The effect of training on two-way shape memory effect of binary NiTi and NiTi based ternary high temperature shape memory alloys

The propensity for various high-temperature shape memory alloys (HTSMA), i.e., Ni28.5Ti50.5Pt21,Ni24.5Ti50.5Pd25 and Ni24.5Ti50Pd25Sc0.5, to exhibit two-way shape memory effect (TWSME) was compared to that of a conventional binary Ni49.9Ti50.1 shape memory alloy (SMA). Thermomechanical training in the form of thermal cycling under constant stress levels was employed to induce two-way shape memory behavior in the various materials. The resulting TWSME was characterized for its magnitude and stability under stress-free conditions, while parameters such as training stress and upper cycle temperature during training were investigated for their influence on this phenomenon. For Ni49.9Ti50.1, a negative correlation was found between an increasing training stress, from 80 MPa to 200 MPa, and the magnitude of the resulting TWSM strain, while a positive correlation was observed for Ni24.5Ti50.5Pd25 and Ni24.5Ti50Pd25SC0.5. The stability of the TWSME for the Ni49.9Ti50.1. measured by the strain evolution of the cold (martensitic) and hot (austenitic) shapes of the samples upon stress-free thermal cycling, was found to depend on the stress and temperature interval during training. Conversely, the stability of the NiTiPd based HTSMAs was much greater and less sensitive to these parameters over the stress and temperature intervals investigated. No TWSME was seen in Ni28.5Ti50.5Pt21 due to the higher upper cycle temperatures required during thermal cycling, which resulted in the recovery of any favorable dislocation structures generated during training. (C) 2012 Elsevier B.V. All rights reserved.