Presence Of Dislocations Research Articles

Synthesis and characterization of tri-aluminide in situ composites

AA5052 tri-aluminide in situ composites with different vol.% of Al3Zr particles were produced by direct melt reaction in situ technique. The in situ composites were characterized by differential thermal analysis for reaction temperature and X-ray diffractometer for phase analysis. Optical, scanning, and transmission electron microscopy were conducted for morphology and distribution of particles. These studies showed grain refinement of matrix with second phase particles, different morphologies of Al3Zr, and their uniform distribution with particle size range. The presence of dislocations was also observed in TEM studies which contribute to hardness and strength of composites. Composite with 10 vol.% of Al3Zr showed maximum strength and toughness while hardness continuously increased up to 30 vol.% Al3Zr. The cumulative mass loss increased with sliding distance, whereas coefficient of friction (COF) showed a fluctuating tendency. Wear rate continuously increased with load while COF showed a decreasing trend. For base alloy and composite with l0 vol.% Al3Zr, wear rate decreases up to 3 m/s sliding velocity and beyond this value a sharp increase in wear rate is observed, whereas composites with higher vol.% of Al3Zr show increasing trend of wear rate from beginning but at much lower rate. COF shows an increasing trend with sliding velocity for all compositions. Wear rate and normalized wear rate continuously decrease with an increase in vol.% of Al3Zr particles while COF shows an increasing trend. All these results have been correlated with roughness (Ra) values and worn surfaces studies.

Simulation of Dislocation Accumulation in Impurity Doped-ULSI Cells and Electric Characteristic Evaluations

The performance of semiconductor devices has improved on introducing increasing refinements to the structures of these devices. This has created various problems at the atomic level. In particular, the presence of dislocations, a type of crystallographic defect, within semiconductor devices poses a major problem. Dislocations accumulated within the device obstruct the movement of electrons and adversely affect the electrical characteristics of the device. However, previous investigations have not sufficiently clarified the relationship between accumulated dislocations and the electrical characteristics of a semiconductor. In this study, we focus on dislocations produced in the fabrication of an impurity-doped ultra-large-scale integration (ULSI) device and, based on a crystal plasticity analysis, perform a simulation of the accumulation of dislocations within the device during the cooling process. We establish an analytical system by which the obtained information on dislocations is applied to a device simulator, in order to evaluate the electrical characteristics by considering the accumulation of dislocations. We investigate the effects that dislocation density and density distribution have on the characteristic current-voltage curve of the device.

Critical thickness investigation of MBE-grown GaInAs/GaAs and GaAsSb/GaAs heterostructures

GaInAs/GaAs and GaAsSb/GaAs heterostructures were grown by molecular beam epitaxy with different In/Sb compositions and thicknesses in order to obtain samples with different amounts of initial strain. High resolution x-ray diffraction was used to extract the alloys composition, specify the presence of dislocations, and determine the extent of relaxation while transmission electron microscopy and x-ray topography were used to observe these dislocations and characterize their type and density. The onset for the formation of misfit dislocations was found to be in agreement with the equilibrium theory. However, the films remained coherently strained for thicknesses far beyond this value. The onset for strain relaxation was found by considering the kinetics of plastic deformation using the approach proposed by Tsao and coworkers [Phys. Rev. Lett. 59, 2455 (1987)]. The mechanism of extended defect creation leading to measurable strain relief is described as a multistage process related with the structural stability and metastability of the epitaxial films.

The role of stress relaxation and creep during high temperature deformation in Ni-base single crystal superalloys – Implications to strain build-up during directional solidification

The study of high temperature deformation of Ni-base superalloys finds a number of applications. In this study we consider how stress/strain resulting from directional solidification can be alleviated through phenomena such as relaxation and how this can be measured. Based on a modelling study, a range of stresses between 550 and 650 MPa were chosen for tensile testing at 900 °C in the Ni-base superalloy, CMSX4. In-situ neutron diffractometry together with ex-situ transmission electron microscopy have been used to study relaxation occurring during high-temperature deformation. The behaviour of (100) and (200) lattice strains with macroscopic stress has been quantitatively analysed for single crystals with axial orientations within 5° from [100]. The (200) γ+γ/ fundamental peak has been used to relate the decay in applied macroscopic stress with lattice strain. At high stresses (above 600 MPa), relaxation was particularly pronounced with an immediate appreciable decay in lattice strain (and stress) within 20 min. At lower stresses, significant relaxation is only observed after prolonged hold. Relaxation occurs in both γ and γ/, as confirmed by presence of dislocations within both phases and it was also with minimal lattice rotation (<3°). Also, the decrease in lattice strain from relaxation was at least two/threefold lower than the creep strain, obtained from sample elongation. It is shown that in modelling of strain during solidification, it is important to consider the relaxation of lattice strain, rather than just creep. Implications of this study to the critical plastic strain for re-crystallisation are addressed.

