On the spacing between dislocation nucleation sources at crack tips

Dislocation nucleation from crack tips in single crystalline silicon is studied. A high temperature holding experiment is carried out. The pre-cracked, four point bending specimens are loaded at high temperature, and held for 20 h. The specimens are then cooled down to room temperature and subsequently fractured. The fracture surfaces are etched to reveal dislocations near the crack front. It is found that dislocations can only nucleated from discrete dislocation nucleation sources along the crack front. There exists a characteristic spacing of the order of 1 μm between these dislocation nucleation sources. The mechanisms that give rise to this characteristic spacing are investigated. It appears that, although the existence of crack front cleavage ledges is the necessary condition for dislocation nucleation, the interaction between emitted dislocations and the crack front is responsible for the characteristic spacing between nucleation sources. The basic mechanism is that the emitted dislocations will shield the neighboring region along the crack front to prevent potential nucleation sources from being activated. By analyzing simple dislocation arrangements at the crack front, it is found that, in order to shield the sources on both sides, dislocation dipoles rather than individual dislocations are needed. The predicted spacing by the analysis agrees with the experimental observations.