Direct atomic-scale imaging about the mechanisms of ultralarge bent straining in Si nanowires

To safely and reliably use nanowires (NWs) for exploring new functions for different nanodevices, the mechanical properties and structural evolution of the nanowires under external stress become highly important. Large strain (up to 14%) bending experiments of Si NWs were conducted in a high-resolution transmission electron microscope at atomic resolution. The direct dynamic atomic-scale observations revealed that partial and full dislocation nucleation, motion, escape, and interaction were responsible for absorbing the ultralarge strain of up to 14% in bent Si nanowires. The prevalent full dislocation movement and interactions induced the formation of Lomer lock dislocations in the Si NWs. Finally, in contrast to the unlock process of Lomer dislocations that can happen in metallic materials, we revealed that the continuous straining on the Lomer dislocations induced a crystal-amorphous (c-a) transition in Si NWs. Our results provide direct explanation about the ultralarge straining ability of Si at the nanometer scale.     

Mechanical characterization of Co/Cu multilayered nanowires

The mechanical deformation properties of (110) Co/Cu multilayered nanowires were studied by Molecular Dynamics under uniaxial tensile and compressive stresses. The potential of the immiscible CoCu system was modeled by a second-moment tight-binding approximation. Stress-strain curves at different conditions were obtained and the elastic modulus and yield stress were analyzed. Both magnitudes are approximately independent of the strain rate, except at high values. They decrease linearly with increasing temperature. Below a volume-to-surface-area ratio, their values drastically increase and diverge from the bulk values. If the thickness of the Cu sublayers increases, the Young's modulus and yield stress decrease, although in a different way. The elastic modulus decreases linearly and the yield stress falls steeply whenever Cu is present in the nanowire, since the lattice distortion takes place firstly and fundamentally in Cu sublayers. The change in the axial stress at the interface is little significant on average and rather localized. Unlike, the transverse stress has a non-uniform distribution along the Cu sublayer, especially at the yield point. The Young's modulus and yield stress are larger in tension than in compression. Under tensile stress, nanowires slip via partial dislocation nucleation and propagation. Unlike, compressive deformation of nanowires takes place via both partial and full dislocations.     

Misfit dislocations and stress in In<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>As/GaAs heterostructures

We have estimated the elastic properties of In_{1 − x} Ga _x As/GaAs heterostructures and the characteristics of misfit dislocations in such heterostructures: misfit dislocation spacing, Burgers vector length in various interfaces, surface density of dangling bonds, film/substrate interface energy, critical film thickness below which pseudomorphic growth is possible without misfit dislocations, elastic strain energy of the film-substrate system, average elastic strain of a thin-film island as a function of its radius, thermal stresses induced by the thermal-expansion and lattice mismatches between the layers in contact, and crack length in the film.     

Configuration and local elastic interaction of ferroelectric domains and misfit dislocation in PbTiO3/SrTiO3 epitaxial thin films

Abstract

We have studied the strain field around the 90° domains and misfit dislocations in PbTiO₃/SrTiO₃ (001) epitaxial thin films, at the nanoscale, using the geometric phase analysis (GPA) combined with high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark field––scanning transmission electron microscopy (HAADF-STEM). The films typically contain a combination of a/c-mixed domains and misfit dislocations. The PbTiO₃ layer was composed from the two types of the a-domain (90° domain): a typical a/c-mixed domain configuration where a-domains are 20–30 nm wide and nano sized domains with a width of about 3 nm. In the latter case, the nano sized a-domain does not contact the film/substrate interface; it remains far from the interface and stems from the misfit dislocation. Strain maps obtained from the GPA of HRTEM images show the elastic interaction between the a-domain and the dislocations. The normal strain field and lattice rotation match each other between them. Strain maps reveal that the a-domain nucleation takes place at the misfit dislocation. The lattice rotation around the misfit dislocation triggers the nucleation of the a-domain; the normal strains around the misfit dislocation relax the residual strain in a-domain; then, the a-domain growth takes place, accompanying the introduction of the additional dislocation perpendicular to the misfit dislocation and the dissociation of the dislocations into two pairs of partial dislocations with an APB, which is the bottom boundary of the a-domain. The novel mechanism of the nucleation and growth of 90° domain in PbTiO₃/SrTiO₃ epitaxial system has been proposed based on above the results.     

