OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 27 No. 3 2025 structure, which was described in detail in [15, 32, 37]. In the investigated as-cast alloy, a characteristic basket weave or banded structure forms in the peripheral regions of dendrites, which is attributed to the spinodal decomposition of the solid solution. No banded structure is observed in the center of dendrites (Fig. 3, a). When the alloy is heated to 900 °C and subsequently cooled, structural heterogeneity in dendrites and the basket weave structure become more pronounced (Fig. 3, b). Furthermore, as mentioned above, σ-phase particles precipitate from the solid solution. According to X-ray structural analysis, after heat treatment, the proportion of the ordered B2 phase decreases, suggesting that the observed contrast within the basket weave structure is due not to the spinodal decomposition of the solid solution into two phases but rather to the heterogeneous segregation of atomic components within the disordered solid solution, as described in [15]. At 1,000 °C, the basket weave structure increases in size and occupies the entire volume of dendrites (Fig. 3, c). Further heating to 1,100 °C causes coarsening (Fig. 3, d). The average microhardness and the microhardness of the structural components of the alloy are presented in Table 3. In all heat treatment conditions, the interdendritic regions demonstrate significantly higher microhardness compared to the dendritic cores. Ta b l e 3 Microhardness of AlCoCrFeNiNb0.25 alloy in the as-cast state and after heat treatment Measurement area Т30 HV Т900 HV Т1000 HV Т1100 HV Dendrites 614 ± 44 582 ± 37 489 ± 53 520 ± 35 Eutectic 640 ± 47 902 ± 66 620 ± 45 636 ± 46 Average value 625 ± 28 730 ± 47 545 ± 52 572 ± 56 The maximum microhardness in dendrites of the as-cast alloy is attributed to the unique structure of the solid solution of the alloy components, which forms during crystallization and cooling. The spinodal decomposition of the disordered solid solution coupled with the precipitation of the ordered B2 phase strengthens the alloy. However, heating of the alloy during heat treatment leads to a partial loss of the order characteristic of the B2 phase, resulting in a decrease in the microhardness of dendrites. Nevertheless, the precipitation of σ-phase particles upon heating to 900 °C allows the microhardness to remain at a high level. When the heating temperature increases to 1,000 °C, the strengthening effect of the σ-phase particles disappears. During heat treatment at 1,100 °C, coalescence of the basket weave structure occurs, leading to the formation of more distinct phase boundaries, possibly due to the increased heterogeneous segregation of atomic components within the solid solution. This, in turn, slightly increases the microhardness in the dendritic zones of the alloy. In the interdendritic space of the as-cast alloy, the microhardness of the eutectic is only slightly higher than that of the solid solution in dendrites. This means that strengthening due to spinodal decomposition is comparable in magnitude to that due to the Laves phase (a solid intermetallic compound) in the eutectic. During heating at 900 °C, as mentioned above, σ-phase particles precipitate from the solid solution and concentrate in the Cr-rich interdendritic space. This significantly increases the microhardness of the eutectic due to strengthening of the solid solution with σ-phase particles in its composition. Dispersed σ-phase particles precipitate in the solid solution of all structural components, significantly enhancing the microhardness in dendrites. Heating to 1,000 °C and 1,100 °C leads to the dissolution of σ-phase particles in the primary phase [11], which contributes to a decrease in the microhardness in the interdendritic space to the values of the initial structure. This is due to the removal of the effect of strengthening due to σ-phase particles. When evaluating the integral microhardness of all structural components of the alloy, the overall trend in the variation of microhardness with heating temperature is retained (Table 3).
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