Effects of vacancy ordering on structure and properties of vanadium carbide

The microhardnesses, electrical conductivities, specific heats at constant pressure (C-p), lattice parameters and microstructures of ordered and disordered vanadium carbides were investigated. It was found that ordered vanadium carbide has a higher microhardness and electrical conductivity than a disordered carbide of the same composition. In a thermodynamic study the C-p and temperatures of the order-->disorder transformations were measured by using differential scanning calorimetry (DSC). The values of latent heats of the disorder-order reactions delta-VC0.87-->V8C7 and delta-VC0.83-->V6C5 were 2354 J/gfw and 2114 J/gfw, respectively. The measured temperatures corresponding to the onset of the transformations were 1095 degrees C for V8C7 and 1163 degrees C for V6C5. Ordered domains could be made visible in microstructural investigations, which showed that ordering in compact vanadium carbide specimens originates at the grain boundaries of the disordered compound. (C) 1997 Elsevier Science S.A.