Abstract
In the construction of nanotwinned (NT) copper, inherent kink-like steps are formed on growth twin boundaries (TBs). Such imperfections in TBs play a crucial role in the yielding mechanism and plastic deformation of NT copper. Here, we used the molecular dynamic (MD) method to examine the influence of kink-step characteristics in depth, including kink density and kink-step height, on mechanical behavior of copper nanowire (NW) in uniaxial tension. The results showed that the kink-step, a stress-concentrated region, is preferential in nucleating and emitting stress-induced partial dislocations. Mixed dislocation of hard mode I and II and hard mode II dislocation were nucleated from kink-step and surface atoms, respectively. Kink-step height and kink density substantially affected the yielding mechanism and plastic behavior, with the yielding stress functional-related to kink-step height. However, intense kink density (1 kink per 4.4 nm) encourages dislocation nucleation at kink-steps without any significant decline in tensile stress. Defective nanowires with low kink-step height or high kink density offered minimal resistance to kink migration, which has been identified as one of the primary mechanisms of plastic deformation. Defective NWs with refined TB spacing were also studied. A strain-hardening effect due to the refined TB spacing and dislocation pinning was observed for defective NWs. This study has implications for designing NT copper to obtain optimum mechanical performance.
Highlights
Nanotwinning is believed to be a feasible strategy for improving the strength and ductility of metals
Growth twins are extensively fabricated via electrodeposition or magnetron sputtering, and two types of TBs are usually observed in growth twins, namely coherent twin boundaries (CTB) and incoherent twin boundaries (ITB)
A kink-step was found to be more effectual than surface atoms in serving as the source for dislocation nucleation
Summary
Nanotwinning is believed to be a feasible strategy for improving the strength and ductility of metals. Strengthening phenomena have been observed in several experiments and MD work [11,12,13,14], in which grain refinement led to a significant improvement in peak stress before the nucleation event. It is well-established that the presence of CTBs that obstruct the transmission of dislocation often favors a substantial increase in strength. Yield strength is inversely proportional to CTB spacing (denoted by λ), where a decrease in λ increases the likelihood of interaction of CTB and dislocations At this stage, the mechanisms of TB-strengthening phenomenon are tentatively understood via existing experiments and simulations. Yield stress can be approximated as the stress required for nucleation of the first dislocation and the yielding mechanism is attributed to Schmid factor analysis based on the loading direction
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