Molecular dynamics study of self-diffusion in bcc Fe

A semiempirical interatomic potential for Fe was used to calculate the diffusivity in bcc Fe assuming the vacancy and interstitial mechanisms of self-diffusion. Point-defect concentrations and diffusivities were obtained directly from molecular dynamics (MD) simulations. It was found that self-diffusion in bcc Fe is controlled by the vacancy mechanism at all temperatures. This result is due to the fact that the equilibrium vacancy concentration is always much larger than the equilibrium interstitial concentration. The predominance of the equilibrium vacancy concentration over the interstitial concentration is explained by the lower vacancy-formation energy at low temperatures and high vacancy-formation entropy at high temperatures. The calculated diffusivity is in good agreement with experimental data. The MD simulations were also used to test the quasiharmonic (QH) approximation for point-defect calculations. It was found that the QH approximation can considerably underestimate variations in point-defect characteristics with temperature.