Strain hardening and softening in nanotwinned Cu

Nanotwinned Cu foils with about 99% coherent twin boundaries (TBs) among all the boundaries were made. These coherent TBs, with an average spacing of 25 nm, were engineered approximately parallel to foil surfaces. Low plane-strain deformation enhances the hardness by refining microstructure and introducing dislocations. High plane-strain deformation results in crystallographic lattice rotation and reaction between dislocations and coherent TBs, and induces incoherent TBs, thus twin coarsening and even diminishing of nanotwins accompanied by recovery and recrystallization, which cause softening.

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