  Vacancy diffusion and clustering processes in body-centered-cubic (bcc) Fe
are studied using the kinetic activation-relaxation technique (k-ART), an
off-lattice kinetic Monte Carlo method with on-the-fly catalog building
capabilities. For monovacancies and divacancies, k-ART recovers previously
published results while clustering in a 50-vacancy simulation box agrees with
experimental estimates. Applying k-ART to the study of clustering pathways for
systems containing from one to six vacancies, we find a rich set of diffusion
mechanisms. In particular, we show that the path followed to reach a
hexavacancy cluster influences greatly the associated mean-square displacement.
Aggregation in a 50-vacancy box also shows a notable dispersion in relaxation
time associated with effective barriers varying from 0.84 to 1.1 eV depending
on the exact pathway selected. We isolate the effects of long-range elastic
interactions between defects by comparing to simulations where those effects
are deliberately suppressed. This allows us to demonstrate that in bcc Fe,
suppressing long-range interactions mainly influences kinetics in the first 0.3
ms, slowing down quick energy release cascades seen more frequently in full
simulations, whereas long-term behavior and final state are not significantly
affected.
