Low energy dislocation structures associated with cracks in ductile fracture

Abstract

Ductile fracture in metals and alloys is preceded by work hardening and necking. Microstructural processes, previously explored by in situ transmission electron microscopy, have been re-examined in the context of interactions between glide dislocations and crack tips. In this, the parameters which significantly influence experimental findings have been critically reviewed and theoretical aspects of ductile fracture have been examined including the principle of energy minimization. Consideration was given to the displacement of the stress singularity ahead of the crack tip on account of the plastic zone and the effects of thermal instabilities during crack growth. Crack propagation occurs in ductile materials mostly by void coalescence. This requires the nucleation of microcracks ahead of the crack tip as was observed experimentally in agreement with theoretical expectations. Microcrack initiation was seen to take place at dislocation cell walls which represent the most frequently formed low energy dislocation configurations in work-hardened metals. As a consequence, too, the formation of a plastic crack and its grorth in a virgin crystal are distinctly different from the nucleation and growth of a crack in a crystal with a well-defined cell structure that was formed through prior deformation. Dislocation multiplication is considered to take place by the supercritical bowing of the longest free link lengths present within the cell walls, as also observed experimentally. The microstructural changes that take place in the plastic zone ahead of the crack tip are found to be more important in fracture than the microscopic processes immediately at the tip itself.