Appearance of Two-Way Strain in Shape Memory Effect of Ti–Ni–Nb Alloy —Influence of Applied Strain on Two-Way Strain-

A promising field for applications of shape memory alloys (SMAs) in the near future is the micro-actuator technology. Especially, two-way shape memory effect (TWSME) is the most suitable to apply in actuators, because a pre-determined response can be obtained very easily by thermal changes against shape memory elements. In this paper, the TWSME in Ti–Ni–Nb alloy was investigated quantitatively by applying various levels of pre-deformation. The deformation in a complete martensite phase was applied by a thermo-mechanical treatment in order to obtain the two-way memory strain. The experimental results indicated that the deformation mechanism in a martensite phase was just the martensite reorientation accompanied by the dislocation slip. The dislocation due to the slip deformation is the origin of the internal stress field that is necessary to generate the two-way memory strain. However, excessive introduction of the dislocation decreases the two-way memory strain. The maximum two-way memory strain observed in this experiment was 2.1% at an applied strain of 18%. In addition, pre-deformation increases the temperature of reverse transformation, but decreases the temperature of martensitic transformation. These experimental results can be explained by using the series-parallel combined model that has been suggested in our previous work.