Grain boundaries (GBs) couple to an applied shear stress by migrating via the movement of dislocation-like defects known as disconnections. This plastic deformation response plays a role for example in nanograined materials, where other plasticity mechanisms are increasingly suppressed, or in stress-driven grain growth. Classical models for GB motion did not incorporate these shear-coupling effects and GB migration models based on disconnection motion are being developed, but they require knowledge of atomistic mechanisms. GBs can exhibit different atomic structures (``GB phases'' or ``complexions''), which might exhibit different disconnection mechanisms.

Two congruent complexions (“pearl” and “domino”) were experimentally observed in $\Sigma$ 19b ⟨111⟩ tilt GBs in elemental copper. Here, we study the effect of these complexions on GB kinetics using molecular dynamics. The shear coupled migration occurs due to the activation of the same disconnection mode with shear coupling factor 0.87 for both the complexions. A closer look at the atomic mechanisms using nudged elastic band calculations, though, revealed that differences in the activation barrier between the complexions exist. The GB migration in pearl is due to the formation of a disconnection pair and its glide along the GB. In domino, the GB motion is due to the simultaneous activation of multiple disconnection pairs of same mode with no significant glide of these disconnection pairs along GB, atleast for the system sizes we investigated. This difference in mechanisms can be explained by the energetics of the defects. A disconnection pair forms a dipole that has an increasing interaction energy the bigger the dipole becomes. The collective nucleation of disconnections also requires an increasing activation energy, the larger the GB area becomes. The interplay between these size effects determines which mechanism occurs, the height of the activation barrier, and therefore the critical stress to induce this GB plasticity.

Acknowledgment: This result is part of a project that has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (Grant agreement No. 787446; GB-CORRELATE).