REACTION-DIFFUSION AND PHASE-EQUILIBRIA IN THE V-N SYSTEM

The formation of phase bands in in situ diffusion couples of the V-N s ystem was studied by the reaction of vanadium sheet with pure nitrogen within the temperature range 1100 degrees C to 1700 degrees C and the nitrogen pressure range 2 to 24 bar. Under these conditions, phase ba nds of beta-V2N and delta-VN1-x develop. The morphology of the beta-V2 N/alpha-V(N) interface depends on the saturation state of the alpha-V( N) core. If the nitrogen content in alpha-V(N) is high, the interface has a jagged appearance, whereas at low nitrogen contents of the alpha -V(N) phase, the interface is planar. Electron probe microanalysis (EP MA) was used to measure the diffusion profiles within the couples. The homogeneity regions of the nitride phases were established and the ph ase diagram accordingly corrected. From the growth rates of the phase bands, the mean composition-independent nitrogen diffusivities in beta -V2N and delta-VN1-x were derived. These diffusivities follow an Arrhe nius equation with activation energies of 2.92 (beta-V2N) and 2.93 eV (delta-VN1-x). By using delta-VN1-x as a starting material and a low n itrogen pressure during annealing, it could be shown that the directio n of nitrogen diffusion can be reversed, i.e., beta-V2N is formed on t he surface of the couple as a result of out-diffusion of nitrogen.