Equilibrium Shapes of Pb Inclusions at 90° Tilt Grain Boundaries in Al

Abstract

Ion implantation of lead in aluminum leads to spontaneous phase separation and formation of dense distributions of nanosized lead inclusions[1]. The inclusions have fee structure, and despite the large lattice mismatch (aA1 = 0.4048 nm and aPb = 0.495 nm) they grow in parallel-cube topotaxy with the matrix. Their shape is cuboctahedral with larger {111} facets and smaller {100} facets which is the minimum- energy shape for an fee crystal in equilibrium with its vapor, as calculated by considering only nearest neighbor bonds. Implantation of polycrystalline aluminum films is accompanied by preferential nucle- ation and enhanced growth of inclusions in the grain boundaries. In adapting their equilibrium shape, grain boundary inclusions will be subject to a larger number of constraints than inclusions in the bulk matrix. This may result in a variety of morphologies characteristic for different types of grain boundaries.In the present study we have used a well-defined bicrystal geometry to study the morphology and structure of lead grain boundary inclusions in mazed bicrystal aluminum films containing mainly 90°<110> tilt boundaries with fixed misorientation but variable inclination[2]. It was found that the shape, size and orientation of the inclusions in the grain boundaries depend on the inclination, i.e. the orientation of the grain boundary plane. Inclusions were all single crystalline and invariably faceted toward one aluminum grain and more rounded toward the other grain (fig.l). Independent of grain boundary inclination, the faceted side was a section of the cuboctahedral equilibrium shape of inclusions in parallel topotaxy with the bulk aluminum matrix. The rounded side, where the inclusions were rotated by 90° with respect to the aluminum lattice, approximated a spherical cap consisting partly of somewhat flatter segments with complex faceting, illustrating the lack of distinctly flat low-energy facets.