From individual dislocation motion to collective behaviour

The dynamical behaviour of dislocations under load is analysed in terms of a balance between mutual interactions and lattice friction, defining a screening distance which is compared to dislocation separation. Large friction stresses or low dislocation densities obviously result in individual dislocation motion; a few examples taken from TEM in situ experiments illustrate how mechanisms recorded at the dislocation scale may help in understanding the macroscopic mechanical behaviour of such materials. The pathologic case of strength anomalies is then analysed: the effect of lattice friction is overwhelmed by a strong strain localisation arising from a very low value of the strain rate sensitivity. The resulting collective and intermittent plastic flow makes difficult any direct analysis of dislocation mechanisms within avalanches, whereas observations made in lower density regions may not be representative of the mechanisms responsible for the strength anomaly. Beyond such transient regimes, the screening distance tends to infinity as the lattice friction vanishes. An obstacle-free and fully collective dislocation motion appears (domino effect), characterised by scale-free avalanche size distributions, in which avalanches of any sizes can occur. Such behaviour is reminiscent of the well known self-organised criticality (SOC), making questionable any micro-macro homogeneization procedure based on a supposed representative elementary volume.     

Investigation of distribution and state of rare-earth elements in pure aluminium by method of internal friction

The dislocation internal friction at room temperature for 99.99% pure Al with different rare-earth (RE) contents (0–3 wt %) was measured with Ke's torsion pendulum in a vacuum. A new method is presented for accurately determining the parameters of Granato-Lück plot and processing the experiment data. It was found that at room temperature the unpinning stress of dislocations and the slope of the G-L plot increase with RE content and tend to change little when RE ? 0.1 %. The binding energy between RE atom and dislocation, and the lattice solubility of RE in Al at room temperature were obtained according to the dependence of unpinning strain on temperature, which are about 0.19 eV and 0.0003 wt%, respectively. The existence of solid solution RE in dislocations and the lattice was confirmed and some behaviour of RE in the interior of the Al grain mentioned in part I was also proved.     

A comparative discrete-dislocation/crystal-plasticity analysis of bending of a single-crystalline microbeam

Bending of a micron-size single-crystalline beam is analyzed using both discrete-dislocation plasticity and crystal-plasticity formulations. Within the discrete-dislocation plasticity formulation, dislocations are treated as infinitely long straight-line defects residing within a linear elastic continuum. Evolution of the dislocation structure during bending is simulated by allowing the dislocations to glide in response to long-range interactions between different dislocations, and between dislocations and the applied stresses, and by incorporating various short-range reactions which can result in dislocation nucleation, annihilation or pinning. At each stage of bending, the stress and deformation fields are obtained by superposing the dislocation fields and the complementary fields obtained as a solution of the corresponding linear-elastic boundary value problem. The results obtained show that there is a continuing accumulation of geometrically necessary dislocations during bending which is expected due to the gradient in the strain throughout the beam height. In addition, it is found that localization of plastic flow into slip bands is a salient feature of materials deformation at the micron-length scale. Within the crystal-plasticity analysis, of beam bending, a small displacement gradient formulation is used and the material parameters selected in such a way that plastic flow localizes into deformation bands at low strains. It is found that, while the global response of the beam predicted by the two approaches can be quite comparable, fine details of the dislocation-based stress and deformation fields cannot be reproduced by the continuum crystal-plasticity model.     

