Energetic and kinetic behaviors of small vacancy clusters near a symmetric Σ5(3 1 0)/[0 0 1] tilt grain boundary in bcc Fe

Using combined techniques of molecular dynamics, molecular statics and the temperature-accelerated dynamics, we investigate the energetic and kinetic behavior of small vacancy clusters near a symmetric tilt grain boundary (GB) Σ5(3 1 0)/[0 0 1] in bcc Fe. We find that, the formation energy of the vacancy cluster (containing 1-9 vacancies) is reduced as it approaches to the GB, which serves as an energetic driving force for the segregation of vacancy clusters. The vacancy clusters in the bulk and near the GB exhibit different configurations. As a typical example, the cluster with four vacancies (V4) undergoes three stages corresponding to the location of the cluster in the bulk, partially at the GB and completely at the GB, indicated by twice steplike decreases in the formation energy, as it moves from the bulk to the GB. In addition, through a dynamical monitor of its transition, we find that the GB enhances the diffusion of V4 by reducing its diffusion barrier near the GB. Meanwhile, there occurs configurational transition for V4 both as the cluster vibrates and moves towards the GB. The present results should help understand the healing processes of radiation-induced defects near the GB in Fe.