First Report on Exact Analysis of Interdiffusion Fluxes in Multicomponent Diffusion with Composition Dependent Partial Molar Volumes

Diffusion couple has been the primary technique for quantitative assessment of diffusion properties of materials and, interdiffusion flux is a critical phenomenological quantity in its analysis. New equations are derived here for determination of interdiffusion fluxes from experimental concentration profiles of multicomponent diffusion couples with composition dependent partial molar volumes. Such equations are absent in the literature so far. The newly developed 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 multicomponent systems may lead to large errors; in the Ni-Cr-Al couples 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. The newly derived expressions for fluxes in number fixed and volume fixed frames of reference are independent of the reference coordinates used and they are also valid for multicomponent multiphase diffusion couples. In the diffusion literature, the interdiffusion analysis so far has been carried out with the assumption of constant partial molar volumes. However, the newly developed equations in the present study and the observed large effect of nonideality in molar volume on the flux analysis are expected to correct this trend.