Triple junction kinetics: Unified atomic mechanism and coordinated grain boundary network evolution

Triple junctions (TJs), as essential components connecting adjoining grain boundaries (GBs), govern the coordinated evolution of the entire GB network in polycrystalline materials under thermomechanical stimulations. Despite decades of research, a comprehensive understanding of TJ kinetics and their contributions to coordinated GB network evolution remains largely elusive, especially in experiments at the atomic scale. Using state-of-the-art in situ nanofabrication-nanomechanical testing with atomic resolution, we present direct evidence that multiple modes of TJ kinetics occur through conservative/non-conservative disconnection activities across neighboring GBs in Au and Pt polycrystals. TJ kinetics can transform mutually between conservative and non-conservative modes, holding significance for enhancing the deformation flexibility and sustaining plasticity of the overall GB network. A unified framework of TJ kinetics is further established by considering the coupling between GB plasticity, intragranular plasticity, and TJ excess volume. These findings are applicable to general TJs with non-coaxial GBs, providing a missing cornerstone for understanding the plasticity of polycrystalline materials.