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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.
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.
As the selection of material for particular engineering properties becomes increasingly important in keeping costs down, methods for evaluating material properties also become more relevant. One such method examines the geometry of grain boundaries, which reveals much about the properties of the material. Studying material properties from their geometrical measurements, The Measurement of Grain Boundary Geometry provides a framework for a specialized application of electron microscopy for metals and alloys and, by extension, for ceramics, minerals, and semiconductors. The book presents an overview of the developments in the theory of grain boundary geometry and its practical applications in material engineering. It also covers the tunneling electron microscope (TEM), experimental aspects of data collection, data processing, and examples from actual investigations. Each step of the analysis process is clearly described, from data collection through processing, analysis, representation, and display to applications. The book also includes a glossary of terms. Exploring both the experimental and analytical aspects of the subject, this practical reference guide is essential for researchers and students involved in material properties, whether in physics, materials science, metallurgy, or physical chemistry.