It was suggested that the so-called ordered-Cu3Au rule could provide a first approximation to the understanding of diffusion phenomena in intermetallic compounds. That is, it permitted estimates to be made of the poorly known diffusion characteristics of compounds, on the basis of the better-known characteristics of diffusion in pure elements. It also permitted predictions to be made concerning the order of phase formation in some diffusion couples. On the other hand, the rule was not universally valid. Examination of those cases where it was expected to apply, but failed to, was in itself instructive. It was concluded that phenomena such as nucleation played only a secondary role in determining which phase grew first.

F.M.D’Heurle, P.Gas, J.Philibert: Solid State Phenomena, 1995, 41, 93-102