A single crystal of silicon approaching 20nm x 20nm x 10nm in size was modelled using up to 200000 atoms on the basis of the ReaxFF first-principles based reactive force field. It was applied here to the examination of the influence of crack-tip heterogeneities upon the nucleation of dislocations from crack tips as the temperature was increased well beyond 1000K. The study aimed to obtain some insight into the mechanism of the brittle-to-ductile transition observed in silicon; which was known to change the behavior of crystals from predominantly brittle at low temperatures to ductile at higher temperatures and was likely to involve a competition between brittle bond-breaking and the emission of dislocations. Firstly, the crystallographic orientations most known to exhibit fracture, e.g. the {111} and {110} planes, were examined. It was found that, even at the very high temperatures considered here (circa 1500K), the material behaved in a completely brittle manner for systems having cracks oriented in the {111} and {110} planes. However, when the crack plane was changed to {100}, the formation of complete dislocation loops was observed. The results reported here were the first direct observations of dislocation loops in silicon, driven by an increase in the temperature, and extended earlier studies that had focussed only upon extremely thin quasi two-dimensional models. It was demonstrated here that the formation of such loops could be linked to geometrical ledges at crack tis with favourable orientations for slip on the {111} planes. The detailed mechanisms leading to the nucleation of dislocations involved changes at the crack tip, with bond rotations and the formation of ledges, and it was found that the emergence of dislocations caused the crack to arrest. However, re-initiation took place and no conclusive nucleation of dislocations and complete arrest of the crack was observed. The thickness effect was considered, together with other sources of ledge formation along the crack front. These findings provided important insights into the development of models for the brittle-ductile transition in silicon and perhaps other materials, and emphasized the significance of thickness effects in simulations of fracture in crystalline slabs.

Atomistic Study of the Effect of Crack Tip Ledges on the Nucleation of Dislocations in Silicon Single Crystals at Elevated Temperature. C.Thaulow, D.Sen, M.J.Buehler: Materials Science and Engineering A, 2011, 528[13-14], 4357-64