The Piezotronic Effect of Zinc Oxide Nanowires Studied by In Situ TEM

Piezotronics is a new field integrating piezoelectric effect into nanoelectronics, which has attracted much attention for the fundamental research and potential applications. In this paper, the piezotronic effect of zinc oxide (ZnO) nanowires, including the response of the electrical transport and photoconducting behaviors on the nanowire bending, has been investigated by in situ transmission electron microscopy (TEM), where the crystal structure of ZnO nanowires were simultaneously imaged. Serials of consecutively recorded current‐voltage (I–V) curves along with an increase of nanowire bending show the striking effect of bending on their electrical behavior. With increasing the nanowire bending, the photocurrent of ZnO nanowire under ultraviolet illumination (UV) drops dramatically and the photo response time becomes much shorter. In addition, the dynamic nanomechanics of ZnO nanowires were studied inside TEM. These phenomena could be attributed to the piezoelectric effect of ZnO nanowires, and they suggest the potential applications of ZnO nanowires on piezotronic devices.     

Mechanical properties of ZnO nanowires under different loading modes

A systematic experimental and theoretical investigation of the elastic and failure properties of ZnO nanowires (NWs) under different loading modes has been carried out. In situ scanning electron microscopy (SEM) tension and buckling tests on single ZnO NWs along the polar direction [0001] were conducted. Both tensile modulus (from tension) and bending modulus (from buckling) were found to increase as the NW diameter decreased from 80 to 20 nm. The bending modulus increased more rapidly than the tensile modulus, which demonstrates that the elasticity size effects in ZnO NWs are mainly due to surface stiffening. Two models based on continuum mechanics were able to fit the experimental data very well. The tension experiments showed that fracture strain and strength of ZnO NWs increased as the NW diameter decreased. The excellent resilience of ZnO NWs is advantageous for their applications in nanoscale actuation, sensing, and energy conversion.      

Strain rate-dependent hardening with dislocation-twin interaction of Fe–Mn–Al–C steel using crystal plasticity

A crystal plasticity finite element model with dislocation-twin interaction was developed to study the strain rate-dependent hardening of Fe–Mn–Al–C twinning-induced plasticity steel. Microstructural state variables including twinning space and dislocation density were incorporated to describe the mechanical twins hindering gliding dislocations. In situ scanning electron microscope tension and electron backscatter diffraction tests were conducted as validation and supplement. Predicted stress and strain hardening rate at various strain rates agree well with the experimental results. The increasing strain hardening stage is attributed to the dynamic competition between deformation twinning and dynamic recovery of dislocations. The intergranular deformation heterogeneity associated with the competitive activities of deformation mechanisms was also studied. The results indicate a larger contribution of slip to overall hardening than twinning.     

Comparison of the Ewald and Wolf methods for modeling electrostatic interactions in nanowires

Ionic compounds pose extra challenges with the appropriate modeling of long‐range coulombic interactions. Here, we study the mechanical properties of zinc oxide (ZnO) nanowires using molecular dynamic simulations with Buckingham potential and determine the suitability of the Ewald (Ann. Phys. 1921; 19) and Wolf (J. Chem. Phys. 1999; 110 (17):8254–8282) summation methods to account for the long‐range Coulombic forces. A comparative study shows that both the summation methods are suitable for modeling bulk structures with periodic boundary conditions imposed on all sides; however, significant differences are observed when nanowires with free surfaces are modeled. As opposed to Wolf's prediction of a linear stress–strain response in the elastic regime, Ewald's method predicts an erroneous behavior. This is attributed to the Ewald method's inability to account for surface effects properly. Additionally, Wolf's method offers highly improved computational performance as the model size is increased. This gain in computational time allows for modeling realistic nanowires, which can be directly compared with the existing experimental results. We conclude that the Wolf summation is a superior technique when modeling non‐periodic structures in terms of both accuracy of the results and computational performance. Copyright © 2010 John Wiley & Sons, Ltd.     

X-ray microbeam measurements of individual dislocation cell elastic strains in deformed single-crystal copper

The distribution of elastic strains (and thus stresses) at the submicrometre length scale within deformed metal single crystals has remarkably broad implications for our understanding of important physical phenomena. These include the evolution of the complex dislocation structures that govern mechanical behaviour within individual grains, the transport of dislocations through such structures, changes in mechanical properties that occur during reverse loading (for example, sheet-metal forming and fatigue), and the analyses of diffraction line profiles for microstructural studies of these phenomena. We present the first direct, spatially resolved measurements of the elastic strains within individual dislocation cells in copper single crystals deformed in tension and compression along <001> axes. Broad distributions of elastic strains are found, with important implications for theories of dislocation structure evolution, dislocation transport, and the extraction of dislocation parameters from X-ray line profiles.     

