Experiments were carried out on the brittle-ductile fracture transition of single crystals. This involved the arrest of cleavage cracks which were made to propagate, on (110) cleavage planes, up a temperature gradient. An activation energy of 1.82eV was determined for the transition process. This was based upon the dependence of the transition temperature upon an averaged crack velocity; as deduced from a jerky mode of crack advance. Dislocation patterns in the arrest zones were studied by means of etch-pitting and Berg-Barrett X-ray topographic imaging, following arrest. The observations indicated that the plasticity of the entire arrest process involved slip activity on a set of 2 symmetrically placed vertical slip planes, in which only one type of dislocation was involved. These planes did not suffer the highest resolved shear stresses, but had 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 which were observable via Nomarski interference contrast on the cleavage surface prior to arrest. A model for crack-tip plasticity was presented which was based upon the Riedel-Rice model of stress relaxation at the tips of cracks in creeping solids. This characterized well all of the non-linear aspects of the arrest process. The results were contrasted with the predictions of a brittle-ductile fracture transition model which was based upon defect-mediated melting, and were found to be inconsistent with that model.

Brittle-to-Ductile Transitions in the Fracture of Silicon Single Crystals by Dynamic Crack Arrest. B.J.Gally, A.S.Argon: Philosophical Magazine A, 2001, 81[3], 699-740