A novel in-situ exothermic assisted sintering high entropy Al2O3/(NbTaMoW)C composites: Microstructure and mechanical properties

In this paper, we have designed a Al2O3/(NbTaMoW)C composite by in-situ exothermic reaction, which opened a new field to explore HEC matrix composites. The Al2O3/(NbTaMoW)C composite was synthesized at 1600 °C using Nb, Ta, W, MoO3, Al and graphite powders as raw materials, and the thermite of Al + MoO3 as heat source provides additional energy for the formation of composites. As-prepared composites with relative density 98.5% only exhibited two phases including a high entropy carbide of (NbTaMoW)C and Al2O3 phases, and Al2O3 randomly distributed within high entropy matrix. Phase boundary between (NbTaMoW)C and Al2O3 was a noncoherent interface, resulting in a moderate interfacial bonding strength which was beneficial to both flexural strength and fracture toughness. Moreover, Al2O3 and HEC form an interesting interlocking structure and a grain size varies from 1 μm to 10 μm due to the inhomogeneous temperature distribution introduced by the in-situ exothermic heat. By taking advantages of these special structures, the composites exhibited significantly enhanced mechanical properties compared to (NbTaMoW)C. The flexural strength of composites was up to 530 MPa, and the fracture toughness was 4.5 MPa m1/2. The main strengthening mechanism is second phase strengthening and the presence interlocking structure releases stress intensity at crack tip.