DISLOCATION LOOPS AT CRACK TIPS - NUCLEATION AND GROWTH - AN EXPERIMENTAL-STUDY IN SILICON

The nucleation of dislocation loops at crack tips and the development of the plastic zone were studied in single-crystal silicon samples precracked at room temperature and loaded at T greater-than-or-equal-to 900 K under well-controlled conditions (mode I loading, constant loading rate). Several crystallographic orientations with different cleavage planes and crack front orientations were investigated. In situ observations by synchrotron X-ray topography were supplemented by chemical etching after fracture. Special attention was paid to the early stages of plastic zone formation. Dislocation nucleation appeared to be very heterogeneous along the crack front and may be favoured at free surfaces and cleavage ledges. Activated slip systems and dislocation arrangements are discussed. It is shown that considerations based on the crack tip stress field do not suffice to account for the observed slip systems. The ledge crack mechanism of Zhou and Thomson has probably been observed but cannot be proved to be the main emission mechanism.