Plastic Deformation through Dislocation Saturation in Ultrasmall Pt Nanocrystals and Its in Situ Atomistic Mechanisms

Separating Geometric and Diffusive Contributions to the Surface Nucleation of Dislocations in Nanoparticles

While metal nanoparticles are widely used, their small size makes them mechanically unstable. Extensive prior research has demonstrated that nanoparticles with sizes in the range of 10-50 nm fail by the surface nucleation of dislocations, which is a thermally activated process. Two different contributions have been suggested to cause the weakening of smaller particles: first, geometric effects such as increased surface curvature reduce the barrier for dislocation nucleation; second, surface diffusion happens faster on smaller particles, thus accelerating the formation of surface kinks which nucleate dislocations. These two factors are difficult to disentangle. Here we use in situ compression testing inside a transmission electron microscope to measure the strength and deformation behavior of platinum particles in three groups: 12 nm bare particles, 16 nm bare particles, and 12 nm silica-coated particles. Thermodynamics calculations show that, if surface diffusion were the dominant factor, the last two groups would show equal strengthening. Our experimental results refute this, instead demonstrating a 100% increase in mean yield strength with increased particle size and no statistically significant increase in strength due to the addition of a coating. A separate analysis of stable plastic flow corroborates the findings, showing an order-of-magnitude increase in the rate of dislocation nucleation with a change in particle size and no change with coating. Taken together, these results demonstrate that surface diffusion plays a far smaller role in the failure of nanoparticles by dislocations as compared to geometric factors that reduce the energy barrier for dislocation nucleation.

In Situ Atomic-Scale Quantitative Evidence of Plastic Activities Resulting in Reparable Deformation in Ultrasmall-Sized Ag Nanocrystals

Permanent structural changes in pure metals that are caused by plastic activity are normally irreparable after unloading. Because of the lack of experimental evidence, it is unclear whether the plastic activity can be repaired as the size of the pure metals decreases to several nanometers; it is also unclear how the metals accommodate the plastic deformation. In this study, the in situ atomic-scale loading and unloading of ∼2 nm Ag nanocrystals was investigated, and three modes of plastic deformation were observed: (i) the phase transition from the face-centered cubic (fcc) phase to the hexagonal close-packed (hcp) phase, (ii) stacking faults, and (iii) deformation twin nucleation. We show that all three modes resulted in structural changes that were reparable, and their generation and restoration during loading and unloading were observed in situ. We discovered that the deformation modes of nanosized metals can be predicted from the ratio of the energy barriers of the fcc-hcp phase transition (ΔγH) and the deformation twin nucleation (ΔγT), which differ from those of the theoretical modes of relatively large-sized metals. The proposed ΔγH/ΔγT criterion provides insights into the deformation mechanism of nanometals.

Chemical inhomogeneity-induced profuse nanotwinning and phase transformation in AuCu nanowires

Nanosized metals usually exhibit ultrahigh strength but suffer from low homogeneous plasticity. The origin of a strength-ductility trade-off has been well studied for pure metals, but not for random solid solution (RSS) alloys. How RSS alloys accommodate plasticity and whether they can achieve synergy between high strength and superplasticity has remained unresolved. Here, we show that face-centered cubic (FCC) RSS AuCu alloy nanowires (NWs) exhibit superplasticity of ~260% and ultrahigh strength of ~6 GPa, overcoming the trade-off between strength and ductility. These excellent properties originate from profuse hexagonal close-packed (HCP) phase generation (2H and 4H phases), recurrence of reversible FCC-HCP phase transition, and zigzag-like nanotwin generation, which has rarely been reported before. Such a mechanism stems from the inherent chemical inhomogeneity, which leads to widely distributed and overlapping energy barriers for the concurrent activation of multiple plasticity mechanisms. This naturally implies a similar deformation behavior for other highly concentrated solid-solution alloys with multiple principal elements, such as high/medium-entropy alloys. Our findings shed light on the effect of chemical inhomogeneity on the plastic deformation mechanism of solid-solution alloys.

