Crystal plasticity of nanotwinned microstructures: A discrete twin approach for copper

This paper presents a discrete twin crystal plasticity (DT-CP) model for the size-dependent mechanics of nanotwinned (nt) metals. Specifically, it considers the length-scale-dependent yield response of nt-Cu [1] which exhibits 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 (TBs) competes with the strengthening arising from dislocation pile-up at TBs [2]. To incorporate the softening mechanism within the DT-CP model, a discrete twin-boundary-affected-zone (TBAZ) of thickness λz is introduced near each TB. This TBAZ is enriched by the kinetics of additional crystallographic slip arising from the profuse slip activity near a TB. The strengthening mechanism within a twin lamella is 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 is 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 are also presented and compared with experiments. The DT-CP computational simulations provide useful insight into the microscopic activities in nt microstructures, which can be corroborated by experiments, and underscores the importance of discrete twin and TBAZ effects that should be accounted for in developing their corresponding homogenized descriptions.