Grain boundary related relaxation processes were studied by internal friction (IF) measurements of annealed and cold worked iron of high purity, cathodically charged with hydrogen to different states, and then degassed. Although hydrogen pretreatment did not affect the grain size and grain boundary chemistry of the material, it did affect the complex IF spectra associated with the grain boundary relaxation. Hydrogen pretreatment influenced the individual components of GB peaks in a different way, being more pronounced in the case of annealed than cold worked iron. Analysis of hydrogen induced modification of IF spectra supported the idea of two relaxation modes in the main process of grain boundary relaxation: a high temperature mode associated with GB sliding, and a low temperature mode associated with motion of near grain boundary dislocations. Hydrogen pretreatment of pure iron even to the content lower than the critical concentration for void formation produced an irreversible change of the grain boundary structure, which can be detected by methods of mechanical spectroscopy. This provides the possibility for application of mechanical spectroscopy to study the initiation and propagation of hydrogen damage.
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