Density-functional theory calculations were made of C dissolution and diffusion in Fe, the latter being a typical example of interstitial diffusion. The Kohn-Sham equations were solved for periodic boundary conditions and within the projector augmented-wave formalism, using the generalized gradient approximation for electron exchange and correlation. With the solution enthalpy as an indication of cell-size convergence, a super-cell was found with 128 Fe atoms and one C atom was sufficient for describing dilute concentrations of C in body-centered cubic Fe. The solution enthalpy of C in an octahedral site in ferrite was predicted to be 0.74eV. That is, the dissolution of C in body-centered cubic ferromagnetic Fe was an endothermic process. Using the Fe128C1 periodic cell, it was found that the minimum-energy path of C diffusion from one octahedral site to another (via a tetrahedral site) has a barrier of 0.86eV, in excellent agreement with the experimental value of 0.87eV. This encouraging benchmark result prompted the investigation of C diffusion in austenite, whose electronic structure was less well-characterized experimentally. Cell-size convergence results showed that a super-cell with 32 Fe atoms and one C atom was sufficient. The calculated solution enthalpy was -0.17eV, which indicated that the dissolution of C in face-centered cubic Fe was exothermic; consistent with the known greater solubility of C in austenite compared to ferrite. The minimum-energy path showed that C moved linearly from an octahedral site to another, contrary to the common assumption of an off-plane diffusion path. The diffusion barrier was calculated to be 0.99eV. Since austenite was modelled as a ferromagnetic high-spin phase, the diffusion barrier obtained was not directly comparable to experiments in which austenite was usually paramagnetic. However, this prediction was relevant to C incorporation into Fe thin films, since ferromagnetic high-spin face-centered cubic Fe could be obtained by epitaxial growth of thin Fe films on a Cu substrate.

Carbon Dissolution and Diffusion in Ferrite and Austenite from First Principles. D.E.Jiang, E.A.Carter: Physical Review B, 2003, 67[21], 214103 (11pp)