Dislocation, Disconnection, Disclination, Step and Facet Associated with Twinning in Metals and Minerals

Due to the small shear vector associated with twinning, deformation twining is often activated in crystalline materials, especially for metals with low symmetry or low stack fault energy and minerals with large lattice constant. Nucleation, propagation and thickening of deformation twins are accomplished through gliding of twinning dislocations associated with atomic shuffles. Twinning dislocation has both dislocation character and step character. It is generally observed that twinning dislocations pile up to form coherency gliding disclination, step and even facet during twinning. The topological model demonstrates that such terraced interfaces, with coherency strains accommodated by arrays of dislocations and disconnections, can migrate without long-range diffusion. In this lecture, our objective is to concentrate on defects relevant to interface motion, and these exhibit dislocation, disclination, or step character, or some combination thereof. We take twinning in hcp metals as many defect examples. The definition of shuffles within the topological model is presented. The concept of anisotropic kinetics for twinning disconnections has recently been elucidated.