Atomistic simulation of crack cleavage and blunting in bcc-Fe

The behavior of crack propagation at different strain rate and temperature in bcc-Fe has been studied by molecular dynamic simulation method with fixed-displacement boundary condition. The results show that at low temperature, the cleavage fracture and the twin (or stacking fault) formation are cooperative processes in brittle fracture, and the shear process induced by the twin formation is favorable for bond breakage at the crack tip. At higher temperature, the twinning becomes weakened, and vanishes at the brittle-to-ductile transition temperature accompanied dislocation nucleation, which is perpendicular to the crack surface. The crack tip is blunted by the plastic deformation due to dislocation nucleation and emission. It can be concluded that the formation of stacking fault and twin at crack tip is particularly important for brittle cleavage. The shear process and the plastic deformation can be taken as, respectively, a feature of brittle fracture and a feature of ductile fracture in bcc-Fe.