INTRINSIC DUCTILITY CRITERION FOR MATERIALS

This paper explores the intrinsic ductility criterion for materials on the basis of two simple models of dislocation emission from a crack in the blunting configuration. Recently, Zhou, Carlsson and Thomson (ZCT) reported a new criterion, based on lattice calculations, which, unlike previous descriptions of the criterion, was independent of the intrinsic surface energy. Because both cleavage and emission events produce surface energy, the surface energy cancels out in the ductility criterion, which involves a ratio of the two separate criteria for emission and cleavage. In order to gain confidence in this prediction, we explore here continuum elastic descriptions of the emission process, which are not expected to be sufficiently precise to yield quantitative information, but which one hopes can give insight into the physical basis for, and limits to, the new criterion. The models confirm the predictions of ZCT, but suggest that at lower values of the intrinsic surface energy than explored by ZCT, there exists a regime where the new criterion breaks down, and the ductility criterion reverts to the older predictions.