Abstract

Molecular dynamics simulations were conducted to investigate the brittle-to-ductile transition in aluminum bicrystals containing edge cracks and grain boundaries (GBs) with varying tilt misorientation angles. This work presents the first atomistic-level study that systematically correlates GB tilt misorientation with a fracture mode transition, integrating mechanical indicators such as fracture toughness, crack-tip opening displacement (CTOD), and local stress triaxiality. The results reveal a sharp increase in fracture toughness for misorientation angles in the range of 20 degrees-25 degrees, associated with a transition from cleavage-dominated fracture to ductile tearing. Bicrystals with GB angles above this threshold exhibit toughness values up to four times higher than those with low-angle boundaries, accompanied by enhanced plastic activity and void formation. In contrast, GBs below 20 degrees show limited plasticity, no tearing, and fracture behavior similar to that of monocrystalline aluminum. This transition correlates strongly with a reduction in crack-tip stress triaxiality, with a critical threshold near 1.5 marking the onset of ductile fracture mechanisms. These findings provide a mechanistic link between GB misorientation, local stress state, and fracture response, offering a physically grounded descriptor for predicting fracture mode transitions and guiding grain boundary engineering in nanostructured aluminum.