Surface Dislocation Nucleation Research Articles

Surface Adatom Diffusion-Assisted Dislocation Nucleation in Metal Nanowires.

We employ a hybrid diffusion- and nucleation-based kinetic Monte Carlo model to elucidate the significant impact of adatom diffusion on incipient surface dislocation nucleation in metal nanowires. We reveal a stress-regulated diffusion mechanism that promotes preferential accumulation of diffusing adatoms near nucleation sites, which explains the experimental observations of strong temperature but weak strain-rate dependence as well as temperature-dependent scatter of the nucleation strength. Furthermore, the model demonstrates that a decreasing rate of adatom diffusion with an increasing strain rate will lead to stress-controlled nucleation being the dominant nucleation mechanism at higher strain rates. Overall, our model offers new mechanistic insights into how surface adatom diffusion directly impacts the incipient defect nucleation process and resulting mechanical properties of metal nanowires.

Solid solution softening in single crystalline metal nanowires studied by atomistic simulations

Solid solution strengthening is a common method used in physical metallurgy to increase the strength of metals. However, it is also possible for solute atoms to reduce the strength of metals, known as the solid solution softening effect. In this paper, atomistic simulations were carried out using molecular dynamics and Monte Carlo simulations to explore the softening phenomenon in single crystalline metal nanowires (MNWs) of different alloy systems. It was found that, for single crystalline MNWs, softening is more prominent than strengthening when solute atoms are introduced, which contrasts with the solid solution strengthening that is usually observed in bulk metals. The reduction of unstable stacking fault energy, increase in atomic size misfit, and solute clustering are responsible for this phenomenon, as they facilitate the surface dislocation nucleation in the alloyed nanowires. Additionally, while the nanowire diameter, orientation, surface segregation, and chemical short-range ordering all influence the yield strength, they do not alter the overall softening trend. It is assumed that the softening mechanisms uncovered in this paper are applicable to metallic structures whose yielding is determined by dislocation nucleation.

Atomistic modeling of surface and grain boundary dislocation nucleation in FCC metals

Dislocation nucleation plays a critical role in the plastic deformation of crystalline materials. However, it is challenging to predict the active mode and associated rate of dislocation nucleation under typical experimental loading conditions through molecular dynamics simulation due to timescale limitations. Here we use the free-end nudged elastic band method to determine the activation energies and activation volumes of dislocation nucleation in four typical face-centered cubic metals of Au, Al, Cu and Ni. We focus on the representative processes of surface and grain boundary dislocation nucleation. The atomistically determined activation volumes of these dislocation nucleation processes are larger than 10b3 (with b being the Burgers vector length) under typical experimental loading conditions. These results are compared with experimentally measured activation volumes in ultrafine-grained and nanocrystalline metals, thereby providing mechanistic insight into their rate-controlling deformation mechanisms.

Reaching near-theoretical strength by achieving quasi-homogenous surface dislocation nucleation in MgO particles

The structural stability of the ceramic nanoparticle (NP) is essential for its versatile applications in broad fields. Due to inevitable imperfections on the surfaces, the NP usually yields at a stress that is far below its theoretical strength. Here we propose an effective way to approach the theoretical strength of the NPs by simply homogenizing the primary imperfections. Electron beam irradiation is utilized to modify the original uneven surface and achieve surface nano-crystallization in MgO NPs, which leads to more homogeneous stress distribution intrinsically as proved by numerical simulations. As a result, quasi-homogeneous surface nucleation of dislocations occurs and raises the compressive strength to be three times higher than the as-received particles with the original surface. The near-theoretical strength gained here indicates that the imperfection in materials can be homogenized to optimize the properties to approach that of perfect crystals.

Site dependence of surface dislocation nucleation in ceramic nanoparticles

The extremely elevated strength of nanoceramics under compression arises from the necessity to nucleate highly energetic dislocations from the surface, in samples that are too small to contain pre-existing defects. Here, we investigate the site dependence of surface dislocation nucleation in MgO nanocubes using a combination of molecular dynamics simulations, nudged-elastic-band method calculations and rate theory predictions. Using an original simulation setup, we obtain a complete mapping of the potential dislocation nucleation sites on the surface of the nanoparticle and find that, already at intermediate temperature, not only nanoparticle corners are favorable nucleation sites, but also the edges and even regions on the side surfaces, while other locations are intrinsically unfavorable. Results are discussed in the context of recent in situ TEM experiments, sheding new lights on the deformation mechanisms happening during ceramic nanopowder compaction and sintering processes.

