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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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