Tailoring thermal and electrical conductivities of a Ni-Ti-Hf-based shape memory alloy by microstructure design

Shape memory alloys (SMAs) exhibit unique properties, including the ability to restore their original shape by temperature variations. One of the grand challenges of the aerospace industry is to develop SMAs with transformation temperatures above 100 ºC with high thermal conductivity. This study focuses on the effects of heat treatments on the microstructure, thermal and electrical conductivities of the Ni_50.9Ti_29.6Hf_19.5 SMA that features good combination between transformation temperatures and physical properties desirable for aerospace applications, such as actuators. We found that heat treatments at 550 °C or 700 °C for 3 or 50 h affect the microstructure significantly, leading to formation of nanometer-size Hf-rich precipitates. As a result, the Martensite-to-Austenite phase transformation temperature is shifted from 100 °C up to 205 °C. Moreover, these heat treatments affect the electrical and thermal conductivities. The maximum room-temperature values of both thermal and electrical conductivities were recorded after heat treatment at 700 ºC for 3 h and are as high as \(\kappa =13.1\pm 0.4\mathrm{ W }{\mathrm{m}}^{-1} {\mathrm{K}}^{-1}\) and \(\sigma =(9\pm 0.27)\times {10}^{3}\mathrm{ S }{\mathrm{cm}}^{-1}\), respectively. The correlation between transformation temperature, microstructure, and thermal and electrical conductivities provides us with useful knowledge required for high temperature SMA design.