Dislocation-based Mechanisms Research Articles

Effect of Zr4+ doping on the creep behavior of the oxygen transport membrane BaFeO3-d

Aiming towards insight into the stability of newly developed advanced ceramic membrane materials, the time-dependent deformation at elevated temperatures is particularly important, where special consideration should be given to application-relevant atmospheres. This study examines the influence of Zr4+ doping on the creep behavior of BaFeO3-δ in air and nitrogen atmospheres at temperatures between 700 ºC and 900°C under compressive stresses ranging from 20 MPa to 60 MPa. BaFeO3-δ creep investigations revealed a transition from primarily diffusive to a combination of diffusive and dislocation-based mechanisms, owing to increasing oxygen deficiencies with increasing temperatures that culminate in a low to high symmetric crystal structure transition. Zr4+ doping improved the creep resistance and altered the creep mechanism to a predominantly dislocation-based mechanism, accompanied by a transition to a cubic crystal structure. Nitrogen atmospheres negatively affected the creep resistance of BaFeO3-δ and BaFe0.9Zr0.1O3-δ, increasing the creep deformation rates for all temperature and stress levels.

Dislocation-based mechanical responses and deformation mechanisms of Al/Cu heterointerfaces: A computational study via molecular dynamics simulations

Layered metal composites have been widely used in various industries fields because of their excellent properties and are responsible for mechanical behavior of materials. This paper focuses on analyzing the deformation mechanism of the (110) interface under different loading states by MD. The results show that there are two yield points in the stress–strain curve under both Z- and Y-axis loading states. The first yield is the nucleation of dislocations at the interface, meantime the slip of dislocations and the extension of stacking faults begin at the Al layer. The second one, the dislocation passes through the interface, nucleates and emits toward the Cu layer at the interface, leading to a stress mutation. It is worth noting that during the stable rheological stage, the deformation mechanisms vary under different loading directions. Under Y-axis tensile loading, the phase transformation of FCC–HCP is present due to the interaction of dislocation movement and stacking fault. On the contrary, there are two twin paths A and B, improving the strength, during Z-axis compression loading. For other loading modes, there are three zones, namely the elastic stage, the release of energy, and strain hardening and dynamic softening. The interface plays the role of nucleation, annihilation and penetration of dislocations, and this interface-dislocation mechanism is reflected in the whole stage of plastic deformation. The results have an insight into the design and control in heterointerface.

Atomistic insights into nanoindentation-induced deformation of [formula omitted]-Al[formula omitted]O[formula omitted] single crystals

The plastic deformability of brittle ceramics, e.g., Al2O3, at a small scale can expand their potential structural applications. In this work, we propose to investigate the nanomechanics and plasticity of α-Al2O3 using molecular dynamics (MD) simulations at room temperature. First, nanoindentation MD simulations are performed with single crystalline α-Al2O3. Four crystallographic orientations are investigated, m11̄00, a21̄1̄0, R1̄012 and c0001, including a detailed analysis of Al2O3 dislocation-based mechanisms. The results show that the O atoms undergo a phase transformation, changing from the hexagonal close-packed structure to face-centered cubic and body-centered cubic structures. Second, during nanoindentation, we focus on pop-in events and the transformation point from elastic to inelastic response during loading forces. The results are discussed in the context of recent transmission electron microscope experiments, possibly opening new doors towards ceramic bulk material processes.

Effects of fluid flow in triple porosity medium on fracture width and its propagation during lost circulation control

During oil and gas well construction, lost circulation is one of the major challenges, and to migrate the problem, lost circulation materials (LCMs) should be used to resist fracture reopening with the hypothesis of fracture tip isolation. A triple porosity medium Darcy flowing model was developed to capture pressure distribution along the fracture surface, coupled with a dislocation-based fracture mechanics model to calculate fracture width and assess its propagation. In the model, mud cake on wellbore and fracture surface was considered as the third type of pore medium. The rest dual pore medium models incorporate the fluid flow through LCMs within fracture and the fluid leakoff from fracture into matrix. Sensitivity analyses of the key parameters, such as triple pore medium permeability, viscosity, wellbore pressure, plugging location are implemented, and the results emphasize that fracturing resistance effect of LCMs was controlled not only by the fluid flow within fractures, but also by the flow through mud cake on wellbore into matrix. To inhibit fracture reopening, in high permeability formations, both reduction of mud cake permeability and LCMs permeability were required while in lower permeability formations, fluid flow within fractures dominates and should be minimized. Besides, tip isolation can be achieved by extending sealing fracture length. However, high wellbore pressure or long operation time will result in elevated distances and unallowable fluid loss. Then, wellbore pressure must be reduced to cure the lost circulation. More viscosity of the fluid and total integrity plugging of LCMs are another major factor extending the time for pressure transfer to fracture tip, resisting unexpected fracture propagation. The method proposed in this study clarifies that tip isolation can be realized by fracture plugging and reduction of fluid filtration from the wellbore to matrix. Moreover, operational insights on LCMs design can also be derived.

