Geometry Of Dislocations Research Articles

Tuning of Single Mixed (Helical) Dislocations in Core-Shell van der Waals Nanowires.

Linear defects (dislocations) not only govern the mechanical properties of crystalline solids but they can also produce distinct electronic, thermal, and topological effects. Accessing this functionality requires control over the placement and geometry of single dislocations embedded in a small host volume to maximize emerging effects. Here we identify a synthetic route for rational dislocation placement and tuning in van der Waals nanowires, where the layered crystal limits the possible defect configurations and the nanowire architecture puts single dislocations in close proximity to the entire host volume. While homogeneous layered nanowires host single screw dislocations, the synthesis of radial nanowire heterostructures (here exemplified by GeS-Ge1-xSnxS monochalcogenide core-shell nanowires) transforms the defect into a mixed (helical) dislocation whose edge/screw ratio is tunable via the core-shell lattice mismatch. The ability to design nanomaterials with control over individual mixed dislocations paves the way for identifying the functional properties of dislocations and harnessing them in technology.

Strengthening via Orowan Looping of Misfitting Plate-like Precipitates

Many common engineering alloys are strengthened by precipitates with plate-like geometries. The mechanics of precipitation strengthening, while well resolved with spherical precipitates, are less well understood for plate-like geometries. In this work, we employ discrete dislocation dynamics simulations to study precipitation strengthening by θ′ precipitates in AlCu. We show that Orowan looping with parallel plate-like precipitates can be fundamentally different from spheres because it is governed by a steady-state glide process rather than a single critical dislocation geometry. We develop a model based on the formation of a dislocation dipole across the precipitate thickness to explain the observed strengthening. Finally, we show that while the precipitate misfit field can lead to both strengthening and weakening, strengthening is more common because of the steady-state process by which Orowan loops form.

GEOMETRY OF RECENT RUPTURE ZONE AT СAPE RYTYI BASED ON GROUND PENETRATING RADAR (NORTHWESTERN COAST OF LAKE BAIKAL)

In here, we present the results of the GPR study of the modern rupture zone discovered earlier in the periphery of the Rita River delta flowing into Lake Baikal. The research was aimed at subsurface imaging of dislocation geometry using the Logis-Geotech OKO-2 radar equipped with the ABDL Triton antenna. As a result, the characterizations have been obtained for six 73 to 197 m long profiles across the rupture zone. All disturbances visible on the aerial photography materials are highlighted on the radargrams by the oblique reflection event due to a decrease in the signal amplitude. The rupture zone has two large segments – northeastern and submeridional, – which differ in zone width, number of discontinuities, dip angles, and displacement amounts. The deformation features can be attributed to different amounts of a plastic loamy aggregate of coarse deposits in different parts of the Rita River delta that is indirectly confirmed by weaker signal amplitudes on the northeastern segment. Based on present and previous studies, we proposed that the M=5.2 earthquake occurred on August 13, 1962, in Lake Baikal initiated the gravitational sinking in the Rita river delta edge by subsidence along gently sloped rupture surfaces on one segment and along steeply sloped ones on the other. It is necessary to identify and monitor such phenomena to prevent the development of emergencies associated with the collapse of the coasts of large water reservoirs.

Structure and evolution features of cutinite with different coal rank from stacking and arrangement of aromatic fringes in HRTEM

The structure and evolutionary features of cutinites isolated from the two samples of different ranks were evaluated by image analysis of aromatic fringes in high resolution transmission electron microscopy (HRTEM) combined with Photoshop and ArcGIS software. The distributions of aromatic fringes (length, orientation, curvature, segmented angle and cumulative angle) were quantified to establish the differences in stacking and spatial arrangement of the aromatic systems in the two cutinites. The bonding states of organic carbon in cutinite, derived from XPS spectra, mainly existed in the form of –CC–, followed by –C–O bonds, whereas the content of π–π bonds was the lowest. The content of oxygen-containing groups decreased and the amount of π–π bonds increased with rank. HRTEM aromatic fringe image analysis showed that short (<0.59 nm) and middle-sized fringes (<1.49 nm) in cutinite were dominant, and that the aggregated structure was mainly 1–2-ring aromatics. The preferred orientations and distribution of aromatic fringes were closely related to the stacking configurations of aromatic systems and to the rank. The larger polyaromatic clusters were observed with higher rank, and the aromatic fringes became more regular. The aromatic systems of cutinite-P (cutinite isolated from Panzhihua area) contained abundant carbon structural layers with screw dislocations, and the contents of Y-type and T-shape arrangements were lower than those of cutinite-L (cutinite isolated from Luquan area). T-shape and Y-type structures were widely distributed in the short fringes (<0.59 nm). The stacking geometry of screw dislocations was observed extensively in medium and long fringes, and the local molecular-oriented domain was isolated by short fringes. Further, the evolution of chemical structure in cutinites in terms of the distribution of orientation, curvature, and cumulative angle of aromatic fringes showed different trends. This present paper is one part of a study that seeks to establish new perspectives on the relationships between coal structure and reactivity.