Surface-Condition-Dependent Deformation Mechanisms in Lead Nanocrystals

Serving as nanoelectrodes or frame units, small-volume metals may critically affect the performance and reliability of nanodevices, especially with feature sizes down to the nanometer scale. Small-volume metals usually behave extraordinarily in comparison with their bulk counterparts, but the knowledge of how their sizes and surfaces give rise to their extraordinary properties is currently insufficient. In this study, we investigate the influence of surface conditions on mechanical behaviors in nanometer-sized Pb crystals by performing in situ mechanical deformation tests inside an aberration-corrected transmission electron microscope (TEM). Pseudoelastic deformation and plastic deformation processes were observed at atomic precision during deformation of pristine and surface-oxidized Pb particles, respectively. It is found that in most of the pristine Pb particles, surface atom diffusion dominates and leads to a pseudoelastic deformation behavior. In stark contrast, in surface-passivated Pb particles where surface atom diffusion is largely inhibited, deformation proceeds via displacive plasticity including dislocations, stacking faults, and twinning, leading to dominant plastic deformation without any pseudoelasticity. This research directly reveals the dramatic impact of surface conditions on the deformation mechanisms and mechanical behaviors of metallic nanocrystals, which provides significant implications for property tuning of the critical components in advanced nanodevices.

Ultrasmall-Scale Brittle Fracture Initiated from a Dislocation in SrTiO3

Crystal defects often lead to an intriguing variety of catastrophic failures of materials and determine the mechanical properties. Here we discover that a dislocation, which was believed to be a source of plasticity, leads to brittle fracture in SrTiO₃. The fracture mechanism, i.e., bond breaking at the dislocation core triggers crack initiation and subsequent fracture, is elucidated from an atomic view by hybrid quantum and molecular simulations and in situ nanomechanical experiments. The fracture strength of the dislocation-included SrTiO₃ was theoretically evaluated to be 8.8-10.7 GPa, which was eminently lower than that of the pristine one (21.7 GPa). The experimental results agree well with the simulated ones. Moreover, the fracture toughness of the ultrasmall crack initiating from the dislocation is quantitatively evaluated. This study reveals not only the role of dislocations in brittle fracture but also provides an in-depth understanding of the fracture mechanism of dislocations at the atomic scale.

Deformation Mechanisms of FCC-Structured Metallic Nanocrystal with Incoherent Twin Boundary

Incoherent twin boundaries (ITBs) can significantly affect the mechanical properties of twin-structured metals. However, most previous studies have focused on the deformation mechanism of the coherent twin boundary (CTB), and metals with ITB-accommodated plasticity still require further investigation. In this study, deformation mechanisms of FCC-structured nanocrystal metals with ITBs were investigated using molecular dynamic (MD) simulations. We revealed that three deformation mechanisms occur in metals with ITBs. The first type of deformation was observed in Au, where the plasticity is governed by partial dislocation intersections with CTBs or reactions with each other to form Lomer–Cottrell (L–C) locks. In the second type, found in Al, the deformation is governed by reversible ITB migration. The third type of deformation, in Ni and Cu, is governed by partial dislocations emitted from the ITB or the tips of the stacking faults (SFs). The observed L–C lock formation, as well as the reversible ITB migration and partial dislocation emission from the tips of SFs, have rarely been reported before.

Timely and atomic-resolved high-temperature mechanical investigation of ductile fracture and atomistic mechanisms of tungsten

Revealing the atomistic mechanisms for the high-temperature mechanical behavior of materials is important for optimizing their properties for service at high-temperatures and their thermomechanical processing. However, due to materials microstructure’s dynamic recovery and the absence of available in situ techniques, the high-temperature deformation behavior and atomistic mechanisms of materials are difficult to evaluate. Here, we report the development of a microelectromechanical systems-based thermomechanical testing apparatus that enables mechanical testing at temperatures reaching 1556 K inside a transmission electron microscope for in situ investigation with atomic-resolution. With this unique technique, we first uncovered that tungsten fractures at 973 K in a ductile manner via a strain-induced multi-step body-centered cubic (BCC)-to-face-centered cubic (FCC) transformation and dislocation activities within the strain-induced FCC phase. Both events reduce the stress concentration at the crack tip and retard crack propagation. Our research provides an approach for timely and atomic-resolved high-temperature mechanical investigation of materials at high-temperatures.

