The origin of the ultrahigh strength and good ductility in nanotwinned copper

In order to make clear the special deformation mechanism of nanotwinned copper, the mechanical properties of two kinds of ultrafine-grained copper, one with nano-scale growth twin in the ultrafine grains and another one without twin, were compared in this study. Though it hardly exhibits work-hardening capacity, the nanotwinned copper shows much higher strength combined with good ductility than the copper without growth twin. The microstructure of nanotwinned copper was separated by twins into nano-scale regions where dislocation intersections and pile-up rarely take place. That leads to the weak work-hardening capacity in nanotwinned copper. The good ductility is attributed to its enhanced strain rate sensitivity which originates from the shortened dislocation segment length. Meanwhile, both the blocking effect of twin boundary on dislocation slipping and the shortened dislocation segment length enhance the strength in nanotwinned copper.