Effect of Material State on the Phase Transformation of NiTi Shape Memory Alloy

The effects of cold-working‚ solution treatment and aging on the phase transformation of near-equi-atomic NiTi shape memory alloy have been investigated systematically by means of DSC. The results show that the microstructure of the cold-worked NiTi wire is fibrous structure and the microstructure of solution treated NiTi alloy is equi axial grains. The defects induced during cold-working restrained the phase transformation of thermoelastic martensite. When aging treatment was taken immediately after cold-working‚, the transformation process of NiTi alloy was P→M. When aging treatment was taken after solution‚ the transformation process of NiTi alloy was P→R→M. Different types of thermoelastic martensite transformation of the NiTi alloy took place under different treatment conditions. It is recognized that the stress‚ dislocation density and precipitates have big influence on the phase transformation behavior of thermoelastic martensite of NiTi alloy.