Shape Memory Effect in Quaternary NiTiHfPd Shape Memory Alloys beyond 200 °C

In this study, the effects of minor palladium (3 and 5 at.%) addition to ternary NiTiHf shape memory alloy (SMA) were investigated to design a high-temperature NiTiHfPd SMA. While most of the reported NiTiHfPd SMAs contain (Ni+Pd)-rich compositions, the (Ti+Hf)-rich compositions have not been studied to the same extent. This study investigates two new NiTiHfPd compositions and their shape memory effect at elevated temperatures. To this end, two Ni_47.3Ti_29.7Hf₂₀Pd₃ and Ni_44.3Ti_31.7Hf₂₀Pd₅ alloys were fabricated and characterized in terms of microstructure, transformation temperatures, and shape memory behavior in as-cast and aged conditions. The results demonstrated that even a minor addition of Pd could drastically alter the operating temperatures and shape memory effect of NiTiHf SMAs. The metallographic studies revealed the presence of a high volume fraction of secondary phases in the microstructure of Ni_44.3Ti_31.7Hf₂₀Pd₅ that consequently created a matrix leaner in (Ti+Hf) compared to Ni_47.3Ti_29.7Hf₂₀Pd₃. As a result, the Ni_44.3Ti_31.7Hf₂₀Pd₅ was found to be more responsive to precipitation hardening. The evaluation of the shape memory effect of the alloys displayed remarkable strength and strain recovery capability of the quaternary NiTiHfPd at a temperature range above 200 °C with thermal hysteresis as small as 25 °C.