Do some virtual bound states carry torsion trace?

This paper presents theoretical arguments that certain virtual bound states carry the trace component of affine torsion. The motivation for this work is that Einstein–Cartan theory, which extends general relativity by including torsion to model intrinsic angular momentum, is becoming more credible. We are not aware of any situation for which there is evidence or substantial argument for the presence of torsion trace, except in the continuum theory of edge dislocations in crystals. The main evidence for the hypothesis consists of analogies between the structure of virtual bound states and (a) geometry of dislocations in crystal lattices, which are modeled with torsion; and (b) modeling of intrinsic angular momentum by torsion in Einstein–Cartan theory and the theory of micro-elasticity. The work focuses on conjectured presence of torsion in para-positronium, which intermediates annihilation of an electron and a positron with opposite z-spins. If the virtual bound state carries torsion, then the local law of conservation of angular momentum can hold over the spacelike separation during annihilation.

Quantitative evaluation of Frank vector on the basis of continuous field theory of dislocation and differential geometry

This study conducts the modeling and numerical analysis of kink deformation on the basis of continuous field theory of dislocation and differential geometry. In particular, we aim to evaluate the existence and nature of disclination which is expected to be formed at the tip of kink band. Plastic deformation due to dislocation is obtained by numerically solving the Cartan first structure equation. This result yields a Riemann - Cartan manifold which equips Riemannian metric and Levi – Civita connection. By introducing the holonomy, i.e., the integration of curvature on a closed interval, we evaluate the magnitude of Frank vector at the kink tip. The result shows quantitative agreement with those expected from the classical dislocation theory.

Line tension induced character angle dependence of dislocation mobility in FCC alloys

We explore the character angle dependence of dislocation-solute interactions in a face-centered cubic random Fe0.70Ni0.11Cr0.19 alloy through molecular dynamics (MD) simulations of dislocation mobility. Using the MD mobility data, we determine the phonon and thermally activated solute drag parameters which govern mobility for each dislocation character angle. The resulting parameter set indicates that, surprisingly, the solute energy barrier does not depend on character angle. Instead, only the zero-temperature flow stress—which is dictated by the activation area for thermal activation—is dependent on character angle. By analyzing the line roughness from MD simulations and the geometry of a bowing dislocation line undergoing thermal activation, we conclude that the character angle dependence of the activation area in this alloy is governed by the dislocation line tension, rather than the dislocation-solute interaction itself. Our findings motivate further investigation into the line geometry of dislocations in solid solutions.

Recombination-enhanced dislocation climb in InAs quantum dot lasers on silicon

We analyze the structure of dislocations in electrically aged InAs quantum dot (QD) lasers on silicon to understand gradual device degradation. We find that misfit dislocations lengthen due to carrier injection, experiencing a combination of recombination-enhanced climb and glide processes constrained by the epitaxial structure. An examination of the dislocation geometry reveals that the climb process involves the addition of atoms to the extra half plane of the dislocation. Spontaneous emission from the QDs is also dimmer after aging. Additionally, the signature of misfit dislocations in the unaged laser, discernible as sharp dark lines in spatially resolved cathodoluminescence, is replaced by finer, more inhomogeneous contrast upon aging. We speculate that this change arises from vacancy clouds expelled from the dislocation during climb. With this insight, we evaluate the driving forces for dislocation climb that could be at play and discuss the origins of slow degradation in QD lasers.