A field theory of strain/curvature incompatibility for coupled fracture and plasticity

The displacement and rotation discontinuities arising between crack surfaces are assigned to smooth areal/tensorial densities of crystal defects referred to as disconnections and rotational disconnections (r-disconnections), through the incompatibility of the smooth strain and curvature tensors. In a dual way, the disconnections and r-disconnections are defined as line defects terminating surfaces where the displacement and rotation encounter a discontinuity. Relationships for the conservation across arbitrary patches of their strength (the crack opening displacement and opening angle) provide a natural framework for crack dynamics in terms of transport laws for the line defect densities. Similar methodology is applied to the discontinuities of the plastic displacement and rotation arising from the presence of dislocations and standard disclinations in the body, which results in the concurrent involvement of the dislocation/disclination density tensors in the analysis. The present model can therefore be viewed as an extension of the mechanics of dislocation and disclination fields to the case where continuity of the body is disrupted by cracks. From the continuity of the elastic strain and curvature tensors, it is expected that the stress/couple stress fields remain bounded everywhere in the body, including at the crack tip and in dislocation/disclination cores. Thermodynamic arguments provide the driving forces for the crystal defects motion, and guidance for the formulation of mobility laws insuring non-negative dissipation. The Peach–Koehler-type forces on dislocations and disclinations are retrieved in the analysis, and similar Peach–Koehler-type forces are defined for the disconnections and r-disconnections. A threshold in the (r-)disconnection driving force vs. (r-)disconnection velocity constitutive relationship provides for a Griffith-type fracture criterion. Application of the theory to a pencil-shaped notch configuration in elastic and elasto-plastic solids through finite element modeling shows that it provides elastic fields with lower energy level around crack tips than conventional singular solutions, and that crack propagation can be consistently described by the transport scheme. Interactions between crack growth and tilt boundary mobility are explored.

Effect of boundary conditions on the kinetics of helium release from structural materials

Gaseous products of nuclear reactions (specifically, helium) play a significant part in altering the material properties upon irradiation. It is known that atoms of inert gases promote the generation and growth of pores in irradiated materials and affect phenomena such as swelling, high-temperature irradiation embrittlement, etc. Therefore, a study of the behavior of helium (its production, accumulation, retention, and release) within structural materials is fairly topical. In order to validate the methods of express imitation of accumulation and retention of helium within structural materials under reactor irradiation, we perform a comparative analysis of the spectra of the rate of gas release from samples of austenitic steel 0Kh16N15M3B that were saturated with helium in different ways, i.e., through irradiation in a cyclotron, a magnetic massseparation setup, the IRT-2000 reactor, the BOR-60 reactor, and using the so-called tritium trick technique. The effect of the presence of dislocations and grain boundaries on the release of helium from materials is evaluated. The results of the research conducted show that the kinetics of helium release from samples saturated with helium through the bombardment with alpha particles of different energies, which ensures the simultaneous introduction of helium and radiation defects (in wide ranges of helium concentration and radiation damage) into the material lattice, is similar to the kinetics of helium release from samples irradiated in reactors.

Diffusion-induced stress and strain energy affected by dislocation mechanisms in a cylindrical nanoanode

Based on diffusion kinetics and dislocation theory, this paper describes an analytical model to analyze the effects of mixed dislocation mechanisms on diffusion-induced stress (DIS) and total strain energy when Li-ions insert a cylindrical nanoanode. Electrochemical Biot number (EBN) coupled into diffusion kinetics is analogous to the Biot number in heat-transfer systems and can govern the whole progress of Li-ion diffusion. The results show that diffusion-induced tensile radial stress can be reduced by dislocation work hardening stress and converted to compressive stress near the electrode surface. The presence of dislocations in the diffused electrode reveals that dislocations can be generated by DIS and in turn partially relieve the radial stress. Under small EBN charging, the degree of dislocation work hardening stress relieving the radial stress is obvious and meanwhile the influence of dislocation strain energy upon total strain energy is more significant. To predict the nucleation and propagation of cracks, it is more necessary to consider dislocation strain energy under small EBN control. These quantitative results also indicate that both EBN and dislocation density can be possibly controlled and coordinated based on nanotechnology so as to prolong the cycle life of batteries.

