Microstructure of ultrafine-grained Ti-40 wt.% Nb alloy after annealing

A.Y. Eroshenko, Y.P. Sharkeev, M.A. Khimich ORCID logo , I.A. Glukhov, P.V. Uvarkin, A.I. Tolmachev, A.M. Mairambekova show affiliations and emails
Received 12 July 2019; Accepted 23 October 2019;
This paper is written in Russian
Citation: A.Y. Eroshenko, Y.P. Sharkeev, M.A. Khimich, I.A. Glukhov, P.V. Uvarkin, A.I. Tolmachev, A.M. Mairambekova. Microstructure of ultrafine-grained Ti-40 wt.% Nb alloy after annealing. Lett. Mater., 2020, 10(1) 54-59
BibTex   https://doi.org/10.22226/2410-3535-2020-1-54-59


Ti-40wt.% Nb alloy in UFG state (structural elements’ average size is 0.28 μm) has multiphase structure, which is represented by β-subgrains, dispersive-strengthened with nanoparticles of nanosized ω-phase, and α-subgrains. Annealing temperature range of 773-973 K is characterized by transformation of alloy’s structure from ultrafingerained state into finegrained, active growth of dispersive-strengthened β-phase’s grains size and significant decrease of microhardness. Transformation occurs according to the α→β scheme.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.

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