TEM-based dislocation tomography: Challenges and opportunities

Dislocation tomography based on transmission electron microscopy (TEM) exhibits excellent capabilities in three dimensional (3D) visualization of various dislocation structures but still suffers from poor quantification and coupling between the geometry and crystallography of dislocations. In the present paper, we review the research on 3D quantitative characterization of dislocation structures using TEM-based dislocation tomography and stereo pair methods, and briefly introduce a novel TEM-based tomographic crystallography method which can simultaneously and quantitatively characterize the geometric and crystallographic features of dislocations. We summarize some technical problems and challenges in the workflow of TEM-based dislocation tomography, including image contrast optimization, irradiation damage, image processing, reconstruction algorithms as well as dislocation segmentation and identification. We further discuss the potential applications of TEM-based dislocation tomography and envisage several promising approaches for developing an advanced four-dimensional (4D) dislocation characterization technique.

Mechanisms of strain accommodation in plastically-deformed zircon under simple shear deformation conditions during amphibolite-facies metamorphism

This study documents the strain accommodation mechanisms in zircon under amphibolite-facies metamorphic conditions in simple shear. Microstructural data from undeformed, fractured and crystal-plastically deformed zircon crystals are described in the context of the host shear zone, and evaluated in the light of zircon elastic anisotropy. Our work challenges the existing model of zircon evolution and shows previously undescribed rheological characteristics for this important accessory mineral.Crystal-plastically deformed zircon grains have <c> axis oriented parallel to the foliation plane, with the majority of deformed grains having <c> axis parallel to the lineation. Zircon accommodates strain by a network of stepped low-angle boundaries, formed by switching between tilt dislocations with the slip systems <100>{010} and <1¯10>{110} and rotation axis [001], twist dislocations with the rotation axis [001], and tilt dislocations with the slip system <100>{001} and rotation axis [010]. The slip system <1¯10>{110} is newly described for zircon. Most misorientation axes in plastically-deformed zircon grains are parallel to the XY plane of the sample and have [001] crystallographic direction. Such behaviour of strained zircon lattice is caused by elastic anisotropy that has a direct geometric control on the rheology, deformation mechanisms and dominant slip systems in zircon. Young's modulus and P wave velocity have highest values parallel to zircon [001] axis, indicating that zircon is elastically strong along this direction. Poisson ratio and Shear modulus demonstrate that zircon is also most resistant to shearing along [001]. Thus, [001] axis is the most common rotation axis in zircon.The described zircon behaviour is important to take into account during structural and geochronological investigations of (poly)metamorphic terrains. Geometry of dislocations in zircon may help reconstructing the geometry of the host shear zone(s), large-scale stresses in the crust, and, possibly, the timing of deformation, if the isotopic systems of deformed zircon were reset.

Elastic strain relaxation in interfacial dislocation patterns: II. From long- and short-range interactions to local reactions

The long- and short-range interactions as well as planar reactions between two infinitely periodic sets of crossing dislocations are investigated using anisotropic elasticity theory in face- (fcc) and body- (bcc) centered cubic materials. Two preliminary cases are proposed to examine the substantial changes in the elastic stress states and the corresponding strain energies due to a slight rearrangement in the internal dislocation geometries and characters. In general, significant differences and discrepancies resulting from the considered cubic crystal structure and the approximation of isotropic elasticity are exhibited. In a third scenario, special attention is paid to connecting specific internal dislocation structures from the previous cases with non-equilibrium configurations predicted by the quantized Frank-Bilby equation for the (111) fcc and (110) bcc twist grain boundaries. The present solutions lead to the formation of energetically favorable dislocation junctions with non-randomly strain-relaxed configurations of lower energy. In particular, the local dislocation interactions and reactions form equilibrium hexagonal-shaped patterns with planar three-fold dislocation nodes without producing spurious far-field stresses.Numerical application results are presented from a selection of cubic metals including aluminum, copper, tantalum, and niobium. In contrast to the fcc materials, asymmetric dislocation nodes occur in the anisotropic bcc cases, within which the minimum-energy paths for predicting the fully strain-relaxed dislocation patterns depend on the Zener anisotropic factor with respect to unity. The associated changes in the dislocation structures as well as the removal of the elastic strain energy upon relaxations are quantified and also discussed.

