Microstructure and Mechanical Behavior of Co40Cr12Nb2Mn6Ni40 Medium-Entropy Alloy

The microstructural features and mechanical behavior of a Co40Cr12Nb2Mn6Ni40 medium-entropy alloy fabricated by vacuum-induction melting were studied. The microstructural studies by optical and scanning electron microscopies revealed the formation of a dendritic architecture with a dendrite size of 150–250 µm. Energy-dispersive spectroscopy confirmed a uniform Co/Ni ratio of ≈1 : 1 in the matrix and the homogeneous distribution of chromium and manganese throughout the sample volume. The presence of fine Nb-rich intermetallic particles with a niobium content of up to 6 at %, which provide dispersion strengthening of the material, was established. A comprehensive study of the mechanical behavior was carried out by instrumental indentation under loads ranging from 15 to 1000 mN and by uniaxial tensile tests. Nanoindentation revealed a pronounced dependence of hardness and elastic modulus on the applied load: the hardness decreased from 3.21 ± 0.61 GPa to 1.18 ± 0.6 GPa and the elastic modulus decreased from 131.7 ± 44.1 to 44.7 ± 1.4 GPa. The ultimate tensile strength was 486 MPa, with a high elongation to failure of 92.6%. The specimens were found to exhibit a mixed-mode fracture characterized chiefly by a ductile dimpled mechanism of fracture with the presence of isolated regions of transcrystalline cleavage. The size of dimples varied from 3 to 20 µm, with large dimples (10–20 µm) constituting approximately 20% of the total count. Spherical inclusions of up to 5 µm in size, which served as micropore nucleation sites, were detected at the bottom of the large dimples.