Transformation And Deformation Characterization Of NiTiHf And NiTiAu High Temperature Shape Memory Alloys

NiTiHf and NiTiAu are potentially viable candidates amongst an emerging class of high temperature shape memory alloys (HTSMAs), exhibiting properties conducive to actuator applications in demanding automotive and aerospace environments. NiTiHf can be tailored to achieve a highly favorable balance of properties, including high strength, stability and work output at temperatures up to 300°C, yet at a reduced cost compared to Pt, Pd, and Au containing counterparts. NiTiAu shows potential for work output at much higher temperatures – where the benefits may offset cost – but additional research is needed. This research focuses on developing a fundamental understanding of the inherent microstructure-property relationships for these NiTi-based HTSMAs. The attractive characteristics seen in many of these systems are strongly influenced by the formation of nanoscale precipitates. Advanced electron characterization techniques are used to explore the interaction of these precipitates with martensite at low temperature, and with dislocations at higher temperature. These insights are incorporated into microstructural modeling frameworks to understand how phase transformations and time-dependent plasticity interact under isothermal deformation and load-biased thermal cycling conditions.