Reactive diffusion and phase equilibria in the V-C, Nb-C, Ta-C AND Ta-N systems

The reactive formation of vanadium, niobium and tantalum carbides as well as tantalum nitrides was investigated by means of diffusion couples. For the preparation of the carbides the compact metals were reacted with graphite powder and the formation of the phases MeCl1-x and Me2C could be observed. The slow growth rate of the Me4C3 phases made it necessary to provide restricted diffusion geometry (wedge-type or plane-sheet samples) or, preferentially. diffusion couples of the type Me2C/graphite. The phases zeta-V4C3-x, zeta-Nb4C3-x and zeta-Ta4C3-x could be observed as distinct diffusion bands below 1900, 1575 and 2170 degrees C. respectively. The compositions of the phase boundaries as a function of temperature were measured by EPMA. The composition of zeta-V4C3-x is 37.9 at.%C (with a narrow homogeneity range of <0.5 at.%C), that of zeta-Nb4C3-x is 40.1-40.7 at.%C and that of zeta-Ta4C3-x is 38.2-39.0 at.%C (standard deviations cn. +/- 0.2 at.%C). For the beta-Me2C phases the carbon content was found to be greater than the values given in recent assessments, whereas the homogeneity regions measured for the delta-MeC1-x phases are consistent with established data, In the Ta-N system the epsilon-TaN and the delta-TaN1-x phases usually form very narrow phase bands. Here again wedge-type diffusion samples were applied in order to enlarge the thickness of the phase bands. delta-TaN1-x undergoes a eutectoid decomposition at 1821 +/- 19 degrees C into epsilon-TaN and beta-Ta2N. The formation of epsilon-TaN occurs at 1888 +/- 15 degrees C at 25 bar N-2, its homogeneity range at 1873 degrees C is smaller than 0.4 at.%N. Re-investigated portions of the phase diagrams of all four systems are given.