A model of dislocation multiplication at a crack tip: influence on the brittle to ductile transition

It has been shown long ago experimentally that primary nucleation of dislocations in silicon takes place heterogeneously on defects along the crack tip. More recently, it has been observed that a source is easily activated at the intersection point of the crack front and of an attracted dislocation. The authors offer and discuss a source multiplication mechanism based on this stimulated emission process. One of the dislocations emitted at a primary source on the plane of maximum resolved shear stress cross-slips to a plane where it is attracted by the crack: the intersection event gives rise to a secondary source. Because shielding is very low at this point, there occurs an emission of a new bundle of dislocations. The process then starts again giving rise to an 'avalanche multiplication' of dislocations which strongly shield the crack. Soft/sharp brittle to ductile transitions (BDT) observed in semi-brittle materials result from such a high shielding rate coupled with a low/high threshold stress intensity factor for the activation of the primary sources. (C) 1999 Elsevier Science S.A. All rights reserved.