Orientational Coherent Effects of High-Energy Particles in a LiNbO3 Crystal.

A bent lithium niobate strip was exposed to a 400-GeV/c proton beam at the external lines of CERN Super Proton Synchrotron to probe its capabilities versus coherent interactions of the particles with the crystal such as channeling and volume reflection. Lithium niobate (LiNbO3) exhibits an interplanar electric field comparable to that of Silicon (Si) and remarkable piezoelectric properties, which could be exploited for the realization of piezo-actuated devices for the control of high-energy particle beams. In contrast to Si and germanium (Ge), LiNbO3 shows an intriguing effect; in spite of a low channeling efficiency (3%), the volume reflection maintains a high deflection efficiency (83%). Such discrepancy was ascribed to the high concentration (10(4) per cm2) of dislocations in our sample, which was obtained from a commercial wafer. Indeed, it has been theoretically shown that a channeling efficiency comparable with that of Si or Ge would be attained with a crystal at low defect concentration (less than ten per cm2). To better understand the role of dislocations on volume reflection, we have worked out computer simulation via dynecharm++ Monte Carlo code to study the effect of dislocations on volume reflection. The results of the simulations agree with experimental records, demonstrating that volume reflection is more robust than channeling in the presence of dislocations.

Emergent gravity and chiral anomaly in Dirac semimetals in the presence of dislocations

We consider the recently discovered Dirac semimetals with two Dirac points ±K. In the presence of elastic deformations each fermion propagates in a curved space, whose metric is defined by the expansion of the effective Hamiltonian near the Dirac point. Besides, there is the emergent electromagnetic field that is defined by the shift of the Dirac point. We consider the case, when the deformations are caused by the dislocations. The dislocation carries singular torsion and the quantized flux of emergent magnetic field. Both torsion singularity and emergent magnetic flux may be observed in the scattering of quasiparticles on the dislocation due to Stodolsky and Aharonov–Bohm effects. We discuss quantum anomalies in the quasiparticle currents in the presence of emergent gauge and gravitational fields and the external electromagnetic field. In particular, it is demonstrated, that in the presence of external electric field the quasiparticles/holes are pumped from vacuum along the dislocation. The appeared chiral imbalance along the dislocation drives the analogue of chiral magnetic effect, that is the appearance of electric current along the dislocation.

A study of phase separation processes in presence of dislocations in binary systems subjected to irradiation

Dislocation-assisted phase separation processes in binary systems subjected to irradiation effect are studied analytically and numerically. Irradiation is described by athermal atomic mixing in the form of ballistic flux with spatially correlated stochastic contribution. While studying the dynamics of domain size growth we have shown that the dislocation mechanism of phase decomposition delays the ordering processes. It is found that spatial correlations of the ballistic flux noise cause segregation of dislocation cores in the vicinity of interfaces effectively decreasing the interface width. A competition between regular and stochastic components of the ballistic flux is discussed.

Emergent geometry experienced by fermions in graphene in the presence of dislocations

In graphene in the presence of strain the elasticity theory metric naturally appears. However, this is not the one experienced by fermionic quasiparticles. Fermions propagate in curved space, whose metric is defined by expansion of the effective Hamiltonian near the topologically protected Fermi point. We discuss relation between both types of metric for different parametrizations of graphene surface. Next, we extend our consideration to the case, when the dislocations are present. We consider the situation, when the deformation is described by elasticity theory and calculate both torsion and emergent magnetic field carried by the dislocation. The dislocation carries singular torsion in addition to the quantized flux of emergent magnetic field. Both may be observed in the scattering of quasiparticles on the dislocation. Emergent magnetic field flux manifests itself in the Aharonov–Bohm effect while the torsion singularity results in Stodolsky effect.

