Metallic and covalent materials are important structural materials. Traditional strategies for strengthening materials compromise their ductility and toughness. Recent experimental results show that twinning can simultaneously improve the strength (hardness) and toughness of copper and diamond; as the inverse relationship between the strength and toughness of materials is broken, this has become a hot research topic. By studying the strengthening mechanism of nanotwinned copper and diamond, methods to simultaneously improve strength and toughness may be found. Herein, this paper presents a comprehensive overview of the recent developments in the experimental and theoretical studies of nanotwinned metals and covalent materials. The microstructures, fabrication methods, and mechanical properties of nanotwinned metals and covalent materials are summarized. Further, the strengthening mechanism of nanotwinned metals and the hardening mechanism of covalent materials are introduced. Finally, the research trend on the mechanical behavior of nanotwinned materials is discussed in detail.
Fig.1 Schematic of the shape changes of the spherical grain above the twin boundary, and illustration of twinning elements (K1—twinning plane, K2—conjugate twinning plane before twinning, —conjugate twinning plane after twinning, η1—twinning direction, η2—conjugate twinning direction, s—magnitude of shear)
Fig.2 Schematics of twin structures of some typical crystals
Fig.3 Schematic of coherent twin boundary (CTB) and incoherent twin boundary (ITB) in twinned materials
Fig.4 Yield strength and ductility of nanotwinned copper[18]
Fig.5 Mechanical properties of nanotwinned covalent materials
Fig.6 Schematics of a model setup and slip modes in nanotwinned copper [49]
Fig.7 Twin-thickness-dependent critical resolved shear stress (CRSS) for different slip modes in nanotwinned Cu[49]
Fig.8 Calculated twin-thickness-dependent yield str-ength for nt-Cu compared with experimental results[49]
Fig.9 Schematics of the nt-diamond microstructure and slip modes[51]
Fig.10 CRSSs for dislocation motion for the three different slip modes in nanotwinned diamond[51]
Fig.11 Calculated hardness compared to experimental data for nt-diamond as a function of twin thickness [51]
Fig.12 Compressive strength as a function of grain size and twin thickness for nc-diamond and nt-diamond samples at 300 K under displacement-controlled compression with Tersoff potential[48]
Fig.13 Diagram of nc-diamond and nc-Cu (nt-diamond and nt-Cu) strength as a function of grain size (twin thickness) (The black line summarizes the main feature of nc-diamond and nc-Cu where a turn from the Hall-Petch effect to reverse Hall-Petch effect (RHPE) occurs at d = dc; the brown patterned area indicates RHPE in nt-Cu with λ < λc (the Hall-Petch effect dominates with λ > λc); the cyan dash line and patterned area emphasize the continuous strengthening of nt-diamond with decreasing λ; dc—critical grain size, λc—critical twin thickness)[48]
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