Atomic-scale intergranular crack-tip plasticity in tilt grain boundaries acting as an effective dislocation source

The intergranular fracture toughness of plastic deformable crystalline materials is strongly controlled by the plastic work ahead of the intergranular crack tip. Therefore, in studies of intergranular fracture toughness, the grain boundaries (GBs) should be regarded as both a cleavage plane and dislocation source. Combining continuum analyses and atomic simulations, this study investigates the atomic-scale mechanism of intergranular crack tip plasticity in aluminum < 112 > tilt GBs as an effective dislocation source. To quantitatively predict the first plastic deformation near the intergranular crack tip, we first model the dislocation emission from the GBs ahead of the intergranular crack tip and analytically derive the critical stress intensity factor. If the predicted first plastic phenomenon is dislocation emission from the GBs, the resulting wedge disclination can shield the stress field near the crack. Dislocation emissions from the crack tip are accompanied by dislocation emissions from the GBs, despite the predicted difficulty of the latter. The lattice defect evolution nucleates a nanograin with a disclination at the triple-junction ahead of the crack tip, which can weaken the mechanical field near the crack tip. Consequently, when improving the intergranular fracture toughness of materials, the role of GBs as dislocation sources cannot be ignored. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.