Dislocations in single hemp fibres—investigations into the relationship of structural distortions and tensile properties at the cell wall level

The relationship between dislocations and mechanical properties of single hemp fibres (Cannabis sativa L. var. Felina) was studied using a microtensile testing setup in a 2-fold approach. In a first investigation the percentage of dislocations was quantified using polarized light microscopy (PLM) prior to microtensile testing of the fibres. In a second approach PLM was used to monitor the dislocations while straining single fibres. The first part of the study comprised 53 hemp fibres with up to 20% of their cell wall consisting of dislocations. For this data set the percentage of dislocations did not affect the mechanical properties. In the second part of the study it was found that dislocations disappeared during tensile testing, and that they did not reappear until several weeks after failure. A strain stiffening effect due to the straightening of the dislocations was not observed. It is possible that the former positions of the dislocations functioned as locations for crack initiation. However, the crack does not propagate transversely all the way trough the dislocation but results in a shear failure between the microfibrils. In rheological studies fibres were strained at constant stress levels, and dislocations that had disappeared did not reappear during that period.     

Toward High Strength and High Electrical Conductivity in Super‐Aligned Carbon Nanotubes Reinforced Copper

The miniaturization of electronic products is drawing higher demand in the strength and conductivity of conductors. This work demonstrates the possibility of substantially increasing the dislocation density in copper to enhance the strength of super‐aligned carbon nanotubes (SACNTs) reinforced copper matrix composites (SACNT/Cu) without compromising the electrical conductivity. High strain is introduced into pure copper and SACNT/Cu by accumulative roll‐bonding (ARB) process up to 16 cycles at ambient temperature. SACNTs with initial laminated distribution turn out to be dispersed uniformly with maintained directional arrangement inside the copper matrix after ARB, which can then effectively block the motion of dislocations. Therefore, large number of dislocations propagated by large strains can be accumulated without subdivision. The accumulated dislocations will result into strain hardening, which is the major strengthening mechanism in SACNT/Cu after ARB. Furthermore, the contribution of dislocations to resistivity increase is little, thus maintaining high electrical conductivity. As a result, a high tensile strength (505 MPa) combined with a high electrical conductivity (90% IACS) is achieved in large‐sized composite sheet.

     

Evolution of the mosaic structure in InGaN layer grown on a thick GaN template and sapphire substrate

The In_xGa_1?xN epitaxial layers, with indium (x) concentration changes between 0.16 and 1.00 (InN), were grown on GaN template/(0001) Al₂O₃ substrate by metal organic chemical vapour deposition. The indium content (x), lattice parameters and strain values in the InGaN layers were calculated from the reciprocal lattice mapping around symmetric (0002) and asymmetric (10–15) reflection of the GaN and InGaN layers. The characteristics of mosaic structures, such as lateral and vertical coherence lengths, tilt and twist angle and heterogeneous strain and dislocation densities (edge and screw dislocations) of the InGaN epilayers and GaN template layers were investigated by using high-resolution X-ray diffraction (HR-XRD) measurements. With a combination of Williamson–Hall (W-H) measurements and the fitting of twist angles, it was found that the indium content in the InGaN epilayers did not strongly effect the mosaic structures’ parameters, lateral and vertical coherence lengths, tilt and twist angle, or heterogeneous strain of the InGaN epilayers.     

Strain hardening in nanolayered thin films

Experimental results indicate that metal–ceramic multilayered thin films have unusual properties such as high strength, measurable plasticity and high strain hardening rate when both layers are nanoscale. Furthermore, the strength and strain hardening rate show a pronounced size effect, depending not only on the layer thickness but also on the layer thickness ratio. We analyze the strain hardening behavior of nanoscale multilayers using a three-dimensional crystal elastic–plastic model (3DCEPM) that describes plastic deformation based on the evolution of dislocation density in metal and ceramic layers according to confined layer slip mechanism. These glide dislocations nucleate at interfaces, glide inside layers and are deposited at interfaces that impede slip transmission. The high strain hardening rate is ascribed to the closely spaced dislocation arrays deposited at interfaces and the load transfer that is related to the layer thickness ratio of metal and ceramic layers. The measurable plasticity implies the plastically deformable ceramic layer in which the dislocation activity is facilitated by the interaction force among the deposited dislocations within interface and in turn is strongly related to the ceramic layer thickness.     

