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instability induced by incoherent twin boundary in nanotwinned copper

1. 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.

2. Atomic-scale studies on the effect of boundary coherency on stability in twinned Cu :

The stored energy and hardness of nanotwinned (NT) Cu are related to interaction between

dislocations and {111}-twin boundaries (TBs) studied at atomic scales by high-angle annular darkfield

scanning transmission electron microscope. Lack of mobile dislocations at coherent TBs

(CTBs) provides as-deposited NT Cu a rare combination of stability and hardness. The

introduction of numerous incoherent TBs (ITBs) reduces both the stability and hardness. While

storing more energy in their ITBs than in the CTBs, deformed NT Cu also exhibits high dislocation

density and TB mobility and therefore has increased the driving force for recovery, coarsening, and

recrystallization 

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