Sample-size-dependent surface dislocation nucleation in nanoscale crystals

The finite-temperature mechanical strength of nanoscale pristine metals at laboratory strain rates may be controlled by surface dislocation nucleation, which was hypothesized to be only weakly dependent on the sample size. Previous studies on surface dislocation nucleation investigated factors such as surface steps, oxidation layers and surface diffusion, while the role of surface stresses and sample size remains unclear. Here we perform systematic atomistic calculations on the activation free energy barriers of surface dislocation nucleation in sub-50 nm nanowires. The results demonstrate that surface stresses significantly influence the activation processes of surface dislocation nucleation. This renders the strength strongly dependent on sample size; whether it is “smaller is stronger” or “smaller is weaker” depends on the combined effects of surface stress and applied axial stress, which can be universally explained in terms of the local maximum resolved shear stress. A linear relation between the activation entropy and activation enthalpy (Meyer-Neldel compensation rule) was found to work well across a range of stresses and sample sizes.

Strain rate effects on the plastic flow in submicron copper pillars: Considering the influence of sample size and dislocation nucleation

Three-dimensional discrete dislocation dynamics (DDD) simulations are performed to investigate the plastic flow behaviors of submicron copper pillars under different loading rates, in which both inertial effect of dislocation motion and surface nucleation are taken into account. It is found that: (1) for pillars with a diameter below ∼400 nm, there is a transition from internal dislocation multiplication to surface dislocation nucleation as the strain rate increases (≥104 s−1); (2) for ∼1 um diameter pillars, stable internal dislocation sources dominate for both low and high strain rates; (3) in general, a larger strain rate, smaller sample size and less internal dislocation sources make it more probable for a surface nucleation process to take the place of dislocation multiplication. Furthermore, a theoretical model is proposed to predict the submicron plastic behavior at different strain rates when internal dislocation sources prevail.

Structural evolution of copper-silver bimetallic nanowires with core-shell structure revealed by molecular dynamics simulations

The molecular dynamics simulation method is used to investigate the tensile deformation mechanism of Cu-Ag core-shell nanowires. The results for the actual structure of the Cu-Ag core-shell nanowires after energy minimization processes indicate that the atoms in shell Ag are reconstructed when the shell thicknesses smaller than 1.0nm, while the shell thickness increases from 1.0nm to 1.5nm, the shape of NWs change into irregular circle. And then the tension strain is performed along the [001] direction under the conditions of varying the shell thickness and the temperature, respectively. It is found that, in low temperature region, <500K, with decreasing the shell thickness from 1.5nm to 0.25nm, the plastic deformation mechanism transforms from the misfit dislocation tube mechanism into the surface dislocation nucleation mechanism. When the temperature increases to the high region, ≥500K, the plastic deformation mechanism then changes into the high temperature influence dislocation nucleation mechanism. A shell thickness-temperature plastic deformation map is proposed to reveal the transition among the three different plastic deformation mechanisms for Cu-Ag core-shell nanowires.

The fracture stabilities of ultrathin gold nanowires

The fracture and corresponding failure of ultrathin gold nanowires have attracted much attention due to its applicable reliability of nanoelectromechanical devices, so understanding the fracture stability is critical especially when the width reduces to ultrathin scale. Here, we studied the fracture stabilities of ultrathin gold nanowires subjected to uniaxial tension. The statistical mechanical properties show that the stabilities result from size effects, in which, the width of 4.5a (“a” means lattice constant, 0.408 nm for gold) is shown as a critical and transitional size. Less than 4.5a, the strong mechanical strength is obviously unstable, attributing to surface dislocation nucleation; Larger than 4.5a, the mechanical stabilities have been enhanced. However, the fracture stabilities are demonstrated to be extrinsic at large sizes because of the dominated action of small aspect ratios. With macro-broken position distributions at the effects of sizes and atomic vacancies, the intrinsic and extrinsic fracture stabilities are further found to be related with the damaged degrees of crystalline lattices and propagated styles of the tensile waves. Lastly, the atomic vacancy sensitivities show the extrinsic fracture stabilities of large sizes are from a competition of bulk atoms and the strong shock induced by small aspect ratio.

Measuring surface dislocation nucleation in defect-scarce nanostructures.

Linear defects in crystalline materials, known as dislocations, are central to the understanding of plastic deformation and mechanical strength, as well as control of performance in a variety of electronic and photonic materials. Despite nearly a century of research on dislocation structure and interactions, measurements of the energetics and kinetics of dislocation nucleation have not been possible, as synthesizing and testing pristine crystals absent of defects has been prohibitively challenging. Here, we report experiments that directly measure the surface dislocation nucleation strengths in high-quality 〈110〉 Pd nanowhiskers subjected to uniaxial tension. We find that, whereas nucleation strengths are weakly size- and strain-rate-dependent, a strong temperature dependence is uncovered, corroborating predictions that nucleation is assisted by thermal fluctuations. We measure atomic-scale activation volumes, which explain both the ultrahigh athermal strength as well as the temperature-dependent scatter, evident in our experiments and well captured by a thermal activation model.

