Microstructure and Mechanical Properties of Ultrafine-Grained CrMnFeCoNi High-Entropy Alloy Prepared via Powder Metallurgy

We studied the microstructural evolution and mechanical properties of ultrafine-grained CrMnFeCoNi high-entropy alloys fabricated by mechanical alloying of various additives and spark plasma sintering. The additives were 1 wt.% process control agent (stearic acid) + 1 wt.% graphene nanofiber (GNF) (PG) or 1 wt.% Y₂O₃ + 1 wt.% GNF (YG) to modify the constituting phase of the sintered alloy. The PG and YG powders exhibited a single FCC phase. The YG powders had a larger powder size and a smaller crystallite size than the PG powders. Ultrafine-grained FCC matrices with average particle sizes of 0.57 μm and 0.71 μm, respectively, were formed through the SPS process of PG and YG powders. The absence of PCA in YG alloys resulted in a bimodal distribution of fine and coarse grains (due to incomplete mechanical alloying) and formation of a lesser and finer Cr₇C₃ phase (due to reduced C content). The sintered PG alloy contained coarse (~60 nm) spinel Mn₃O₄ oxides along grain boundaries, whereas the YG alloy exhibited coarse Mn₃O₄ and fine (~17 nm) Y₂O₃ oxide particles along grain boundaries. Additionally, the YG alloy contained tiny (~5 nm) Y₂O₃ oxide particles with a cube-on-cube orientation relationship within the FCC matrix. YG alloy exhibited higher hardness and compressive yield strength than PG alloy, mainly due to the oxide dispersion strengthening of finely dispersed Y₂O₃ particles. The addition of Y₂O₃ reinforcing particles had a minimal effect on the ultimate compressive strength and fracture strain of the sintered alloy.