A molecular-dynamics simulation of crack-tip extension: The brittle-to-ductile transition

A brittle-to-ductile transition via dislocation nucleation is directly observed by molecular-dynamics simulation of the propagation of a sharp crack in a single crystal under mode-I loading. The crack-tip system is modelled using an embedded-atom method (EAM)-type interatomic potential function for alpha -Fe (BCC) and a new border condition designed to permit spontaneous nucleation of dislocations and their migration away from the crack tip. The present model, with three crack-tip orientations, shows brittle cleavage at zero temperature; the observed critical stress intensity factor agrees with the Griffith prediction to within an accuracy of about 5%. As the temperature is raised, a ductile-to-brittle transition is observed in all three orientations, the transition temperature in each case being in the range between 200 and 300 K. A specific nucleation event occurring during ductile response is characterized by detailed analysis of the atomic trajectories using a method based on the disregistry of atomic coordinates; it is shown that the deformation mechanism associated with the onset of ductility is the activation of (111)(011) slip.