OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 4 2022 The presence of columnar dendrites indicates liquation in the b-solid solution during remelting [15]. The X-ray microanalysis study showed that the ingots after remelting had a high degree of uniformity of the distribution of alloying elements (niobium and zirconium) by volume. The concentration of niobium over the cross-section of the ingot was in the range of 41.2-43.1 wt. %, while the zirconium concentration was 6.8-7.3 wt. %. A characteristic feature of the ingot microstructure was the developed dendritic structure in its upper part and a coarse-grained structure with grains dimensions of 200‑500 μm based on a solid solution of titanium and/or niobium in its lower part. According to the TEM data, the main phase in the alloy is the b-phase based on a solid solution (Fig. 2, c). Before deformation impact, the alloy was subjected to quenching, which consisted in holding at a temperature of 1,000 °C for 3 hours, followed by cooling in water at room temperature. The optical image of the microstructure of the alloy after quenching is shown in Fig. 2, d. The microstructure is homogeneous over the cross-section of the ingot. In the structure, equiaxed grains of the b-phase and plates of the martensitic a″-phase are observed, which are characteristic of the structure after quenching. Formation of the martensite a″-phase is characteristic of titanium-based b-alloys due to the high niobium content. Thus, for the Ti–Nb system, the formation of a martensitic a″-phase is observed in hardened alloys containing niobium in the concentration range from 30 to 40 wt % [15, 16]. The average grain size of the b-phase was 100 µm. The microstructure of the hardened alloy after multi-pass rolling is presented in Fig. 3. Rolling leads to the formation of a strip character of the microstructure. In bright-field images, the “stripe” fragments with cross-sectional dimensions of 0.2‑0.8 µm and a length of 0.2‑0.7 µm are observed. This corresponds to the UFG state according to the classification given in [17]. In the stripe fragments, the formation of a dislocation substructure is observed. The stripe fragments consist of a b-phase based on a solid solution of titanium and niobium (Fig. 3, a, b) On the bright‑field images in the local areas, there are precipitates of the second a″-phase in the form of 10 nm wide plates, which are localized inside the subgrains of the matrix β-phase (Fig. 3, c). The microdiffraction pattern (Fig. 3, c) is represented by point reflexes. Moreover, Fig. 3, b shows the scheme of microdiffraction pattern identification, in which reflexes corresponding to the nanodispersed w-phase particles were distinguished in the grid of the b-phase reflexes. On the dark-field image obtained in reflexes from the b-and w-phases, the nanoparticles of the w-phase with a size of 10 nm are visible inside the b-phase bands (Fig. 3, d). Figure 4 a, b shows the TEM images of the microstructure of the Ti-42Nb-7Zr alloy subjected to abcpressing followed by rolling (the second scheme). The microstructure has a less pronounced “band” character (Fig. 4, a). As a result of combined SPD, non-equiaxed subgrains of the β-phase are formed, in which there are dispersed nanoparticles of the w-phase (Fig. 4, b). In the β-phase subgrains the developed dislocation substructure with an increased density of dislocations is observed. The reflexes in the microdiffraction pattern are located in circles, which indicates the significant refinement of the structure after deformation, as well as the presence of high-angle grain boundaries. Subgrains of the b-phase have sizes in the range of a b c d Fig. 2. Optical (a, d), SEM (b) and TEM images with corresponding microdiffraction patterns (c) of Ti-42Nb-7Z alloy microstructure: cast (a, b, c); quenched (d) states
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