On intrinsic brittleness and ductility of intergranular fracture along symmetrical tilt grain boundaries in copper

The intrinsic brittleness and ductility of intergranular fracture along a number of symmetrical [1 1 0] tilt grain boundaries (GBs) in Cu are investigated via combined atomistic and continuum studies of dislocation nucleation from an atomically sharp crack tip. In all cases investigated, the classical model of Rice predicts a directional anisotropy in that, along a given GB, brittle cleavage is favored for crack propagation in one direction while dislocation emission from the crack tip is preferred in the opposite direction. This prediction is validated by atomistic simulations of crack propagation along coherent GBs, including Sigma 3(1 (1) over bar 1) and Sigma 11 (1 (1) over bar 3). However, for incoherent GBs such as Sigma 9(2 (2) over bar 1), Sigma 9(1 (1) over bar 4) and Sigma 11(3 (3) over bar 2), such directional anisotropy in intrinsic ductility is not observed; instead, we show that dislocation emission is favored in both crack propagation directions. The reason for this discrepancy is shown to be dislocation emission at a distance ahead of the crack tip along an incoherent GB, which violates the assumption in Rice's model that dislocation emission occurs directly at the crack tip. (C) 2010 Published by Elsevier Ltd. on behalf of Acta Materialia Inc.