INTERNAL-FRICTION EFFECTS DUE TO GRAIN-BOUNDARY SLIDING IN LARGE-GRAINED AND SMALL-GRAINED ALUMINUM

The grain boundary peaks observed in the internal friction of bulk and thin-film aluminium are critically compared. It is concluded that the underlying relaxation mechanism in thin films is a pure grain boundar y sliding motion, while for bulk material, grain boundary migration is dominant. This is due to the grain size being several orders of magni tude larger in bulk Al as compared to thin films, which leads to diffe rent microstructures of the grain boundary. Additionally, in the large r samples a higher number of impurities is attached to the grain bound ary. Impurity dragging then results in a retardation of the grain boun dary migration. This effect explains the puzzling observation that int ernal friction experiments on bulk Al yield an activation energy of 1. 4 eV, while for thin films 0.6 eV is obtained.