Direct atomic-scale observation of thermally driven grain-boundary migration associated with triple-junction motion

The internal structure of metals is defined by a complex network of grain boundaries (GBs) and their intersections---triple junctions (TJs). Heat or stress forces GBs to migrate, thus altering many physical properties of the material, and TJs play a key role in this migration as they control GB edges. The GB-TJ interaction strongly depends on the local crystallography, so it can only be properly described at the atomic scale. Although recent direct atomic-scale imaging of GB migration provides some insight into their dynamic behavior, the nanoscale details of the coupled motion of TJs and GBs in a natural nanocrystalline system remain elusive. Here, using in situ transmission electron microscopy (TEM) imaging, we describe the dynamics of the GB-TJ system in a nanocrystalline Co film with mixed fcc and hcp crystal phases during thermal annealing at both the mesoscale and the atomic scale. Our observations reveal that GB evolution proceeds through destabilization and displacement of TJs followed by GB relaxation within the new TJ geometry, which modifies the dihedral angles. This process is mediated by atomic-scale disconnection dynamics, where TJs serve as their sources and sinks. These findings advance our understanding of grain evolution in metal nanostructures, which is crucial for engineering their properties.