Segregation, precipitation, and phase decomposition behavior of a carbon-doped non-equiatomic nanocrystalline CoCrFeMnNi high entropy alloy

Elemental segregation at grain boundaries significantly influences the mechanical and functional properties of materials through structural and compositional changes. This phenomenon is especially common in high entropy alloys (HEAs) composed of multi-principal elements. Understanding the behavior and evolution of elemental segregation at grain boundaries is essential for tailoring material properties. In this study, we investigated the segregation, precipitation, and phase decomposition behavior of a nanocrystalline non-equiatomic CoCrFeMnNi HEA with intentional doping C interstitials, subjected to isochronal annealing treatments. Microstructure characterization using electron microscopies and atom probe tomography suggests that the nanocrystalline FCC solid solution decomposed during annealing at 500 °C, leading to the formation of CoFe B2 and NiMn FCC phases together with significant decoration of grain boundaries by NiMn and CCr co-segregations. Furthermore, annealing at higher temperatures accelerated the precipitation of Cr carbides, CoFe B2 and NiMn FCC phases. However, the intermetallic particles were not observed after annealing at the evaluated temperatures, while the carbides persisted. It is proposed that diffusion processes were accelerated in the nanocrystalline HEA due to the high density of lattice defects. These findings provide detailed insight into the sequence and mechanisms of decomposition, from initial elemental segregation to precipitation in HEAs.