Dislocation Spacing Research Articles

Modified Young's equation for equilibrium dihedral angles of grain boundary grooves in thin films at the nanoscale

We derive the modified Young's equation for the equilibrium dihedral angle at the triple junction of the grain boundary groove by taking into account the discrete structure of the low angle grain boundary. For low angle grain boundaries, the geometric relation that the misorientation of the bicrystal is inversely proportional to the dislocation spacing naturally gives rise to the variation in the misorientation when the grain boundary length changes (holding the number of dislocations constant). The fact that the grain boundary energy increases as the grain boundary length decreases due to a smaller dislocation spacing leads to a larger dihedral angle compared to that of the classical theory. Two atomistic continuum modelling tools, namely the phase field crystal model and the amplitude equations, are used to simulate the equilibrium dihedral angle. The numerical results are in quantitatively good agreement with the derived modified Young's equation. Furthermore, the amplitude equations are employed to investigate the kinetics of the grain boundary grooving. The time-independent groove shape as predicted by Mullins is observed for bicrystal with high misorientation, and the groove width scales with time approximately as a power law t1/4. For bicrystals with small misorientations, the groove root exhibits a stick-slip motion due to volatile dislocation motion toward the liquid phase as it is close to the triple junction.

Interface characteristics of cubic AlN film on MgO (100) substrate

Highly oriented cubic AlN film was deposited on MgO (100) substrate by laser molecular beam epitaxy (LMBE) technique, and the interface characteristics were investigated. Cubic AlN film and MgO substrate show the well-defined orientation relationship of AlN [001]//MgO [001] and AlN (100)//MgO (100). The AlN/MgO interface is characterized by coherent regions separated by periodically spaced misfit dislocations, forming a semi-coherent interface. The experimentally measured interface dislocation spacing is slightly smaller than that calculated from the lattice mismatch, implying that the lattice mismatch between cubic AlN film and MgO substrate is accommodated mainly by interface misfit dislocations.

Mechanical interactions between semicoherent heterophase interfaces and free surfaces in crystalline bilayers

A force formalism is proposed to investigate the interactions between a semicoherent heterophase interface and free surfaces in anisotropic finite-thickness bilayers. The present approach enables the estimation of the mechanical forces acting on interfaces, which arise from inhomogeneous elastic fields between the neighboring materials. These short-range fields are produced by the superposition of coherency strains and the elastic strain fields of dislocation patterns, where the individual Burgers vectors are defined by requiring the global force balance on any internal plane perpendicular to the interfaces. Predictions in heterogeneous free-standing bilayers are compared to results for the homogeneous elasticity problem (i.e., both materials having the same stiffness) and the limiting case of infinite bicrystals. Examples of applications to pure misfit interfaces are given with controlled misfit dislocation spacings by continuously varying the alloy composition. The effect of dislocation density on the stability of semicoherent interfaces interacting with free surfaces is discussed in terms of the change in elastic strain energy of Pt/Pd bilayer systems.

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.

Strain analysis of misfit dislocations in α-Fe2O3/α-Al2O3 heterostructure interface by geometric phase analysis

The α-Fe2O3/α-Al2O3 heterostructure interfaces have been studied using transmission electron microscopy (TEM). The interface exhibited coherent regions separated by equally spaced misfit dislocations. The misfit dislocations were demonstrated to be edge dislocations with dislocation spacing of ∼4nm. The strain fields around the misfit dislocation core were mapped using a combination of geometric phase analysis and high-resolution transmission electron microscopy images. The strain measurement results were compared with the Peierls–Nabarro dislocation model and the Foreman dislocation model. These comparisons show that the Foreman model (a=2) is the most appropriate theoretical model to describe the strain fields of the dislocation core.

Termination chemistry-driven dislocation structure at SrTiO3/MgO heterointerfaces.

