Competition and coordination of disconnection modes in shear-coupled migration of asymmetric tilt grain boundaries

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Grain boundaries play a critical role in determining the mechanical properties of polycrystalline materials, and their shear‑coupling behavior is essential for stress‑driven boundary migration and plastic deformation. Asymmetric tilt grain boundaries (ATGBs) are ubiquitous in polycrystals, yet their shear‑coupling mechanisms remain much less understood than those of symmetric tilt grain boundaries (STGBs). Here, we combine molecular dynamics (MD) simulations and theoretical modeling to systematically compare the shear‑coupling behavior of Σ5 and Σ25 〈001〉 ATGBs, revealing how different combinations of 〈100〉 and 〈110〉 migration modes govern their migration dynamics. Based on the disconnection‑mediated migration model, a three-dimensional geometric superposition model is developed to describe the shear‑coupling factor of ATGBs. Phenomenological parameters are further introduced to account for finite-temperature effects and deviations from ideal faceted migration. MD simulations reveal that Σ5 ATGBs exhibit a divergent coupling factor, corresponding to a sliding‑dominated transition, whereas Σ25 ATGBs show a smooth variation of the coupling factor with values lower than those of their constituent STGBs, indicating that the coexistence of 〈100〉 and 〈110〉 modes accelerates migration of the grain boundary. These findings suggest that intersecting disconnection modes do not necessarily lead to kinetic blocking. The disconnection‑superposition model and the dislocation‑coordination mechanism uncovered here provide a theoretical tool for grain boundary engineering to tailor shear‑coupling behavior by tailoring misorientation and inclination angles.