Grain boundary related relaxation processes were studied by means of internal friction measurements of annealed and cold-worked high-purity material that had been cathodically charged with H to various degrees and then degassed. It was found that, although H pre-treatment did not affect the grain size and grain boundary chemistry of the material, it affected the complex internal friction spectra that were associated with grain boundary relaxation. The use of H pre-treatment affected the individual components of grain boundary peaks in various manners; being more pronounced in the case of annealed than cold-worked material. An analysis of the H-induced modification of internal friction spectra supported the concept of 2 relaxation modes in the overall process of grain boundary relaxation. These were a high-temperature mode that was associated with grain boundary sliding, and a low-temperature mode that was associated with the motion of near-boundary dislocations. The H pre-treatment of pure Fe, even to contents that were lower than the critical concentration for void formation, produced an irreversible change in the grain boundary structure. This could be detected by means of mechanical spectroscopy. This suggested the possibility of applying mechanical spectroscopy to the study of the initiation and propagation of H damage.

E.Lunarska, Y.Jagodzinski, J.Ososkov: Physica Status Solidi A, 1996, 156[2], 343-51