Nanosized lead inclusions in aluminum-magic sizes and aspect ratios

Aluminum-lead alloys containing dense distributions of nanosized lead precipitates have been made by rapid solidification and ion implantation. Embedded precipitates grow in parallel-cube alignment with the aluminum matrix; they have a cuboctahedral shape resulting from a minimization of the total interface energy, and atomically smooth {111} and {100} facets. The smallest precipitates, less than about 20 nm in size, adopt a series of magic sizes determined by periodic minima in the residual strain energy. In this size range the energy of the cuboctahedral edges becomes significant. This leads to changes in the aspect ratio of the precipitates so that the smaller precipitates are less truncated. Fitting the experimental data to an analytical expression gives a value of 1.15 for the ratio between the {100} and {111} interface energies and a value of 2.06.a(Pb) for the ratio of energies between the <110> edges and {111} interfaces.