OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 2 2024 cal b′-phase globules and b1-phase lamellas. The average size of b′-phase precipitates is 0.2 µm, while the length and width of the b1-phase lamellas vary from 0.6 to 0.7 and from 0.02 to 0.05 µm, respectively (Fig. 3, i). Eutectic β-phase particles are well-defined rhombs, as it is shown in Fig. 3, j. Annealing at 450 °С provides further coarsening of Mg24Y5 intermetallic particles. At higher annealing temperatures, b′-, and b1-phases particles become larger or are replaced by the stable β-phase according to b′ → β or b1→ β phase transformations [24, 27–30]. After annealing at 525 °С, the alloy microstructure undergoes transformation. According to optical and SEM microscopy the structure becomes more homogeneous (Fig. 4, а, b). The average size of the base a-phase reaches 32 µm. There are no textured bands consisting of magnesium fine grains, which indicates intensive recrystallization processes. Bright field TEM images demonstrate four types of intermetallic inclusions, namely Mg24Y5 particles (Fig. 4, c) and b-, b′-, and b1-phases (Fig. 4, d, е) similar to those observed after annealing at 450 °С. The medium size of Mg24Y5 particles is 1.4 µm, and its shape is a regular tetrahedron. The width of the subgrain b-phase boundary expands and ranges between 0.6 and 1.2 µm. The length and width of b1-phase lamellas vary from 1.1 to 6.2 and from 0.4 to 1 µm, respectively. The average size of b′-phase precipitates is 0.3 µm. c d e Fig. 4. Optical, SEM and TEM images of Mg-2.9Y-1.3Nd alloy microstructure after annealing at 525°С: a, b – optical and SEM images; c–e – bright field TEM images with corresponding microdiffraction pattern a b The dependence of the average size of structural elements of various phases on the annealing temperature is shown in Fig. 5. When annealing in the temperature range 100–450 °C, the average grain size of the a-phase does not change, but at the same time there is a slight increase in the particle size of Mg24Y5 particles and precipitates of b-, b′-, and b1-phases, which indicates its thermal instability at the above temperatures. At a temperature of 525 °C, there is a noticeable increase in both the grain size of the matrix a-phase of magnesium, and Mg24Y5 particles, and precipitates of b-, b′-, and b1-phases. Note that the Mg24Y5 particles and precipitates of b-,b′-, and b1-phases are present at fairly high temperatures, up to 525 °C. It should be noted that Mg24Y5 particles and b-, b′-, and b1-phases in the Mg-Nd-Y system alloy, are thermally stable at rather high annealing temperatures reaching 525 °С. In Fig. 6, one can see fragments of XRD patterns of the extruded Mg-2.9Y-1.3Nd alloy and annealed at temperatures of 100–525 °С.
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