Nanoscale lead-tin inclusions in aluminium

Nanoscale lead-tin alloy inclusions have been made by sequential ion implantation of lead and tin in aluminium targets at 150 and 200°C. The alloy inclusions with sizes in the range of 2-20 nm form spontaneously during the ion implantation, independent of whether lead or tin is implanted first. Alloys with nominal compositions of Pb:Sn equal to 1:1 and 1:3, respectively, have inclusion microstructures consisting of segments of a lead-rich fcc phase and a tin-rich tetragonal phase attached to each other along internal interfaces that are often close to (111)_fcc. The overall morphology of the inclusions is cuboctahedral-like and most of the inclusions are bicrystalline. Some inclusions, however, have multicrystalline morphology where one or two slabs of lead are attached between two segments of tin or vice versa, resembling a lamellar eutectic structure of nanoscale dimensions. The lead-rich fcc phase grows in parallel cube alignment with the matrix while the orientation relationship of the tin-rich phase varies. Many inclusions have the {111}_Pb planes parallel to the {100}_sn planes and in this common plane both the 〈001〉_Sn and 〈011〉_Sn directions have been found to be parallel to 〈110〉_Al. Nanoprobe Energy Dispersive X-ray (EDX) analysis on the two-phase inclusions with sizes in the range of 10-20 nm shows that both phases are supersaturated, and their concentrations are considerably larger than given by the phase diagram, at around 100°C, where equilibrium can still be attained by diffusion. Inclusions less than about 5-10 nm in size nearly always display a single phase fcc structure with tin concentrations that can be as high as 50 at.%.