Microstructure evolution and grain refinement mechanism in undercooled (CoFe)₅₀Si₅₀ multi-principal element intermetallics

Multi-principal element alloys (MPEAs), as a novel alloy design idea, have been proved to be promising materials and gained increasing attention recently. In this study, the single-phase (CoFe)₅₀Si₅₀ multi-principal element intermetallics (MPEIs) was synthesized and subject to rapid solidification via the melt fluxing technique. A maximum undercooling of 314 K was achieved and the microstructure of as-solidified samples with different undercooling was characterized. An ordered B2 phase was found to dominate the phase constitution despite the formation of minor Fe-rich BCC metastable phase in highly undercooled samples. The microstructure of undercooled (CoFe)₅₀Si₅₀ MPEIs shows a clear morphological evolution from dendrites to equiaxed grains and then to refined dendritic seaweed-like morphology with undercooling. At low undercooling, spontaneous grain refinement can be described by the dendrite fragmentation and dendrite remelting mechanisms. At high undercooling, detailed EBSD analysis reveals that the formation of subgrains and subsequent recrystallization play a significant role, but the inherent characteristics of (CoFe)₅₀Si₅₀ MPEIs (such as high stacking fault energy) prevent the occurrence of complete recrystallization and hence complete grain refinement. This study could provide a reference for controlling the non-equilibrium microstructure of MPEIs and understanding the microstructure evolution from binary intermetallic compounds to MPEIs.