Room temperature stability, structure and mechanical properties of cubic tungsten carbide in flash sintered products

This work explores the possibility to obtain the metastable cubic tungsten carbide phase (WC1-x) during the rapid and the ultrarapid consolidation of hexagonal WC nanopowders. Spark plasma sintering and flash sintering techniques are implemented to study the formation and the stability of WC1-x during sintering. A biphasic ceramic material, corresponding to cubic-WC/hex-WC (10/90 vol%), can be obtained uniquely with the flash sintering process. The ultrafast sintering of powders containing Cr and V impurities results in a peculiar cubic WC quaternary phase (W0.87Cr0.12V0.09)C0.71 which retains the crystalline structure, Fm3m {225}, and the metastable character of WC1-x, decomposing into the thermodynamically stable hexagonal phases (WC and W2C) after vacuum annealing at 700 °C and 1100 °C. Structural features of the composite are analysed by HRTEM and Transmission Kikuchi Diffraction (TKD), which pointed out the activation of room-temperature plastic deformation mechanisms in cubic WC grains. Such behaviour is connected to stacking faults interacting with partial dislocations on {111} planes, similarly to FCC metals. Nanomechanical indentations were also used to map the elastic modulus and the hardness of the cubic phase, which shows a noticeable softer character with respect to the hexagonal matrix. These results suggest flash sintering as a new strategy for producing WC/WC1-x composite materials with tailored elastic modulus and hardness/toughness ratio.