It was recalled that, in a numerical description of multi-component diffusion processes, coefficients of the Onsager diffusion matrix were required. A comparison was made of a number of models which related these parameters to experimental quantities such as tracer diffusion coefficients. Because the models involved various degrees of approximation, differences in the Onsager parameters which were deduced from tracer diffusion data were investigated. An ideal solution alloy was studied using Monte Carlo methods in order to determine the Onsager coefficients directly. By measuring the atomic fluxes and site fraction gradients, this simulation method was more closely related to a real physical experiment than were the usual simulation methods based upon generalized Einstein equations. Onsager's variation principle for the calculation of the kinetic coefficients was extended, and it was shown that a reasonable description - even of a simple alloy system - required the consideration of a non-diagonal dissipation matrix in the derivation of the diffusion equations.

Onsager's Coefficients and Diffusion Laws - a Monte Carlo Study. M.A.Hartmann, R.Weinkamer, P.Fratzl, J.Svoboda, F.D.Fischer: Philosophical Magazine, 2005, 85[12], 1243-60