Two sets of published experimental data on high-diffusivity paths were fitted by using an analytical solution of a continuum double-diffusivity model. The values of the parameters which were used were consistent with those obtained from independent diffusion measurements. Values of the lattice diffusivity were obtained from diffusion experiments on monocrystalline specimens. The grain-boundary or pipe diffusivities were not easy to calculate directly. Grain-boundary diffusivities in bicrystals represented only an approximation to the corresponding diffusivities in polycrystals. The values of the required diffusivities could be deduced from discrete models by fitting experimental data while assuming an ideal geometrical configuration for the distribution of high-diffusivity paths. A continuum model did not have any such constraints, but instead required the use of a probability factor and a volume fraction for treating the high-diffusivity paths. Two free parameters were used in the calculations, and diffusion experiments for the determination of these parameters were chosen so as to represent 2 different cases. One was grain-boundary diffusion with a finite thin-film source, and the other was pipe diffusion with a constant thick-film source.

On the Experimental Validation of the Double Diffusivity Model. D.A.Konstantinidis, I.E.Eleftheriadis, E.C.Aifantis: Scripta Materialia, 1998, 38[4], 573-80