Ab initiomodeling of quasielastic neutron scattering of hydrogen pipe diffusion in palladium

A recent quasielastic neutron scattering (QENS) study of hydrogen in heavily deformed fcc palladium provided the first direct measurement of hydrogen pipe diffusion, which has a significantly higher diffusivity and lower activation barrier than in bulk. While ab initio estimates of hydrogen diffusion near a dislocation corroborated the experimental values, open questions remain from the Chudley-Elliott analysis of the QENS spectra, including significant nonmonotonic changes in jump distance with temperature. We calculate the spherically averaged incoherent scattering function at different temperatures using our ab initio data for the network of site energies, jump rates, and jump vectors to directly compare to experiment. Diffusivities and jump distances are sensitive to how a single Lorentzian is fit to the scattering function. Using a logarithmic least squares fit over the range of experimentally measured energies, our diffusivities and jump distances agree well with those measured by experiment. However, these calculated quantities do not reflect barriers or distances in our dislocation geometry. This computational approach allows for validation against experiment, along with a more detailed understanding of the QENS results.

Computation of the lattice Green function for a dislocation.

Modeling isolated dislocations is challenging due to their long-ranged strain fields. Flexible boundary condition methods capture the correct long-range strain field of a defect by coupling the defect core to an infinite harmonic bulk through the lattice Green function (LGF). To improve the accuracy and efficiency of flexible boundary condition methods, we develop a numerical method to compute the LGF specifically for a dislocation geometry; in contrast to previous methods, where the LGF was computed for the perfect bulk as an approximation for the dislocation. Our approach directly accounts for the topology of a dislocation, and the errors in the LGF computation converge rapidly for edge dislocations in a simple cubic model system as well as in BCC Fe with an empirical potential. When used within the flexible boundary condition approach, the dislocation LGF relaxes dislocation core geometries in fewer iterations than when the perfect bulk LGF is used as an approximation for the dislocation, making a flexible boundary condition approach more efficient.

Scale-Free Phase Field Theory of Dislocations

According to recent experimental and numerical investigations, if a characteristic length (such as grain size) of a specimen is in the submicron size regime, several new interesting phenomena emerge during the deformation. Since in such systems boundaries play a crucial role, to model the plastic response it is crucial to determine the dislocation distribution near the boundaries. In this Letter, a phase-field-type continuum theory of the time evolution of an ensemble of parallel edge dislocations with identical Burgers vectors, corresponding to the dislocation geometry near internal boundaries, is presented. Since the dislocation-dislocation interaction is scale free (1/r), apart from the average dislocation spacing the theory cannot contain any length scale parameter. As shown, the continuum theory suggested is able to recover the dislocation distribution near boundaries obtained by discrete dislocation dynamics simulations.

Discontinuities without discontinuity: The Weakly-enforced Slip Method

Tectonic faults are commonly modelled as Volterra or Somigliana dislocations in an elastic medium. Various solution methods exist for this problem. However, the methods used in practice are often limiting, motivated by reasons of computational efficiency rather than geophysical accuracy. A typical geophysical application involves inverse problems for which many different fault configurations need to be examined, each adding to the computational load. In practice, this precludes conventional finite-element methods, which suffer a large computational overhead on account of geometric changes. This paper presents a new non-conforming finite-element method based on weak imposition of the displacement discontinuity. The weak imposition of the discontinuity enables the application of approximation spaces that are independent of the dislocation geometry, thus enabling optimal reuse of computational components. Such reuse of computational components renders finite-element modeling a viable option for inverse problems in geophysical applications. A detailed analysis of the approximation properties of the new formulation is provided. The analysis is supported by numerical experiments in 2D and 3D.

Theory and simulation of the diffusion of kinks on dislocations in bcc metals

Isolated kinks on thermally fluctuating (1/2)<111> screw, <100> edge and (1/2)<111> edge dislocations in bcc iron are simulated under zero stress conditions using molecular dynamics (MD). Kinks are seen to perform stochastic motion in a potential landscape that depends on the dislocation character and geometry, and their motion provides fresh insight into the coupling of dislocations to a heat bath. The kink formation energy, migration barrier and friction parameter are deduced from the simulations. A discrete Frenkel-Kontorova-Langevin (FKL) model is able to reproduce the coarse grained data from MD at a fraction of the computational cost, without assuming an a priori temperature dependence beyond the fluctuation-dissipation theorem. Analytic results reveal that discreteness effects play an essential r\^ole in thermally activated dislocation glide, revealing the existence of a crucial intermediate length scale between molecular and dislocation dynamics. The model is used to investigate dislocation motion under the vanishingly small stress levels found in the evolution of dislocation microstructures in irradiated materials.