A discrete dislocation dynamics framework for modeling polycrystal plasticity with hardening

A two-dimensional discrete dislocation dynamics (DDD) framework is extended to model plastic deformation in polycrystalline materials having realistic grain morphology in finite geometries. The formulation includes constitutive rules to represent intra-grain dislocation mechanisms as well as dislocation/slip transmission across grain boundaries. Rules for intra-granular interactions mimic key three-dimensional dislocation mechanisms, such as line tension and dynamic junction and source formation, in addition to other short-range dislocation interactions. Various effects of idealized three-dimensional dislocation mechanisms on the mechanical response in tension are explored. The scaling of flow strength with grain size is investigated with and without slip transmission at grain boundaries. To aid in the analysis, spatial mappings of relevant stress components and geometrically necessary dislocation density are produced, which bring out competing dislocation-based strengthening mechanisms leading to the observed grain size effect in polycrystals.

Linear complexions directly modify dislocation motion in face-centered cubic alloys

Linear complexions are defect phases that form in the presence of dislocations and thus are promising for the direct control of plasticity. In this study, atomistic simulations are used to model the effect of linear complexions on dislocation-based mechanisms for plasticity, demonstrating unique behaviors that differ from classical dislocation glide mechanisms. Linear complexions impart higher resistance to the initiation and continuation of dislocation motion when compared to solid solution strengthening in all of the face-centered cubic alloys investigated here, with the exact strengthening level determined by the linear complexion type. Stacking fault linear complexions impart the most pronounced strengthening effect, as the dislocation core is delocalized, and initiation of plastic flow requires a dislocation nucleation event. The nanoparticle and platelet array linear complexions impart strengthening by acting as pinning sites for the dislocations, where the dislocations unpin one at a time through bowing mechanisms. For the nanoparticle arrays, this event occurs even though the obstacles do not cross the slip plane and instead only interact through modification of the dislocation's stress field. The bowing modes observed in the current work appear similar to traditional Orowan bowing around classical precipitates but differ in a number of important ways depending on the complexion type. As a whole, this study demonstrates that linear complexions are a unique tool for microstructure engineering that can allow for the creation of alloys with new plastic deformation mechanisms and extreme strength.

Small-scale mechanical properties and deformation mechanisms of a laser welded CrCoNi medium-entropy alloy joint

Attributed to deformation twins, laser welded CrCoNi medium-entropy alloy joints exhibit best combination of strength and ductility among the joints of Cantor alloy and its derivatives. However, the nano/microscale mechanical properties and associated deformation mechanisms of the welded CrCoNi alloy joint remain mysterious. Here, we fabricated cylindric pillars with diameter from 500 nm to 2000 nm in a single crystal of the fusion zone, and then compressed these pillars in a nano-indenter. Results showed that the yield stress and flow stress of these pillars were size-dependent, with a “smaller is stronger” trend. Via examination and analysis of the deformed microstructure using electron backscattered diffraction, transmission electron microscope, and large-scale atomistic simulations, we found that deformation mechanism in the single crystal at nano/microscales was the formation of nanoscale stacking-fault networks. Large-scale atomistic simulations further revealed that dislocation-based mechanisms, such as entanglements, and annihilation from the free surface of the pillar, dominated the compressive deformation. Finally, a dislocation-based theoretical model was successfully utilized to predict the critical resolved shear stress and the size effect, which matched well with our experimental results.

