Multicomponent Interstitial Diffusion in and Thermodynamic Characteristics of the Interstitial Solid Solution ε-Fe3(N,C)1+x : Nitriding and Nitrocarburizing of Pure α-Iron

Abstract

A series of gas nitriding and gas nitrocarburizing experiments was performed at 823 K (550 °C) to investigate the growth kinetics of e-Fe3(N,C)1+x /γ′-Fe4N1-z -double layers on pure α-iron substrates. The growth rate and composition of the (sub)layers were determined by (sub)layer-thickness measurements using light optical microscopy and electron-probe microanalyses (EPMA), respectively. Models for the growth of bilayers into a substrate, controlled by the interstitial diffusion of two elements (N and C), were applied to the experimental data to determine the intrinsic diffusion coefficients of N and C in e-Fe3(N,C)1+x as well as the self-diffusion coefficient of N in γ′-Fe4N1−z . For e-Fe3(N,C)1+x , it was found that the four components of the diffusion matrix, \( D_{\text{NN}}^{\varepsilon } ,\,D_{\text{CC}}^{\varepsilon } ,\,D_{\text{NC}}^{\varepsilon } \) and \( D_{\text{CN}}^{\varepsilon } \), are all positive. The significant values of the off-diagonal diffusivities \( D_{\text{NC}}^{\varepsilon } \) and \( D_{\text{CN}}^{\varepsilon } \) indicate profound interaction of both interstitial species. Thereby, additional information is obtained about the thermodynamic properties of the e phase in the ternary Fe-N-C system.