Ultra-High Temperature Shape Memory Behavior in Ni-Ti-Hf Alloys

Shape memory behavior in stoichiometric Ni-Hf-Ti shape memory alloys with high Hf was evaluated. Five alloy compositions with a hafnium content from 30 to 50 at.% were arc melted, homogenized and tested to reveal microstructure and shape memory properties. Transformation temperatures increased linearly with Hf addition, reaching a maximum austenite finish temperature of 1190 °C at 50Hf, measured using differential scanning calorimetry (DSC). The low temperature stable microstructures were composed of a majority B33 orthorhombic phase, with traces of B19′ monoclinic structure below the martensite finish temperature, as revealed by X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM). These microstructures convert to a B2 cubic structure at higher temperature. Macroscopically, specimens were tested isothermally at room temperature, and endured stresses as high as 1 GPa in compression. Strain recovery decreased from nearly 100% recovery in the 30Hf alloy, to nearly 0% at 50Hf alloy, as plasticity mechanisms dominated at high temperatures in the higher Hf alloys. Uniaxial constant-force thermal cycling (UCFTC) experiments revealed limited work output at high temperatures due to creep-dominant mechanisms simultaneously occurring during the phase transformation process.