Effect of heat treatment on the two-way recovery stress of tube-shaped NiTi

The effect of annealing temperatures on the thermomechanical behavior of NiTi in terms of transformation temperatures, mechanical properties, two-way shape memory strain, and two-way recovery stress was investigated experimentally to develop a NiTi tubular actuator with high two-way recovery force. Five tubular specimens were used to obtain the two-way shape memory strain and two-way recovery stress by varying heat treatment conditions. Annealing heat-treated temperatures were 400°C, 500°C, 600°C, 700°C, and 800°C. A two-way shape memory effect was introduced in the shape memory alloy tube by thermomechanical treatment after annealing heat treatment. According to the results, the two-way strain induced by shape memory cycling was independent of the annealing temperature. However, the two-way recovery stress generated by the two-way strain depended on the annealing temperature, even though the values of the two-way strain were similar in all specimens. Considering all heat treatment results, we concluded that the optimum annealing temperature to acquire two-way recovery stress and two-way strain for an actuator is 600°C.