Numerical modeling of interstitial diffusion in binary systems. Application to iron nitriding

The purpose of the present work is to provide a numerical model for nitrogen diffusion in iron accounting for the concentration dependence of the intrinsic diffusivity of nitrogen. Fickʹs second equation is solved using a finite volume method for the typical example of nitriding of pure iron. The non-linearity of the problem is treated without any specific assumption concerning the interface position. The thermodynamic factor is determined by means of a regular solution model. The surface boundary conditions are not required to be constant. Comparison of the results of the present work with experimental data available in the literature reveals a number of discrepancies. In particular, the nitrogen depth-concentration profiles for the ε-Fe2N1−x and γ′-Fe4N phases are significantly different from those predicted by other models. The mobility and self-diffusion coefficient of nitrogen in ε-Fe2N1−x and γ′-Fe4N, respectively, have therefore been re-evaluated. It is found that MN(ε)=4.0×10−14 exp(−66 400/RT) mol m2 s−1 J−1 and that DN(γ′)*=2.63×10−10 exp(−83 500/RT) m2 s−1.