Hierarchy of defects in near-Σ5 tilt grain boundaries in copper studied by length-scale bridging electron microscopy

Abstract

Grain boundaries (GBs) are material imperfections that significantly impact material properties. Understanding how their atomic structure deviates from ideal symmetric orientations is crucial for establishing fundamental structure–property relationships. In this study, we utilized aberration-corrected scanning transmission electron microscopy, geometric phase analysis and nanobeam electron diffraction (NBED) to examine the structure of a series of near-$\Sigma 5\left(310\right)\left[001\right]$ tilt grain boundaries in copper and to explore the formation of GB defects and their associated strain field evolution on different length scales. Globally, the GB appears flat with no noticeable defects, as confirmed by NBED strain mapping. On the atomic-scale, however, various types of GB defects are observed. When a slight deviation in the misorientation is introduced, a patterning emerges featuring characteristic structural units from the $\Sigma 5\left(310\right)\left[001\right]$ and $\Sigma 5\left(210\right)\left[001\right]$ tilt boundaries. This pattern can be interpreted as secondary GB dislocations, a conclusion that is supported by GB structure prediction. Since these defects are confined to within the GB core, their associated strain field does not extend into the adjacent bulk grains. The structural landscape of the GB becomes more complex when GB plane inclination is also present, such as a wavy morphology or staircase-like architecture. The wavy morphology shows an unusual V-shape of the expansion and compression zones of the GB facet junctions that continue to extend into the bulk crystals for several nanometers. Our investigation into GB structure, particularly its inherent defects, is a prerequisite towards gaining atomic-scale insights into their potential impact on material properties.