Molecular-dynamics studies of grain-boundary diffusion. I. Structural properties and mobility of point defects

The structural relaxation of a high-angle grain boundary at elevated temperatures has been simulated by molecular dynamics with the use of a bicrystal model composed of 399 atoms. The system studied was a $\Sigma =5\mathrm{}$ (36.9°), [001] tilt boundary with interatomic interactions given by the empirical Johnson potential for $\alpha -\mathrm{F}\mathrm{e}$. In the presence of an extrinsic vacancy, the boundary structure was found to be stable up to temperatures of at least two-thirds the melting temperature. Vacancy jumps, confined preferentially to within the grain-boundary core, were observed. Also observed were the thermal activation of vacancy-interstitial pairs, and with increasing temperatures a variety of more complicated vacancy-jump sequences. The simulation data are relevant to the understanding of fast diffusion along grain boundaries, the kinetics of which is analyzed and discussed in the following paper.