Influence of defects on the irreversible phase transition in Fe–Pd ferromagnetic shape memory alloys

In Fe–Pd ferromagnetic shape memory alloys a face-centered tetragonal (fct) martensite can be obtained when cooling the face-centered cubic austenite through the martensite transition. Nevertheless, further irreversible transformation on cooling into a body-centered tetragonal (bct) martensite needs to be prevented in order to retain the shape memory properties. Differential scanning calorimetry experiments demonstrate that high temperature thermal treatments stabilize the fct phase, reducing the fct–bct transformation temperature. A large misfit between the cell parameters of fct and bct phases was determined by neutron diffraction, pointing to the critical role of dislocations in the accommodation of both phases. The presence of dislocations and its dynamics was analyzed by mechanical spectroscopy, and a relaxation peak at ∼443K related to the dislocation movement was identified. The driving force of the relaxation process can be proposed as a dislocation dragging mechanism controlled by the migration of vacancies without break-away. Defects such as dislocations and vacancies have been shown to play an important role in changing the irreversible phase transformation temperature. A reduction in the dislocation density reduces the irreversible transformation temperature and so increases the stability range of the alloy.

Boron Diffusion in Silicon in the Presence of Grain Boundaries and Voids

We have investigated the behavior of one B impurity in the presence of dislocations, nanovoids and grain boundaries (GBs) in Si. Our study has been motivated by recent experiments on highly B-doped polycrystalline Si where a very significant thermoelectric power factor enhancement was observed. We study the influence of nanovoids on B diffusivity and activation in Si. We compare with the grain-boundary of pure tilt Si[001](120) poly-Si, where the presence of dislocations is dominant, similarly as in the case of nanovoids in c-Si. We performed DFT calculations to investigate minimum energy configurations, energy barriers and activation or deactivation of one B impurity in the proximity of the above structures. We found that, B preferably occupies substitutional lattice sites at the edge of the cavities. Thus the presence of small voids reduces its diffusivity and mobility, but the electrical activation of B is not significantly influenced. In the case of Si[001](120) GB, B is found to form Boron interstitials clusters in the close vicinity of a GB. The formation energy of those clusters is lower when the cluster is closer to center of the GB. Our results are in agreement with previously reported experimental data. It is provided insight to the effect of structural inhomogeneity on the dopant properties in Si structures that are currently of interest for thermoelectric applications.

Suppressing dislocations in normalized hypercubic smearing

Normalized hypercubic smearing improves the behavior of dynamical Wilson-clover fermions, but has the unwanted side effect that it can occasionally produce spikes in the fermion force. These spikes originate in the chain rule connecting the derivative with respect to the smeared links to the derivative with respect to the dynamical links, and are associated with the presence of dislocations in the dynamical gauge field. We propose and study an action designed to suppress these dislocations. We present evidence for improved performance of the hybrid Monte Carlo algorithm. A side benefit is improvement in the properties of valence chiral fermions.

The Influence of Mg and Ag on the Precipitation Kinetics and the Formation of the T&lt;sub&gt;1&lt;/sub&gt; Phase in Al-Cu-Li Alloys

Al-Cu-Li alloys are extensively used for aerospace applications. The main hardening phase is the T1 phase that precipitates as thin platelets on {111}Al planes. To facilitate its nucleation, different minor alloying elements are added and dislocations are introduced by cold deformation before the ageing treatment. The impact of these additions in combination with the presence of dislocations on precipitate nucleation and growth needs a deeper understanding. In this work, we investigated the precipitation kinetics of the T1 phase in alloys containing a common content of Cu and Li and different contents of minor solutes (Mg, Ag) where these elements are present either together or independently. A general overview on the precipitation kinetics was achieved by in-situ small-angle X-ray scattering and hardness measurements. The evaluation of precipitation kinetics reveals that magnesium plays an important role during precipitation by enhancing nucleation kinetics. Additionally, a smaller yet measureable effect of Ag, both in the presence and absence of Mg has been evidenced.

Real-time observation of local strain effects on nonvolatile ferroelectric memory storage mechanisms.

We use in situ transmission electron microscopy to directly observe, at high temporal and spatial resolution, the interaction of ferroelectric domains and dislocation networks within BiFeO3 thin films. The experimental observations are compared with a phase field model constructed to simulate the dynamics of domains in the presence of dislocations and their resulting strain fields. We demonstrate that a global network of misfit dislocations at the film-substrate interface can act as nucleation sites and slow down domain propagation in the vicinity of the dislocations. Networks of individual threading dislocations emanating from the film-electrode interface play a more dramatic role in pinning domain motion. These dislocations may be responsible for the domain behavior in ferroelectric thin-film devices deviating from conventional Kolmogorov-Avrami-Ishibashi dynamics toward a Nucleation Limited Switching model.

