STUDY OF LOW-TEMPERATURE THERMOMECHANICAL BEHAVIOR OF THE Ti-18Zr-15Nb SUPERELASTIC ALLOY UNDER DIFFERENT TEMPERATURE-RATE CONDITIONS

The biomedical alloy Ti-18Zr-15Nb (at.%) with shape memory was subjected to a true deformation e = 0.7 in three different modes: in the temperature range from 20°C to 600°C at deformation rate ξ = 0. 1 s-1; at temperatures 250°C and 300°C with strain rates ξ = 0.1, 1 and 10 s-1; deformation at 300°C and strain rate ξ = 0.1 s-1 after annealing at 300°C (τ = 10, 60, 300, 600 and 1200 s). It is established that with increasing temperature the conditional yield strength σ0.2 continuously decreases, at that in the temperature range of deformation temperatures 250°С-300°С the increase of maximum stress σmax is observed. In the temperature range from 200°С to 400°С fluctuations are observed on the flow curves, the amplitude of which increases with increasing temperature. The change σ0.2 and σmax, as well as the presence of oscillations on the strain diagrams are connected with the course of dynamic strain aging accompanied by the release of excess ω-phase particles at temperatures 200°С - 400°С. Increasing the strain rate at temperatures of 250°С - 300°С has a strong effect on the deformation behavior of the alloy due to a significant increase in temperature during the deformation process. Thus, increasing the strain rate up to ξ = 10 s-1 leads to a stepwise decrease in stress, starting from e ≈ 0.3, after which the plastic flow curve acquires a wave-like form with a low frequency of stress fluctuations. The main phase after all modes of thermomechanical tests is the BCC β-phase. At annealing of 300°С and an exposure of more than 300 s weak ω-phase lines are observed, and at deformation after aging significantly broadened ω-phase lines are observed only after prolonged exposure (1200 s).