Exploiting the promise of nanocomposite oxides necessitates a detailed understanding of the dislocation structure at the interfaces, which governs diverse and technologically relevant properties. Here we report atomistic simulations demonstrating a strong dependence of the dislocation structure on the termination chemistry at the SrTiO3/MgO heterointerface. The SrO- and TiO2-terminated interfaces exhibit distinct nearest neighbour arrangements between cations and anions, leading to variations in local electrostatic interactions across the interface that ultimately dictate the dislocation structure. Networks of dislocations with different Burgers vectors and dislocation spacing characterize the two interfaces. These networks in turn influence the overall stability of and the behaviour of oxygen vacancies at the heterointerface, which will dictate vital properties such as mass transport at the interface. To date, the observed correlation between the dislocation structure and the termination chemistry at the interface has not been recognized, and offers novel avenues for fine-tuning oxide nanocomposites with enhanced functionalities.

Comparison between dislocation dynamics model predictions and experiments in precipitation-strengthened Al–Li–Sc alloys

Precipitate distributions were quantitatively measured by local electrode atom-probe (LEAP) tomography in several age-hardenable Al-based alloys to provide input to both analytical models and dislocation dynamics simulations of critical resolved shear strength, for comparison with experimentally measured Vickers microhardness values. The method of reconstructing precipitate-containing volumes from LEAP tomography, then importing these data to dislocation dynamics simulations, is explained in detail in the supplementary material available in the online version of this paper. Two alloys were studied: Al–2.9Li–0.11Sc at.% (Al–Li–Sc) and Al–6.3Li–0.07Sc–0.02Yb at.% (Al–Li–Sc–Yb). Heat treatment of these alloys produced nanometer-scale α′-Al3(Li,Sc,Yb)(L12) precipitates after isothermal aging at 325°C. In some cases δ′-Al3Li(L12) shells were formed on these precipitates after subsequent isothermal aging at 170°C. Dislocation dynamics results and experimental measurements were combined to define empirical strengthening superposition rules for the cases of contributions from: (i) Li in solid-solution plus α′-Al3(Li,Sc,Yb)(L12) precipitates; (ii) α′-Al3(Li,Sc,Yb)(L12) precipitates plus δ′-Al3Li(L12) shells in doubly aged Al–Li–Sc–Yb. Simulations of aged Al–Li–Sc overpredict the strength if a single dislocation is used, and underpredict the strength if instead a cooperative dislocation pair is considered. For simulations of dislocation pairs in Al–Li–Sc (single-phase precipitates), the precipitate bypass mechanism depends on the aging condition of the alloy. At peak age, precipitate shearing occurs mainly by pairs of closely spaced dislocations moving cooperatively. As overaging progresses, Orowan looping increasingly dominates and the distance between the leading and trailing dislocations increases. For dislocation pairs in doubly aged Al–Li–Sc–Yb with some core/shell precipitates, the measured and simulated strength values agree to within their uncertainties.

Crystal Plasticity Analysis of Mechanical Response and Size Effect in Two Phase Alloys with Dispersion of Fine Particles

Plastic deformation and dislocations accumulation in a steel alloy dispersed with vanadium carbide particles is numerically analyzed by a crystal plasticity finite element technique and work hardening characteristics are discussed. Increment of dislocation density that contributes to work hardening is calculated from the mean free path of dislocations. The mean free path is defined by the spacing of forest dislocations and the average spacing of dispersed particles. Obtained yield stress and work hardening characteristics was close to that of experimental result, except that the value of work hardening rate was higher than that of experimental one.

Evaluation of thickness and strain of thin planar layers of InAs on GaAs(001) using spectroscopic ellipsometry

We develop a technique for accurately measuring thickness of planar InAs films grown on (001) GaAs by spectroscopic ellipsometry, using bulk optical constants. We observe that the critical point structure for the E1 and E1 + Δ1 transitions extracted from the measured dielectric properties varies with strain in the layer. Transmission electron microscopy confirms the extracted thickness and measures the residual strain based on the dislocation spacing in the film. At small thickness, the E1 critical point is seen to markedly deviate from the dependence predicted by deformation potential theory and appears to be consistent with additional quantum confinement effects.

