Theoretical study of atomic transport via interstitials in dilute Fe-P alloys

By combining density functional theory, empirical potential, and atomic transport model approaches, we investigate the energetics and the diffusion properties of P interstitials in dilute Fe-P alloys. Although P is a substitutional impurity in α-iron, when a self-interstitial atom (SIA) approaches a substitutional P, the P atom becomes interstitial with an energy gain of up to 1.0 eV. The octahedral and the ⟨110⟩ mixed dumbbell are the lowest-energy configurations with similar stabilities. The P atoms are highly mobile in both configurations. The transitions between these two configurations also require low activation energies. The most likely mechanisms leading to long-distance diffusion of a P interstitial are proposed by ab initio calculations. The resulting effective diffusion energy estimated by the transport model is 0.19 eV, which agrees with the result from resistivity recovery experiments, suggesting that the Fe-P mixed dumbbells are more mobile than the SIAs. The fast-migrating P interstitial can be deeply trapped by a substitutional P atom. The resulting complexes are very stable with a binding energy of around 1.0 eV. Their mobilities are investigated by means of the dimer method using an Fe-P empirical potential. A comparison between the present predictions and existing experimental results is also discussed.