Microstructural evolution and micromechanical properties of gamma-irradiated Au ball bonds

The effect of gamma radiation on the mechanical and structural properties of gold ball bonds was investigated. Gold wires from thermosonic wire bonding were exposed to gamma rays from a Cobalt-60 source at a low dose (5Gy). The load–depth curve of nanoindentation for the irradiated gold wire bond has an apparent staircase shape during loading compared to the as-received sample. The hardness of the specimens calculated from the nanoindentation shows an increase in value from 0.91 to 1.09GPa for specimens after exposure. The reduced elastic modulus for irradiated specimens significantly increased as well, with values from 75.18 to 98.55GPa. The change in intrinsic properties due to gamma radiation was investigated using dual-focused ion beam and high-resolution transmission electron microscope analysis. The dual-focused ion beam and high-resolution transmission electron microscope images confirmed the changes in grain structure and the presence of dislocations. The scanning electron microscope micrographs of focused ion beam cross sections showed that the grain structure of the gold became elongated and smaller after exposure to gamma rays. Meanwhile, high-resolution transmission electron microscopy provided evidence that gamma radiation induced dislocation of the atomic arrangement.

A field theory of distortion incompatibility for coupled fracture and plasticity

The displacement discontinuity arising between the crack surfaces is assigned to smooth areal/tensorial densities of crystal defects referred to as disconnections, through the incompatibility of the continuous distortion tensor. In a dual way, the disconnections are defined as line defects terminating surfaces where the displacement encounters a discontinuity. A conservation argument for their strength (the crack opening displacement) provides a natural framework for their dynamics in the form of a transport law for the disconnection densities. Similar methodology is applied to the discontinuity of the plastic displacement arising from the presence of dislocations in the body, which results in the concurrent involvement of the dislocation density tensor in the analysis. The present model can therefore be viewed as an extension of the mechanics of dislocation fields to the case where continuity of the body is disrupted by cracks. From the continuity of the elastic distortion tensor, it is expected that the stress field remains bounded everywhere in the body, including at the crack tip. Thermodynamic arguments provide the driving forces for disconnection and dislocation motion, and guidance for the formulation of constitutive relationships insuring non-negative dissipation. The conventional Peach–Koehler force on dislocations is retrieved in the analysis, and a Peach–Koehler-type force on disconnections is defined. A threshold in the disconnection driving force vs. disconnection velocity constitutive relationship provides for a Griffith-type fracture criterion. Application of the theory to the slit-crack (Griffith–Inglis crack) in elastic and elasto-plastic solids through finite element modeling shows that it allows recovering earlier results on the stress field around cracks, and that crack propagation can be consistently described by the transport scheme. Shielding/anti-shielding of cracks by dislocations is considered to illustrate the static/dynamic interactions between dislocations and disconnections resulting from the theory. Sample size effects on crack growth are evidenced in solids encountering plastic yielding.

Cracking in Brittle Material at Presence of a Dislocation

The present study evaluate the stress field during the cracking caused by applying pressure loading to this ends of material. Our problem formulated by using the brittle material (concrete, ceramic, glass,...) having; elasticity modulus E and Poissons ratio (υ).The fundamental idea of this work rests on the determination of the influence of the dislocation presented near of the main crack on this cracking. This problem can be treated by finite element method while using Software (ABAQUS). One uses a square plate from element fissured at his end (semi-infinite) and a dislocation in the vicinity of the main crack, with various distances between the two cracks (main crack-dislocation), for each distance we have stresses can be found at the end of the main crack. After the determination of these latter, we deduced that the presence of dislocation influences largely on the propagation of cracking by amplification or reduction of this propagation.

Boundary integral equations for 2D elasticity and its application in discrete dislocation simulation in finite body: 2. Numerical implementation

In the previous paper by Yu and Diab (2013), several sets of boundary integral equations are derived for general anisotropic materials and corresponding equations for materials with different classes of symmetry are deduced. The work presented herein implements two sets of boundary element schemes to numerically solve the stress field. The integration on the element that has the singular point of the kernel is bounded and can be evaluated analytically. Four benchmark elastic problems are solved numerically to show the advantage of the two schemes over the conventional boundary element formulation in eliminating the boundary layer effect. The one with the weaker singularity has better convergence and gives more accurate results. The presented formulation also provides a direct approach to solve for stress field in a finite solid body in the presence of dislocations. Combined with discrete dislocations dynamics, boundary value problems with dislocations in finite bodies can be solved. Two examples, bending of a single crystal beam and pure shearing of a polycrystalline solid, are simulated by discrete dislocation dynamics using the scheme that has the weaker singularity. The comparisons with the published results using the well-established superposition technique validate the proposed formulation and show its quick convergence.