Melting, growth, and faceting of lead precipitates in aluminum

Aluminum single crystals cut in the 〈111〉 direction were implanted with 2×{}10²⁰ m^-2 Pb⁺ ions at 75 or 150 keV. The implanted insoluble lead precipitated as epitaxially oriented crystallites in the aluminum matrix. The precipitates were studied by x-ray diffraction at Riso?, DESY, and Brookhaven National Laboratory, and showed large superheating as well as supercooling during repeated heating cycles. The as-implanted precipitates had a characteristic size of {~}140 {}, which grew to 210--260 {} during repeated heating cycles. A detailed annealing study shows that the growth rate of the precipitates changes discontinuously at the onset of melting. This shows that significant precipitate growth takes place by coalescence. The diffracted x-ray intensities showed characteristic truncation rods (streaks), indicating that the solid precipitates were octahedra limited by {111} planes and truncated at the corners by {100} facets.