Grain boundary diffusion: fundamentals to recent developments

Diffusion along grain boundaries (GBs) often controls the evolution of structure and properties of engineering materials at elevated temperatures. A knowledge of diffusion characteristics of GBs and deep fundamental understanding of this phenomenon are critical to many materials applications. The purpose of the present review is twofold. First, a comprehensive overview of boundary diffusion theory with emphasis on the interpretation of concentration profiles measured in diffusion experiments is given. The most important situations encountered in boundary diffusion experiments are considered, such as diffusion in the B and C regimes, diffusion in the presence of segregation, diffusion in multiple GBs, diffusion in moving GBs, etc. The discussion of such situations is accompanied by easy to use recipes for the determination of boundary diffusion characteristics from the concentration profiles. Second, the most recent advances in the basic understanding of GB diffusion are reviewed. The focus is placed on diffusion-segregation effects, diffusion in polycrystals with moving GBs, diffusion in oriented bicrystals, and other areas of increased research activity. Recent progress in the atomistic interpretation of GB diffusion is also discussed. It is demonstrated that many problems in the area can only be solved by the combination of new theoretical models with precise measurements using novel experimental techniques; and that diffusion measurements can be used as a tool to study the structure and physical properties of GBs.