The effect of post-deformation aging on superelastic properties of Ni50.9Ti thin wires attaining micro and nano-substructure

Superelastic properties of Ni50.9Ti shape memory wires were studied after cold drawing and post-deformation annealing at 450 °C. Characteristic transformation temperatures were determined using differential scanning calorimetry. Microstructural investigations were performed using optical and transmission electron microscopy. Results indicate that deformations more than 0.4 of true strain yield in high stress and high strain values of upper plateau. On the other hand, deformations less than 0.4 result in work hardening and reduce plateau strain. Post-deformation heat treatment at 450 °C leads to precipitation of Ni4Ti3 particles and development of recovered microstructure in slightly cold drawn wires. Post-deformation annealing of wires with cold work value of 0.6 in true strain develop nanocrystalline microstructure and hindered the formation of Ni4Ti3 precipitates. Precipitation of Ni4Ti3 particles improves the superelastic properties of not cold drawn wires. However, in comparison with annealed and aged wires, severely deformed wires attain better superelastic properties after annealing at 450 °C without any Ni4Ti3 precipitates.

► Precipitation of Ni4Ti3 during aging at 450 °C is hindered after severe cold work.
► Nanostructure formation improves the superelastic behavior of Ni-rich NiTi.
► Deformations less than 0.4 result in work hardening and reduce plateau strain.
► Deformations more than 0.4 yield in high stress and strain values of upper plateau.