Intergranular fracture in bicrystals

Abstract

Fracture experiments, at elevated temperatures, were carried out on bicrystals of copper containing silica particles. Fracture in each instance was intergranular. In all cases except one, fracture was caused by cavitation. The cavities grew either by diffusional transport or by plastic flow. In the rate equation for time to fracture, diffusional growth led to a linear stress dependence and an activation energy equal to that for boundary self-diffusion. Growth by plastic flow was manifested by a non-linear stress dependence and a higher activation energy. In one instance, when fracture occurred in only a few seconds, grain boundary separation was caused by the incompatibility of matrix slip at the grain boundary. Data in the literature indicates that the diffusional mechanism for the growth of cavities is suppressed in polycrystals; an explanation for this anamolous behavior is given. It is postulated that diffusional growth of cavities is possible in polycrystals in a hydrostatic state of stress.