Diffusion in fine-grained materials: Theoretical aspects and experimental possibilities

Abstract

Polycrystals with small grain size demonstrate a much larger variety of possible kinetic regimes than massive polycrystals commonly used in grain boundary ( GB) diffusion experiments. Based on the kinetic analysis, a complete classification of diffusion regimes in fine-grained materials is developed, depending of the grain size, diffusion temperature and time, the GB segregation level in the case of impurity diffusion and other parameters. It is established that, with respect to their diffusion behavior, polycrystalline materials should be subdivided into three classes depending on interrelation between the average grain size d and the critical diffusion lengths L′ = sδ( D b/4 D) 1 2 and L″= sδ(D b/2D), where s is the segregation factor, δ the boundary width, D b and D the coefficients of boundary and lattice diffusion. Each class demonstrates its own sequence of development of the diffusion regimes in time. For each of the diffusion regimes the shape of the diffusion profile measured in sectioning experiments, the diffusion parameters that can be determined experimentally, and the mechanism controlling the total mass absorption by the specimen are established. Due to the grain growth, occurring even at room temperature, the GBs in nanocrystalline materials often move in the course of the diffusion anneal. The GB motion is capable of having a drastic effect on the diffusion kinetics and the shape of diffusion profiles and can lead to essentially underestimated characteristics of boundary diffusion when the GB motion is not taken into account. In small-grained polycrystals this effect can be even more pronounced and, additionally, sensitively depends on the diffusion time.