Migration kinetics of the self-interstitial atom and its clusters in bce Fe

Self-interstitial atoms (SIAs) and SIA clusters are produced in displacemen t cascades during irradiation of a material with high-energy particles. The migration kinetics of such defects are a critical factor in controlling mi crostructure evolution and the ensuing changes in mechanical properties. In this study, extensive molecular dynamics (MD) simulations were performed o n the diffusion of the SIA and its clusters in bcc Fe. Diffusivities were c alculated for various SIA cluster sizes. It was found that, although the di ffusivity itself decreases as the SIA cluster size increases, their activat ion energy for migration is very small and does not increase with size, in contrast with previous assumptions. Based on previous results obtained by W irth et al. and the current calculations, we study the mechanism of single SIA diffusion by a kinetic Monte Carlo technique. The resulting model is co nsistent with experiments. An important conclusion of this study is that th e 'effective' migration energy of the single SIA (0.17 eV in the present MD study) is smaller than the activation energy for stage I-E recovery. The p roposed model explains all the details of the low temperature recovery stag es, I-D and I-E, Of bcc Fe without the need to invoke the existence of two independent interstitial configurations.