Facile synthesis of high entropy carbide-nickel based cermets by in-situ carbo-thermal reduction of transition metal oxides

Equimolar and non-equimolar high entropy carbide-nickel based cermets were successfully prepared by in-situ carbo-thermal reduction of constituent metal oxides in a single pressureless vacuum sintering cycle at 1420°C. The carbon content in quinary (Ti0.2V0.2Nb0.2Ta0.2W0.2)Cx-10 wt % Ni and senary (Ti0.2V0.2Nb0.2Ta0.2W0.1Mo0.1)Cx-10 wt % Ni high entropy carbide cermets was varied (x = 0.6-1.0). Two-phase HEC-Ni cermets were formed at x=0.75-0.85, whereas M6C (η-phase) and graphite phase coexisted at lower and higher carbon contents (x = 0.6, 0.9 and 1.0) in both HEC 5 and HEC 6 systems. In HEC 5, submicrometric WC precipitates were observed, which were attributed to the limited solubility of hexagonal-WC within the in-situ formed HEC 5 phase, contrasting with HEC 6 where no such precipitations were formed across the entire carbon content range. The partial replacement of W with Mo in the (Ti0.2V0.2Nb0.2Ta0.2W0.1Mo0.1)Cx-10 wt % Ni cermets promoted solid solution formation at lower temperature, and the grain size and hardness decreased and the fracture toughness increased. Both HEC-Ni cermets showed a homogeneous distribution of elements within the carbide phase, resulting in (HV30) hardness around 16 GPa and a fracture toughness around 8 MPa.m1/2. The phase evolution was systematically investigated by interrupting the sintering cycle at 600-1400°C, employing XRD analysis. Thermodynamic calculations were performed to predict the critical reaction temperatures for carbo-thermal reduction. To evaluate the intrinsic mechanical properties of each phase and establish the correlation with microstructural features, micromechanical mapping was performed utilizing the high-speed nano-indentation technique.