Multicomponent Interstitial Diffusion in and Thermodynamic Characteristics of the Interstitial Solid Solution ε-Fe_3(N,C)_(1+x) Nitriding and Nitrocarburizing of Pure α-Iron

摘要

A series of gas nitriding and gas nitrocarburizing experiments was performed at 823 K (550 °C) to investigate the growth kinetics of ε-Fe_3(N,C)_(1+x)/γ'-Fe_4N_(1-z)-double layers on pure α-iron substrates. The growth rate and composition of the (sub)layers were determined by (sublayer-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 ε-Fe_3(N,C)_(1+x) as well as the self-diffusion coefficient of N in γ'-Fe_4N_(1-z). For ε-Fe_3(N,C)_(1+x), it was found that the four components of the diffusion matrix, D_(NN)~ε D_(CC)~ε, D_(NC)~ε and D_(CN)~ε, are all positive. The significant values of the off-diagonal diffusivities D_(NC)~ε and D_(CN)~ε indicate profound interaction of both interstitial species. Thereby, additional information is obtained about the thermodynamic properties of the ε phase in the ternary Fe-N-C system.