Two softening stages in nanotwinned Cu

This study focuses on the deformation mechanisms of nanotwinned Cu with hierarchical twins (HTs) by virtue of a series of large-scale molecular dynamics (MD) simulations. For the same grain size dG and the same secondary twin spacing L₂, two softening stages and one strengthening stage are discovered, accompanied by the reduction of the primary twin spacing L₁. This special phenomenon is firstly found in nanotwinned Cu with HTs by MD simulations. In addition, totally different deformation mechanisms are observed along with the variation of L₁: (1) The transition of full dislocation to partial dislocation-dominated plastic deformation as the primary twin spacing L₁ decreases until L₁ reaches the first critical primary twin spacing in the first softening stage. (2) The transition of partial dislocation to dislocation blockage-dominated plastic deformation leads to strengthening when the primary twin spacing L₁ is between the first critical primary twin spacing and second critical primary twin spacing and in the strengthening stage. (3) Partial dislocations parallel to the twin boundaries (TBs), detwinning and partial dislocation motion-induced thickening/thinning of TBs result in the second softening stage when the primary twin spacing L₁ is smaller than the second critical primary twin spacing.