Enhancement of solute self-diffusion in fcc alloys

Using a new simulation approach to the calculation of correlation factors in very dilute solutions, the present paper reports on results of the first calculations of the first and second solute enhancement factors for solute self-diffusion in the fourteen-frequency model postulated by Bocquet for describing the diffusion kinetics of paired and unpaired solute atoms in dilute fcc alloys. Bocquet's assumption that correlation effects do not contribute to solute self-diffusion enhancement is shown to have a fairly limited range of validity. Specifically, it is shown that this assumption is reasonable when the solvent–vacancy exchange frequency far from the solute is high compared with the others or when all of the solute–vacancy exchange frequencies are small compared with the others. The assumption is also reasonable when the vacancy is not attracted to the solute atoms and when the vacancy does not often alternate between two solute atoms. The present study would strongly suggest that many of the ratios of exchange frequencies that have been determined from experimental data on the basis of this assumption are likely to be in error.