Microstructure and mechanical properties of oxide-dispersion-strengthened CrMnFeCoNiC_0.2O_0.2 high-entropy alloy fabricated by mechanical alloying and spark plasma sintering

We investigated the microstructural and mechanical properties of an CrMnFeCoNiC_0.2O_0.2 high-entropy alloy (HEA) fabricated by high-energy ball milling (HEBM), spark plasma sintering (SPS), and solution and annealing heat treatment. Nanocrystalline HEA powder with a face-centered cubic (FCC) structure was successfully fabricated using HEBM. Ultrafine-grained HEA containing a Cr₂₃C₆ phase and nanosized oxide particles was obtained using HEBM followed by SPS. The application of HEBM reduced the size of the FCC grains, Cr₂₃C₆ phase, and oxide particles of the sintered alloy, while transforming the oxide particles from Mn₃O₄ into MnCr₂O₄. After appropriate HEBM, the sintered alloy exhibited an excellent hardness of ∼4.9 GPa, owing to strengthening by the ultrafine-grained FCC matrix and nanosized oxide particles. Solution treatment at 1100 °C caused significant coarsening of the FCC grains, and the Cr₂₃C₆ and MnCr₂O₄ phases by a factor of ∼2, thus reducing the hardness of the alloy to ∼2.9 GPa owing to weaker grain boundary strengthening and oxide dispersion strengthening. The solution-treated alloy showed excellent thermal stability with minimal microstructural coarsening up to the annealing temperature of 800 °C due to the uniformly distributed MnCr₂O₄ oxide particles that exert a sufficiently strong pinning effect. Annealing at temperatures above 900 °C led to the coarsening of the FCC grains and oxide particles, leading to a slight reduction in the hardness of the alloy.