) was induced into the TiNi shape memory alloys (SMAs) spring by thermomechanical training after annealing treatment, which has promising application in micro-actuating fields. The TWSME spring can contract upon heating and extend upon cooling. The results show that there is an increase of the recovery ratio up to a maximum TWSME of 45%. During the training procedure, transformation temperatures and hysteresis were measured by different scanning calorimetry (DSC). The results show that A_s (reverse transformation start temperature) and A_f (reverse transformation finish temperature) shift to lower temperature after training. The intervals of A_f-A_s and M_s-M_f (M_s and M_f are the martensite start and finish temperatures, respectively) increase and the heat of transformation decreases after training. The electrothermal driving characteristics of the TWSME springs were also investigated with alternating current density of 3.2-14.7 A/mm~2. It is found that the time response and the maximum contraction ratio greatly depend on the magnitude of the electrical current density.