A discrete twin crystal plasticity model was presented for the size-dependent mechanics of nanotwinned metals. Specifically, it considers the length-scale-dependent yield response of nanotwinned Cu which exhibited a strengthening–softening transition of the yield strength below a critical twin thickness. The softening arising from source-governed preferential dislocation nucleation in the vicinity of the twin boundaries competes with the strengthening arising from dislocation pile-up at twin boundaries. To incorporate the softening mechanism within the discrete twin crystal plasticity model, a discrete twin-boundary-affected-zone of thickness λz was introduced near each twin boundary. This twin-boundary-affected-zone was enriched by the kinetics of additional crystallographic slip arising from the profuse slip activity near a twin boundary. The strengthening mechanism within a twin lamella was incorporated through internal stress on each slip system related to the average gradient of the excess dislocation density, introducing additional length-scale lb which mimics the effectiveness of dislocation pile-up. With this framework, the orientation-dependent yield behavior of single grains with discrete twins was probed. These simulations qualitatively capture the experimental observations of the transition of the yield behavior from strengthening to softening as a function of λ. Some results for polycrystals with random orientations were also presented and compared with experiments. The discrete twin crystal plasticity computational simulations provide useful insight into the microscopic activities in nt microstructures, which could be corroborated by experiments, and underscores the importance of discrete twin and twin-boundary-affected-zone effects that should be accounted for in developing their corresponding homogenized descriptions.

Crystal Plasticity of Nanotwinned Microstructures: a Discrete Twin Approach for Copper. H.Mirkhani, S.P.Joshi: Acta Materialia, 2011, 59[14], 5603-17