Transformation behaviors and superelasticity of Ti50Ni48Fe2 shape memory alloy subjected to cold-rolling and subsequent annealing

The effects of annealing on the phase transformation behavior and superelasticity of cold-rolled Ti50Ni48Fe2 shape memory alloy were extensively investigated. Curves of temperature dependence of electrical resistivity reveal that both the cold-rolled and annealed specimens exhibit a B2 -> R -> B19' two-stage martensitic transformation upon cooling and a B19' -> B2 one-stage transformation upon heating, although the austenitic transformation temperature decreases with the increase of the annealing temperature. Tensile stress-strain curves show the critical stress for stress-induced martensite (sigma (SIM)) of Ti50Ni48Fe2 alloys decreases with the increase of annealing temperature due to the decrement of dislocation density caused by the recrystallization. As a result, the sigma (SIM) decreases. Upon a cold-rolling and annealing at 623 K for 30 min, the Ti50Ni48Fe2 alloy exhibits excellent superelasticity with the maximum recoverable strain of 5.8 % at a loading strain of 7 %. In such a case, a complete superelasticity of 5 % can be obtained in the Ti50Ni48Fe2 alloy after deformation increasing to 15 cycles.