Effects of cooling rate on martensitic transformation behavior and shape memory property for Ni-rich NiTiHf-Nb high-temperature shape memory alloy

The NiTiHf-Nb alloy exhibits good workability as well as high-temperature shape memory property. In this work, effects of cooling rate on martensitic transformation behavior and shape memory property for Ni-rich (Ni50.1Ti34.9Hf15)85Nb15 alloy were investigated. For comparison, the martensitic transformation behavior and shape memory property of Ni50.1Ti34.9Hf15 alloy were also investigated. As the cooling rate decreased from water cooling to furnace cooling after solution treatment, the martensitic transformation temperatures (TTs) of (Ni50.1Ti34.9Hf15)85Nb15 alloy decreased gradually, whereas the decrease was not obvious for Ni50.1Ti34.9Hf15 alloy. The increased valence electron concentration (cv) should account for the decrease in TTs of (Ni50.1Ti34.9Hf15)85Nb15 alloy. The shape memory effect strain increased with the decrease in the cooling rate for both two alloys, and the largest shape memory effect strain of 2.6% can be obtained in furnace-cooled (Ni50.1Ti34.9Hf15)85Nb15 alloy. This enhancement in the shape memory property should be ascribed to the precipitation of H-phase in furnace-cooled (Ni50.1Ti34.9Hf15)85Nb15 alloy.