Surface dislocation nucleation controlled deformation of Au nanowires

We investigate deformation in high quality Au nanowires under both tension and bending using in-situ transmission electron microscopy. Defect evolution is investigated during: (1) tensile deformation of 〈110〉 oriented, initially defect-free, single crystal nanowires with cross-sectional widths between 30 and 300 nm, (2) bending deformation of the same wires, and (3) tensile deformation of wires containing coherent twin boundaries along their lengths. We observe the formation of twins and stacking faults in the single crystal wires under tension, and storage of full dislocations after bending of single crystal wires and after tension of twinned wires. The stress state dependence of the deformation morphology and the formation of stacking faults and twins are not features of bulk Au, where deformation is controlled by dislocation interactions. Instead, we attribute the deformation morphologies to the surface nucleation of either leading or trailing partial dislocations, depending on the Schmid factors, which move through and exit the wires producing stacking faults or full dislocation slip. The presence of obstacles such as neutral planes or twin boundaries hinder the egress of the freshly nucleated dislocations and allow trailing and leading partial dislocations to combine and to be stored as full dislocations in the wires. We infer that the twins and stacking faults often observed in nanoscale Au specimens are not a direct size effect but the result of a size and obstacle dependent transition from dislocation interaction controlled to dislocation nucleation controlled deformation.

The influence of residual stress on incipient plasticity in single-crystal copper thin film under nanoindentation

The incipient plastic deformation of single-crystal copper thin film with in-plane residual stress under Hertzian nanoindentation is studied using molecular dynamics simulation. The result reveals that the residual stresses significantly influence the surface strength and dislocation nucleation behavior of material: (i) the indentation hardness decreases with tensile residual stress, while it increases with moderate compressive residual stress, but it may drop down under higher compressive residual stress; (ii) the dislocation nucleation depth and its slip direction also vary with different residual stress state. This demonstrates that residual stress may influence incipient plasticity not only on the threshold value but also on the initial dislocation nucleation behavior under indentation.

Modeling dislocation nucleation strengths in pristine metallic nanowires under experimental conditions

The nature of dislocation sources in small-scale metals is critical to understanding and building models of size-dependent crystalline strength at the nanoscale. Pre-existing dislocations with one pinning point can be readily described as truncated Frank–Read sources, and modeling their operation strengths is straightforward. In contrast, simple and accurate models describing surface dislocation nucleation processes remain elusive. Here, we develop a computationally simple model of heterogeneous dislocation nucleation from free surfaces by combining a continuum description of the nucleation process with the atomistic inputs where accuracy is critical. This model is used to derive the upper strength limits of single-crystalline face-centered cubic nanopillars as a function of size, material and crystal orientation for uniaxial compression and tension. The output parameter space of this model is critically compared against direct atomistic simulations, as well as experiments on tension of pristine gold nanowires and compression of copper nanopillars to highlight its virtues and limitations.

Surface dislocation nucleation by wedge indenter contacts

Dislocation nucleation from a surface in contact with a rigid wedge indenter is studied in the framework of plane strain. First, the equivalent driving force on a singularity is derived in compact form. Using the result obtained, the driving force is then expressed in the form of complex integral formulae in terms of the Kolosov–Muskhelishvili potentials for wedge contacts. Then, by the Rice–Thomson model, a dislocation nucleation criterion is proposed for a surface loaded in contact, from which we obtain the critical contact size (equivalently, the critical contact load) and the dislocation emission angles at the different wedge indenter apex angles. Finally, the results are discussed, and conclusions are drawn.

Surface dislocation nucleation mediated deformation and ultrahigh strength in sub-10-nm gold nanowires

The plastic deformation and the ultrahigh strength of metals at the nanoscale have been predicted to be controlled by surface dislocation nucleation. In situ quantitative tensile tests on individual 〈111〉 single crystalline ultrathin gold nanowires have been performed and significant load drops observed in stress-strain curves suggest the occurrence of such dislocation nucleation. High-resolution transmission electron microscopy (HRTEM) imaging and molecular dynamics simulations demonstrated that plastic deformation was indeed initiated and dominated by surface dislocation nucleation, mediating ultrahigh yield and fracture strength in sub-10-nm gold nanowires. Open image in new window

