It was recalled that faceting could be considered to be a phase transition in which the original surface or grain boundary dissociated into flat segments whose energy was less than that of the original surface or grain boundary. In order to investigate grain boundary faceting, a cylindrical Cu bicrystal with an island grain was grown by using the Bridgman technique. Grain 1 of this bicrystal was completely surrounded by grain 2. The dissociation, Σ9 → Σ3 + Σ3, proceeded during growth of the bicrystal. Twins appeared instead of a {111}1/{115}2 or (110)Σ9CSL facet. Grain-boundary faceting was studied at 1293, 1073 and 873K. The profiles of the grain boundary thermal groove were analyzed by means of atomic force microscopy. Wulff-Herring plots and grain boundary phase diagrams were constructed for the Σ3, Σ9 and Σ9 + Σ3 grain boundaries. With increasing temperature, facets with low-density CSL planes disappeared from the grain boundary shape. A grain-boundary roughening phase transition was suggested to be responsible for this phenomenon.

Faceting of Σ3 and Σ9 Grain Boundaries in Copper. B.B.Straumal, S.A.Polyakov, E.Bischoff, W.Gust, E.J.Mittemeijer: Interface Science, 2001, 9[3-4], 287-92