Effect of Heat Treatment Temperature on Martensitic Transformation and Superelasticity of the Ti(49)Ni(51) Shape Memory Alloy

The martensitic transformation and superelasticity of Ti(49)Ni(51) shape memory alloy heat-treatment at different temperatures were investigated. The experimental results show that the microstructures of as-cast and heat-treated (723 K) Ni-rich Ti(49)Ni(51) samples prepared by rapidly-solidified technology are composed of B2 TiNi phase, and Ti(3)Ni(4) and Ti(2)Ni phases; the microstructures of heat-treated Ti(49)Ni(51) samples at 773 and 823 K are composed of B2 TiNi phase, and of B2 TiNi and Ti(2)Ni phases, respectively. The martensitic transformation of as-cast Ti(49)Ni(51) alloy is three-stage, A→R→M(1) and R→M(2) transformation during cooling, and two-stage, M→R→A transformation during heating. The transformations of the heat-treated Ti(49)Ni(51) samples at 723 and 823 K are the A↔R↔M/A↔M transformation during cooling/heating, respectively. For the heat-treated alloy at 773 K, the transformations are the A→R/M→R→A during cooling/heating, respectively. For the heat-treated alloy at 773 K, only a small thermal hysteresis is suitable for sensor devices. The stable σ(max) values of 723 and 773 K heat-treated samples with a large W(d) value exhibit high safety in application. The 773 and 823 K heat-treated samples have large stable strain–energy densities, and are a good superelastic alloy. The experimental data obtained provide a valuable reference for the industrial application of rapidly-solidified casting and heat-treated Ti(49)Ni(51) alloy.