Tensile shape memory behavior of Ni50.3Ti29.7Hf20 high temperature shape memory alloys

The effects of heat treatment on the shape memory characteristics of a polycrystalline Ni50.3Ti29.7Hf20 alloy were studied via thermal cycling under stress and isothermal stress cycling experiments in tension. It was revealed that transformation temperatures could be increased above 100 °C with aging at temperature above 500 °C and in particular were stabilized against stress-free thermal cycling after aging at 500 °C. Recoverable strain of ~ 5% was observed for the as-extruded samples and decreased to ~ 4% after aging due to the formation of non-transformable precipitates. The aged alloys demonstrated near perfect shape memory effect under tensile stresses as high as 700 MPa and perfect superelasticity at temperatures up to 230 °C. Finally, the tension-compression asymmetry observed in NiTiHf alloys was discussed.