Effect of Heat Treatment Temperature on Martensitic Transformation and Superelasticity of the TiNi Shape Memory Alloy.

The martensitic transformation and superelasticity of TiNi 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 TiNi samples prepared by rapidly-solidified technology are composed of B2 TiNi phase, and TiNi and TiNi phases; the microstructures of heat-treated TiNi samples at 773 and 823 K are composed of B2 TiNi phase, and of B2 TiNi and TiNi phases, respectively. The martensitic transformation of as-cast TiNi alloy is three-stage, A→R→M and R→M transformation during cooling, and two-stage, M→R→A transformation during heating. The transformations of the heat-treated TiNi 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 values of 723 and 773 K heat-treated samples with a large 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 TiNi alloy.