Microstructure and Mechanical Properties of Low-Density, B2-Ordered AlNbZrTix Multi-Principal Element Alloys

Low-density multi-principal element alloys (MPEAs) combining a high specific strength and considerable ductility have remained a research hotspot, due to their promising prospects for energy-saving industrial applications. Light Ti-containing AlNbZrTix (x = 1−3) MPEAs were designed and prepared by induction melting and annealing. As the Ti content increases, the microstructure of these MPEAs evolves from dual phase (B2-ordered and Zr5Al3-type structure) into a single-phase B2-ordered structure, while the density reduces by ~8.7%, from ~5.85 g·cm−3 (x = 1) to ~5.34 g·cm−3 (x = 3). Unexpectedly, the AlNbZrTix (x = 1, 2, 3) alloys possess high specific yield strengths of ~270 kPa·m3·kg−1, ~221 kPa·m3·kg−1, >208 kPa·m3·kg−1, along with excellent fracture strains of ~17.8%, 21.8%, and >50%, respectively. These combined compressive properties are superior to the reported data of most BCC/B2-dominant MPEAs. The deformation mechanism of the B2-ordered structure is explained as a dislocation-based mechanism, accompanied by antiphase domains. Here, the effect of Ti on the microstructure and compressive properties of AlNbZrTix MPEAs was investigated, providing scientific support for the development of advanced low-density materials.

Tensile Failure and Fracture Width of Partially Permeable Wellbores with Applications in Lost Circulation Material Design

SummaryEstimation of near-wellbore fracture widths remains central to designing the particle size distribution (PSD) and composition of lost circulation material (LCM) blends. Although elastic rock models are often used for this purpose, they fall short in capturing the substantial effect of pore fluid pressure on the fracture width. The problem is addressed in this paper by incorporating the poroelastic back stress in width estimation of axial fractures nearby an inclined wellbore. The poroelastic back stress is caused by a nonideal drilling fluid filter cake allowing for fluid pressure communication between the wellbore and pore space of the rock surrounding the wellbore. In this aspect, a proper definition of the filter-cake efficiency is made in terms of the wellbore pressure, far-field pore fluid pressure, and pore fluid pressure of the rock surrounding the wellbore. The value of this parameter is estimated from the standard drilling fluid filtration test results, as well as the formation rock permeability. The filter-cake efficiency is next used to develop the long-time, asymptotic analytical solution for the poroelastic stress of an inclined wellbore. By accounting for the obtained poroelastic back stress, an improved estimation of the wellbore tensile limit that depends on the filter-cake efficiency parameter is developed. For wellbore pressures beyond the wellbore tensile limit, the width of the near-wellbore fractures is estimated. The fracture width estimation is made through an analytical, dislocation-based fracture mechanics solution to the integral equation describing the nonlocal stress equilibrium along the fracture faces. The commonly practiced scheme for geometric design of LCM blends is enhanced by using the presented improvement in estimation of the near-wellbore fracture width. A case study is used to demonstrate the substantial effect of drilling fluid filtration properties and the resulting poroelastic back stress on the wellbore tensile limit, estimated fracture width, and consequently, composition of the recommended LCM blend.

A novel deformation mechanism in Ti-V binary metastable β-Ti alloys: Deformation kinking promoted by dislocation accumulation

Unlike stress-induced martensitic transformation, deformation twinning as well as dislocation gliding, deformation kinking is an uncommon deformation mechanism in metastable β-titanium (Ti) alloys. In this study, the unique deformation mechanism was reported in Ti-V binary β-Ti alloys for the first time. It was found that a large number of kink bands were distributed in β-grains after cold forging at a strain rate of ~103 s−1. Microstructural characterization manifests that the appearance of kink bands is frequently accompanied by slip bands, whilst dense dislocations are accumulated around the kink bands on a microscopic scale. Such local plastic deformation is quite strong so that the pre-existing athermal ω-precipitates in the initial microstructure is completely destroyed within the kink bands and instead fresh deformation-induced single variant of ω-precipitates is produced. This deformation localization mainly originates from the impediment of dislocation activity caused by both pre-existing athermal ω-precipitates and the intrinsic slip retardation in BCC crystal structures. The deformation kinking is thus suggested to follow a dislocation-based formation mechanism, whilst the resulting Taylor axis of lattice rotation was deduced to be<011>β on basis of extensive crystallographic analysis. These findings enrich fundamental understanding on deformation kinking, and provide helpful information for utilizing the unique deformation mechanism to tune mechanical properties of β-Ti alloys.

