Melting, growth, and faceting of lead precipitates in aluminium

Aluminum single crystals cut in the < 111 > direction were implanted with 2 x 10 exp 20 m exp --2 Pb exp + ions at 75 or 150 keV. The implanted insoluble Pb precipitated as epitaxially oriented crystallites in the Al matrix. The precipitates were studied by X-ray diffraction at Riso, DESY, and Brookhaven National Laboratory, and showed large superheating and supercooling during repeated heating cycles. The as-implanted precipitates had a characteristic size of approx 140 A, which grew to 210-260 A 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.