MELTING, GROWTH, AND FACETING OF LEAD PRECIPITATES IN ALUMINUM

Aluminum single crystals cut in the <111> direction were implanted with 2 x 10(20) 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 approximately 140 angstrom, which grew to 210-260 angstrom 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.