High-temperature deformation of materials is challenging to evaluate. Here the authors develop a novel device that allows atomic resolved in situ high temperature mechanical tests inside a transmission electron microscope and reveal ductile fracture of a single crystal tungsten deformed at 973 K.

Emission of Dislocations from Grain Boundaries and Its Role in Nanomaterials

The Frank-Read model, as a way of generating dislocations in metals and alloys, is widely accepted. In the early 1960s, Li proposed an alternate mechanism. Namely, grain boundary sources for dislocations, with the aim of providing a different model for the Hall-Petch relation without the need of dislocation pile-ups at grain boundaries, or Frank-Read sources inside the grain. This article provides a review of his model, and supporting evidence for grain boundaries or interfacial sources of dislocations, including direct observations using transmission electron microscopy. The Li model has acquired new interest with the recent development of nanomaterial and multilayers. It is now known that nanocrystalline metals/alloys show a behavior different from conventional polycrystalline materials. The role of grain boundary sources in nanomaterials is reviewed briefly.

Unraveling the Origin of Tribomagnetization in Ferromagnetic Materials

Tribomagnetization, a natural phenomenon known for decades, has many engineering applications today. However, its intrinsic origin remains controversial and the mechanistic-based model has not yet been developed to predict the tribomagnetization behavior. In this work, the origin of tribomagnetization is investigated via both the experimental and modeling approaches. For the first time, the expansion of surface plastic deformation to the subsurface layer (i.e., dislocation strain gradient) is characterized in conjunction with the tribological behavior of the interface. By performing the reciprocating sliding experiment in a pin-on-block contact configuration, the tribomagnetization evolution in different wear regimes is followed, the geometrically necessary dislocations (GNDs) developed in the subsurface are measured, and the formation of characteristic magnetic domains is revealed. A tribomagnetization model is proposed on the basis of the sliding-induced plastic deformation mechanism with GNDs, bringing the missing link with classical tribology and ferromagnetism. The model reveals that the surface plastic strain and width of a wear scar are two crucial factors in determining the surface magnetic field changes. We believe that the tribomagnetization originates from the magnetic distortion accompanied by the progression of GNDs in the subsurface. Our hypothesis is supported by the good agreement between the model prediction and experimental observation.

Mechanical behavior of SiNC layers on PDMS: effects of layer thickness, PDMS modulus, and SiNC surface functionality

Thin layers of nanomaterials on stretchable substrates have the potential to enable stretchable, bendable optoelectronic devices, wearable diagnostics, and more. Recently, our group reported on a novel method for finding the neo-Hookean coefficient of thin layers of silicon nanocrystals (SiNCs) on polydimethylsiloxane (PDMS). Here we elaborate on that initial study by examining the effects of the SiNC layer thickness, PDMS neo-Hookean coefficient, and SiNC surface functionality on the neo-Hookean coefficient of the SiNC layers. We found that, while the layer thickness and PDMS neo-Hookean coefficient influence the behavior of the SiNC layers, layers of surface-functionalized SiNCs do not exhibit disparate behavior from layers of bare SiNCs.

Experimental/theoretical estimations of the neo-Hookean coefficients of SiNC layers on PDMS show a dependence on layer thickness as well as on the modulus of the PDMS, but not on the surface functionality of the SiNCs.

