Influence of Prior Thermomechanical Treatment on the Training Behaviour of a NiTi Alloy

It is known that two-way memory effect (TWME) can be obtained in shape memory alloys by various types of thermomechanical cycling (training). Although much uncertainty still exists on the physical origin of the TWME, it is well recognised that dislocation arrays developed during training can have an important influence on the TWME. The investigation presented in this paper was designed to provide more information on the influence of initial dislocation structure on the TWME training behaviour of a near equiatomic NiTi alloy. Differences in initial dislocation structure were created by cycling specimens through austenite t+ martensite uansformations in different modes: thermal cycling between 200 K and 370 K, symmemc mechanical cycling to + 5% shear strain, and combined thermal-mechanical cycling. Subsequent TWME training was performed in tension at a temperature just above Ms by deforming to 5% strain followed by heating. It is found that, whereas the deformation behaviour during training was different for specimens of different prior treatments, the final TWME after training was always the same.