Effect of surface energy on size-dependent deformation twinning of defect-free Au nanowires

Effects of radiation damage on the yielding and fracture of nanowires

Since free surfaces act as perfect sinks for radiation-induced defects, nanowires, owing to their high surface-to-volume ratio, are considered to be radiation tolerant. But the question remains on how this tolerance translates to their yielding and fracture behavior. Atomistic simulations of irradiated gold nanowires reported here show the existence of a size regime where the yield stress is affected by the accumulation of radiation damage. Our analysis also shows that, regardless of the diameter of the nanowire, early on during tensile loading, much of the radiation-induced defect content initially present in the wire is rapidly cleaned by the motion of pre-existing dislocations as well as dislocations emitted from the surface of the wire. This defect removal process resets the crystallographic configuration of the wire which subsequently deforms and fractures via the same mechanisms that occur in pristine, un-irradiated nanowires. We conclude that the fracture behavior of nanowires in the size and dose regimes tested is unaffected by radiation damage.

Room-temperature super-elongation in high-entropy alloy nanopillars

Nanoscale small-volume metallic materials typically exhibit high strengths but often suffer from a lack of tensile ductility due to undesirable premature failure. Here, we report unusual room-temperature uniform elongation up to ~110% at a high flow stress of 0.6-1.0 GPa in single-crystalline <110>-oriented CoCrFeNi high-entropy alloy nanopillars with well-defined geometries. By combining high-resolution microscopy and large-scale atomistic simulations, we reveal that this ultrahigh uniform tensile ductility is attributed to spatial and synergistic coordination of deformation twinning and dislocation slip, which effectively promote deformation delocalization and delay necking failure. These joint and/or sequential activations of the underlying displacive deformation mechanisms originate from chemical compositional heterogeneities at the atomic level and resulting wide variations in generalized stacking fault energy and associated dislocation activities. Our work provides mechanistic insights into superplastic deformations of multiple-principal element alloys at the nanoscale and opens routes for designing nanodevices with high mechanical reliability.

Methods to evaluate the twin formation energy: comparative studies of the atomic simulations and in-situ TEM tensile tests

Deformation twinning, one of the major deformation modes in a crystalline material, has typically been analyzed using generalized planar fault energy (GPFE) curves. Despite the significance of these curves in understanding the twin nucleation and its effect on the mechanical properties of crystals, their experimental validity is lacking. In this comparative study based on the first-principles calculation, molecular dynamics simulation, and quantitative in-situ tensile testing of Al nanowires inside a transmission electron microscopy system, we present both a theoretical and an experimental approach that enable the measurement of a part of the twin formation energy of the perfect Al crystal. The proposed experimental method is also regarded as an indirect but quantitative means for validating the GPFE theory.

Molecular Dynamics Simulations of Deformation Mechanisms in the Mechanical Response of Nanoporous Gold

The mechanical behaviour of nanoporous gold has so far been the subject of studies for bicontinuous morphologies, while the load transfer between ligaments is the primary challenge for using nanoporous structures—especially membranes with nanopores—in single-molecule sensors. This work studies the pore shape effect on deformation mechanisms of nanoporous gold membranes through molecular dynamics simulations. Tension and compression tests are carried out for nanoporous gold with circular, elliptical, square and hexagonal pore shapes. A significant pore shape effect on the mechanical properties is observed with distinct load transfer capabilities. A uniform stress transfer between ligaments constitutes a distinguished set of mechanical responses for structures with the hexagonal pore shape under tension, while a unique stress distribution in nanoporous with the circular pore shape introduces a high strength and ductile structure under compression. Further to shed light on the existing experimental observations, this work provides a comprehensive study on load transfer capabilities in the mechanical behaviour of nanoporous gold for sensing applications.

Transition of Deformation Mechanisms in Single-Crystalline Metallic Nanowires

Twinning and dislocation slip are two competitive deformation mechanisms in face-centered cubic (FCC) metals. For FCC metallic nanowires (NWs), the competition between these mechanisms was found to depend on loading direction and material properties. Here, using in situ transmission electron microscopy tensile tests and molecular dynamics simulations, we report an additional factor, cross-sectional shape, that can affect the competition between the deformation mechanisms in single-crystalline FCC metallic NWs. For a truncated rhombic cross-section, the extent of truncation determines the competition. Specifically, a transition from twinning to localized dislocation slip occurs with increasing extent of truncation. Theoretical and simulation results indicate that the energy barriers for twinning and dislocation slip depend on the cross-sectional shape of the NW. The energy barrier for twinning is proportional to the change of surface energy associated with the twinning. Thus, the transition of deformation modes can be attributed to the change of surface energy as a function of the cross-sectional shape.

