Structure and morphology of nanosized lead inclusions in aluminum grain boundaries

Grain boundary lead inclusions formed by ion implantation of mazed bicrystal aluminum films have been investigated by transmission electron microscopy. The vapor-grown bicrystal films contained mainly 90 degrees [110] tilt boundaries with fixed misorientation but variable inclination, as well as some growth twins with 70.5 degrees [110] symmetrical tilt boundaries and a few small-angle boundaries. It was found that the shape, size and orientation of the inclusions in the grain boundaries depend on the orientation of the aluminum grain boundary plane. Inclusions at 90 degrees [110] tilt boundaries were invariably sharply faceted toward one aluminum grain and more rounded toward the other grain. The faceted side was a section of the cuboctahedral equilibrium shape of bulk lead inclusions in parallel topotaxy with the aluminum matrix. The rounded side, where the aluminum grain was rotated by 90 degrees with respect to the lead lattice, approximated a spherical cap. At specific low-energy segments of the grain boundary where a (100) plane in grain 1 meets an (011) plane in grain 2, only two of several possible shapes were observed. One of these was preferred in as-implanted samples while both types were found after melting and re-solidification of the lead inclusions. The observations are discussed in terms of a modified Wulff construction.