Atomistic simulation of structure evolution at a crack tip in bcc-iron

The structure evolution of the crack tip in bcc iron at low temperatures is investigated by atomistic simulation. Several types of crack are selected in our study, while the crack plane and the crack front are different. Twinning and recrystallisation are observed at the crack tip during crack propagation, and new grain nucleation at the crack tip with the typical close-packed hexagonal (hcp) structure is observed. The calculation results indicate that the energy of the hcp phase is higher than that of the original bcc phase, thus the phase transformation can only occurs at a higher loading level. The stress concentration at the crack tip is relaxed by twinning and new grain nucleation, and the external work transforms into the increase of surface energy, the elastic strain energy owing to twin formation, the phase transition energy and the boundary energy.