Estimation of the ultimate tensile strength and yield strength for the pure metals and alloys by using the acoustic wave properties

In this paper, the acoustic impedance property has been employed to predict the ultimate tensile strength (UTS) and yield strength (YS) of pure metals and alloys. Novel algorithms were developed, depending on three experimentally measured parameters, and programmed in a MATLAB code. The measured parameters are longitudinal wave velocity of the metal, density, and crystal structure. 19-samples were considered in the study and divided into 3-groups according to their crystal structure; 7-FCC, 6-BCC, and 6-HCB. X-ray diffraction was used to examine the crystal structure of each sample of each group, while longitudinal wave velocity and metals' density were measured experimentally. A comparison between mechanical properties predicted by the model and the ASTM standards was done to investigate the validity of the model. Furthermore, predicted stress-strain curves were compared with corresponding curves in the pieces literature as an additional validation check. The results revealed the excellence of the model with 85-99% prediction accuracy. The study also proved that if metals are grouped according to their crystal structure, a relation between UTS, YS, and modulus of elasticity (E) properties and wave pressure transmission coefficient (Tr) could be formulated.

Functional Materials Under Stress: In Situ TEM Observations of Structural Evolution

The operating conditions of functional materials usually involve varying stress fields, resulting in structural changes, whether intentional or undesirable. Complex multiscale microstructures including defects, domains, and new phases, can be induced by mechanical loading in functional materials, providing fundamental insight into the deformation process of the involved materials. On the other hand, these microstructures, if induced in a controllable fashion, can be used to tune the functional properties or to enhance certain performance. In situ nanomechanical tests conducted in scanning/transmission electron microscopes (STEM/TEM) provide a critical tool for understanding the microstructural evolution in functional materials. Here, select results on a variety of functional material systems in the field are presented, with a brief introduction into some newly developed multichannel experimental capabilities to demonstrate the impact of these techniques.

In situ atomic-scale observation of grain size and twin thickness effect limit in twin-structural nanocrystalline platinum

Twin-thickness-controlled plastic deformation mechanisms are well understood for submicron-sized twin-structural polycrystalline metals. However, for twin-structural nanocrystalline metals where both the grain size and twin thickness reach the nanometre scale, how these metals accommodate plastic deformation remains unclear. Here, we report an integrated grain size and twin thickness effect on the deformation mode of twin-structural nanocrystalline platinum. Above a ∼10 nm grain size, there is a critical value of twin thickness at which the full dislocation intersecting with the twin plane switches to a deformation mode that results in a partial dislocation parallel to the twin planes. This critical twin thickness value varies from ∼6 to 10 nm and is grain size-dependent. For grain sizes between ∼10 to 6 nm, only partial dislocation parallel to twin planes is observed. When the grain size falls below 6 nm, the plasticity switches to grain boundary-mediated plasticity, in contrast with previous studies, suggesting that the plasticity in twin-structural nanocrystalline metals is governed by partial dislocation activities.

The deformation mechanisms of micron-sized twinned metals are well-understood, but it is not so for twinned nanocrystalline metals. Here, the authors use high resolution microscopy to image the deformation of nanocrystalline twinned platinum and show that grain boundary behaviors dominate plasticity below 6 nm.

Atomistic and dynamic structural characterizations in low-dimensional materials: recent applications of in situ transmission electron microscopy

In situ transmission electron microscopy has achieved remarkable advances for atomic-scale dynamic analysis in low-dimensional materials and become an indispensable tool in view of linking a material's microstructure to its properties and performance. Here, accompanied with some cutting-edge researches worldwide, we briefly review our recent progress in dynamic atomistic characterization of low-dimensional materials under external mechanical stress, thermal excitations and electrical field. The electron beam irradiation effects in metals and metal oxides are also discussed. We conclude by discussing the likely future developments in this area.

