Kinetic and molecular dynamics analysis of carbon diffusion in austenite

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A kinetic 10-frequency model for interstitial diffusion via octahedral interstices in the fcc lattice is developed. In this model, the specific role of the transition probabilities during association and dissociation of the first nearest neighbour interstitial pairs through the second nearest neighbour sites is considered. Application of the model is made to carbon diffusion in austenite. Molecular dynamics is used to investigate carbon interstitial diffusion in austenite at low carbon contents. The assumption that carbon atoms can interact with each other only indirectly (via neighbouring iron atoms) is used. The Arrhenius parameters of interstitial carbon jump frequencies consistent with the 10-frequency model are determined. Comparison of the molecular dynamics results with experimental data at 1273 K in the context of the 10-frequency model is performed. It is shown that a small direct repulsion between carbon atoms at first nearest neighbours should be included. It is found that the initial increase (with increasing carbon content) in both the tracer and the chemical diffusion coefficients is shown to be a result of increased rates of dissociation of carbon from first and second nearest neighbour pairs to third nearest neighbour sites.