Room temperature aging behavior of Ti-Nb-Mo-based superelastic alloys

The effect of room temperature (RT) aging on the superelasticity of Ti-Nb-Mo-based superelastic alloys is investigated. The results show that annealing at relatively low temperatures such as 973 K after severe cold rolling results in poor resistance to the effect of RT aging. The transformation stress increases considerably due to the formation of an isothermal ω phase at RT. Addition of Sn is partially effective in suppressing the RT aging effect in the specimens annealed at 973 K. The RT aging effect is suppressed by increasing the annealing temperature, due to the annihilation of lattice defects or non-equilibrium vacancies introduced during cold rolling, which are responsible for accelerating the diffusion process, however, superelasticity is reduced by annealing at higher temperatures, due to a decrease in the critical stress for slip deformation (σ_CSS). The specimen annealed at 1173 K followed by aging at 773 K exhibits stable superelasticity with a high resistance to the effect of RT aging. Annealing at 1173 K causes the annihilation of lattice defects or non-equilibrium vacancies, while aging at 773 K induces precipitation of the α phase, which in turn causes an increase in σ_CSS, and further enhances the resistance to the RT aging effect by enriching the matrix with β-stabilizing elements.