Regularities in the formation of dislocation networks on the boundary of bonded Si(001) wafers

The dislocation networks in structures with hydrophilically bonded Si (001) wafers are investigated by transmission electron microscopy. Networks with differing geometry and type of dominant dislocations are observed. One type of networks, which is typical of bonded structures, is formed on the basis of a square network of screw dislocations and contains a system of unidirectional 60° zigzag-shaped dislocations. It is established that such dislocation networks are flat in structures with an azimuthal misorientation of wafers exceeding 2°, whereas they are three-dimensional at smaller misorientation angles. A unique network of another type is formed only by 60° dislocations, the majority of which are extended along one direction, which does not coincide with the 〈110〉 directions in the boundary plane and has a number of specific features, the explanation of which is impossible within the framework of conventional representations.

A distributional approach to the geometry of 2D dislocations at the continuum scale

This paper develops a geometrical model of dislocations and disclinations in single crystals at the continuum scale, with use of the distribution theory to represent concentrated effects in the defect lines which in turn form the branching lines of the multiple-valued elastic displacement and rotation fields. Fundamental identities relating the incompatibility tensor to the dislocation and disclination densities are proved in the case of locally countably many parallel defect lines, under global 2D strain assumptions relying on the geometric measure theory. Our approach provides a new understanding of the continuum theory of defects and introduces the appropriate objective internal variables and the required mathematical framework for a rigorous homogenization of all relevant fields from mesoscopic to macroscopic scale.

Fault geometry, coseismic-slip distribution and Coulomb stress change associated with the 2009 April 6, Mw 6.3, L’Aquila earthquake from inversion of GPS displacements

The 2009 April 6, Mw= 6.3 L’Aquila earthquake occurred within a complex system of NW-SE trending normal faults in the Abruzzi Central Apennines (Italy). We analyse the coseismic deformation as measured by >70 global positioning system (GPS) stations, both from continuous and survey-mode networks, providing unprecedented details for a moderate normal faulting earthquake in Italy from GPS measurements. We use rectangular, uniform-slip, dislocations embedded in an elastic, homogeneous and isotropic half-space and a constrained, non-linear optimization algorithm, to solve for the best-fitting rectangular dislocation geometry and coseismic-slip distribution. We use a bootstrap approach to investigate uncertainties in the model parameters and define confidence bounds for all the inverted parameters. The rupture occurred on a N129°E striking and 50° southwestward dipping normal fault, in agreement with geological observations of surface breaks along the Paganica fault. Our distributed slip model exhibits a zone of relatively higher slip (>60 cm) between ~1.5 and ~11 km depth, along a roughly downdip, NW-SE elongated patch, confined within the fault plane inverted assuming uniform-slip. The highest slip, of the order of ~1 m, occurred on a ~16 km2 area located at ~5 km depth, SE of the mainshock epicentre. The analysis of model resolution suggests that slip at depth below ~5 km can be resolved only at a spatial scale larger than 2 km, so a finer discretization of different asperities within the main patch of coseismic-slip is not allowed by GPS data. We compute the coseismic Coulomb stress changes in the crustal volume affected by the major aftershocks, and compare the results obtained from the uniform-slip and the heterogeneous-slip models. We find that most of the large aftershocks occurred in areas of Coulomb stress increase of 0.2-13 bar and that a deepening of the slip distribution down to a depth greater than 6 km in the SE part of the fault plane, in agreement with the inverted slip model, can explain the deepest, April 7, Mw 5.3 aftershock.

Microstructure of Heteroepitaxial ZnTe Grown by Molecular Beam Epitaxy on Si(211) Substrates

The interface of ZnTe/Si(211) grown by molecular beam epitaxy was investigated by high-resolution transmission electron microscopy. Several types of defects such as misfit dislocations, stacking faults, agglomerations of vacancies, and precipitates were observed and studied by electron microscopy at the ZnTe/Si interface. The distribution of misfit dislocations at the interface was revealed with the assistance of the fast Fourier transformation filtering technique. A stick-and-ball interface model including misfit dislocation geometry is proposed. The possible origins of the stacking faults, vacancies, and precipitates are discussed.