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. Fick's 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 epsilon -Fe2N1-x and gamma'-Fe4N phases are significantly different from those predicted by other models. The mobility and self-diffusion coefficient of nitrogen in epsilon -Fe2N1-x and gamma'-Fe4N, respectively, have therefore been re-evaluated. It is found that M-N((epsilon)) = 4.0 x 10(-14) exp(-66 400/RT) mol m(2) s(-1) J(-1) and that D-N((gamma')) = 2.63 x 10(-10) exp(- 83 500/RT) m(2) s(-1). (C) 2001 Elsevier Science B.V. All rights reserved.