Fifty-year study of grain-boundary relaxation

The present report attempts to make a historical review of the progress of the study of grain-boundary relaxation since 1947 to the present time. The outcomes of scientific experiments are gathered mainly from the measurements of internal friction and the accompanying anelastic effects in polycrystalline and bicrystal metals. Emphasis is placed on the information provided about the structure of the grain boundary, especially at elevated temperatures. The study was started with the confirmation of the viscous behavior of grain boundary in polycrystalline specimens, and a macroscopic viscous sliding model of grain-boundary relaxation was suggested. Analysis on the data concerning the activation energy associated with grain-boundary relaxation pointed out that the grain-boundary relaxation is correlated to an atomic diffusion process. Thus, the models of periodic grain-boundary structure consisting of "good" and "bad" regions were suggested.