Inelastic Deformation Mechanisms in Shock Compressed Polycrystalline Pure Magnesium at Temperatures Approaching Melt

In this paper, we investigate inelastic deformation mechanisms in polycrystalline commercially-pure (99.9%) magnesium samples shock-compressed using reverse-geometry normal plate-impact experiments at elevated temperatures. The favorability of orientation for the possible deformation modes (i.e. basal slip, prismatic slip, pyramidal I & II slip and extension/contraction twinning) is investigated using Schmid Factor analysis using the EBSD data. The study is an extension of a recent study by the authors [Wang et al. in J Dyn Behav Mater 3:497–509, 2017], and is motivated by the need to better understand the underlying inelastic deformation mechanisms operative in shock-compressed Mg at incipient plasticity at near-melt temperatures. Electron Backscatter Diffraction analysis of the as-received polycrystalline magnesium samples at room temperature reveal that the $$\langle 10\bar{1}0\rangle$$ directions (normal to the prismatic planes) are parallel to the impact direction, while the [0001] poles are aligned with the radial directions (RD). Because of the unfavorable orientation for basal slip and the high Critical Resolved Shear Stresses (CRSS) for the other non-basal slip systems, extension twinning is observed to be the preferred deformation mode in the samples. In addition, results from samples tested at 400 °C, 500 °C, 610 °C and 630 °C, indicate progressive grain coarsening and a notable increase in extension twin activity at the two highest test temperatures used in the study, i.e. 610 °C and 630 °C. In general, for polycrystalline hcp metals, an increase in grain size and test temperatures are understood to promote dislocation-mediated slip and not dislocation twinning; however, in the present experiments, at the two highest test temperatures a notable increase in extension twin area fraction is observed, indicating an increased resistance (suppression) of dislocation slip at incipient inelasticity. These results, along with the experimentally measured particle velocity profiles at the two highest test temperature experiments (610 °C and 630 °C), provide evidence for transition in dislocation-based slip mechanisms from being thermally activated to viscous drag mediated under the normal shock compression and elevated temperature conditions of the present experiments.

Twin nucleation from a single dislocation in hexagonal close-packed crystals

Twinning plays an important role in governing the balance between strength and ductility in hexagonal-close-packed (HCP) metals. Here, we report a combined experimental and theoretical study of twin nucleation from a single <c+a> dislocation in HCP crystals. Specifically, high-resolution transmission electron microscopy has been used to identify {112¯1} twin nuclei in HCP rhenium, providing evidence of their nucleation from a <c+a> dislocation. The favorability of this dislocation-based nucleation mechanism is rationalized by an anisotropic elasticity model of <c+a> dislocation dissociation, parametrized by density functional theory calculations, which suggests the conditions for disconnection nucleation and propagation, under which this {112¯1} twinning mechanism is expected to be effective. The analysis serves to advance our understanding of the origin of the unique predominance of {112¯1} twinning in rhenium, which correlates with the high strength and ductility featured by this metal. It also provides new insights into design strategies that may be effective in activating this twinning mode and enhancing the balance between strength and ductility in HCP alloys more broadly.

An energy-based stability analysis of misfit dislocations in two-phase electrode particles: A planar model and implications for LiFePO4

It is well known that cycling induced capacity fading in phase transforming intercalation materials is critically linked to the severity of misfit strain between the phases which could result in the emergence of a variety of defects in the material. Here, adopting a planar model and making use of a well-established energy-based framework, we examine stability of a misfit dislocation lying at the phase boundary within a two-phase electrode particle. Within this framework, the stability criterion emerges as a result of competition between the work that must be done against the image forces with the work that is supplied by the phase transformation induced stresses, as the dislocation is pulled from free surface of the solid toward interior. A numerical model accounting for anisotropy of both misfit strain between the phases and elastic properties of the solid is developed. The model is in particular applied to the electrode particles of LiFePO4, a highly promising cathode material for lithium-ion battery applications in which misfit dislocations as a result of phase transformation have also been experimentally observed. The analysis is revealing of size effects on the stability of a misfit dislocation in the electrode particle. Most notably, it is shown that below a critical particle size, no misfit dislocation at the interphase can remain stable within the particle as it becomes energetically more favorable for the dislocation to be driven out of the particle through absorption by the free surfaces. Numerical estimates of the critical size are presented and discussed with reference to available experiments. Through providing a guideline for suppressing misfit dislocations, results of this work could have potentially important implications for disabling a wide range of dislocation-based fatigue mechanisms which could otherwise become active and result in capacity fading upon a large number of cycles.

