Tuning the thermal cyclic stability of martensitic transformation in Ni50.3Ti29.7Hf20 high temperature shape memory alloy

Abstract

Thermal stimulus-responsive NiTi-based shape memory alloys, which undergo reversible, diffusion-less martensitic phase transformation, have emerged as important functional materials. Amongst them, Ni-rich NiTiHf high temperature shape memory alloys exhibit martensitic phase transformation temperatures well above 100 °C, making them suitable for high temperature applications. However, during repeated thermal cycles their transformation temperatures show instabilities which limit their practical applications. Here, we perform thermal cycling experiments at fixed as well as variable heating/cooling rates on the Ni50.3Ti29.7Hf20 alloy which is heat-treated at different temperatures. We successfully minimized the variations in phase transformation temperatures to an acceptable range by aging the alloy below 500 °C for 3 h. We further show that this minimization correlates well with high activation energy required for the phase transformation elucidating the physical reason behind this observation. Our results pave the way forward for designing Ni-rich NiTiHf shape memory alloys with a stable functional response.