EVOLUTION OF LEAD PRECIPITATES IN ION-IMPLANTED ALUMINUM

Aluminium-lead alloys are characterized by extremely low miscibility in the solid phase across the entire equilibrium phase diagram. Ion implantation of lead into aluminium at room temperature therefore results in spontaneous phase separation and formation of nm-size lead precipitates growing in topotactical alignment (epitaxy in three dimensions) with the aluminium matrix with a cube/cube orientation relationship. Transmission electron microscopy (TEM), Rutherford backscattering spectrometry (RBS) and channeling techniques were used to analyze the formation and evolution of lead precipitates during implantation and subsequent annealing. Implantations at room temperature induce a microstructure characterized by very small (2-5 nm) precipitates embedded in a heavily damaged matrix. Only approximately 50% of the implanted lead atoms are retained in precipitates while the rest are in supersaturated solution or in small (< 1 nm) Pb clusters. Annealing of the samples results in an increase in the size of the inclusions. With RBS/channeling analysis we observe two different stages in this growth: (1) incorporation of the initially dispersed fraction of lead atoms and, (2) coalescence of precipitates. The larger crystallites were found to improve channeling in the lead. A similar change in the microstructure of the inclusions was found after implantation at elevated temperatures.