Controlled fabrication of gold nanotip arrays by nanomolding-necking technology

The fabrication of nanotips has been driven by the increasing industrial demands in developing high-performance multifunctional nanodevices. In this work, we proposed a controlled, rapid as well as low cost nanomolding-necking technology to fabricate gold nanotips arrays. The geometries of gold nanotips having cone angle range of ∼28–77° and curvature radii of <5 nm can be prepared by tailoring the diameters of raw nanorods in nanomolding process or modulating the necking temperature. Molecular dynamics simulation reveals that the formation of the nanotip geometry is determined by the interplay between dislocation-based and diffusion-based deformation mechanisms, intrinsically arising from the nonlinear dependence of atom diffusion on temperature and sample size. The good controllability, mass production and low cost of the developed nanomolding-necking technology make it highly promising in developing nanodevices for a wide range of applications, such as probing, sensing, antireflection coating and nanoindentation.

Mechanical Properties of Laser Shock-Peened Regions of SS316LN and SS304 Studied by Nanoindentation

Nanoindentation has been used to evaluate various mechanical properties of affected regions of laser shock-peened (LSP) stainless steels, SS316LN and SS304 plates. Depending on the applied laser parameters (laser energy, pulse width), the hardness was varied in the range 3.5–6.2 GPa in comparison with the untreated sample hardness of 2.93 GPa for SS316LN. Similarly in SS304, the hardness has been improved to 5.73 GPa in comparison with the untreated specimen hardness of 2.57 GPa. The improvement in the hardness for the laser-treated stainless steel targets is attributed to various factors viz., increment in internal frictional stress, dislocation–dislocation interaction and solute–dislocation interactions. Effect of laser energy is not appeared in the hardness values but a difference in hardness values with the pulse width has been observed. The deformation characteristics such as strain rate sensitivity and activation volume were evaluated from the indentation creep data. Notable change in the strain rate sensitivity is not observed for the laser-treated samples in comparison with the untreated samples and the range of values measured between 0.01 and 0.02. The activation volumes for both the treated and untreated samples were in the range, 100–200 b3. There was a slight decrease in activation volume in the laser-treated samples. Several rate-controlling deformation mechanisms appear to be operative in these treated samples, in addition to the dislocation-based mechanisms.

Dislocation cross slip in molecular crystal cyclotetramethylene tetranitramine (β-HMX)

In this work, we explore the mechanism of cross-slip in the low symmetry molecular crystal cyclotetramethylene tetranitramine (β-HMX)—a secondary explosive. Cross-slip is well studied and understood in high symmetry crystals but virtually uninvestigated in molecular crystals. To this end, we use molecular simulations and observe that only screw dislocations with the [100] Burgers vector may cross-slip effectively. The process involves the (011), (010), (001), and (011¯) planes and takes place in both the positive and negative directions of dislocation motion in each of the respective slip systems. Resolved shear stresses larger than ∼0.6 of the critical resolved shear stress are necessary in at least two of the planes in order to activate cross-slip. The application of pressure does not prevent cross-slip from taking place. The phenomenon occurs at elevated pressures in the same slip systems as at zero pressure. However, due to the limited number of slip systems involved, cross-slip does not appear to be of central importance in β-HMX and, of course, remains relevant only as long as the dislocation-based mechanism of plasticity is not replaced by the shear localization mode, which becomes dominant at high pressure, under strong shock conditions.

Mechanical properties of stabilized nanocrystalline FCC metals

In this perspective, recent advances and current research challenges concerning the mechanical properties of stabilized nanocrystalline face-centered cubic (FCC) metals are discussed. First, a brief review of key experiments and modeling efforts over the last two decades is provided, with a focus on elucidating the mechanisms associated with plastic yield, hardening, and microstructure stabilization in nanocrystalline metals. This prior work has provided an understanding of the transition between dislocation-based and grain boundary-mediated mechanisms in plasticity and has identified several strategies to mitigate temperature or stress driven grain growth. Yet, the consequence of various stabilization methods on mechanical properties is not well understood. Future research challenges are presented in order to address this scientific gap, most critically the need to include grain boundary chemistry or grain boundary phases resulting from stabilization methods in new mechanistic theories for mechanical properties of nanocrystalline FCC metals.

