Molecular-dynamics studies of grain-boundary diffusion. II. Vacancy migration, diffusion mechanism, and kinetics

Vacancy jumps in a bicrystal model of $\Sigma =5\mathrm{}$ (36.9°) [001] tilt boundary in bcc Fe have been simulated at temperatures of 1300, 1400, and 1500 K by molecular dynamics with the use of the empirical Johnson potential. The results confirm the dominance of a vacancy mechanism in grain-boundary diffusion. An activation energy of 0.51 eV for vacancy migration has been obtained along with a reasonable value of the jump-attempt frequency. Analysis of the jump directions shows a preferential bias along the tilt axis. The relation between diffusivity and atomic mean-square displacement is examined. It is suggested that the structure dependence of grain-boundary diffusion may be expressed through a matrix of transition probability for vacancy jumps among the various discrete sites.