Energy minimization mechanisms of semi-coherent interfaces

In this article, we discussed energy minimization mechanisms of semi-coherent interfaces based on atomistic simulations and dislocation theory. For example, of {111} interfaces between two face centered cubic (FCC) crystals, interface comprises of two stable structures (normal FCC stacking structure and intrinsic stacking fault structure), misfit dislocations, and misfit dislocation intersections or nodes (corresponding to the high energy stacking fault (HESF) structure). According to atomistic simulations of four interfaces, we found that (1) greater spacing between misfit dislocations and/or larger slopes of generalized stacking fault energy at the stable interface structures leads to a narrower dislocation core and a higher state of coherency in the stable interfaces; (2) the HESF region is relaxed by the relative rotation and dilation/compression of the two crystals at the node. The crystal rotation is responsible for the spiral feature at the vicinity of a node and the dilation/compression is responsible for the creation of the free volume at a node; (3) the spiral feature is gradually frail and the free volume decreases with decreasing misfit dislocation spacing, which corresponds to an increase in lattice mismatch and/or a decrease in lattice rotation. Finally, the analysis method and energy minimization mechanisms explored in FCC {111} semi-coherent interfaces are also applicable for other semi-coherent interfaces.

Revisión en dos tiempos del reemplazo total de cadera infectado

<div class="page" title="Page 1"><div class="layoutArea"><div class="column"><p><strong>In­tro­duc­ción: </strong><span>La infección de un reemplazo total de cadera es una de las complicaciones más graves, y el recambio protésico, aspecto fundamental del tratamiento, puede realizarse en uno o dos tiempos quirúrgicos. El objetivo de este trabajo fue evaluar, en forma prospectiva, el tratamiento del reemplazo total de cadera infectado mediante la revisión protésica en dos tiempos quirúrgicos. </span></p><p><strong>Materiales­ y­ Métodos: </strong><span>Se analizaron 72 revisiones en dos tiempos quirúrgicos de reemplazos totales de cadera infectados (subagudos y crónicos), en 71 pacientes, con un seguimiento promedio de 6,4 años (1-11 años). Siempre se utilizó un espaciador de cemento con antibióticos y un esquema de antibióticos empírico inicial, hasta definir el tratamiento específico, según el resultado de los cultivos de las muestras intraoperatorias.</span></p><p><strong>Resultados:­ </strong><span>Se logró curar la infección en el 96% de los casos (69 caderas). Se utilizaron 59 espaciadores “artesanales” fabricados en quirófano (82%) y 13 prefabricados. El germen más frecuente fue </span><span>Staphylococcus </span><span>coagulasa negativo (23 casos). Ocho pacientes sufrieron complicaciones: 4 luxaciones del espaciador, 2 fracturas periespaciador y 2 roturas del espaciador. </span></p><p><span><strong>Conclusión: </strong>­</span><span>Las revisiones en dos tiempos muestran excelentes porcentajes de curación de la infección. Su éxito depende de una adecuada limpieza quirúrgica, del uso de un espaciador con antibióticos y de un adecuado esquema farmacológico posoperatorio. </span></p></div></div></div><p> </p>

Theoretical study on the dislocation structure of parent–martensite interface in a magnetic shape memory alloy

Ferromagnetic shape memory alloys have received increasing interest and have great potential for actuator and sensor applications, the mobility of the parent–martensite interphase interface in such alloys is determined by its interfacial structure and the migration mechanism; therefore, a thorough understanding of its nature is essential. In the present paper, the parent–martensite interface in a Ni2MnGa alloy is studied in the light of the topological model of martensite transformation crystallography, where the habit plane is envisaged comprising coherent terraces and steps. The coherency strains arising on the terrace plane are accommodated by a network of interfacial dislocations, e.g., twinning dislocations originating in the martensite phase, and disconnections. The topological parameters of these defects, i.e., the Burgers vectors, line directions, and dislocation spacings are quantified via rigorous crystallographic analysis and matrix algorithm based on the Frank-Bilby equation. Consequently, martensite transformation crystallographic features, e.g., the habit plane index and the orientation relationship, in the Ni2MnGa alloy are determined and found to be in good agreement with the results predicted by the well-established phenomenological theory.

