Microstructure of ultrafine-grained ti-40 wt.% nb alloy after annealing

The results of studies on the evolution of the microstructure and phase composition of ultrafine-grained Ti-40 wt.% Nb alloy during annealing in the temperature range of 673 –1073 K are presented. The ultrafine-grained structure in the Ti-40 wt.% Nb alloy was formed by a combined severe plastic deformation (SPD) method, which includes three-cycled abc-forging with a sequential temperature decrement in the range of 773 – 673 K, multi-pass rolling in grooved rollers at room temperature, and subsequent recrystallization annealing at 573 K. After SPD, the Ti-40 wt.% Nb alloy had a microstructure represented by the β-phase subgrains with ellipsoidal particles of the ω-phase localized in the bulk of the β-grains, and the α-phase subgrains. The average size of structural elements (grains, subgrains and fragments) was 0.28 μm. After annealing in the range of 673 – 873 K, the microstructure consisted of the dispersion-strengthened ω-phase, β-subgrains and α-subgrains, similarly to the initial UFG state. At the same time, a redistribution of the volume fraction of the α-phase occurred. In the range of 773 – 973 K, the transformation of the ultrafine-grained (β + α + ω)-structure into a fine-grained structure consisting of β-and ω-phases with phase transformation according to the α → β scheme was observed. At temperatures above 973 K, active recrystallization occurred, which was accompanied by the rapid growth of a dispersive-strengthened β-phase grain size. This was also accompanied by the transformation of the alloy into a сoarse-grained state and a significant decrease of microhardness. Change in the concentration of niobium in the range of 40 – 45 wt.% for the titanium-niobium alloy in the ultrafine-grained state does not have a significant effect on the structural-phase transformations during annealing.