Effect of severe ausforming via equal channel angular extrusion on the shape memory response of a NiTi alloy

Abstract

The effects of severe plastic deformation via equal channel angular extrusion (ECAE) and cold drawing followed by low temperature annealing on the shape memory behavior of the Ti50.27Ni49.73 alloy were comparatively investigated. The ECAE billets were processed at 300°C using a 90° angle ECAE tool while 30% cold drawing was performed at room temperature followed by a low temperature annealing treatment. Transformation temperatures, and transformation and irrecoverable strain levels were revealed using thermal cyclic experiments under various constant tensile stress levels. Considerable improvement in the thermal cyclic stability of transformation temperatures and transformation strains and reduction in irrecoverable strains under high stress levels were achieved in the ECAE processed material. More interestingly, the thermal hysteresis was notably lower in the ECAE case. This was attributed to the formation of favorable dislocation substructure and microstructural refinement due to the deformation twinning in austenite. The results showed that the severe ausforming via ECAE has certain advantageous over cold deformation followed by annealing process in enhancing the shape memory properties of NiTi alloys.