Phase transformations in non-stoichiometric vanadium carbide

Abstract

X-ray diffraction, differential scanning calorimetry and reflected light microscopy studies are made of disorder-order phase transformations in the region of homogeneity of a non-stoichiometric cubic vanadium carbide, (0.66 < y < 0.88). It is found that according to the composition of the carbide, both an ordered phase with monoclinic or trigonal symmetry and a cubic ordered phase may form in this carbide at a temperature below 1450 K. Consideration is given to the effect that non-stoichiometry and the ordering of structural vacancies have on the heat capacity of the carbide. Temperatures and heats of reversible equilibrium disorder-order transitions are determined. The ordering transformations and are shown to be first-order phase transitions. An equilibrium phase diagram of the V-C system is constructed which allows for the formation of ordered phases in a non-stoichiometric vanadium carbide. The order parameter functional method is used for calculation of phase equilibria in the region of ordering of non-stoichiometric cubic carbide.