Size effects of primary/secondary twins on the atomistic deformation mechanisms in hierarchically nanotwinned metals

A series of large-scale molecular dynamics simulations have been performed to investigate the tensile properties of nanotwinned (NT) copper with hierarchically twinned structures (HTS). For the same grain size d and the same spacing of primary twins λ1, the average flow stress first increases as the spacing of secondary twins λ2 decreases, reaching a maximum at a critical λ2, and then decreases as λ2 becomes even smaller. The smaller the spacing for λ1, the smaller the critical spacing for λ2. There exists a transition in dominating deformation mechanisms, occurring at a critical spacing of λ2 for which strength is maximized. Above the critical spacing of λ2, the deformation mechanisms are dominated by the two Hall-Petch type strengthening mechanisms: (a) partial dislocations emitted from grain boundaries (GBs) travel across other GBs and twin boundaries (TBs); (b) partial dislocations emitted from TBs travel across other TBs. Below the critical spacing of λ2, the deformation mechanism is dominated by the two softening mechanisms: (a) Partial dislocations emitted from boundaries of the primary twins travel parallel to the TBs of the secondary twins, leading to detwinning of the secondary twins; (b) Boundaries of the primary twins shift entirely, leading to thickening in one part of primary twins and thinning in the other part of primary twins. The present results should provide insights to design the microstructures for reinforcing the mechanical properties in the NT metals with HTS.