Recovery and facets for deformation twins in minerals and metals

Type II and IV twins with irrational twin boundaries are studied by high-resolution transmission electron microscopy in two plagioclase crystals. The twin boundaries in these and in NiTi are found to relax to form rational facets separated by disconnections. The topological model (TM), amending the classical model, is required for a precise theoretical prediction of the orientation of the Type II/IV twin plane. Theoretical pre-dictions also are presented for types I, III, V, and VI twins. The relaxation process that forms a faceted structure entails a separate prediction from the TM. Hence, faceting provides a difficult test for the TM. Analysis of the faceting by the TM is in excellent agreement with the observations.

Automated summary

Type II and IV twins with irrational twin boundaries were studied in plagioclase crystals using high-resolution transmission electron microscopy. It was observed that the twin boundaries relaxed and formed rational facets separated by disconnections, similar to what was seen in NiTi. The topological model (TM) was required for precise theoretical prediction of the orientation of the Type II/IV twin plane, and predictions were also made for other types of twins. The faceting process provided a challenging test for the TM, but the analysis showed excellent agreement with the observations.