Influence of copper additions on the basal cleavage in beryllium single crystals at room temperature

Abstract

In order to study the influence of Cu-alloying on the susceptibility of Be to cleavage along the basal planes, a series of single phase alloys containing up to 5 at. pct Cu in Be were prepared and tested at room temperature by the DCB technique. Copper additions were found to influence strongly the plastic events induced by the shear stresses acting at the crack tip. The crack propagation energy increases with Cu content up to about 1.2 at. pct Cu and decreases with further additions. The maximum value attained at 1.2 at. pct Cu is about seven times that of pure Be. This remarkable initial increase in the crack propagation energy can be accounted for on the basis of the higher energy needed to activate basal and prism slip due to solid solution hardening. In alloys containing more than about 1.2 at. pct Cu, twinning is found to be the most prominent deformation mechanism. The propensity to twin is observed to increase with increasing Cu content, and it is assumed that this reflects a corresponding decrease in the twinning energy. Since twin boundaries are highly susceptible to cleavage cracking, a decrease in the crack propagation energy results which has the effect of increasing the tendency of Be to fracture in a brittle manner in the alloys with higher Cu contents.