Room-temperature superplasticity in Au nanowires and their atomistic mechanisms

We report experimental observation of room-temperature superplasticity and the distinct nanosize effect on the deformation mechanisms of Au nanowires. The Au nanowires were subjected to in situ tensile straining in a transmission electron microscope by using a home-made strain actuator, and a super large plastic strain with ∼150% uniform elongation and ∼260% total strain were observed before fracture. The plastic deformation started through full dislocation slip and was followed by the activities of stacking fault ribbons (or dissociated full dislocations) that were generated from surface sources and disappeared at the other end surfaces. With the reduction of the diameter of Au nanowires, the deformation changed to the twinning mode through partial dislocation emissions from sample surfaces. The mechanisms behind the observed phenomena are discussed in detail. These results shed light on the size-controlled plasticity of nano-metals.

Ultrastrong Al0.1CoCrFeNi high-entropy alloys at small scales: effects of stacking faults vs. nanotwins

Metastability engineering opens a new avenue to design high-entropy alloys (HEAs) originally proposed to benefit from phase stabilization. Meanwhile, boundary engineering via embedding planar defects such as stacking faults and nanotwins into the matrix of metals provides them with unique mechanical properties. In this work, for the first time, we combine the above two strategies to prepare Al0.1CoCrFeNi HEA pillars populated with a high density of stacking faults and nanotwins. It is uncovered that the stacking faulted (SF) Al0.1CoCrFeNi HEA pillars manifest ultrahigh strength exceeding 4.0 GPa and considerable compressive plasticity over 15%, much superior to their nanotwinned (NT) counterparts. Compared with the nanotwins undergoing detwinning during plastic deformation, the stacking faults in Al0.1CoCrFeNi high-entropy alloy thin films (HEAFs) are quite stable to hinder dislocation motion. Our findings not only endow the Al0.1CoCrFeNi HEAs with a predominant combination of strength and compression deformability, but also shed light on a new perspective for overcoming the strength and ductility trade-off in structural materials.

Twin and dislocation mechanisms in tensile W single crystal with temperature change: a molecular dynamics study

Molecular dynamics simulations are performed to investigate the orientation and temperature dependence of tensile response in single crystal W. It is found that W single crystal exhibits distinct temperature-dependent deformation behaviors along different orientations. With increasing temperature, the yield strain in the [001] orientation increases, while those in [110] and [111] orientations first increase and then decrease. The tensile deformations along orientations close to [001] are found to be dominated by twinning; the nucleation and growth of twins are accomplished through the nucleation and glide of ⅙111 partial dislocations on {112} planes. In contrast, the deformations along orientations close to [110] and [111] are found to be dominated by the slip of ½111 full dislocations, which move in a stay-and-go fashion. Moreover, intermediate deformation behaviors, which may become unstable at high temperatures, are observed for some intervening orientations. The distinct deformation behaviors of W along different orientations are rationalized based on the twinning-antitwinning asymmetry of ⅙111 partial dislocations on {112} planes.

Highly Flexible and Transparent Ag Nanowire Electrode Encapsulated with Ultra-Thin Al2O3: Thermal, Ambient, and Mechanical Stabilities

There is an increasing demand in the flexible electronics industry for highly robust flexible/transparent conductors that can withstand high temperatures and corrosive environments. In this work, outstanding thermal and ambient stability is demonstrated for a highly transparent Ag nanowire electrode with a low electrical resistivity, by encapsulating it with an ultra-thin Al₂O₃ film (around 5.3 nm) via low-temperature (100 °C) atomic layer deposition. The Al₂O₃-encapsulated Ag nanowire (Al₂O₃/Ag) electrodes are stable even after annealing at 380 °C for 100 min and maintain their electrical and optical properties. The Al₂O₃ encapsulation layer also effectively blocks the permeation of H₂O molecules and thereby enhances the ambient stability to greater than 1,080 h in an atmosphere with a relative humidity of 85% at 85 °C. Results from the cyclic bending test of up to 500,000 cycles (under an effective strain of 2.5%) confirm that the Al₂O₃/Ag nanowire electrode has a superior mechanical reliability to that of the conventional indium tin oxide film electrode. Moreover, the Al₂O₃ encapsulation significantly improves the mechanical durability of the Ag nanowire electrode, as confirmed by performing wiping tests using isopropyl alcohol.

Effect of RGO deposition on chemical and mechanical reliability of Ag nanowire flexible transparent electrode

The role of RGO in chemical and mechanical reliability was studied for Ag nanowire/RGO hybrid electrode. RGO deposition can be effective in reducing the oxidation while maintaining the superior mechanical reliability under cyclic bendings.