Solidification, microstructure and elemental partitioning in the FeMnNi medium entropy alloy

Fe-Mn-Ni alloys represent a core subsystem of the widely studied high-entropy Cantor alloy family and offer provide an ideal platform to explore the solidification behaviour, and the impact of elemental partitioning on the microstructural stability and properties. In this work, the microstructure and elemental segregation was systematically investigated in an equimolar FeMnNi medium entropy alloy (MEA). Samples were sectioned from as-cast 400x200x10 mm slabs to examine the microstructure, elemental behaviour and distribution as this alloy upon solidification. Melting took place in vacuumed-furnace ceramic crucible and casting was done in a heat-resistant tool steel rectangular mould. Optical and electron microscopy revealed a predominantly coarse dendritic microstructure with chemical segregation between Fe-rich dendritic cores and Mn-enriched interdendritic regions. EDS chemical mappings and EPMA analysis depicted the elemental segregation: Fe (melting point 1538 ^oC) was mostly concentrated within the coarse grains and arms of the dendrite, while Mn (melting point 1246 ^oC) was segregated towards the interdendritic structure. Ni (melting point 1455 ^oC) Ni was enriched in regions where higher concentrations of Mn were detected, i.e. interdendritic regions. The effects of the elements’ physical properties and thermodynamic parameters including the atomic size, enthalpy of mixing (∆H_mix) and electron state on segregation behaviour during solidification are discussed. The results highlight the potential of as-cast FeMnNi alloys as a model system for understanding metastability-driven deformation in medium entropy alloys, while also pointing to their promise for structural applications requiring robust ductility and toughness, particularly under cryogenic conditions.