Experiments have been conducted on the brittle-to-ductile transition of fracture in silicon single crystals through the arrest of cleavage cracks made to propagate on the (110) cleavage planes up a temperature gradient. An activation energy of 1.82 eV has been determined for the transition process based on the dependence of the T-BD on an averaged crack velocity, inferred from a jerky mode of crack advance. The dislocation patterns in the arrest zones have been studied in detail by a combination of etch pitting and Berg-Barrett X-ray topographic imaging after the arrest. These observations indicated that the plasticity of the entire arrest process is accomplished by slip activity on a set of two symmetrically placed vertical slip planes in which only one type of dislocation was involved. These planes do not have the highest resolved shear stresses but have the advantage of a very low energy barrier to the nucleation of dislocations from crack tip cleavage ledges. A close correspondence was noted between the spacing of dislocation sources along the crack tip and the density of cleavage ledges observable by Nomarski interference contrast on the cleavage surface prior to arrest. A homogenized model of crack tip plasticity is presented that is based on the Riedel-Rice model of stress relaxation at tips of cracks in creeping solids which serves to characterize well all nonlinear aspects of the arrest process. The results have also been contrasted with the predictions of a brittle-to-ductile fracture transition model based on defect mediated melting and were found to be uniformly inconsistent with that model. [References: 44]
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