Behavior and effect of Ti2Ni phase during processing of NiTi shape memory alloy wire from cast ingot

Binary NiTi alloy is one of the commercially successful shape memory alloys (SMAs). Generally, the NiTi alloy composition used for thermal actuator application is slightly Ti-rich. In the present study, vacuum arc melted alloy of 50.2Ti-Ni (at.%) composition was prepared and characterized using optical, scanning and transmission electron microcopy. Formation of second phase particles (SPPs) in the cast alloy and their influence on development of microstructure during processing of the alloy into wire form has been investigated. Results showed that the present alloy contained Ti2Ni type SPPs in the matrix. In the cast alloy, the Ti2Ni particles form in varying sizes (1-10 μm) and shapes. During subsequent thermo-mechanical processing, these SPPs get sheared/fragmented into smaller particles with low aspect ratio. The presence of SPPs plays a significant role in refinement of the microstructure during processing of the alloy. During deformation of the alloy, the matrix phase around the SPPs experiences conditions similar to that observed in severe plastic deformation of metallic materials, leading to localized amorphisation of the matrix phase.