Revealing the size effect mechanism of reversible grain boundary migration in nanocrystalline coppers: Molecular dynamics simulations and a refined disconnection model

Abstract

Reversible grain boundary (GB) migration is of great significance for promoting cyclic deformability and has been observed in nanocrystalline metals. However, GBs as dislocation sources generally emit more dislocations under deformation with increasing grain sizes. The size effect mechanism of reversible GB migration is thus crucial but remains unknown. We demonstrate that the reversibility of the representative Σ11(113) GB migration in copper (Cu) bicrystals under cyclic shear strongly depends on their grain sizes using molecular dynamics (MD) simulations. Atomic observation of the GB plane reveals the transition mechanism from the fully reversible disconnection-mediated GB migration to the disconnection pair grooving (DPG) induced irreversible structural damage with increasing grain size (d), where the disconnections at the groove are proved to be screw disconnections with Burgers vectors of 1/22[332¯] using Burgers circuit and lattice analysis. A refined disconnection model considering grain size is developed to predict disconnection-mediated GB migration behavior and interpret the forming mechanism of DPG. When satisfying d > 1.6651Gb/πτ (shear modulus G, Burgers vector b, and shear stress τ), the DPG mode is triggered, while the single disconnection sliding provokes the reversible GB migration when d ≤ 1.6651Gb/πτ. This study holds implications for nanocrystalline metals towards endurable cyclic deformability by controllable grain sizes.