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 disloc ations in silicon takes place heterogeneously on defects along the crack ti p. 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 t his stimulated emission process. One of the dislocations emitted at a prima ry source on the plane of maximum resolved shear stress cross-slips to a pl ane where it is attracted by the crack: the intersection event gives rise t o a secondary source. Because shielding is very low at this point, there oc curs an emission of a new bundle of dislocations. The process then starts a gain giving rise to an 'avalanche multiplication' of dislocations which str ongly shield the crack. Soft/sharp brittle to ductile transitions (BDT) obs erved in semi-brittle materials result from such a high shielding rate coup led with a low/high threshold stress intensity factor for the activation of the primary sources. (C) 1999 Elsevier Science S.A. All rights reserved.