The diffusivity of C in the carbides was investigated at temperatures ranging from 1700 to 2200C (figure 3), and the diffusivity of N in the nitrides was investigated at temperatures ranging from 1700 to 1950C (figure 2). The concentration-independent diffusion coefficients were obtained in each case by investigating enhanced layer growth in wedge-shaped specimens. In the case of the non-metal rich phases with a broad homogeneity range (δ-TaC, β-Ta2C, β-Ta2N), the concentration-dependent diffusion coefficients were calculated and were compared with the concentration-independent diffusion coefficients. The calculation of the concentration-dependent diffusion coefficients was carried out by fitting a modified error function to the measured concentration profiles, while assuming that the non-metal diffusivity was an exponential function of the non-metal concentration. A marked dependence of the diffusion coefficient upon the non-metal concentration was found for δ-TaC, whereas the non-metal diffusion coefficients were almost independent of concentration in β-Ta2C and β-Ta2N.

D.Rafaja, W.Lengauer, H.Wiesenberger: Acta Materialia, 1998, 46[10], 3477-83