In situ transmission electron microscopy observations of alloying of nanoscale Pb inclusions by implantation with Cd ions

Dense distributions of nanosize inclusions of lead and cadmium can be made in aluminum by ion implantion. Both elements are insoluble in aluminum but together they form a simple eutectic system. Sequential ion implantation leads to formation of alloy inclusions which are bicrystalline and made up of a part of a cuboctahedrally shaped lead crystal with larger {111} and smaller {100} facets, and a disk-shaped cadmium slab attached to one of the {111}(Pb) facets. The use of sequential ion implantation for in situ alloying offers the opportunity to observe directly the nucleation and growth processes occurring during implantation of the second element, which will spontaneously attach to the existing inclusions. In order to investigate these processes, Cd+ ions were implanted into aluminum samples already containing lead inclusions. Implantations below the lead/cadmium eutectic temperature of 521 K showed nucleation and growth of solid cadmium slabs on {111}(Pb/Al) interfaces, while implantations above the eutectic temperature induced gradual melting of the inclusions after the solubility of cadmium in lead was exceeded. Melting was seen initially to nucleate on {100}(Pb/Al) interface facets, followed either by continuous melting or by melting in abrupt steps within a fairly limited concentration range.