The structure and behavior of grain boundaries, an important class of defects widely exist in materials, largely determine physical, chemical and mechanical properties of polycrystalline materials. For more than a century, the study of grain boundary structure and behavior has been a focus of materials science. Although the development of transmission electron microscopy (TEM) has pushed materials research to sub-angstrom resolution, due to the complexity of grain boundary structure and the limitation of two-dimensional imaging nature of conventional transmission electron microscopy, the understanding of grain boundary structure is still limited. Therefore, it is of great significance to realize three-dimensional atomic-resolution imaging of grain boundaries experimentally.
Conventional TEM techniques (including both TEM and STEM imaging) can be used to image the morphology and atomic structure of samples thin as hundreds of nanometers or less. Although the spatial resolution of TEM has been improved to sub-angstrom level by the aberration correction technique, the information limited by the two-dimensional projection often leads to misunderstanding of real 3D structure of materials. Therefore, direct capturing and analyzing 3D structure of materials are desired at nanometer or atomic resolution through 3D imaging. Electron tomography provides such an opportunity by combining electron microscopy with computational image processing. In recent years, with the rapid development of electron microscope and image processing, electron tomography has been more and more widely applied to fields of biology, chemistry and materials science.
Recently, the research group of quantitative electron microscopy from Shenyang National Laboratory for Materials Science and collaborators have realized 3D atomic-resolution electron tomography of grain boundaries, including high-angle (structural-unit type) and low-angle (dislocation type) grain boundaries. Their paper entitled "Three-Dimensional Atomic Structure of Grain Boundaries Resolved by Atomic-Resolution Electron Tomography" is published inMatter.
Unlike conventional descriptions of grain boundaries, whereby they are either planar or curved planes with one-dimensional translational symmetry, Professor Kui Du and his colleagues have found that high angle grain boundaries completely lose translational symmetry due to undulated curvature related to the configuration of structural units. Statistical distribution of grain boundary coordination numbers, structural units and curvature are correlatively analyzed. The undulated grain boundary curvature is found to be closely related to the distribution of different types of structural units. Moreover, by thoroughly deciphering the 3D atomic configuration of kinks and jogs on dislocations, they provide direct experimental evidence for the dislocation kink-and-jog model proposed over half a century ago. The three-dimensional atomic structure of grain boundaries obtained by atomic-resolution electron tomography provides helpful guidelines for future experimental study and computational simulation of grain boundaries.
Figure 1 Atomic-resolution electron tomography of bi-crystal and polycrystals.
Figure 2 3D atomic structure, atomic coordination and structural-unit distribution of a high-angle structural-unit-type grain boundary.
Figure 3 3D atomic structure and configuration of kinks and jogs in a low-angle dislocation-type grain boundary.