Determination of the dislocation-based mechanism(s) responsible for the anomalous ductility of a class of B2 intermetallic alloys

A class of stoichiometric, ordered MR alloys (where M denotes a metal and R either a rare earth or group IV refractory metal) with B2 (CsCl) crystal structure, were discovered to possess anomalous ductility by researchers at Ames Laboratory in the early 2000’s. Despite active research on the topic for over 15 years, a holistic explanation for the anomalous ductility of these metal alloys has not yet been accepted by the scientific community. The reason for this appears to be a failure to account for all relevant length scales. Researchers either focus on the atomistic length scale relevant to dislocation core structures, the microscopic length scale of dislocations and dislocation ensembles, the mesoscopic length scale of grains and their interactions, or the macroscopic scale of mechanical testing samples with millimetre dimensions or larger. Focused studies at each of these length scales have provided essential, but insufficient information to provide a complete answer to the question. The insight provided by in-situ diffraction studies, along with crystal plasticity modelling as an interpretive tool, and a novel grain-by-grain line profile analysis technique for assessing dislocation densities in polycrystals is highlighted. In addition to the primary <100> dislocations, which are most easily activated in all of these anomalous MR alloys, the results show that dislocations with large Burgers vectors (e.g., <110> and <111>) are present in large numbers beyond a transition in the strain hardening response. The motion of these large Burgers vector dislocations is experimentally observed to occur at stress levels consistent with atomistic modelling predictions of the Peierls resistance and simultaneously provides a satisfying explanation for the anomalous ductility and other experimental results.

Macroscopic to nanoscopic in situ investigation on yielding mechanisms in ultrafine grained medium Mn steels: Role of the austenite-ferrite interface

Ultrafine austenite-ferrite duplex medium Mn steels often show a discontinuous yielding phenomenon, which is not commonly observed in other composite-like multiphase materials. The underlying dislocation-based mechanisms are not understood. Here we show that medium Mn steels with an austenite matrix (austenite fraction ∼65 vol%) can exhibit pronounced discontinuous yielding. A combination of multiple in situ characterization techniques from macroscopic (a few millimeters) down to nanoscopic scale (below 100 nm) is utilized to investigate this phenomenon. We observe that both austenite and ferrite are plastically deformed before the macroscopic yield point. In this microplastic regime, plastic deformation starts in the austenite phase before ferrite yields. The austenite-ferrite interfaces act as preferable nucleation sites for new partial dislocations in austenite and for full dislocations in ferrite. The large total interface area, caused by the submicron grain size, can provide a high density of dislocation sources and lead to a rapid increase of mobile dislocations, which is believed to be the major reason accounting for discontinuous yielding in such steels. We simultaneously study the Lüders banding behavior and the local deformation-induced martensite forming inside the Lüders bands. We find that grain size and the austenite stability against deformation-driven martensite formation are two important microstructural factors controlling the Lüders band characteristics in terms of the number of band nucleation sites and their propagation velocity. These factors thus govern the early yielding stages of medium Mn steels, due to their crucial influence on mobile dislocation generations and local work hardening.

On the ductile damage of nanotwinned copper crystal with prolate void defect at the twin boundary

Void growth and coalescence is one of the primary causes that are responsible for the ductile damage of metals. Extensive literature works have been conducted for exploring the dislocation-based damage mechanisms due to cylindrical or spherical voids embedded in single or polycrystals. In this paper, molecular dynamics simulations were performed for elucidating the ductile damage mechanisms of nanotwinned copper crystal containing prolate void defects sitting along the twin boundary. The effects of length scale, strain rate, temperature and void fraction on the strength, stiffness and dislocation evolution of the proposed nanotwinned model are studied in detail. Starting at the critical yield stress, the successive nucleation, propagation and reaction of Shockley partial dislocations are responsible for the continuous void growth process and result in a dramatic stress drop immediately following the ultimate strength. An obvious dependence on length scale is found for both the yield and the peak stress. The continuum Lubarda model that was developed for characterizing the critical stress of spherically voided nanocrystals is found to appreciably underestimate the strength of nanotwinned copper embedded with prolate voids. Simulation results also reveal that twin boundaries function as a dislocation barrier that is able to prevent dislocations from penetrating through. It is by this mechanism that twin boundaries affect the strength of nanotwinned crystals.