Microstructure and martensitic transformation of NiTiHfSc high temperature shape memory alloys

In present work, microstructure and martensitic transformation of Ni49Ti36-xHf15Scx(x = 0, 0.5, 1, 2 at.%) alloys were investigated. The results show that the Sc addition significantly influences the microstructure of the alloy. After the addition of Sc, Sc2O3 phase presents besides B19′ martensite and (Ti,Hf)2Ni phase at room temperature. When the Sc content is no less than 1 at.%, the chain-like morphology of (Ti,Hf)2Ni phase changes to spherical-like one. Upon cooling and heating, these alloys show single-step martensitic transformation and its reverse transformation. With increasing Sc content, the transformation temperatures are reduced at a rate of 21 °C per 1 at.%Sc resulting from the reduced (Ti+Hf+Sc)/Ni ratio. The addition of Sc is effective in improving the thermal cycling stability of martensitic transformation because of the solid-solution strengthening of Sc. After ten cycles, the change of transformation peak temperature of Ni49Ti35.5Hf15Sc0.5 alloy is only 4.7 °C.