Observation of an elementary cuboctahedron of Xe nanocrystal in an Al matrix

摘要

When a noble gas element such as Xe is implanted in an fcc metal matrix such as211u001eAl at room temperature, a fine dispersion of precipitates forms. The precipitates 211u001eare elementary fcc crystals up to diameters of several nanometers (for Xe in Al, 211u001e8-10 rim), above which they are non-crystalline. The precipitates exhibit a cube-211u001eon-cube orientation relation with the matrices and have lattice parameters which 211u001eare much larger than those of the matrices (a(sub Xe) = 1.5a(sub Al)). Thus the 211u001einterphase interfaces are incommensurate though the lattices are isotactic. The 211u001eprecipitates assume the shape of matrix cavities; for an Al matrix, at 211u001eequilibrium this is a cuboctahedron, a (r-brace)111(l-brace) octahedron truncated 211u001eat the corners on (l-brace)100(r-brace). Fig. 1 is a sketch of a dispersion of 211u001esuch cuboctahedra, viewed approximately along a (l-angle)110(r-angle). For this 211u001estudy specimens were prepared in the HVEM-Tandem Facility at Argonne National 211u001eLaboratory by implanting 35 keV Xe to a dose of 4x10(sup 19) m(sup (minus)2) into 211u001ewell-annealed 5N Al discs which had been thinned by jet electropolishing. The 211u001erange of the implant is approximately 25 nm. Specimens were examined at high 211u001eresolution in the JEOL ARM-1000 high voltage electron microscope (HVEM) at the 211u001eHigh Resolution Beam Station of the National Research Institute for Metals 211u001e(NRIM), Tsukuba, Japan. The HVEM was operated at 1 MeV with a LaB(sub 6) electron 211u001esource. A series of studies of electron irradiation effects in this material have 211u001ebeen conducted, which have revealed a number of irradiation-induced phenomena 211u001eincluding migration within the matrix, changes in shape, faulting, melting, 211u001ecrystallization and coalescence of Xe precipitates. In this presentation, the 211u001estructure of the smallest possible cuboctahedral Xe nanocrystal will be discussed 211u001eand its apparently random migration under the influence of the electron 211u001eirradiation will be demonstrated.