Diffusion dynamics of defects in Fe and Fe-P systems

The dimer method with the Ackland EAM potential has been used to determine the diffusion mechanisms of isolated defects in the bulk of α-Fe. Three defect systems were studied, an isolated vacancy, a P-vacancy complex and a P interstitial defect. Using an event table consisting of the transitions found using the dimer method, the kinetic Monte Carlo method has been used to simulate the diffusion of these defects. Periodic boundary conditions were used to simulate Fe crystals with finite concentrations of P atoms between 0.006 at. % and 0.038 at. %. At lower temperatures of around 350 K, substitutional P atoms in Fe act as centers of attraction for vacancy defects, such that the defect moves as a P-vacancy complex for most of the time. However, as the temperature is increased, the phosphorus atom and the vacancy spend greater amounts of time dissociated. We found that P interstitial defects can also diffuse through the lattice. Diffusion constants have been calculated for these systems at various temperatures and P concentrations. These showed that an Fe-P dumbbell is the most mobile of these defect systems and a P-vacancy complex the least mobile. For the isolated vacancy and P interstitial defect systems, the diffusion constant was found to satisfy the Arrhenius relation; the P-vacancy complex, however, showed a deviation from this relation.