Stress-driven triple junction reconstruction facilitates cooperative grain boundary deformation

Triple junctions (TJs), essential components linking neighboring grain boundaries (GBs), are of great significance for the deformation of entire GB networks in polycrystalline materials. However, kinetic behaviors of TJs and their coupling with GB plasticity remain largely unexplored, especially at atomic scale. Using atomistic in situ nanomechanical testing, we reveal a regime of dynamic TJ reconstruction for accommodating the coordinated deformation of GB network in gold and platinum polycrystals, proceeding through different modes of structural transformations, including disordered atomic arrangement, subgrain, dense stacking faults, and nanotwins. Such TJ reconstruction preferentially nucleates at TJs predicted with strong dragging effect, which serves as an effective route to facilitate the cooperative motion of neighboring GBs, in contrast to the widely-believed TJ deformation in steady state. This reconstruction-coordinated TJ kinetics provides novel insights into complicated GB network evolution and calls for a revisit of TJ roles in polycrystalline materials.