Low amplitude cyclic deformation and crack nucleation in copper and copper-aluminum bicrystals

Abstract

This investigation was undertaken to study the compatability of deformation across the grain boundary in bicrystals subjected to cyclic deformation. Copper and α-copper-aluminum alloy birystals possessing a longitudinal planar grain boundary were grown from the melt, and cycled axially between reversed strain limits in a closed-loop testing machine. Hysteresis behavior was recorded in conjunction with optical microscopic studies of slip striations. The study shows that the active slip systems are determined by the effects of elastic and plastic anisotropy and by compatibility requirements instead of by the resolved external stress. Elastic anisotropy is as important as plastic incompatibility in controlling slip and crack formation in the vicinity of the grain boundary, although incompatibility effects did not influence the cyclic hardening behavior as much as alloy content did. Cracks formed within the boundary-affected regions of incompatible bierystals but throughout the bands of primary slip in compatible specimens. Cracks in the grain boundary appear as a result of propagation between slip band cracks in the adjacent crystals.