It is now widely accepted that the VHCF life of metals is mainly determined by processes at the atomic-, meso-, and micro-scales, i.e., localized and irreversible dislocation motion, crack nucleation, and the overcoming of microstructural barriers by the embryonic crack. Recent ultrasonic fatigue experiments on low carbon steels with systematically varied carbon contents showed the localization of plastic deformation in favourably oriented ferrite grains prior to the nucleation of cracks at ferrite-ferrite grain boundaries (GBs) or ferrite-pearlite phase boundaries.
To understand the atomic-scale details of nucleation and propagation of cracks at GBs as well as their interaction with microstructural defects, we performed large-scale 3D atomistic simulations using recently developed Embedded Atom Method potentials for various bcc metals. The influence of absorbed dislocations or vacancies as well as dislocation pile-ups was studied in a bicrystal setup. Crack propagation along GBs was systematically studied by varying grain misorientation, GB planes and symmetry, as well as the crack propagation direction. The simulation results highlight the importance of locally varying bonding situations as the crack is trapped at individual atomic bonds leading to a strong variation of the fracture resistance within the same GB. Counter to standard thermodynamic reasoning, this allows the GB to withstand higher applied stress intensity factors as compared to correspondingly oriented single crystals. The asymmetrical orientation relationship between crack plane and possible slip planes further leads to a strong direction dependence of the fracture behaviour. The first 3D simulations of penny-shaped grain boundary cracks showed a higher propensity for ductile crack tip blunting than for cracks with straight crack fronts, which could be related to an increase in available slip systems as well as dislocation cross slip. The simulation results are discussed in the context of recent fatigue experiments and meso-scale models for fatigue fracture.