Relating tracer and chemical diffusion coefficients in intermetallic compounds taking the DO3 and Al5 structures

In this article we employ computer simulation to explore the validity of the Darken/Manning relation between the chemical diffusion coefficient and the tracer diffusion coefficients of the components in stoichiometric intermetallic compounds A(3)B taking the DO3 and A15 structures at vanishingly small vacancy contents. The analysis centres on the validity of Manning's random alloy expression for the vacancy wind factor. The models for both the DO3 and A15 structures use eight atom-vacancy exchange frequencies. For the DO3 structure it is found that the actual vacancy wind factor is usually somewhat larger than that predicted by Manning but overall the agreement is good. At worst the use of Manning's expression would underestimate this factor by about 30 or 40%. For the A15 structure a similar result is found except when diffusion along the chains in the structure is rapid. Then Manning's expression fails badly when a constant geometrical tracer correlation factor is employed. Tn both the structures if the geometrical correlation factor is varied to reflect the structure actually explored by the atoms (mainly the majority atoms A) the agreement is improved very dramatically. (C) 1999 Elsevier Science Ltd. All rights reserved.