Phase equilibria and multiphase reaction diffusion in the Cr-C and Cr-N systems

The phase formation in the Cr-C and Cr-N systems was investigated using reaction diffusion couples. The carbides were prepared by reaction of chromium metal with graphite powder in the range 1143 to 1413 °C in argon atmosphere; the nitride samples by reaction of the metal with N₂ (≤31 bar) in the range 1155 to 1420 °C. While the carbide samples showed the three chromium carbide phases in form of dense diffusion layers between 1100 and 1400 °C, porosity occurred at temperatures above 1400 °C. The composition of the phase bands was measured by the means of electron probe microanalysis. For the Cr₂₃C₆ phase, a slightly higher C composition was found than given in the literature. In Cr-N diffusion couples both the δCrN_1−x and βCr₂N formed phase bands at T≥1150 °C. Because decomposition processes occurred upon cooling, quenching experiments were carried out in the range 1370 to 1420 °C at 31 bar N₂ to stabilize the phases. The EPMA investigations of the homogeneity ranges yielded a large increase of the homogeneity range for δCrN_1−x with increasing temperature. The nonmetal diffusion coefficients in all phases of both systems were calculated from layer growth and/or from concentration profiles. In δCrN_1−x the N diffusivity was found to be strongly dependent on the composition. The Vickers microhardnesses of the various phases were obtained by measuring the diffusion layers.