Faceting and roughening of the asymmetric twin grain boundaries in zinc

Faceting of grain boundaries (GBs) or surfaces can be considered as a phase transition when the original surface or GB dissociates onto flat segments whose energy is less than that of original surface or GB. Zn [11 (2) over bar0] flat single crystals were grown using the modified Bridgman technique from Zn of 99.999 wt% purity. Individual elongated twin plates having very uniform thickness were produced with the aid of slight deformation of single crystals. Parallel elongated sides of the twin plate are formed by the coherent symmetric twin (1 (1) over bar0 (2) over bar)(1)parallel to(1 (1) over bar0 (2) over bar)(2) grain boundary (STGB) facets. Due to its optical anisotropy, zinc allows one to study the shape of the GB with the aid of polarised light. The stationary shape of the slowly migrating tip of the twin plate has been studied in situ. The hot stage of optical microscope was used. The temperature interval from 592 to 692 K was investigated. Below 632 K the twin tip contains only one plane facet 1 which is nearly parallel to the ((1) over bar 10 (2) over bar)(2) plane and has the angle of 84degrees with the coherent STGB. Above 632 K the second facet 2 appears at the tip of the twin plate. This facet is nearly parallel to the (1 (1) over bar 00)(1) plane and has the angle of 46degrees with the coherent STGB. Between 632 and 682 K both 84degrees and 46degrees facets coexist, and 84degrees facet gradually disappear with increasing temperature. Above 682 K only 46degrees facet is present in the twin tip. The indications of the GB roughening phase transition were also observed, namely the edges of the facets become smoother with increasing temperature. The GB phase diagram for the twin GBs in zinc containing the lines of two GB faceting phase transitions has been constructed. Schematic Wulff-Herring diagrams explaining these transitions are presented.