Atomistic simulation of stacking fault formation in bcc iron

We present large scale atomistic simulations of crack growth in iron under quasistatic loading in mode I. We show that long cracks display a brittle character of extension, while the growth of smaller cracks is accompanied by emission of partial dislocations from the crack tip and subsequent transformation of the stacking faults behind the dislocations to multilayer twins. The competing shear processes at a crack tip are characterized in terms of the relative sliding of up to four adjacent atomic planes emanating from the crack tip region. The results are in agreement with a global energy balance derived from perfect samples, and with experimental observations that twinning and fracture are cooperating processes under sufficiently large quasistatic loading at low temperatures.