Effect of cold rolling ratio on the microstructure and recovery properties of Ti-Ni-Nb-Co shape memory alloys

The Ti_44.5Ni_44.5Nb₉Co₂ alloys with different cold rolling ratios were annealed at 650 °C to investigate the influence of cold rolling ratio on the microstructure and recovery properties. The change in cold rolling ratio remarkably affected the distribution of β-Nb phase and the density of dislocations and precipitates. The density of dislocations increased with the increase of cold rolling ratio, which facilitated preferential nucleation and growth of Ti₂Co precipitates. Meanwhile, high density of dislocations and precipitates retarded the recovery and recrystallization process. Owing to the strengthening effect of dislocations and nanoscale precipitates, the 40% cold rolled Ti_44.5Ni_44.5Nb₉Co₂ alloy annealed at 650 °C after 16% pre-strain exhibited 25% enhancement in the total recovery strain and over 50% improvement for the recovery stress compared to the typical Ti-Ni-Nb alloys.