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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.

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By the neutron diffraction structure analysis method, an ordered phase of non-stoichiometric tantalum carbide with the base B1(NaCl) structure has been detected. It is shown that the superstructural reflections observed correspond to an incommensurate superstructure close to type. With allowance for ordering of , the phase diagram of the Ta - C system has been calculated and constructed. The effects of non-stoichiometry and ordering on the period of a base B1-type lattice of tantalum carbide and on the superconducting critical temperature have been studied.

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