High-Resolution-Electron-Microscopy Investigation of Nanosize Inclusions
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also to their shape and interface structure. In this article, we focus on the characterization of the equilibrium shape for small Pb inclusions in Al and their dependence on size and location (bulk or grain boundary) in the Al matrix. The work described here is part of an ongoing investigation of Pb and Pb-alloy inclusions in Al, and further details appear in the original references.7"12 Using highresolution electron microscopy, we find that, unlike the equilibrium shape of free particles, the shape of inclusions changes with size, and surprisingly, certain "magic size" dimensions are preferred over others. This behavior is a result of an oscillatory residual strain energy. Similarly the more complex composite equilibrium shape of grain boundary inclusions changes with boundary orientation and can be understood using a modification of the Wulff construction.1314
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Figure 1. High-resolution micrographs of "magic-size" nanoscale Pb inclusions with different sizes and shapes embedded in an Al matrix. Moire fringes arise from interference between overlapping Pb and Al lattices. The inclusion in (a) has two symmetrical {100} truncations, while that in (b) is truncated asymmetrically and that in (c) has two equal truncations but has different extent of its {111} facets. The inclusion in (d) is highly symmetrical but has no {100} truncations at all. (These are projections of three-dimensional shapes, and not all facets are visible.)
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High-Resolution-Electron-Microscopy Investigation of Nanosize Inclusions
Background Al-Pb is a good model alloy, and a significant amount of work has already been reported on Pb inclusions in bulk Al.1521 Both components are face-centeredcubic, leading to a particularly simple crystallography. Pb and Al have negligible mutual solid solubility and do not react, resulting in especially simple thermodynamic conditions. Pb has a low melting point, and shape equilibrium can readily be obtained by melting and resolidifying the inclusions. This is unusual for solid inclusions because in most cases—such as age-hardening alloys— precipitates either grow, shrink, or coarsen, and their shapes are usually kinetically determined. Pb inclusions in Al thus offer a rare opportunity to study inclusion shapes under equilibrium conditions and as a function of size and location. Experimental Nanosized Pb inclusions were produced by ion implantation of Pb into 1,000-A-thick Al films. The films were grown by vapor deposition on singlecrystal Si substrates at 280°C and subsequently implanted from the Al surface with 40-keV Pb ions to an average concentration of about 1% at temperatures between 150 and 250°C. The films were back-thinned from the Si side to electron transparency and examined by highresolution electron microscopy (Berkeley Atomic Resolution Microscope operating at 800 kV). To determine the optimum visibility conditions as a function of defocus, foil thickness, incl
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