Effect of HEA addition on the microstructure evolution and mechanical properties of SPSed ultrafine-grain WC-6Co cemented carbide

In this work, ultrafine-grain WC-6Co/AlCoCrCuFeNi cemented carbides have been fabricated by spark plasma sintering method. The sintered samples are mainly composed of WC and the binder phase. With AlCoCrCuFeNi high-entropy alloy (HEA) increasing from 0 to 6 wt%, the binder phase transforms from Co to FCC solid solution phase rich in Co, Cu, Fe and Ni. Additionally, (Al, Cr)₂O₃ oxides and η phase form among the WC grains. Average equivalent diameter of WC grains decreases gradually from 358.2 to 271.7 nm. This is mainly because the solubility of W and C in HEA binder phase is much lower and their diffusion can be better hindered due to the sluggish effect. Thus, the dissolution-precipitation-growth process of WC can be effectively inhibited. On the other hand, the formation of (Al, Cr)₂O₃ oxides and η phase along WC grain boundaries can impede their abnormal growth. Consequently, the hardness can be improved while the fracture toughness decreases. WC grain refinement, higher hardness of HEA and formation of hard phases ((Al, Cr)₂O₃ oxides and η phase) account for the hardness improvement. The decreased fracture toughness is attributed to the weak interface bonding between WC and oxides, formation of brittle η phase and lower relative density. Furthermore, the transversal rupture strength (TRS) decreases with HEA addition while compressive strength (CS) can be improved. By comparison, WC-2Co4HEA sample shows relatively better comprehensive mechanical properties, with hardness, fracture toughness, TRS and CS of 2185.75 HV₃₀, 10.47 MPa m^1/2, 1717.12 MPa and 3054.25 MPa, respectively.