Dislocation structures ahead of advancing cracks

Previous transmission electron microscope (TEM) observations of the dislocation structure in the vicinity of a crack suggested that the region immediately ahead of a crack is devoid of dislocations. In the present paper, the results ofin situ TEM deformation experiments in numerous systems are described. The dislocation configurations are generally complex, with dislocations extending from the crack tip(i.e., no dislocation-free zone (DFZ)) and forming complex arrangements in the plastic region in front of the crack tip. Crack advance was accompanied by the emission of dislocations from both the crack tip and nearby sources. These observations are summarized, and the theory of dislocation configurations in front of a crack is reconsidered. This paper is based on a presentation made in the symposium “Interface Science and Engineering” presented during the 1988 World Materials Congress and the TMS Fall Meeting, Chicago, IL, September 26–29, 1988, under the auspices of the ASM-MSD Surfaces and Interfaces Committee and the TMS Electronic Device Materials Committee.