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.