OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 2 2024 The optical image shows two types of structural elements, namely coarse (~17 µm) and ultra-fine (~1 µm) grains, the latter represent textured bands. The volume fraction of ultra-fine grains is 50 % of the total volume. Intermetallic particles are additionally deduced from the energy-dispersive X-ray (EDX) analysis of the elemental composition shown in Fig. 1, c, d. EDX spectra in this figure are obtained at different points of thin specimen. Mg24Y5 particles are high in yttrium (30 wt.%) and locate mostly inside grains (Fig. 1, e). In the extruded alloy, Mg24Y5 particles are mostly irregular polyhedrons with the average size of 0.6 µm. According to TEM images, the volume fraction of these particles is not over 2 %. The eutectic equilibrium β-phase is localized along the grain boundaries as a network of precipitates up to 0.3–0.4 µm thick. This phase is also irregular polyhedrons and, to a lesser extent, regular tetrahedrons presented in Fig. 1, f. The medium size of b′-phase globules is 0.2 µm. The length and width of the b1-phase vary within the range of 0.06–0.30 µm and 0.03–0.04 µm, respectively (Fig. 1, g). Note that b1-phase lamellas are positioned in the same direction. In the b-phase, yttrium and neodymium amount to (3.54–7.18) wt. % and (2.26–9.59) wt. %, respectively. In the b′-phase, the content ranges between 3.21 and 5.39 wt. % for yttrium and between 1.83 and 2.07 wt. % for neodymium. In b1-phases, yttrium and neodymium range between (3.32–5.27) wt. % and (1.75–8.46) wt. %, respectively. The b′-phase contributes to the greatest extent to the increase in the mechanical properties of Mg-Y-1Nd alloys due to its strain hardening [24]. Figure 2 presents optical images of the Mg-2.9Nd-1.3Y alloy microstructure after annealing in the temperature range of 100–450 °С. As can be seen in this figure, the bimodal microstructure does not change after annealing (Fig. 2, a-d). Finer grains of the a-phase range in size from 0.2 to 5.0 µm. After annealing, its medium size does not change and is equal to 1 µm. а b c d Fig. 2. Optical images of extruded Mg-2.9Y-1.3Nd alloy microstructure after annealing at different temperatures: a – 100 °С; b – 300 °С; c – 350 °С; d – 450 °С Note that the formation of the fine-grained structure in the extruded Mg-2.9Y-1.3Nd alloy significantly improves its yield strength and tensile strength up to 150 and 230 MPa, respectively. For the recrystallized structure obtained after 525 °С annealing for 6 hours, these parameters are 220 and 340 MPa, respectively [22]. The alloy plasticity also increases from 12 to 21 %. Figure 3 contains TEM images of the alloy microstructure after annealing at different temperatures. On bright field images, one can see four types of intermetallic inclusions after 100 °С annealing, namely Mg24Y5 particles (Fig. 3, а) and b-, b′- and b1-phases (Fig. 3, b), as in the extruded structure. Unlike the extruded structure, the medium size of Mg24Y5 particles grows up to 0.9 µm, and there is a slight increase in the width of the subgrain b-phase boundary, which varies in the range (0.4–0.5) µm (Fig. 3, a, b). Linear dimensions of secondary b′- and b1-phase precipitates do not change. The temperature growth up to 300 °С leads to a further increase in the medium size of Mg24Y5 particles from 0.9 to 1.2 µm and morphology transformation of some particles from irregular polyhedrons to regular tetrahedrons, as presented in Fig. 3, c. This indicates the occurrence of recrystallization process. The microstructure in Fig. 3, d–f, includes all types of secondary non-equilibrium phases described above. The network width of the grain boundary increases up to 1.2–1.7 µm and consists of the eutectic β-phase (Fig. 3, d). In Fig. 3, e, f, one can see b′-phase globules and b1-phase lamellas. A significant growth in b1-
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