Sequential slip transfer of mixed-character dislocations across Σ3 coherent twin boundary in FCC metals: a concurrent atomistic-continuum study

Sequential slip transfer across grain boundaries (GB) has an important role in size-dependent propagation of plastic deformation in polycrystalline metals. For example, the Hall–Petch effect, which states that a smaller average grain size results in a higher yield stress, can be rationalised in terms of dislocation pile-ups against GBs. In spite of extensive studies in modelling individual phases and grains using atomistic simulations, well-accepted criteria of slip transfer across GBs are still lacking, as well as models of predicting irreversible GB structure evolution. Slip transfer is inherently multiscale since both the atomic structure of the boundary and the long-range fields of the dislocation pile-up come into play. In this work, concurrent atomistic-continuum simulations are performed to study sequential slip transfer of a series of curved dislocations from a given pile-up on Σ3 coherent twin boundary (CTB) in Cu and Al, with dominant leading screw character at the site of interaction. A Frank-Read source is employed to nucleate dislocations continuously. It is found that subject to a shear stress of 1.2 GPa, screw dislocations transfer into the twinned grain in Cu, but glide on the twin boundary plane in Al. Moreover, four dislocation/CTB interaction modes are identified in Al, which are affected by (1) applied shear stress, (2) dislocation line length, and (3) dislocation line curvature. Our results elucidate the discrepancies between atomistic simulations and experimental observations of dislocation-GB reactions and highlight the importance of directly modeling sequential dislocation slip transfer reactions using fully 3D models. Three-dimensional modelling has uncovered important mechanistic clues to strengthening polycrystalline metals through plastic deformation. A team led by David McDowell at the Georgia Institute of Technology in the US used concurrent atomistic-continuum (CAC) simulations to investigate the challenging problem of how curved dislocations pile-up and interact with special ‘twin’ grain boundaries in copper and aluminum when the metals are subjected to mechanical strain. These line defects move until they meet barriers such as grain boundaries separating crystalline regions, where they ‘pile-up’ behind the leading defect and may inhibit further defects from forming, a process known as work hardening. The multiscale CAC technique coarse grains the lattice using 3-D rhombohedra, and then applies an integral form finite element method to describe dislocation motion between elements, critical for understanding work hardening.