OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 25 No. 3 2023 Fig. 7. X-ray diffraction patterns obtained by surfacing the powder mixtures with different CrB content According to the results of the energy dispersive analysis and microstructural studies, it can be assumed that borides of the composition (Cr,Mn,Fe)2B were formed in the coatings under investigation. An increase in the chromium boride amount in the surfacing mixture to 20 % and 30 % led to the appearance of reflections from borides with orthorhombic (Cmcm) and tetragonal (I4/mcm) crystal lattices in the diffraction patterns. In the system under consideration, CrB, FeB, and MnB are borides with a lattice related to the Cmcm space group. As mentioned above, in the sample obtained by surfacing the mixture with 30 % CrB, regions with composition close to the original boride that substantially did not contain other elements were observed. Thus, it can be assumed that the reflections from the phase that belongs to the Cmcm space group correspond to CrB borides crystallized from the melt or original borides undissolved during surfacing. Such borides as Mn2B, Co2B, Ni2B, and Fe2B belong to the I4/mcm space group. Previously, it was shown that the structure of coatings with more than 20 % CrB usually exhibits the skeleton morphology of eutectic instead of lamellar. At the same time, in the skeleton eutectic, the concentration of Cr and Fe is much lower than that of Co, Mn, and Ni. The results obtained allow us to conclude that the boride found in the eutectic is a (Ni,Co,Mn)2B solid solution based on chemical compound. On the basis of the conducted studies, one can assume the following scheme for the formation of the phase composition of coatings obtained by surfacing powder mixtures with different CrB content. At a CrB concentration of 5 % (a hypoeutectic structure is formed): – crystallization of the melt bath starts with the formation of dendrites based on fcc solid solution; – increasing the concentration of boron in the melt brings its composition closer to the eutectic; – in the interdendritic space, the eutectic consisting of (Cr,Mn,Fe)2B and the fcc phase crystallizes. At a CrB concentration of 10 % (a hypereutectic structure is formed): – primary borides (Cr,Mn,Fe)2B precipitate; – the eutectic consisting of (Cr,Mn,Fe)2B and the fcc phase solidifies. At CrB concentrations of 20 and 30 %: – primary precipitation of CrB crystals with a higher melting point than that of Cr2B phase from the melt bath occurs; – (Cr,Mn,Fe)2B phase around CrB crystals forms; – the eutectic consisting of (Ni,Co,Mn)2B and fcc solid solution crystallizes. It is assumed that the change in the type of eutectic from the (Cr,Mn,Fe)2B + fcc phase to the (Ni,Co,Mn)2B + fcc phase is due to a decrease in the chromium content in the melt as a result of the primary precipitation of chromium-rich phases, such as CrB and (Cr,Mn,Fe)2B.
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