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 Ni 50.3 Ti 29.7 Hf 20 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. [ABSTRACT FROM AUTHOR]