Ductile vs brittle behavior of crystals

Aspects of the ductile vs brittle response of crystals are examined. Models are proposed for production of dislocations from crack tips in such ways that after expansion of the dislocations under external stress field, automatically sharp cracks are blunted by atomic planes. Various forces operating between cracks and dislocations are considered in two dimensions, and the activation energy is estimated for formation of dislocation half loops from cracks under the influence of these forces when energy barriers exist. An evaluation of experimental results is included. (JRD)