Tensile shape memory behavior of Ni 50.3 Ti 29.7 Hf 20 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 700MPa and perfect superelasticity at temperatures up to 230°C. Finally, the tension-compression asymmetry observed in NiTiHf alloys was discussed.