In Situ Atomic-Scale Study on the Ultralarge Bending Behaviors of TiO2-B/Anatase Dual-Phase Nanowires

It is challenging but important to understand the mechanical properties of one-dimensional (1D) nanomaterials for their design and integration into nanodevices. Generally, brittle ceramic nanowires (NWs) cannot withstand a large bending strain. Herein, in situ bending deformation of titanium dioxide (TiO₂) NWs with a bronze/anatase dual-phase was carried out inside a transmission electron microscopy (TEM) system. An ultralarge bending strain up to 20.3% was observed on individual NWs. Through an in situ atomic-scale study, the large bending behavior for a dual-phase TiO₂ NW was found to be related to a continuous crystalline-structure evolution including phase transition, small deformation twinning, and dislocation nucleation and movements. Additionally, no amorphization or crack occurred in the dual-phase TiO₂ NW even under an ultralarge bending strain. These results revealed that an individual ceramic NW can undergo a large bending strain with rich defect activities.

Investigation of reorganization of a nanocrystalline grain boundary network during biaxial creep deformation of nanocrystalline Ni using molecular dynamics simulation

In this paper, simulated biaxial creep deformation behaviour for nanocrystalline (NC) nickel (Ni) has been performed at various applied load (i.e. 1 GPa, 1.4 GPa, 2 GPa, 2.5 GPa and 3 GPa) for a particular temperature (i.e. 1210 K) using molecular dynamics (MD) simulation to investigate underlying deformation mechanism based on the structural evolution during biaxial creep process. Primary, secondary and tertiary stages of creep are observed to be exhibited significantly only at 3 GPa applied stress. While, only primary and secondary stages of creep are exhibited at 1 GPa applied stress. Atomic structural evaluation, dislocation density, shear strains, atomic trajectory, inverse pole figures and grain orientation with texture distribution have been carried out to evaluate structural evolution. Stress exponent (m) for NC Ni is analysed for a particular creep temperature (i.e. 1210 K) and obtained m value is 1.30. According to shear strains counter plot, accumulation of higher shear strains are observed at grain boundary (GB) during biaxial creep deformation. It is found that dislocation density during biaxial creep is increased with the progress of creep process. Grain rotation and texture evaluation during biaxial creep process are studied using grain tracking algorithm (GTA). Grain rotation in ultrafine-grained NC Ni specimen during biaxial creep deformation is happened and exhibits almost distinct distribution, which is occurred due to the atomic shuffling within the GBs. Grain growth of ultrafine grained NC Ni is observed during biaxial creep deformation which is caused by mechanical stress.

In Situ Nano-thermomechanical Experiment Reveals Brittle to Ductile Transition in Silicon Nanowires

Silicon (Si) nanostructures are widely used in microelectronics and nanotechnology. Brittle to ductile transition in nanoscale Si is of great scientific and technological interest but this phenomenon and its underlying mechanism remain elusive. By conducting in situ temperature-controlled nanomechanical testing inside a transmission electron microscope (TEM), here we show that the crystalline Si nanowires under tension are brittle at room temperature but exhibit ductile behavior with dislocation-mediated plasticity at elevated temperatures. We find that reducing the nanowire diameter promotes the dislocation-mediated responses, as shown by 78 Si nanowires tested between room temperature and 600 K. In situ high-resolution TEM imaging and atomistic reaction pathway modeling reveal that the unconventional 1/2⟨110⟩{001} dislocations become highly active with increasing temperature and thus play a critical role in the formation of deformation bands, leading to transition from brittle fracture to dislocation-mediated failure in Si nanowires at elevated temperatures. This study provides quantitative characterization and mechanistic insight for the brittle to ductile transition in Si nanostructures.

Atomic-scale investigation on the ultra-large bending behaviours of layered sodium titanate nanowires

A study on the mechanical properties of one-dimensional layered titanate nanomaterials is crucial since they demonstrate important applications in various fields. Here, we conducted ex situ and in situ atomic-scale investigation on the bending properties of a kind of ceramic-layered titanate (Na2Ti2O4(OH)2) nanowire using transmission electron microscopy. The nanowires showed flexibility along the 100 direction and could obtain a maximum bending strain of nearly 37%. By analysing the defect behaviours, the unique bending properties of this ceramic material were found to correlate with a novel arrangement of dislocations, an active dislocation nucleation and movement along the axial direction resulting from the weak electrostatic interaction between the TiO6 layers and the low b/a ratio. These results provide a pioneering and key understanding on the bending behaviours of layered titanate nanowire families and potentially other one-dimensional nanomaterials with layered crystalline structures.

