Bulk Nanostructured Metal from Multiply-Twinned Nanowires.

Using chemically synthesized silver nanowires with 5-fold twinning planes as a model system, a bottom-up process to generate a bulk nanostructured metal has been demonstrated. Although the nanowires would be shortened and deformed during densification, they are chosen as a model system because they are currently the most scalable and convenient way to obtain Ag particles with high twinning densities. Direct cold pressing of a silver nanowire filter cake did not generate a sufficiently cohesive sample, while hot pressing at 190 °C for 8 h resulted in extensive sintering, eliminating the nanowire morphology. Copper was then electroplated on the silver nanowires as a binder and filler to increase the densification upon hot pressing; despite nonuniform plating across the thickness of the filter cake, the thermal stability of the nanowires was increased, allowing hot pressing at 390 °C. Finally, a uniform copper coating on silver nanowires was achieved by electroless plating, leading to cohesive bulk metal after hot pressing.

Nanotwinning and tensile behavior in cold-welded high-entropy-alloy nanowires.

Since the fabrication technique for high-entropy alloy (HEA) nanowires/nanopillars is still in its infancy, neither experimental nor modeling analyses of their cold-welding performance have been reported. Based on insights accumulated in our previous experiments and simulations regarding cold-welded metallic nanowires, in this study, the cold-welding performance of HEA nanowires is probed by atomistic simulations. Among different materials, our simulations reveal that extensively twinned structures are formed in CoCrMnFeNi samples, but not in CoCrCuFeNi or Ni samples. The larger fracture strain in certain HEAs is due to the improved ductility around the fracturing area as well as multiple twinning. Unlike in Ni samples, the fracture strains in HEA samples, regardless of being cuboid or cylindrical, are improved by shrinking the sample size. Among different orientations, the [010]-direction monocrystalline nanowires fail at a strain over 0.6, which is almost double that of the [111] direction. The fracture strains in polycrystalline HEA samples are, on average, larger than those in polycrystalline Ni samples. Furthermore, fracture strains in randomly generated polycrystalline HEA samples are more predictable than those in polycrystalline Ni samples with identical grain configurations. As previously reported, dislocation emission is still a prerequisite to fracture in all cold-welded samples.

Plastic Deformation and Strengthening Mechanisms of Nanopolycrystalline Diamond.

Bulk nanopolycrystalline diamond (NPD) samples were deformed plastically within the diamond stability field up to 14 GPa and above 1473 K. Macroscopic differential stress Δσ was determined on the basis of the distortion of the 111 Debye ring using synchrotron X-ray diffraction. Up to ∼5(2)% strain, Debye ring distortion can be satisfactorily described by lattice strain theories as an ellipse. Beyond ∼5(2)% strain, lattice spacing d111 along the Δσ direction becomes saturated and remains constant with further deformation. Transmission electron microscopy on as-synthesized NPD shows well-bonded grain boundaries with no free dislocations within the grains. Deformed samples also contain very few free dislocations, while density of {111} twins increases with plastic strain. Individual grains display complex contrast, exhibiting increasing misorientation with deformation according electron diffraction. Thus, NPD does not deform by dislocation slip, which is the dominated mechanism in conventional polycrystalline diamond composites (PCDCs, grain size >1 µm). The nonelliptical Debye ring distortion is modeled by nucleating 12⟨110⟩ dislocations or their dissociated 16⟨112⟩ partials gliding in the {111} planes to produce deformation twinning. With increasing strain up to ∼5(2)%, strength increases rapidly to ∼20(1) GPa, where d111 reaches saturation. Strength beyond the saturation shows a weak dependence on strain, reaching ∼22(1) GPa at >10% strain. Overall, the strength is ∼2-3 times that of conventional PCDCs. Combined with molecular dynamics simulations and lattice rotation theory, we conclude that the rapid rise of strength with strain is due to defect-source strengthening, whereas further deformation is dominated by nanotwinning and lattice rotation.