Effective diffusion coefficient

The effective diffusion coefficient of a diffusant (atoms of a material which are diffusing in another material) in atomic diffusion of solid polycrystalline materials like metal alloys is often represented as a weighted average of the grain boundary diffusion coefficient and the lattice diffusion coefficient. Diffusion along both the grain boundary and in the lattice may be modeled with an Arrhenius equation.

Source: Wikipedia — Effective diffusion coefficient (CC BY-SA 4.0)