Abstract
Metals with a high density of nanometre-scale twins have demonstrated simultaneous high strength and good ductility, attributed to the interaction between lattice dislocations and twin boundaries. Maximum strength was observed at a critical twin lamella spacing (∼15 nm) by mechanical testing; hence, an explanation of how twin lamella spacing influences dislocation behaviours is desired. Here, we report a transition of dislocation nucleation from steps on the twin boundaries to twin boundary/grain boundary junctions at a critical twin lamella spacing (12–37 nm), observed with in situ transmission electron microscopy. The local stress concentrations vary significantly with twin lamella spacing, thus resulting in a critical twin lamella spacing (∼18 nm) for the transition of dislocation nucleation. This agrees quantitatively with the mechanical test. These results demonstrate that by quantitatively analysing local stress concentrations, a direct relationship can be resolved between the microscopic dislocation activities and macroscopic mechanical properties of nanotwinned metals.
Highlights
Metals with a high density of nanometre-scale twins have demonstrated simultaneous high strength and good ductility, attributed to the interaction between lattice dislocations and twin boundaries
The mechanical properties are associated with interactions between lattice dislocations and the coherent twin boundaries (TBs), as TBs serve as strong barriers for the motion of dislocations[1,3,4,5,6,7]
The dislocation activities in nanotwinned metals can be divided into three types[9,10,11]: Hard mode I, dislocations slip along the {111} planes that cross the twin lamellae; Hard mode II, threading dislocations traverse through the twin lamellae where the slip planes are inclined to TBs; and Soft mode, dislocations initiate from TB/GB junctions and slip along the coherent TBs
Summary
Metals with a high density of nanometre-scale twins have demonstrated simultaneous high strength and good ductility, attributed to the interaction between lattice dislocations and twin boundaries. The local stress concentrations vary significantly with twin lamella spacing, resulting in a critical twin lamella spacing (B18 nm) for the transition of dislocation nucleation This agrees quantitatively with the mechanical test. A strong dependence of twin lamella spacing on the local stress concentration is detected; a critical twin lamella spacing is determined as B18 nm for the transition from strengthening to softening in nanotwinned Cu. The result agrees quantitatively with the experimentally measured critical spacing (15 nm). The result agrees quantitatively with the experimentally measured critical spacing (15 nm) These data provide insights into the direct relationship between the local dislocation activities and macroscopic mechanical properties of bulk materials with geometrically designed nanostructures such as nanotwins
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