Revisiting ultra-high temperature shape memory behavior in NiHf-X and RuNb-X alloys

High temperature shape memory alloys based on Ni-Ti-Hf formulations have been shown to exhibit transformation temperatures as high as 330 °C, with Hf addition between 20 and 30 at.%. There is a need for even higher activation temperatures to satisfy warmer systems in aeronautics (e.g., engine related applications, supersonic inlets, hypersonic skin morphing), space applications (e.g., solar orbiters, Venus probes), and potential uses in other fields. Limited solutions for achieving this capability include alloys such as TiPt or multi-component alloys based on NiPt-TiHfZr alloys, both exhibiting low to no work output. In this work, higher ternary Hf additions between 30-50Hf were explored to reveal microstructural features and shape memory properties.

It was found that transformation temperatures increased linearly with Hf addition, reaching a maximum austenite finish temperature of 1190 °C at 50 at.% Hf. The low temperature stable microstructures were composed of a majority B33 orthorhombic phase, with traces of B19′ monoclinic structure below the martensite finish temperature. These microstructures convert to a B2 cubic structure at higher temperature. Macroscopically, specimens endured stresses as high as 1 GPa in compression, with strain recovery which decreased from nearly 100% recovery in the 30Hf alloy, to nearly 0% at 50Hf alloy. The latter alloy revealed limited work output at high temperatures due to creep-dominant mechanisms simultaneously occurring during the phase transformation process. Another alloy system comprised of RuNb formulations was explored to address the actuation stability and work outputs at elevated temperatures. Initial results on both alloy systems are discussed.