Dislocation nucleation and multiplication at crack tips in silicon

The brittle-ductile transition (BDT) has been studied in silicon single cry stals of different orientations and purities. It is shown that the BDT temp erature at a given loading rate can be significantly varied depending on st ructural parameters, especially the density of cleavage defects (steps,...) . The critical BDT temperature is raised when the cleavage defect density i s lowered. This is explained by observations at crack tips which prove that dislocation nucleation is highly inhomogeneous. A first preliminary attemp t to identify nucleation sites by AFM is reported. In crystals containing h ighly perfect cleavage cracks, dislocation formation prior to fracture in m ode I loading could be suppressed. In such cases, a very small number of di slocations created on purpose from remote sources, sufficed to trigger the formation of a plastic zone as soon as they touched the crack front. Experi mental results are compared to theoretical models and recent numerical comp utations.