A semi-empirical interatomic potential for Fe was used to calculate the diffusivity in body-centered cubic Fe assuming the vacancy and interstitial mechanisms of self-diffusion. Point-defect concentrations and diffusivities were obtained directly from molecular dynamics simulations. It was found that self-diffusion in body-centered cubic Fe was controlled by the vacancy mechanism at all temperatures. The results could be described by:

vacancies: D(cm2/s) = 7.87 x 10-3exp[-0.60(eV)/kT]

interstitials: D(cm2/s) = 5.34 x 10-4exp[-0.15(eV)/kT]

This result was due to the fact that the equilibrium vacancy concentration was always much larger than the equilibrium interstitial concentration. The predominance of the equilibrium vacancy concentration over the interstitial concentration was explained by the lower vacancy-formation energy at low temperatures and high vacancy-formation entropy at high temperatures. The calculated diffusivity was in good agreement with experimental data. The molecular dynamics simulations were also used to test the quasi-harmonic approximation for point defect calculations. It was found that the quasi-harmonic approximation could considerably underestimate variations in point-defect characteristics with temperature.

Molecular Dynamics Study of Self-Diffusion in BCC Fe. Mendelev, M.I., Mishin, Y.: Physical Review B, 2009, 80[14], 144111