Ti60合金保温过程中片状组织的粗化行为

Due to its high thermal strength, oxidation resistance and good creep resistance, Ti60 alloy, as a multi-element reinforced near-alpha high-temperature titanium alloy with independent intellectual property rights designed and developed by China, had been widely studied in terms of its processing technology, microstructure and mechanical performance. The coarsening of the lamellar structure of titanium alloy often occurred during solid solution or high-temperature aging, which affected the ratio of the equiaxed microstructure and the lamellar microstructure, and ultimately had a significant impact on the mechanical properties of the parts. Therefore, it was essential to systematically study the coarsening behavior of the lamellar structure of Ti60 alloy during heat treatment. The microstructure evolution and coarsening behavior of the lamellar structure in Ti60 titanium alloy heat treated at α+β two-phase (965 ℃) with different holding time were investigated. The phase structures of as-received Ti60 alloy, the microstructures of lamellae after heat treatment, and chemical composition of particle/rod-shaped precipitates were analyzed by X-ray diffraction (XRD), optical microscope (OM), scanning electron microscope (SEM) and energy dispersive spectrometer (EDS), respectively. The thickness of the lamellae was measured by using SEM processed with Image-Pro Plus software, and quantitative analysis was performed using the parallel equally spaced raster line truncation method. In addition, the coarsening mechanism of β phase of Ti60 alloy during heat treatment was also explored. The results showed that after β annealing treatment and furnace cooling, a large number of particles precipitated in Ti60 alloy. During heated at α+β two-phase region, the morphology of lamellar α/β phase interface was no longer smooth and β phase appeared the branch structure with different holding time. The particles which precipitated in Ti60 alloy after furnace cooling gradually dissolved with increasing holding time. Moreover, the lamellar α and β phases gradually became thicker and coarser as the holding time increased, and the thickness of the lamellar structure increased faster when the holding time was shorter. With the increase of holding time, the growth rate of the lamellar thickness decreased and tended to be stable eventually, and the thickness of the coarse lamellae in the branch structure gradually increased and the thin lamellae gradually disappeared. During the heat treatment process, β phase on the branch continued to migrate toward the trunk. The migration of β phase did not stop until β phase on the branch totally filled into the trunk so that the concave surface disappeared, resulting in the coarsening of β phase. While β phase was coarsening, α phase between the branch and the trunk in the branch structure migrated into the adjacent lamellar α phase in the direction opposite to that of β phase migration, which led to the coarsening of the lamellar α phase. The branch structure of β phase caused a curvature difference between α/β phases. The chemical potential of the substance on the convex surface was always greater than that on the concave surface, resulting in a chemical potential gradient, that was, the potential energy difference caused by the curvature difference, which provided a driving force for the migration of solute atoms and made a continuous growth in thicknesses of both the lamellar α and β. This was a typical Oswald ripening phenomenon, which described a non-uniform structure that changed over time, causing the smaller size structure to dissolve and redeposit onto a larger one, resulting in the gradual growth of a large-size one. The holding time of 6 h was not sufficient to reach the complete equilibrium state, so there were still some undissolved branch structures. Analysis of the chemical composition of particle precipitate showed that in the part passing through the precipitated particle, strong fluctuations in elemental content were produced, in which the content of Ti decreased sharply, the content of Al decreased slightly, and the content of Si, Sn, Zr, Nb and Ta increased significantly. It showed that there was a certain concentration gradient of each element around the precipitate, and the precipitated particle was the second phase precipitated from β phase by the long-range diffusion and enrichment of each solute atom. Since the outer electronic structures of Zr, Ta and Ti were similar and Nb was in the adjacent subgroup of Ti, there was a substitution of Ti by Zr, Ta and Nb in the precipitate. However, only a small amount of Ti was replaced since Nb had a certain difference in electronic structure from Ti, and the large enrichment of Si elements in the precipitates also aggravated the decrease of the proportion of Ti elements. Based on the above analysis, it was found that the precipitate was a silicide. Since the dissolution temperature of silicide in Ti60 alloy was 990 ℃, a small amount of particle precipitate still existed after heated at 965 ℃ for 6 h. As a β stable element, the Si element was mainly concentrated in β phase, so the silicide was usually precipitated in β phase, and its chemical formula could be approximately expressed as (Ti, Zr)_xSi_y. According to the atomic percentage data, x/y≈2.5 was between 2 and 3, so the precipitate could be a mixture of Ti₆Si₃ and Ti₃Si.