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 th e Darken/Manning relation between the chemical diffusion coefficient and th e tracer diffusion coefficients of the components in stoichiometric interme tallic compounds A(3)B taking the DO3 and A15 structures at vanishingly sma ll vacancy contents. The analysis centres on the validity of Manning's rand om alloy expression for the vacancy wind factor. The models for both the DO 3 and A15 structures use eight atom-vacancy exchange frequencies. For the D O3 structure it is found that the actual vacancy wind factor is usually som ewhat larger than that predicted by Manning but overall the agreement is go od. At worst the use of Manning's expression would underestimate this facto r by about 30 or 40%. For the A15 structure a similar result is found excep t when diffusion along the chains in the structure is rapid. Then Manning's expression fails badly when a constant geometrical tracer correlation fact or is employed. Tn both the structures if the geometrical correlation facto r is varied to reflect the structure actually explored by the atoms (mainly the majority atoms A) the agreement is improved very dramatically. (C) 199 9 Elsevier Science Ltd. All rights reserved.