New Analysis for Interdiffusion Fluxes Determined From Experimental Concentration Profiles in a Multicomponent Diffusion Couple with Composition Dependent Partial Molar Volumes

New equations are derived for determination of interdiffusion fluxes from experimental concentration profiles of multicomponent diffusion couples with composition dependent partial molar volumes of the diffusing components. The newly derived equations are applied to determine interdiffusion fluxes in experimental diffusion couples of FCC Ni-Cr-Al system. It is shown that for the case of varying partial molar volumes, the expressions discerned from the popular works of Sauer- Freise and Wagner are valid only for binary systems. Their use in determination of interdiffusion fluxes in ternary and higher order systems may lead to large errors; in the Ni-Cr-Al couple studied here, a mere 3% variation in deviation from ideality of molar volume is seen to give as high as -40% errors in interdiffusion fluxes, which may propagate to greater than 100% errors in interdiffusion coefficients determined from such fluxes. The newly derived expressions for fluxes in number fixed and volume fixed frames of reference are independent of the reference coordinate and they are also valid for multicomponent multiphase diffusion couples. In the absence of data on composition dependent molar volumes in ternary and higher order systems, the interdiffusion analysis so far has been carried out with assumption of constant molar volume. However, the newly developed equations in the present study and the observed large effect of molar volume on the flux analysis are expected to correct this trend.