Study of Low-Temperature Thermomechanical Behavior of the Ti–18Zr–15Nb Superelastic Alloy under Different Temperature-Rate Conditions

Biomedical shape-memory Ti–18Zr–15Nb alloy (at %) was subjected to upsetting with a true strain e = 0.7 in three different regimes: within a temperature range from 20 to 600°C at a deformation rate ξ = 0.1 s^–1; at temperatures of 250 and 300°C at deformation rates ξ = 0.1, 1, and 10 s^–1; and deformation at a temperature of 300°C and a rate ξ = 0.1 s^–1 after annealing at a temperature of 300°C for different times (τ = 10, 60, 300, 600, and 1200 s). It has been established that the conditional yield stress σ_0.2 continuously decreases with increasing temperature and, at the same time, the maximum stress σ_max is observed to grow within this deformation temperature range of 250–300°C. In the region of temperatures from 200 to 400°C, fluctuations with an amplitude growing with an increase in the temperature are observed in the yield curves. The change in σ_0.2 and σ_max and the presence of fluctuations in the strain diagrams are produced by dynamic strain aging accompanied by the precipitation of excessive ω-phase particles at temperatures of 200–400°C. An increase in the deformation rate at temperatures of 250–300°C has a strong effect on the deformation behavior of this alloy due to considerable additional deformation-induced heating. Thus, an increase in the deformation rate to ξ = 10 s^–1 leads to a jump-like decrease in the stress starting from e ≈ 0.3, afterwards the plastic yield curve takes a wavy shape with a low stress fluctuation frequency. The body-centered cubic (BCC) β-phase is the major phase after all the regimes of thermomechanical tests. Some weak ω-phase lines are observed after annealing at 300°C with exposure for more than 300 s, and essentially broadened ω-phase lines appear for the aged alloy after deformation only with long-term exposure (1200 s).