Faceting of Σ3 and Σ9 grain boundaries in Cu-Bi alloys

The faceting of Σ3 and Σ9 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 Σ3(1 0 0), Σ9(1 0 0), Σ9(−1 1 0), and Σ9(−1 2 0) facets and non-CSL Σ3 82° 9R facet were observed, where Σ is the inverse density of coincidence sites. The ratio between GB energy, σGB, and surface energy, σsur, 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 Σ9(1 1 0) facet to 15 times larger for the Σ3(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 σGB/σsur values.