Shape recovery behavior under thermo-mechanical loads in TiNi alloys

摘要

The measurements of shape memory effect of the Ti_(50.08)Ni_(49.92) alloy were conducted by observing the progressive changes in shape recovery with increasing number of thermo-mechanical cycles. Two different thermo-mechanical protocols were studied using a bending apparatus. In the first one, the temperature cycling was performed under bending deformation for cooling and subsequent heating without external load. After each heating path the shape recovery was evaluated as a result of reverse transformation. In the second protocol the temperature cycling was executed under bending deformation both for cooling and heating. Here, the shape recovery was measured under load and as the load was released right after heating. The results for the second protocol showed that the degree of shape recovery decreases continuously with thermo-mechanical cycling while in the first protocol it is observed to be high almost stable. It was found that the changes in shape recovery behaviour depend on the nature of the transformation cycle, in particular, on whether a stress is applied during transformation. The reverse transformation in the second protocol occurred under load; therefore, such a loading process may inhibit to some extent the shape recovery by an uncompleted transformation (retained martensite). The results also suggested that the microstructural changes in the actuated material are more pronounced for the thermo-mechanical cycling where the load was maintained during both cooling and heating in comparison to the cycling with heating under no load.