Faceting of (sigma)3 and (sigma)9 grain boundaries in Cu-Bi alloys

The faceting of (sigma)3 and (sigma)9 tilt grain boundaries (GBs) has been studied in bicrystals of pure Cu and Cu-Bi alloys containing 2.5 × 10^-3, 10 × 10^-3 and 16 × 10^-3 at.% Bi. The (sigma)3(1 0 0), (sigma)9(1 0 0), (sigma)9(-1 1 0), and (sigma)9(-1 2 0) facets and non-CSL (sigma)3 82° 9R facet were observed, where (sigma) is the inverse density of coincidence sites. The ratio between GB energy, (sigma)GB, and surface energy, (sigma)sur, was measured by atomic force microscopy using the GB thermal-groove method. The GB energy and thermal-groove deepening rate increased slightly between 0 and 10 × 10^-3 at.% Bi for all facets studied. However, between 10 × 10^-3 and 16 × 10^-3 at.% Bi the GB energy increased dramatically [from a factor 2 for the (sigma)9(1 1 0) facet to 15 times larger for the (sigma)3(1 0 0) facet]. The thermal-groove deepening rate also increased by a factor of 10 in this concentration range. This change corresponds well with the GB solidus line (i.e., the formation of a stable layer of a liquid-like GB phase called GB prewetting) observed previously. Wulff diagrams were constructed using measured (sigma)GB/(sigma)sur values.