A novel approach to finding mechanical properties of nanocrystal layers

Flexible, bendable, stretchable devices represent the future of electronics for a wide range of real-world applications. Due to the fact that these technologies deviate significantly from traditional wafer technologies there is a need to understand and engineer material systems that allow large elastic deformations present in such devices, which requires knowledge about the mechanical properties of these material systems. Here we evaluate the mechanical properties of a bilayer polydimethylsiloxane (PDMS)/silicon nanocrystal system. By observing the formation of instabilities due to finite bending deformation and applying theoretical modeling, we estimated the neo-Hookean coefficient (analogous to shear modulus at low stress/strain) of the silicon nanocrystal film to be 345 ± 23 kPa. The method used here represents a novel approach to evaluating these properties and is widely applicable to many different combinations of systems of nanocrystals and elastomers.

Surface Energy Driven Liquid-Drop-Like Pseudoelastic Behaviors and In Situ Atomistic Mechanisms of Small-Sized Face-Centered-Cubic Metals

The elastic strain of conventional metals is usually below ∼1%. As the metals' sizes decrease to approximate a few nanometers, their elastic strains can approach ∼8%, and they usually exhibit pseudoelastic strain that can be as large as ∼35%. Previous studies suggested that the pseudoelastic behaviors of nanocrystals were attributed to distinctive mechanisms, including the release of stored elastic energies, the temperature-enhanced surface diffusion, etc. However, the atomistic mechanisms remain elusive. In this study, through large numbers of in situ atomic-scale tensile-fracture experiments, we report liquid-drop-like pseudoelastic behaviors of face-centered-cubic fractured single-crystalline nanowires with diameters varying from 0.5 to 2.2 nm. The ultralarge liquid-drop-like pseudoelastic strain ranged from 31.4% to 81.0% after the nanowire fracture was observed. The in situ atomic-scale investigations revealed that the atomistic mechanisms resulted from surface energy driven plastic deformation including surface diffusion mixed with shear plastic deformation as well as the release of true elastic energy. As the nanowires' diameters decrease below a critical value, the surface pressure can approach the ideal strength of metals. This ultralarge surface pressure drives atoms to diffuse mixed with dislocation nucleation/propagation, which ultimately leads to the fractured nanowires exhibiting liquid-drop-like pseudoelastic phenomena.

Nucleation-Controlled Plasticity of Metallic Nanowires and Nanoparticles

Nanowires and nanoparticles are envisioned as important elements of future technology and devices, owing to their unique mechanical properties. Metallic nanowires and nanoparticles demonstrate outstanding size-dependent strength since their deformation is dislocation nucleation-controlled. In this context, the recent experimental and computational studies of nucleation-controlled plasticity are reviewed. The underlying microstructural mechanisms that govern the strength of nanowires and the origin of their stochastic nature are also discussed. Nanoparticles, in which the stress state under compression is nonuniform, exhibit a shape-dependent strength. Perspectives on improved methods to study nucleation-controlled plasticity are discussed, as well the insights gained for microstructural-based design of mechanical properties at the nanoscale.

In situ atomic scale mechanisms of strain-induced twin boundary shear to high angle grain boundary in nanocrystalline Pt

Twin boundary can both strengthen and soften nanocrystalline metals and has been an important path for improving the strength and ductility of nano materials. Here, using in-lab developed double-tilt tensile stage in the transmission electron microscope, the atomic scale twin boundary shearing process was in situ observed in a twin-structured nanocrystalline Pt. It was revealed that the twin boundary shear was resulted from partial dislocation emissions on the intersected {111} planes, which accommodate as large as 47% shear strain. It is uncovered that the partial dislocations nucleated and glided on the two intersecting {111} slip planes lead to a transition of the original <110> symmetric tilt ∑3/(111) coherent twin boundary into a <110> symmetric tilt ∑9/(114) high angle grain boundary. These results provide insight of twin boundary strengthening mechanisms for accommodating plasticity strains in nanocrystalline metals.

