Temperature influence on the faceting of (sigma)3 and (sigma)9 grain boundaries in Cu

The faceting of a tube-like tilt grain boundary (GB) in Cu bicrystals has been studied in the temperature interval from 0.5 to 0.95 of the melting temperature Tm (Tm = 1356 K). The grains of the bicrystals form the coincidence site lattice (CSL) with inverse density of coincidence sites (sigma) = 3 and (sigma) = 9. No rounded edges between facets were observed up to 0.95Tm. With decreasing temperature new facets of increasingly higher CSL indices appear. At 0.5Tm six crystallographically different (sigma) = 3 facets exist simultaneously. The appearance of high-index CSL facets can be explained by the roughening phase transition of a KosterlitzThouless type. The ratio of GB energy (sigma)GB and surface energy (sigma)sur of the specimen was measured by applying atomic force microscopy to the profile of the GB thermal groove formed upon additional annealing. The Wulff–Herring diagrams were constructed using measured (sigma)GB/(sigma)sur values.