Application of the electron channeling contrast imaging technique to the study of dislocations associated with cracks in bulk specimens

The application of the electron channeling contrast imaging (ECCI) technique to the study of dislocation behavior in the region of crack tips in high-purity stoichiometric single crystal NiAl is presented. Because the ECCI technique, which is based on the dependence of the intensity of backscattered electrons on the incident beam orientation with respect to the crystal lattice and defects near the specimen surface, allows near-surface defects to be imaged in bulk specimens, it has specific advantages over transmission electron microscopy approaches. The technique eliminates the tedious task of thin foil preparation and avoids many of the constraints imposed by the thin foils. Dislocations associated with crack propagation were observed by examining bulk 4-point bend specimens deformed in situ in a field emission gun scanning electron microscope. Dislocations were directly imaged in the crack tip region as well as along the crack paths. Crack deflection and blunting were observed to occur primarily by plastic zone formation in the immediate region of the crack tip.