DIFFUSION IN NANOSTRUCTURED METALS

Nanostructured materials are polycrystals with a crystal size of a few (typically 20 or less) nm and hence consist of about equal volume fractions of grain (interphase) boundary cores and crystals. Due to the large volume fraction of incoherent interfaces, the diffusional processes in these materials seem to differ from the ones in single or coarse grained polycrystals with the same chemical composition. The diffusion data available today for nanostructured metals suggest that self and solute diffusion is enhanced relative to lattice as well as grain boundary diffusion, provided the solute atoms are substitutionally dissolved in the host lattice. This enhancement seems to originate from the free volume in the boundary cores. The diffusivities of atoms that are interstitially dissolved in the lattice of the host metals (e.g. B in Fe, H in Pd) are comparable to the corresponding lattice diffusivities.