Some studies on the V-C system

Ghaneya, Abdel
Major Professor
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Materials Science and Engineering

The present study consists of a comprehensive critical review of the data in the literature on the V-C system together with experimental work on some areas of uncertainty in the V-C diagram. The combined results are the basis for a proposed revised V-C phase diagram;The (zeta) phase carbide was prepared by annealing a V-39.67 at. % C alloy at 1300(DEGREES)C for 220 h in a vacuum furnace. A hexagonal crystal structure with a = 2.9185 (+OR-) 0.002 (ANGSTROM) and c = 27.788 (+OR-) 0.03 (ANGSTROM) was obtained for this phase from its x-ray powder diffraction data. The peritectoid decomposition temperature was determined as 1320 (+OR-) 5(DEGREES)C by differential thermal analysis. From x-ray, metallographic, and chemical analyses, the stoichiometric composition of the (zeta) phase was determined as V(,4)C(,2.62) or (TURNEQ) V(,3)C(,2);The solvus curve for the V-rich end of the V-C system was determined by equilibrating vanadium with carbon powder at temperatures between 1100 and 1500(DEGREES)C. The maximum solubility of C in V was obtained from the results of the solubility studies as 4.5 at. % C at 1665(DEGREES)C;The composition and temperature of the VC-C eutectic alloy were experimentally determined to be 49.2 at. % C and 2645 (+OR-) 25(DEGREES)C, respectively, in agreement with published data;No evidence to support the postulated existence of an ordered V(,64)C phase was found from TEM, microscopic, or electron diffraction analyses of the matrix and precipitate phases;An attempt was made to calculate the contribution of carbon to the residual resistivity of vanadium. A value for (rho)' of 4.0 (+OR-) 1.9 x 10('-10) ohm-cm/at. ppm was obtained from the measurement of residual resistivity ratios (RRR) as a function of carbon concentration;Crystal structures data and thermodynamic properties of the reported phases of the V-C system available from the published literature are tabulated and evaluated.