Microstructural, mechanical and strain hardening behaviour of NiTi alloy subjected to constrained groove pressing and ageing treatment

Equi-atomic NiTi alloy is a versatile Shape Memory Alloy (SMA) with diverse applications emerging in the aerospace and biomedical sectors. Mechanical properties of NiTi can be transformed by severe plastic deformation (SPD) and heat treatments for a wide range of applications. In the present work, NiTi is subjected to severe plastic deformation by constrained groove pressing (CGP) followed by short term ageing treatments at 350 degrees C, 450 degrees C and 550 degrees C. Microstructural evolution confirms the grain refinement by CGP whereas post ageing at different temperatures leads to nucleation, growth, and decay of Ni4Ti3 and Ni3Ti precipitates affecting the mechanical properties and strain hardening behaviour. CGP leads to an improvement of yield strength (YS) and ultimate tensile strength (UTS) by 200 % and 61 % respectively whereas the post-CGP ageing marks the yield strength and ultimate tensile strength improvement in the range of 112 % to 73 % and 25 % to 12 % respectively. Microhardness shows a similar trend of improvements in the CGP processed NiTi (92 %) and post-CGP aged NiTi (67 % to 22 %). The role of shearable and non-shearable precipitates in the strain hardening behaviour of NiTi is also captured by the modified Kock-Mecking hardening model. In addition to that, their role in changing the mode of fracture from ductile to ductile-brittle in NiTi and CGP processed NiTi is presented using fractography analysis. Finally, CGP and post-ageing treatment at 350 degrees C has been found to be very effective in improving the overall microstructural and mechanical properties. Thus, the enhanced and appropriately modulated mechanical properties of NiTi alloy after CGP and ageing treatments may enable to widen the domain of existing applications in aerospace and biomedical sector.