Ms. Michal Keret Klainer, M.Sc. Candidate
Shape memory alloys (SMAs) exhibit unique properties, including the ability to restore their original shape by temperature variations. These alloys, therefore, have been used in a wide range of applications such as sensors and actuators. One of the grand challenges is to develop SMA with Martensite-to-Austenite (M→A) transformation temperatures greater than 100ºC with high thermal conductivity and controllable electrical conductivity. These requirements cannot be addressed by the classical binary NiTi compounds. This study focuses on the effects of heat treatments on the microstructure, thermal and electrical conductivities of the Ni-Ti-20at.%Hf SMA features a good combination between transformation temperatures and physical properties. We perform a series of heat treatments at 550, 700, and 800ºC for 3 and 50h, and investigate the resulting microstructure, transformations temperatures, thermal and electrical conductivities. We found that heat treatment at 550ºC or 700ºC for 3 or 50h affects the microstructure significantly, leading to the formation of nanometer-size Hf-rich precipitates. As a result, the M→A phase transformation temperature is shifted from 100ºC up to 205ºC and electrical and thermal conductivities changed accordingly. Interestingly, thermal diffusivity data exhibit hysteresis, which corresponds to the M→A and A→M transitions. The correlation between transformation temperature, microstructure, and thermal and electrical conductivities provides us with useful knowledge required for high-temperature SMA design.