GRAIN-BOUNDARY RELAXATION OF HYDROGEN PRETREATED PURE IRON

Grain boundary related relaxation processes were studied by internal f riction (IF) measurements of annealed and cold worked iron of high pur ity, 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 compl ex IF spectra associated with the grain boundary relaxation. Hydrogen pretreatment influenced the individual components of GB peaks in a dif ferent way, being more pronounced in the case of annealed than cold wo rked iron. Analysis of hydrogen induced modification of IF spectra sup ported the idea of two relaxation modes in the main process of grain b oundary relaxation: a high temperature mode associated with GB sliding , and a low temperature mode associated with motion of near grain boun dary dislocations. Hydrogen pretreatment of pure iron even to the cont ent lower than the critical concentration for void formation produced an irreversible change of the grain boundary structure, which can be d etected by methods of mechanical spectroscopy. This provides the possi bility fur application of mechanical spectroscopy to study the initiat ion and propagation of hydrogen damage.