On diffusion kinetics in ordered binary alloys

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Abstract
It is shown that for self–diffusion by means of vacancies, in an ordered binary alloy with the B2 (CsCl) structure the diffusion coefficients can be decomposed into three relevant factors : the average correlation factor, the average vacancy–availability factor and the average atom–vacancy exchange rate. Within a model of only nearest–neighbour interaction the full procedure to obtain reasonably reliable estimates of all these factors is described. In this treatment Monte Carlo computer simulation serves as a criterion. The pair approximation of Kikuchi and Sato (1969, 1972} is found to give good results for vacancy availability and exchange frequency. An interpretation is given of the theory of Manning (1967, 1971) for the random alloy and it is demonstrated how a modification can be applied to calculate correlation factors for diffusion in ordered structures. The results always turn out to be better than the values obtained by Kikuchi and Sato's path–probability calculations. In a large majority of cases the agreement with the results of computer simulation is satisfactory. Only under the condition of extreme ratios of exchange frequencies of both species, do discrepancies occur. However, those situations are traceable.