NONLINEAR MECHANICAL RELAXATION ASSOCIATED WITH THE VISCOUS SLIDING OF GRAIN-BOUNDARIES IN ALUMINUM BICRYSTALS

The internal friction of high-purity aluminium bicrystals with a [110] symmetric tilt boundary having a misorientation angle of 129.5 degree s was measured with a forced-vibration method. A pronounced internal f riction peak (versus temperature) was observed around 200 degrees C wi th frequency of vibration about 1 Hz. The activation energy concerned was found to be 0.88 eV. Similar experiments were also performed with bicrystals having misorientation angle 60 degrees and the activation e nergy was found to be 0.92 eV. An amplitude effect was observed within the temperature range of the internal friction peak. This shows that the relaxation process associated with the grain boundary relaxation o f bicrystals is non-linear. It is considered that the basic process as sociated with the observed internal friction peak is viscous sliding o f the boundary, which drags the dislocation substructure in the vicini ty of the boundary. This dragging process controls the viscous sliding along the boundary, so that an internal friction peak can appear. How ever, by virtue of the interaction between the dislocation substructur e and the boundary of the bicrystal during the boundary sliding, the c ontrolling action is complicated and the relaxation process exhibits a non-linear behaviour.