It was noted that polycrystals with a small grain size exhibited a much wider variety of kinetic regimes than did the massive polycrystals which were usually used in grain-boundary diffusion experiments. On the basis of a kinetic analysis, a complete classification of diffusion regimes in fine-grained materials was developed. This depended upon the grain size, diffusion temperature and time, and the grain-boundary segregation level. It was established that, with respect to their diffusion behavior, polycrystalline materials could be sub-divided into 3 classes; depending upon the relationship between the average grain size and 2 critical diffusion lengths: L = s(Db/4D)½ and L = s(Db/2D), where s was the segregation factor,  was the boundary width, and Db and D were the coefficients of boundary and lattice diffusion, respectively. Each class exhibited its own individual sequence of development steps, of the diffusion regime, as a function of time. For each of the diffusion regimes, the shape of the diffusion profile as measured in sectioning experiments, the diffusion parameters which could be determined experimentally, and the mechanism which controlled the total mass absorption by the specimen were established. Due to grain growth, which occurred even at room temperature, the grain boundaries in nano-crystalline materials often moved during diffusion annealing. Such motion could have a marked effect upon the diffusion kinetics and the shapes of diffusion profiles, and could lead to underestimates of boundary diffusivity if boundary motion were not taken into account. This effect could be even more pronounced in small-grained polycrystals, and depended sensitively upon the diffusion time.

Y.Mishin, C.Herzig: Nanostructured Materials, 1995, 6[5-8], 859-62