Interlayer electrical resistivity of rotated graphene layers studied by in-situ scanning electron microscopy

Interlayer electrical transport between two-dimensional atomic crystals can be strongly modulated by the rotational misalignment between them. However, the experimental study on the interlayer electrical transport between rotated two-dimensional atomic crystals with variable rotation angles is challenging. Here, an in-situ scanning electron microscopy method is developed to study the interlayer electrical transport between rotated graphene layers. We employ nanoprobes installed in a scanning electron microscope to function as both "fingers" to induce interlayer rotation of a microfabricated metal-graphite-metal sandwiched island and also electrical probes to measure interlayer electrical resistivity of the rotated graphene layers. Interlayer electrical resistivity of the rotated graphene layers is found to increase monotonically by three orders of magnitude from ∼0.1 to ∼100 Ω cm when the rotational misalignment angle increases from 0° to 30°. This phenomenon can be well described by phonon-mediated electrical transport model. The large-magnitude tunability of interlayer electrical resistivity by mechanical rotation implies the potential applications of rotated graphene layers in nanoelectromechanical systems. Our results also provide a method for studying and tuning interlayer electrical transport between rotated two-dimensional atomic crystals.

Superplastic Creep of Metal Nanowires from Rate-Dependent Plasticity Transition

Understanding the time-dependent mechanical behavior of nanomaterials such as nanowires is essential to predict their reliability in nanomechanical devices. This understanding is typically obtained using creep tests, which are the most fundamental loading mechanism by which the time-dependent deformation of materials is characterized. However, due to existing challenges facing both experimentalists and theorists, the time-dependent mechanical response of nanowires is not well-understood. Here, we use atomistic simulations that can access experimental time scales to examine the creep of single-crystal face-centered cubic metal (Cu, Ag, Pt) nanowires. We report that both Cu and Ag nanowires show significantly increased ductility and superplasticity under low creep stresses, where the superplasticity is driven by a rate-dependent transition in defect nucleation from twinning to trailing partial dislocations at the micro- or millisecond time scale. The transition in the deformation mechanism also governs a corresponding transition in the stress-dependent creep time at the microsecond (Ag) and millisecond (Cu) time scales. Overall, this work demonstrates the necessity of accessing time scales that far exceed those seen in conventional atomistic modeling for accurate insights into the time-dependent mechanical behavior and properties of nanomaterials.

Atomistic Simulation of the Rate-Dependent Ductile-to-Brittle Failure Transition in Bicrystalline Metal Nanowires

The mechanical properties and plastic deformation mechanisms of metal nanowires have been studied intensely for many years. One of the important yet unresolved challenges in this field is to bridge the gap in properties and deformation mechanisms reported for slow strain rate experiments (∼10^-2 s^-1), and high strain rate molecular dynamics (MD) simulations (∼10⁸ s^-1) such that a complete understanding of strain rate effects on mechanical deformation and plasticity can be obtained. In this work, we use long time scale atomistic modeling based on potential energy surface exploration to elucidate the atomistic mechanisms governing a strain-rate-dependent incipient plasticity and yielding transition for face centered cubic (FCC) copper and silver nanowires. The transition occurs for both metals with both pristine and rough surfaces for all computationally accessible diameters (<10 nm). We find that the yield transition is induced by a transition in the incipient plastic event from Shockley partials nucleated on primary slip systems at MD strain rates to the nucleation of planar defects on non-Schmid slip planes at experimental strain rates, where multiple twin boundaries and planar stacking faults appear in copper and silver, respectively. Finally, we demonstrate that, at experimental strain rates, a ductile-to-brittle transition in failure mode similar to previous experimental studies on bicrystalline silver nanowires is observed, which is driven by differences in dislocation activity and grain boundary mobility as compared to the high strain rate case.