Grain-boundary transformations: A crystallographic study of boundary faceting in bismuth-doped copper polycrystals

Grain-boundary faceting is a structural 2-D transformation from a flat or curved boundary into a configuration composed of two or more types of facets. Changes of composition at the boundary have been reported to induce such type of boundary reconstruction. This reconstruction occurs into very-well developed crystallographic orientations; however, there is little information on this topic. Bi-doped copper was used as a model system to study the general characteristics of grain boundary faceting and to determine whether boundary crystallography is an important factor for faceting. Specifically, the study was focused to $\Sigma$=3 boundaries because these boundaries were present in a large proportion and also exhibited a strong tendency to facet. Polycrystalline specimens of Cu-12atppm Bi were aged at 600$\sp\circ$C for 15min, 6h, 24h or 72h. Specimens were aged at 500$\sp\circ$C for 48h to observe the effect of temperature. Electron microscopy techniques were used to characterize the faceted boundaries. It was found that the transformation progresses with aging. The general evolution was from unfaceted boundary to partially faceted to completely faceted boundary. The analysis of facet orientations in $\Sigma$=3 faceted boundaries showed that there are two transformations occurring in the $\Sigma$=3 boundaries. The first one from an unfaceted to a faceted structure followed by an evolution of the faceted structure. The first transformation was found to be driven by the formation of the $\{$111$\}$/$\{$111$\}$ facets, where the coexisting facets serve as mere links. The second transformation was found to be driven by the formation of $\langle$110$\rangle$ tilt link facets. A mechanism for facet orientation rotations involving lateral ledge migration and formation of submicron steps is proposed. The preference for $\langle$110$\rangle$ tilt orientations is discussed as possibly related to either the planar density of coincidence sites, formation of 2-D Bi coherent with the facet structure or formation of a BiCu$\sb2$ compound commensurate with the $\Sigma$=3 coincidence site lattice