Invariant Interdiffusion Coefficients in the Two-Phase Zone

The above suggested scheme of interdiffusion in the two-phase zone can be formulated also in terms of invariant coefficients [37, 38, 52] (see also [53-57]). The explicit 2x2 matrix of interdiffusion coefficients in a ternary system depends on the solvent chosen, but the matrix eigenvalues D remain invariant at such transformation in the concentration space. These values are the elements of the diagonalized matrix (D(° = a Da ) and the diagonalizing matrix a gives linear combinations of concentrations 

Formally, one can put down the usual expressions for fluxes in the two-phase zone as well [10]  

But unlike the one-phase case, the coefficients are no longer independent. The point is that we may rewrite Equation 10.29 taking into account 

In the two-phase zone at quasiequiUbrium diffusion, the fluxes caimot depend on dpa/dx. Therefore, the coefficients in Equation 10.31 at dpa/dx are equal to zero. Hence we obtain the relations 

From the determinant of the matrix D being equal to zero one gets 

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