The grain boundary diffusion coefficient is the diffusion coefficient (during atomic diffusion) of a diffusant along a grain boundary in a polycrystalline solid. At lower temperatures in metals and metal alloys, this diffusion term may dominate the effective diffusion rate. The grain boundary diffusion coefficient is the diffusion coefficient (during atomic diffusion) of a diffusant along a grain boundary in a polycrystalline solid. At lower temperatures in metals and metal alloys, this diffusion term may dominate the effective diffusion rate.