An Atomic-Resolution Evaluation of the Mechanism by which Segregated Solute Influences Grain Boundary Structures

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9317680 Babcock This research program explores the ways in which grain boundary segregated elements affect the atomic structure of grain boundaries in metals and oxides. Characterization techniques include high resolution, phase-contrast transmission electron microscopy (TEM) imaging, high resolution Z-contrast scanning TEM imaging, high spatial resolution x-ray and electron energy loss spectroscopy, and high speed position-sensitive atom probe microscopy to determine: (a) the atomic-scale distribution of the segregated solute both parallel and perpendicular to the grain boundary, and (b) the influence of the segregated solute on the atomic packing units of the host grain boundary. These studies are conducted on bicrystalline samples of controlled grain boundary crystallography, controlled doping, and simple, but representative, geometry. Lead- doped nickel oxide and gold-doped iron have been chosen for the initial experiments. These two alloy systems are model materials whose behavior represent that of a broad range of compounds and alloys. The experimental results will be compared with contemporary models for grain boundary structure. The thrust of these analyses is to determine whether or not a fundamental structural mechanism exists by which segregated atoms influence grain boundary properties. %%% Such knowledge is fundamental to understanding the mechanisms by which grain boundary segregated solutes control a broad range of grain boundary and, therefore, bulk properties in engineering materials. ***