Xe Precipitates in Aluminum
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Xe Precipitates in Aluminum Robert C. Birtcher†, Stephen E. Donnelly*, Ian Morrison*, Charles W. Allen† , Kazuo Furuya‡, Minghui Song‡, Kazutaka Mitsuishi‡ and Ulrich Dahmen§ † Materials Science Division, Argonne National Laboratory, Argonne IL 60439, USA * Institute for Materials, University of Salford, Greater Manchester M5 4WT,UK ‡National Institute for Materials Science, 3–13 Sakura, Tsukuba 305, Japan §National Center for Electron Microscopy, LBNL, Berkeley, CA 94720, USA ABSTRACT Real space, high-resolution transmission electron microscopy observations of Xe confined in nanometer size faceted cavities in Al yield information on both the inert gas and the matrix in which it is confined. At room temperature, Xe in such cavities can be liquid or an fcc solid. In larger cavities, Xe within can undergo melting and recrystallization. The Al surface energy can be deduced from the largest Xe nanocrystal at 300 K by setting the corresponding calculated Laplace pressure equal to the equilibrium pressure for melting of Xe, obtained from empirical bulk compression data. These surface energy values are 1.05 J m-2 for {111} facets and 1.10 J m-2 for {200} facets. Because of the weak interactions, these values correspond to the surface tensions for Al at 300 K. At room temperature, fluid Xe confined in small faceted cavities in aluminum has up to three ordered layers of Xe atoms at the Al interface. Conceptually in a three-dimensionally confined system of sufficiently small size, complete three-dimensional ordering of the fluid may occur. Molecular dynamics simulations have revealed that such ordering would result in fluid Xe confined to a small tetragonal volume solidifying as a body-centered cubic phase on compression.
This research was supported by the U. S. Department of Energy, Office of Science, BESMaterials Science, under Contract No. W-31-109-Eng-38.
INTRODUCTION The physics of precipitates in metals is complex. This complexity is reduced in the case of inert gases that are insoluble in metals and form nanometer size precipitates. Xe when injected into a crystalline matrix, such as Al, precipitates within faceted cavities[1]. Below a critical size, inert gas precipitates are observed by TEM to be solid at room temperature. Room temperature precipitates in which the inert gases Ar, Kr and Xe were under sufficiently high pressures to be in the solid phase have been identified by electron diffraction[1, 2] and lattice imaging[3]. The existence of such condensed phases of noble gases at ambient temperatures (the triple point of Xe is 161 K) is ascribed to the Laplace pressure associated with the size of the cavity in which the precipitate is located. The state of the inert gas provides a probe of its host. For example, a spherical precipitate at thermal equilibrium will be under a pressure given approximately by P = 2γ/R,
(1)
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where γ is the surface energy of the cavity and R is its radius[4]. For cavities with radii of the order of a few nanometres, equation 1 yields Gigapascal pressures in typical
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