When a noble gas element such as Xe is implanted in an fee metal matrix such as Al at room temperature, a fine dispersion of precipitates forms. The precipitates are elementary fee crystals up to diameters of several nanometers (for Xe in Al, 8-10 nm), above which they are non-crystalline. The precipitates exhibit a cube-on-cube orientation relation with the matrices and have lattice parameters which are much larger than those of the matrices (axe ≃ 1-5aAl). Thus the interphase interfaces are incommemsurate though the lattices are isotactic. The precipitates assume the shape of matrix cavities; for an Al matrix, at equilibrium this is a cuboctahedron, a {111} octahedron truncated at the corners on {100}. Fig. 1 is a sketch of a dispersion of such cuboctahedra, viewed approximately along a <110>.

For this study specimens were prepared in the HVEM-Tandem Facility at Argonne National Laboratory by implanting 35 keV Xe to a dose of 4x1019 m−2 into well-annealed 5N Al discs which had been thinned by jet electropolishing.