A statistical calculation of the interstitial diffusion flux in a face-centered cubic lattice containing one dilute interstitial component and one substitutional component leads to a relation between the on-diagonal and off-diagonal diffusion coefficients,[Formula: see text]where the [Formula: see text]'s are pair interaction energies and N is the mole fraction. Alternatively, we may combine the thermodynamic relation obtained by neglecting thermodynamic forces due to off-diagonal chemical potential gradients,[Formula: see text]with a nearest-neighbor, regular solution calculation of the thermodynamic activities to obtain the same result.The latter thermodynamic relation is put to the experimental test for the ternary systems Fe–C–Si and Fe–C–Mn by substituting empirical thermodynamic data and comparing with values of D12/D11 obtained by means of a "transient equilibrium" diffusion experiment. In both systems the deviations lie within the experimental and analytical errors.
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Kirkaldy et al. (1962) studied this question.