Twinning is a critically important deformation mode in hexagonal close-packed metals. Twins are three-dimensional (3D) domains, whose growth is mediated by the motion of facets bounding the 3D twin domains and influences work hardening in metals. An understanding of twin transformations therefore necessitates that the atomic-scale structure and intrinsic mobilities of facets be known and characterized. The present work addresses the former point by systematically characterizing the boundary structures of 3D {1¯012} twins in magnesium using high-resolution transmission electron microscopy (HRTEM). Eight characteristic facets associated with twin boundaries are reported, five of which have never been experimentally observed before. Further, molecular dynamics simulations suggest that the formation and motion of these facets is associated with the accumulation of twinning dislocations. This work provides insights into understanding the structural character of 3D twins and serves to develop strategies for modulating twin kinetics by modifying twin boundaries, such as solute segregation.

孪晶是六方密排金属中极其重要的变形模式。孪晶是三维 (3D) 域，其生长由 3D 孪晶域边界的面的运动介导，并影响金属的加工硬化。因此，对孪生变换的理解需要了解和表征面的原子尺度结构和固有迁移率。目前的工作通过系统地表征 3D 的边界结构来解决前一点{1￣012}使用高分辨率透射电子显微镜 (HRTEM) 观察镁中的双胞胎。报道了与孪生边界相关的八个特征面，其中五个以前从未通过实验观察到。此外，分子动力学模拟表明这些面的形成和运动与孪晶位错的积累有关。这项工作为理解 3D 孪生的结构特征提供了见解，并有助于开发通过修改孪生边界（例如溶质偏析）来调节孪生动力学的策略。