Order-disorder phase transformations and specific heat of nonstoichiometric vanadium carbide

AbstractA study has been made of the order-disorder phase transformations in the homogeneity region of the VC y nonstoichiometric cubic vanadium carbide (0.66< y <0.88). It has been established that an ordered V 6 C 5 phase with monoclinic (space group C 2/ m ) or trigonal ( P 3 1 ) symmetry, and a cubic (space group P 4 3 32) ordered V 8 C 7 phase can form in the VC y carbide below 1450 K, depending on the actual composition. The effect of off-stoichiometry and structural vacancy ordering on the specific heat of the VC y carbide has been investigated. The temperatures and heats of the reversible order-disorder equilibrium transitions have been determined. The ordering in the VC y carbide is shown to be a first-order phase transition. An equilibrium diagram of the V-C system taking into account ordering in the nonstoichiometric vanadium carbide has been constructed.