Strengthening and plasticity in nanotwinned metals
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Introduction The exceptional macroscopic tensile strength and high ductility of nanotwinned (NT) metals,1 comprised of coherent twin boundaries (CTBs) with nanometer-scale spacing, are now well established. Growth and deformation nanotwins have been shown to form ubiquitously in many materials, including pure elements and engineering alloys of various shapes and sizes, from bulk to thin films, and nanowires (NWs).2 Elucidating the microscopic origin of strength, plasticity, and twin-size effects in NT metals has become an important objective for fundamental research and engineering applications. New NT metals and alloys have been synthesized, and new experiments have flourished in recent years. In parallel, new highresolution diffraction tools and more sophisticated atomistic simulations have been developed to probe the microstructure of deformation in NT metals at the atomic scale. This article highlights recent important advances enabled by these experimental techniques and atomistic models. First, we discuss the phenomenon of strain hardening in low-dimensional metals such as twinned face-centered-cubic (fcc) NWs where, in the absence of grain boundaries (GBs), strengthening and enhanced plasticity emerge from the complex interplay between twin boundaries (TBs) and free surfaces. Next, we focus on the microscopic mechanisms of plasticity in columnar-grained NT Cu films with a particular emphasis on the role of TB defects in strengthening and softening processes. We further extend
this discussion to the mechanisms responsible for the excellent plastic flow stability of this material under severe rolling deformation. Last, we present the results of an innovative experimental approach using bundles of nanotwins induced by dynamic plastic deformation (DPD) (i.e., deformation with high strain rates) to dramatically strengthen coarse-grained Fe-based and Cu-based alloys.
Strain hardening in one-dimensional nanotwinned metals Metallic NWs usually exhibit ultrahigh strength but low tensile ductility owing to their limited strain-hardening capability.3–5 Single-crystalline NWs of fcc metals (Figure 1a) deform via dislocation-mediated plasticity6–9 or via deformation twinning and lattice reorientation,10–12 without exhibiting pronounced hardening. To promote strain hardening, one can engineer TBs into NWs to act as barriers to dislocation motion. Figure 1b–d shows schematics of different-ordered arrangements of TBs in NWs, including horizontal, inclined, and vertical (fivefold twinned) TBs. Several groups have studied NWs with horizontal TBs (Figure 1b).13–16 In periodically twinned NWs, strain-hardening behavior was only evidenced in low stacking-fault energy (SFE) metals such as NT Au and Ag.17 In contrast, no strain hardening was observed in high SFE metals like NT Cu pillars.13 This fundamental difference is attributable to the
F. Sansoz, School of Engineering, The University of Vermont, USA; [email protected] K. Lu, Institute of Metal Research, Chinese Academy of Sciences, China; [email protected] T. Zhu, George W. Wo
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