A controversy concerning the description of twinning, whether it is shear-shuffle or pure glide-shuffle or pure shuffle, has developed. There is disagreement about the interpretation of transmission electron microscopic observations, atomistic simulations and theories for twin growth. In this article, we highlight the atomic-level, characteristic, equilibrium and non-equilibrium boundaries and corresponding boundary defects associated with the three-dimensional ‘normal’, ‘forward’ and ‘lateral’ propagation of growth/annealing and deformation twins. Although deformation twin boundaries (TBs) after recovery exhibit some similarity to growth/annealing TBs because of the plastic accommodation of stress fields, there are important distinctions among them. These distinctions distinguish among the mechanisms of twin growth and resolve the controversy. In addition, a new type of disconnection, a glide disclination, is described for twinning. Synchroshear, seldom considered, is shown to be a likely mechanism for twinning. GRAPHICAL ABSTRACT IMPACT STATEMENT A controversy concerning twinning in hcp metals has developed. We present comprehensive understanding of interface structures and twinning mechanisms of growth/annealing and deformation twins at the atomic level.

Given the infinite diversity of microstructural inhomogeneity, the variation in spatial distribution of local strain could be infinite. However, this study finds that the statistical distribution of local strain universally follows a lognormal distribution irrespective of phase content and deformation mechanism. Moreover, this universal law is proved conditional upon the macroscopic homogeneity of deformation on the statistical window scale, equivalent to the equality between the macrostrain calculated from the displacements at the window corners and the average of the local strain. The discovery of a lognormal distribution law suggests the existence of a minimum statistical representative window (MSRW) size that is characteristic for each material. Explorations on the dependence of MSRW size on the microstructure, deformation mechanism, and strain magnitude are expected to add new dimensions to understanding of the relationship between microstructure and mechanical properties.