Effect of Heusler Phase Precipitation on the Transformation Behaviors of a NiTiHf-Based Shape Memory Alloy

Abstract Nanoscale precipitates have been proved to affect the transformation behavior of shape memory alloys (SMA) significantly. In this work, the effects of Heusler precipitates (Ni 2 TiAl) on the Ni 50 Ti 24.5 Hf 22.5 Al 3 SMA were systematically investigated. Density functional theory (DFT) calculations showed that coherent Heusler precipitates can mechanically stabilize the austenite phase, indicating the decrease of transformation temperatures. This trend was further confirmed experimentally. The depression of transformation temperatures with the increase of heat treatment time was observed as nanoscale coherent precipitates nucleated and grew. This was followed by an abrupt increase in transformation temperatures when aging time exceeded 200 hours at 700 °C. We attribute this increase to the loss of coherency between the Heusler precipitates and matrix which weakened their mechanical coupling. Detailed microstructure characterization by high-resolution transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) were provided to support the finding.