It was pointed out that the faceting of grain boundaries or surfaces could be considered to be a phase transition in which the original surface or grain boundary dissociated into flat segments whose combined energies were lower than that of the original surface or grain boundary. Here, flat [11•0] single crystals were grown from 5N-purity material by using the modified Bridgman technique. Individual elongated twin plates, having very uniform thicknesses, were produced with the aid of a slight deformation of single crystals. Parallel elongated sides to the twin plate were formed by coherent symmetrical twin grain boundary facets. Due to its optical anisotropy, the Zn permitted the study of the shape of the grain boundary with the aid of polarized light. The stationary shape of the slowly migrating tip of the twin plate was studied in situ on the hot stage of an optical microscope at 592 to 692K. Below 632K, the twin tip contained only one plane facet which had an angle of 84° with respect to the coherent symmetrical twin grain boundary facets. Above 632K, a second facet appeared at the tip of the twin plate. This facet had an angle of 46° with respect to the coherent symmetrical twin grain boundary facets. Between 632 and 682K, both 84° and 46° facets coexisted, and the 84° facet gradually disappeared with increasing temperature. Above 682K, only the 46° facet was present in the twin tip. Indications of a grain-boundary roughening phase transition were also observed, in that the edges of the facets became smoother with increasing temperature. The phase diagram for twin grain boundaries in Zn containing the lines of 2 grain-boundary faceting phase transitions was constructed. Schematic Wulff-Herring diagrams could explain the transitions.

Faceting and Roughening of the Asymmetric Twin Grain Boundaries in Zinc. B.B.Straumal, V.G.Sursaeva, S.A.Polyakov: Interface Science, 2001, 9[3-4], 275-9