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 rea ction diffusion couples. The carbides were prepared by reaction of chromium metal with graphite powder in the range 1143 to 1413 degrees C in argon at mosphere; the nitride samples by reaction of the metal with N-2 (less than or equal to 31 bar) in the range 1155 to 1420 degrees C. While the carbide samples showed the three chromium carbide phases in form of dense diffusion layers between 1100 and 1400 degrees C, porosity occurred at temperatures above 1400 degrees C, The composition of the phase bands was measured by th e means of electron probe microanalysis. For the Cr23C6 phase, a slightly h igher C composition was found than given in the literature. In Cr-N diffusi on couples both the delta CrN1-x and beta Cr2N formed phase bands at T grea ter than or equal to 1150 degrees C, Because decomposition processes occurr ed upon cooling, quenching experiments were carried out in the range 1370 t o 1420 degrees C at 31 bar N-2 to stabilize the phases. The EPMA investigat ions of the homogeneity ranges yielded a large increase of the homogeneity range for delta CrN1-x with increasing temperature. The non-metal diffusion coefficients in all phases of both systems were calculated from layer grow th and/or from concentration profiles. In delta CrN1-x the N diffusivity wa s found to be strongly dependent on the composition. The Vickers microhardn esses of the various phases were obtained by measuring the diffusion layers .