Structural and photoluminescent properties of low temperature InAs buffer layer grown by MOVPE on GaAs substrates

Epitaxial growth of InAs on semi-insulating GaAs was a subject of various attempts to reduce the influence of the ~7% lattice mismatch on the InAs layer properties. The most cost effective and promising method appears to be the growth of low temperature buffer (LTB) InAs layer at ~400°C followed by a thick InAs layer at ~600°C. There is a scarcity of available information about the structural properties of the LTB layers, their background conductivity type and level of doping. We have found that a predominant part of the threading dislocations generated at the interface annihilate within the first 400nm.The average misfit dislocation spacing is 6.15nm, proving that the LTB InAs/GaAs interface is nearly completely relaxed. XRD measurements have revealed a well pronounced deformation decrease in the LTB layers for thicknesses above 300nm. The LTB InAs layer is n-type with carrier concentration of the order of 5×1016cm−3 and can be additionally doped with Te and hence can serve as a bottom contact layer. The morphology of unintentionally doped InAs layer grown at 600°C upon the LTB shows sub-nanometer flatness and carrier concentration of the order of 5×1015cm−3.

Mechanisms of hydrogen-enhanced localized plasticity: An atomistic study using α-Fe as a model system

Atomistic simulations of the effects of H on edge dislocation mobility and pile-ups are performed to investigate possible nanoscale mechanisms for hydrogen-enhanced localized plasticity (HELP). α-Fe is used as a model system because H diffusion is fast enough to capture kinetics within the time scales of molecular dynamics and because edge dislocation glide in α-Fe is similar to glide in face-centered cubic metals. Results over a wide range of H concentrations sufficient to generate a range of sizes in the Cottrell atmospheres show that the Cottrell atmospheres follow the moving dislocations, leading to a resistance to dislocation motion that is consistent with solute drag theory; thus, H reduces the dislocation mobility. Furthermore, once motion stops and a pile-up is established, the H Cottrell atmospheres do not affect the equilibrium spacing of dislocations in the pile-up; thus, the H atmosphere provides no “shielding” of dislocation–dislocation interactions. This result is consistent with conclusions from previous continuum calculations. Two oft-proposed mechanisms for HELP (H-induced increase in true dislocation mobility and decrease in dislocation–dislocation pile-up interactions) are therefore not supported by the present simulations. A mechanistic understanding of HELP phenomena observed in various experiments thus requires evaluation of more complex H–dislocation interactions.

Micro-/macro-responses of a ferroelectric single crystal with domain pinning and depinning by dislocations

Phase field simulations are conducted to investigate the micro-structural signature and the macro-response of a ferroelectric single crystal with domain pinning and depinning phenomena by dislocation arrays. It is shown that due to the presence of the dislocation arrays, a domain with polarizations antiparallel to an applied field can survive under the small amplitude of applied field. The residual domain serves as a pre-existing nucleus during the following macroscopic switching via only domain wall motion. The pinned domain will be depinned when the external electric field amplitude exceeds a critical value, which highly depends on the dislocation spacing in the dislocation array. Due to the pinning and depinning effect, an asymmetric hysteresis loop of polarization versus electric field might appear when a bias field is applied.

Ab initio prediction of the critical thickness of a precipitate

Segregation and precipitation of second phases in metals and metallic alloys is an important phenomenon that has a strong influence on the mechanical properties of the material. Models exist that describe the growth of coherent, semi-coherent and incoherent precipitates. One important parameter of these models is the energy of the interface between matrix and precipitate. In this work we apply ab initio density functional theory calculations to obtain this parameter and to understand how it depends on chemical composition and mechanical strain at the interface. Our example is a metastable Mo–C phase, the body-centred tetragonal structure, which exists as a semi-coherent precipitate in body-centred cubic molybdenum. The interface of this precipitate is supposed to change from coherent to semi-coherent during the growth of the precipitate. We predict the critical thickness of the precipitate by calculating the different contributions to a semi-coherent interface energy by means of ab initio density functional theory calculations. The parameters in our model include the elastic strain energy stored in the precipitate, as well as a misfit dislocation energy that depends on the dislocation core width and the dislocation spacing. Our predicted critical thickness agrees well with experimental observations.

