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This report outlines some preliminary experiments on grain boundaries, grain boundary ledges, and the apparent emission of dislocations from grain boundary ledges, as part of an attempt to directly observe the emission of dislocations from grain boundary ledge sources in-situ by high-voltage transmission electron microscopy. Observations of grain boundary ledges and dislocation emission profiles in strained sheets of 304 stainless steel are described. Preliminary results indicate that the number of dislocation profiles per unit length of grain boundary are related to the engineering strain, as are the mean profile lengths. In addition, ledge density increases with increasing strain, and grain boundary structure changes with increasing strain.
The microstructures attendant to specific peak strains along the strain axis of the stress-strain diagram for type 304 stainless steel and nickel have been examined and compared by transmission electron microscopy from epsilon = 0.05% to 55% in the former and from epsilon = 0.05% to 35% in the latter. The onset of flow is characterized by the emission of dislocations from grain boundary ledge source which form emission profiles resembling dislocation pileups in the stainless steel, and a random distribution of dislocations with evidence for very short emission profiles near the grain boundaries in nickel. At the engineering yield point (0.2%) every grain in the stainless steel shows evidence for dislocation emission profiles, while in the nickel every grain contains some dislocations distributed within the grain interior.