On the Processability and Scale-Up of NiTi-20Hf High Temperature Shape Memory Alloys

NiTi-20Hf (at.%) high temperature shape memory alloys (HTSMAs) have shown initial promise for high-force actuator applications at temperatures above 100 °C. These alloys have been developed as a low cost alternative compared to NiTi-X (X = Pt, Pd, Au), and were found to exhibit superior properties and dimensional stability through compositional and aging control. This was accomplished by developing a slightly Ni-rich composition with 20 at.% Hf, capable of being precipitation strengthened when subjected to an aging heat treatment to produce fine, nanometer size precipitates. Nonetheless, processing and scalability of these materials are challenging due to their extreme sensitivity to composition and the difficulty reproducing properties heat-to-heat.

In this work, several large heats (~ 60lbs) of Ni_50.3Ti_29.7Hf₂₀ (target composition) were produced using a vacuum induction skull melting (VISM) process. The goal was to determine the variability in properties between and within each heat and define appropriate processing specifications for future heats. This was done by correlating the mechanical and thermal properties, composition, and microstructure of each heat in the as-extruded and aged conditions. Lessons learned and processing effects on functional properties will be discussed.