Dislocation interaction of layers in the Ge/Ge-seed/GexSi1−x/Si(0 0 1) (x ∼ 0.3–0.5) system: Trapping of misfit dislocations on the Ge-seed/GeSi interface

High-resolution electron microscopy has been applied to study the dislocation redistribution between Ge and GeSi layers at the atomic scale. Ge0.3Si0.7 (30nm in thickness) and Ge0.5Si0.5 (10nm) buffer layers buried between the Si(001) substrate and the plastically relaxed Ge layer 0.5μm thick remain in a metastable (stressed) state during the growth of Ge/Ge-seed/GexSi1−x/Si(001) (x∼0.3–0.5) heterostructures, though the buffer layer thickness is several times greater than the critical value for insertion of misfit dislocations (MDs). An ordered grid of edge MDs is observed only on the Ge/GeSi interface; the mean distance between the MDs is ∼10nm (which is close to the equilibrium value for the non-stressed Ge/Si system). After 30min of annealing at 700°С, the Ge0.3Si0.7 buffer layer still remains in a metastable state, and the edge MDs are located only on the Ge/GeSi interface with the same dislocation spacing of ∼10nm. At the same time, approximately one-half of MDs in the structure with the Ge0.5Si0.5 buffer layer passes through the Ge/GeSi interface to the GeSi/Si(001) interface, and the buffer layer plastically relaxes by almost 100%. An assumption is put forward that there exists a barrier for the MD transition from the Ge layer to the GeSi layer, which results in MD trapping on this interface. The magnitude of this barrier depends on the difference in the compositions of the main Ge (x=1) film and the GexSi1−x buffer layer, and increases with increasing this difference.

Effect of dislocation walls on the polarization switching of a ferroelectric single crystal

Phase field simulations were conducted to study the influence of dislocation walls on the domain configuration and polarization switching behavior of a ferroelectric single crystal. The simulation results show that the domain configuration and polarization switching behavior depend highly on the dislocation spacing in the dislocation wall. The ferroelectric properties can be greatly improved by reducing the coercive field and meanwhile enhancing the remanent polarization if an appropriate density of dislocations is introduced at high temperature. The phase field simulations also put insights into the mechanism of polarization switching.

Intergranular Corrosion of Σ9 Copper Bicrystal with Gradually Changed Misorientation in Young’s Dislocation Etchant

The allowable deviation to preserve the “specialness” of a coincident site lattice (CSL) boundary was shown using a pure copper bicrystal which was designed to have gradually changed deviation around 〈110〉 axis from near-ideal Σ9{221} to a random relation. The grain boundary preserved immunity to grain boundary corrosion in Young’s dislocation etchant until the deviation reached the angle where the misfit dislocation spacing is supposed to decrease to CSL dimension. This underlies the common principle of the Brandon-type criteria of CSL.

A neutron diffraction study of lattice distortion, mismatch and misorientation in a single-crystal superalloy after different heat treatments

Two-dimensional neutron diffraction measurements of superlattice and fundamental lattice reflections from a single-crystal superalloy confirm the existence of angular distortion and shear stress in the γ′ phase, and reveal its correlation with the morphology and deformation of the γ′ phase during heat treatment. The tetragonal distortions with c/a>1 are developed and retained during subsequent heat treatments, whereas the angular distortions are enhanced for the γ′ phase during the aging treatments. The evolution of the lattice mismatch and anisotropy during the subsequent heat treatments are consistent with the transformation of certain macroscopic anisotropic compositional and stress distributions in the as-cast sample to the microscopic level at the γ/γ′-interface upon reprecipitation and the growth of the γ′ from the supersaturated γ-matrix. The high negative γ/γ′ lattice mismatch of the alloy is consistent with the high levels of refractory elements, in particular Cr, in the alloy. Comparison between superlattice and fundamental lattice reflections is revealing. Firstly, the existence of lattice misorientation at the γ/γ′-interface and the discrete misorientations from the splitting of coherently aligned aggregates of γ′-precipitates on the smoothly distributed γ -matrix is shown. Secondly, the measured lattice misorientation correlates with the spacing of dislocations at the γ/γ′-interface and evidenced by the transmission electron microscopy observation. Furthermore, it indicates the anisotropic distribution of dendrites along crystal growth axis and the presence of small misoriented higher-order dendrite arms in variously heat-treated samples.