Evolution of the structural phase state, deformation behavior, and fracture of ultrafine-grained near-β titanium alloy after annealing

In the paper, we study the evolution of the structural phase state, deformation behavior, and fracture of ultrafine-grained near-β titanium alloy Ti–5Al–5Mo–5V–1Cr–1Fe after annealing in the temperature range 773–1073 K (0.4–0.55 T_m). The duration of the strain-hardening stage under tensile conditions is shown to be almost independent of the annealing temperature. The character of the strain-softening stage is largely determined by the alloy structure formed after annealing. It is found that annealing at the temperatures 773 and 873 K does not change the deformation behavior of the ultrafine-grained alloy under tension at room temperature. Deformation and fracture of the specimens localize in shear bands. Recrystallization occurring at the annealing temperature 973 and 1073 K leads to the transition of grain boundaries to a more equilibrium state and to a sharp decrease in strength and softening rate of the titanium alloy. It also affects the neck formation prior to fracture, giving a developed neck. Fracture surfaces are indicative of ductile dimple fracture of the alloy in all states. The dimple size depends on the size of structural elements after heat treatments. Based on the experimental data, the σ₀ value and Hall–Petch coefficient k are determined. In the investigated grain size range (0.17–1.25 μm) the values are found to be, respectively, 680 MPa and 0.36 MPa m^1/2.