Microstructure and Hardness of Non–Equiatomic Nb–Co–Cr–Ni–Mn Medium–Entropy Alloys

Non–equiatomic Nb–Co–Cr–Ni–Mn medium–entropy alloys were designed based on thermodynamic modeling to achieve solid solution strengthening while maintaining a stable FCC structure. Five compositions (Nb2Co40Cr26–12Ni30–40Mn2–6) were fabricated by arc melting and characterized using XRD, SEM–EDS, TEM, and nanoindentation. XRD confirmed FCC crystal structures in all alloys, though peak broadening suggested potential phase complexity. SEM–EDS revealed dendritic microstructures with systematic Nb segregation to interdendritic regions (4.9–7.9 at. %) and Nb–rich intermetallic phases in two compositions. Molecular dynamics simulations using the PET–MAD potential predicted Nb–Mn short–range ordering. Nanoindentation measurements showed hardness values of 112–142 HV1, comparable to the baseline CoCrNi alloy. The results indicate that while non–equiatomic compositions can enhance Nb solubility compared to equiatomic alloys, solidification segregation and secondary phase formation remain challenges requiring homogenization treatment.