Near-Ideal Strength in Gold Nanowires Achieved through Microstructural Design

The ideal strength of crystalline solids refers to the stress at elastic instability of a hypothetical defect-free crystal with infinite dimensions subjected to an increasing load. Experimentally observed metallic wires of a few tens of nanometers in diameter usually yield far before the ideal strength, because different types of surface or structural defects, such as surface inhomogeneities or grain boundaries, act to decrease the stress required for dislocation nucleation and irreversible deformation. In this study, however, we report on atomistic simulations of near-ideal strength in pure Au nanowires with complex faceted structures related to realistic nanowires. The microstructure dependence of tensile strength in face-centered cubic Au nanowires with either cylindrical or faceted surface morphologies was studied by classical molecular dynamics simulations. We demonstrate that maximum strength and steep size effects from the twin boundary spacing are best achieved in zigzag Au nanowires made of a parallel arrangement of coherent twin boundaries along the axis, and {111} surface facets. Surface faceting in Au NWs gives rise to a novel yielding mechanism associated with the nucleation and propagation of full dislocations along {001}110 slip systems, instead of the common {111}112 partial slip observed in face-centered cubic metals. Furthermore, a shift from surface dislocation nucleation to homogeneous dislocation nucleation arises as the twin boundary spacing is decreased below a critical limit in faceted nanowires. It is thus discovered that special defects can be utilized to approach the ideal strength of gold in nanowires by microstructural design.

Surface dislocation nucleation from frictional sliding contacts

We examine the conditions for dislocation nucleation beneath a plane strain rectangular indenter using an analytical model. Firstly, the equivalent driving force on a singularity is derived in a compact form. With this general result, the driving force is expressed in the form of complex integral formulae in terms of Muskhelishvili’s complex potentials for rectangular edged contacts. Then, adopting the Rice–Thomson model, we propose a dislocation nucleation criterion for stress contacted surface. The validity of the criterion is compared with a simple experimental result and shows good agreement. The results are discussed and conclusions are drawn.

Temperature and Strain-Rate Dependence of Surface Dislocation Nucleation

Dislocation nucleation is essential to the plastic deformation of small-volume crystalline solids. The free surface may act as an effective source of dislocations to initiate and sustain plastic flow, in conjunction with bulk sources. Here, we develop an atomistic modeling framework to address the probabilistic nature of surface dislocation nucleation. We show the activation volume associated with surface dislocation nucleation is characteristically in the range of 1-10b3, where b is the Burgers vector. Such small activation volume leads to sensitive temperature and strain-rate dependence of the nucleation stress, providing an upper bound to the size-strength relation in nanopillar compression experiments.

Dislocation Nucleation and Segregation in Nano-scale Contact of Stepped Surfaces

ABSTRACTA myriad of engineering applications involve contact between two surfaces, which induces localized plastic deformation near the surface asperities. As a generic problem in studying nanometer scale plastic deformation of solid surfaces, a unit process model of dislocation formation near a surface step under contact loading of a flat rigid surface is considered. The driving force on the dislocation is calculated using conservation integrals. The effect of surface adhesion, step size and lattice resistance on the dislocation driving force are analyzed in a continuum dislocation model, while the nucleation process is simulated atomistically. The driving force formula is used for a dislocation nucleation criterion and to get the equilibrium distance traveled by the dislocation away from the surface step. Results of the unit process model show that under a normal contact load dislocations nucleated in certain slip planes can only stay in a thin layer near the surface, while dislocations nucleated along other slip planes easily move away from the surface into the bulk material. The former dislocation is named anti-load dislocation and the latter dislocation is called pro-load dislocation. Embedded atom method (EAM) is utilized to perform the atomistic simulation of the unit-process model. As predicted by the continuum dislocation model, the atomistic simulations also indicate that surface adhesion plays significant role in dislocation nucleation process. Varying the surface adhesion leads to three different regimes of load-deflection instabilities, namely, just dislocation nucleation instability for no adhesive interaction, two distinct surface adhesion and dislocation nucleation instabilities for weak adhesive interaction and a simultaneous surface adhesion and dislocation nucleation instability for strong adhesive interaction. The atomistic simulations provide additional information on dislocation nucleation and growth near the surface steps. The results of dislocation segregation predict existence of a thin tensile-stress layer near the deformed surface and the results on the adhesion effect provides a cold-welding criterion.

Surface instability and dislocation nucleation in strained epitaxial layers

We have studied numerically the stability and defect nucleation in epitaxial layers on a substrate with lattice mismatch. Stress relaxation and energy barriers for misfit dislocation nucleation are estimated using modern methods for saddle point search based on a combination of activation with local repulsive potential and the Nudged Elastic Band method. Stress relaxation processes correspond to different transition paths from coherent to incoherent states of the epitaxial layer.Using a two-dimensional atomistic model with Lennard-Jones interacting potential, we and different equilibrium critical thickness and activation energy behavior for dislocation nucleation of epitaxial films under tensile and compressive strain. For tensile strain, the energy barrier decreases with thickness while it reaches a constant value for compressive strain.