OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 27 No. 3 2025 The existence of the Laves phase is characteristic of AlFeNiCoCrNb alloys where the Nb concentration corresponds to a molar ratio of 0.25 or higher. In this case, Nb not only dissolves in the primary BCC phase but also promotes the formation of the secondary – Laves – phase, which forms a eutectic mixture with the BCC phase [29]. According to the findings of [32], during cooling of the AlFeNiCoCr alloy, the primary crystallized BCC phase may incoherently separate into a mixture of an unordered BCC phase enriched in Cr-Fe and an ordered B2 phase enriched in Al-Ni, which is evidenced by the presence of a peak corresponding to the B2 phase in the XRD pattern. Subsequent heat treatment of the alloy leads to the following changes in the XRD patterns. As the heating temperature increases, the intensity of the B2-phase peak decreases, while the intensity of the peak corresponding to the Laves phase slightly increases. In addition, changes occur in the lattice of the primary BCC phase. Fig. 1, b presents an enlarged view of the (110) peak of the BCC phase. It is evident that, with an increase in the heating temperature, the peak shifts towards smaller angles, indicating an increase in the lattice parameter of the BCC solid solution, which suggests changes in the composition of the solid solution. After heat treatment at 900 °C, peaks of a new phase emerge, which is identified as the tetragonal σ phase composed of Cr and Fe. The σ phase is absent at higher heating temperatures. The phenomenon of the σ-phase precipitation and dissolution in the BCC phase within a similar temperature range was also observed previously [29]. The XRD analysis of the AlCoCrFeNiNb0.25 alloy throughout the entire heating temperature range shows that the primary phase remains an unordered BCC solid solution. However, upon heating of the AlFeNiCoCr alloy without Nb, part of the material transforms into an FCC solid solution [29]. Thus, the addition of Nb helps stabilize the BCC phase and maintain a predominantly single-phase structure in the high-entropy alloy. Fig. 2 depicts the microstructure of the AlCoCrFeNiNb0.25 alloy both in the as-cast state and after heat treatment. The alloy consistently exhibits a dendritic morphology with hypoeutectic characteristics. The microstructure comprises primary dendritic and interdendritic eutectic regions. Dendritic regions consist of a BCC phase, while the eutectic structure is a mixture of the BCC and Laves phases. In the as-cast state, dendritic segregation results in compositional heterogeneity: dendritic cores (BCC phase) are enriched in Ni and Al, whereas the dendritic periphery and eutectic regions are enriched in Cr and Fe. Nb partially dissolves in the BCC phase, but most of it enters the composition of the Laves phase [29]. Results of the elemental analysis in different zones of the as-cast alloy are detailed in Table 2. The same pattern of formation of the dendritic structure in the alloy was reported in [15, 29]. The dendritic structure that exhibits a dark contrast after etching is surrounded by lighter layers of the Laves phase, which represents a secondary phase. The secondary Laves phase forms along the solid solution boundaries during dendritic growth and is attributed to the reduced solubility of niobium in the solid solution of the principal components during cooling. The enrichment of the peripheral regions of dendrites with niobium and chromium creates conditions for the formation of the secondary Laves phase based on these components. Fig. 2, b shows a defective eutectic structure. Grains of the Laves phase are divided into fragments with random crystallographic orientations. Since the eutectic, which includes the Laves phase, forms in the interdendritic space, it is structurally impossible to distinguish between the secondary Laves phase and the Laves phase present in the eutectic. Heat treatment at 900–1,100 °C does not alter the dendritic structure of the alloy (Fig. 2). The dendrite width along the secondary axes ranges from 11 to 15 μm. Increasing the heating temperature from 900 °C to 1,100 °C leads to changes in the structure of the eutectic, this can be observed in high-magnification metallographic images (Figs. 2, f, h). At the heating temperature of 900 °C, no noticeable changes in the eutectic structure occur. However, at 1,000 °C, Laves phase fragments begin to align, which is typical of the eutectic, and the formed lines alternate with the solid solution. At 1,100 °C, eutectic grains are well defined in the interdendritic space (Fig. 2, h).The XRD analysis confirms the eutectic transformation: the peak intensity of the Laves phase increases with temperature. This may arise from either the coalescence of
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