Grain-Boundary Slide-Hardening in Zinc Bicrystals

Zinc bicrystals containing high-angle tilt boundaries, and others containing twist boundaries were subjected to constant shear stresses parallel to the boundary at temperatures in the range 20 to 300 degrees C. At stresses both above and below that necessary to cause macroscopic slip in the constituent crystals, the grain-boundary sliding rate decreased with time. This "slide-hardening" at the grain-boundary was identified with the formation of asperities in the grain-boundary which were generated first near the intersection of boundary and free surface. Removal of the layer containing these asperities restored the boundary behaviour to that of a virgin bicrystal in certain cases. This leads to the conclusion that when slide-hardening occurs, the rate-limiting process for grain-boundary sliding is that associated with the deformation of the crystal regions which have been introduced into the original boundary plane by grain-boundary migration.