Formation of chiral branched nanowires by the Eshelby Twist

Manipulating the morphology of inorganic nanostructures, such as their chirality and branching structure, has been actively pursued as a means of controlling their electrical, optical and mechanical properties. Notable examples of chiral inorganic nanostructures include carbon nanotubes, gold multishell nanowires, mesoporous nanowires and helical nanowires. Branched nanostructures have also been studied and been shown to have interesting properties for energy harvesting and nanoelectronics. Combining both chiral and branching motifs into nanostructures might provide new materials properties. Here we show a chiral branched PbSe nanowire structure, which is formed by a vapour-liquid-solid branching from a central nanowire with an axial screw dislocation. The chirality is caused by the elastic strain of the axial screw dislocation, which produces a corresponding Eshelby Twist in the nanowires. In addition to opening up new opportunities for tailoring the properties of nanomaterials, these chiral branched nanowires also provide a direct visualization of the Eshelby Twist.     

Mechanical properties of Si nanowires as revealed by in situ transmission electron microscopy and molecular dynamics simulations

Deformation and fracture mechanisms of ultrathin Si nanowires (NWs), with diameters of down to ~9 nm, under uniaxial tension and bending were investigated by using in situ transmission electron microscopy and molecular dynamics simulations. It was revealed that the mechanical behavior of Si NWs had been closely related to the wire diameter, loading conditions, and stress states. Under tension, Si NWs deformed elastically until abrupt brittle fracture. The tensile strength showed a clear size dependence, and the greatest strength was up to 11.3 GPa. In contrast, under bending, the Si NWs demonstrated considerable plasticity. Under a bending strain of <14%, they could repeatedly be bent without cracking along with a crystalline-to-amorphous phase transition. Under a larger strain of >20%, the cracks nucleated on the tensed side and propagated from the wire surface, whereas on the compressed side a plastic deformation took place because of dislocation activities and an amorphous transition.     

Micro-scale modeling of interface-dominated mechanical behavior

A micro-scale interface dislocation dynamics approach to model the mechanical behavior of crystalline nanolaminates is presented. To circumvent the exhaustive atomistic modeling of interfaces and dislocations in nanolaminates, an atomistically informed dislocation dynamics model was developed in which interfaces are categorized using a geometrical interface classification scheme and the interface-dominated mechanical response is related to nucleation, glide, and reactions of lattice and interface dislocations at/within/across interfaces. We show that such a scheme is effective in mapping the structure–property relations of various types of interfaces.     

Nanoscale size dependence parameters on lattice thermal conductivity of Wurtzite GaN nanowires

A detailed calculation of lattice thermal conductivity of freestanding Wurtzite GaN nanowires with diameter ranging from 97 to 160 nm in the temperature range 2–300 K, was performed using a modified Callaway model. Both longitudinal and transverse modes are taken into account explicitly in the model. A method is used to calculate the Debye and phonon group velocities for different nanowire diameters from their related melting points. Effect of Gruneisen parameter, surface roughness, and dislocations as structure dependent parameters are successfully used to correlate the calculated values of lattice thermal conductivity to that of the experimentally measured curves. It was observed that Gruneisen parameter will decrease with decreasing nanowire diameters. Scattering of phonons is assumed to be by nanowire boundaries, imperfections, dislocations, electrons, and other phonons via both normal and Umklapp processes. Phonon confinement and size effects as well as the role of dislocation in limiting thermal conductivity are investigated. At high temperatures and for dislocation densities greater than 10¹⁴ m^?2 the lattice thermal conductivity would be limited by dislocation density, but for dislocation densities less than 10¹⁴ m^?2, lattice thermal conductivity would be independent of that.