Grain boundary migration and shear accommodation controlled by disconnection mobility

The disconnection mechanism has been established as a powerful descriptor for grain boundary (GB) behaviors, including GB migration, sliding, triple junction migration, and GB-bulk defect exchange. However, the relative contributions from disconnection nucleation and migration to mode selection and the effective GB mobility are yet to be fully resolved. Disconnection nucleation barriers are also typically very high, calling into question how disconnections can feasibly mediate GB migration outside of large driving forces and small-scale bicrystal simulations where periodic boundary conditions reduce the effective nucleation radius. This work illustrates the role of disconnection mobility in mode selection and GB mobility using the Σ5(310) symmetric tilt GB as a model interface. Under the application of shear stress, high mobility disconnections form and propagate along the GB. Due to this high mobility, the GB remains flat during most of migration. Under a synthetic driving force, a different mode of low mobility disconnections instead form and accumulate in the boundary, leading to a dense, persistent population of defects. These low-mobility disconnections serve as nucleation sites for more disconnections, greatly reducing the effective barrier to migration. The evolution of GB mobility with accumulating disconnections implies that GB behavior is not only sensitive to the present conditions on the GB, but also past migration and defect absorption, thereby potentially deviating from predictions based on initially flat bicrystals.