Fracture toughness and bond trapping of grain boundary cracks

The fracture toughness of materials in which brittle cracks initiate and propagate along grain boundaries (GBs) depends not only on the energy of the GB, but also on its microscopic structure. Exactly how and to what degree brittle GB fracture is affected by the local atomic structure at the crack tip has not yet been studied in detail. Here, we use molecular static simulations to study the atomic-scale fracture behavior of six large-angle tilt GBs in tungsten bicrystals. The fracture toughness depends critically on the propagation direction and on the position of the crack tip within the structural units of the GB. Furthermore, the GB fracture toughness can be significantly larger than for single crystals in the same orientation. These results cannot be explained by the usual thermodynamic approach in continuum-scale fracture mechanics but can be understood by considering the effect of bond trapping of GB cracks.