Structure-property relationships in a precipitation strengthened Ni-29.7Ti-20Hf (at%) shape memory alloy

The martensitic transformation temperatures, load-biased thermomechanical properties, and microstructure (characterized by transmission electron microscopy and atom probe tomography) were investigated for a Ni-29.7Ti-20Hf (at%) alloy aged at 550 °C for 0-300 h. Aging for three hours and longer resulted in the precipitation of a face-centered orthorhombic phase, previously denoted as the H-phase. The number density, size, and composition of this phase did not change significantly upon aging from 3 to 30 h. However, continued aging to 300 h resulted in a decrease in the number density and significant coarsening of the precipitates at 550 °C. The alloy exhibited near optimum response for shape memory behavior and dimensional stability after aging for three hours, though transformation temperatures continued to increase with aging time.