Dislocation Length Research Articles

Remodeling of the acetabulum after experimental hip joint dislocation--an animal experiment study of the rabbit

Little is known about the remodeling potential of the acetabulum after dislocation and relocation of the hip joint. Most clinical data rely on radiographic measurements in a clinical setting, with only few singular reports on histologic changes at the time of surgery. This study was undertaken to examine the remodeling potential of the rabbit acetabulum after different length of hip dislocation and to gain insight into the histology of the remodeling process involved. The right knee joint was immobilized in 28 matched New Zealand white rabbits, causing dislocation of the associated hip joint. Assessment of the joint status was made by serial X-rays in a reproducible standardized way, allowing measurement of the Acetabular Index (AI). The untreated left hip joints served as controls in the histologic sections and also allowed to develop an age matched scale of normal acetabular development for radiographic assessment. Dislocation was unsuccessful in 2 rabbits, 9 animals were sacrificed with the joint dislocated, and in 13 animals a closed reduction was performed and maintained in a Petrie type position for 10 days. Reduction was unsuccessful in 4 cases. The reduced group was sacrificed after 4 to 12 weeks, the point of harvesting being determined by no further change in the serial standardized radiographic measurements of the AI. All surgical procedures were done under the supervision of a veterinary surgeon and followed an approved protocol. In the time observed, the normal values of the Acetabular Index drop from 30 degrees to 20 degrees over a time of 3 months. In animals with a dislocated hip the AI increased to 40 degrees, which was completely reversible and reached normal values in 4 weeks, if reposition was done at this time (group A). Later reposition did improve the AI, but compared to normal age matched values, the AI remained elevated (> 30 degrees), demonstrating a residual dysplasia (group B). The histologic sections of the control left joints showed normal hyaline cartilage with physiologic vertical stacking of the cartilage zones. In the dislocated group the vertical stacking was lost, with chondrocytes demonstrating a fibro-cartilaginous metaplasia. There was hypocellularity with loss of nuclear staining. These changes were completely reversed in the group A, but remained in group B. This study demonstrates that an early reduction will not only allow for a normalisation of the radiographic values, but that the histologic changes as they develop during dislocation can completely be reversed. Long standing dislocation will lead to a permanent tissue metaplasia similar to the one seen in compression testing of joint cartilage, therefore loosing the potential for remodeling. Similar processes may occur in the human situation and the findings here underline the necessity for early reduction to allow for a complete remodeling of the hip joint.

Total Hip Arthroplasty in Patients with Bone Deficiency of the Acetabulum

Total hip replacement (THR) requires revision in only a minority of cases (approximately 17% of prosthetic hips fail), but when THR failures occur there may be significant acetabular bone deficiency. There is a variety of surgical hardware and strategies available to address this problem. The causes of primary THR revision include aseptic loosening or particle disease, infection, recurrent dislocation, implant failure, periprosthetic fracture, and leg length discrepancy. Almost all patients who need THR revision undergo a standard radiographic evaluation of the pelvis and hip. In general, CT is an excellent tool for evaluating loosening of the prosthesis caused by either mechanical reasons or infection, and MR imaging is best suited for evaluating the soft tissues surrounding the prosthesis. Nuclear medicine studies are performed when results of CT and MR imaging are inconclusive. When patients are evaluated for revision THR, radiologists must check for acetabular cup loosening, the amount and type of bone stock loss, the amount of component migration, and the presence or absence of liner wear. Before revision hardware is placed, bone stock loss must be repaired, either by using bone grafting or by placing accessory acetabular hardware such as cups, rings, or cages. The long-term success of revision acetabular surgery varies; there is acetabular cup presence at 5 years after surgery in 60%-94% of cases. Complications include postoperative infections, repeat liner wear, bone graft failure, periprosthetic or prosthetic fractures, dislocation, vascular injury, and nerve injury.

High-Resolution Methods for Characterizing Mesoscale Dislocation Structures

It has become apparent through experimentation at the micro- and nanolevels that the crystalline defects known as dislocations have significant effects on a material’s properties. Accordingly, a complete material state description must include the characterization of the dislocated state. However, this characterization presents a twofold problem: resolution and representation. In general, high-resolution microscopy techniques are only useful for considering a few dislocations at a time, but automated high-speed methods are only capable of resolving dislocation densities well above the average density in a typical annealed metal. The second challenge is developing a method of representation for the dislocated state. Unlike most state quantities, dislocation lengths are not conserved and complete representation would require tracking of position, momentum, and interactions, as well as the creation and annihilation of dislocations. Such a scheme becomes unwieldy when considering the large numbers of dislocations involved in common crystal plasticity. In 1970, Kröner (“Initial Studies of a Plasticity Theory Based Upon Statistical Mechanics,” Inelastic Behaviors of Solids (Materials Sciences and Engineering), M. F. Kanninen et al., eds., McGraw-Hill, New York, pp. 137–147), a pioneer in the continuum representation of dislocations, proposed a statistical method using n-point correlations to classify the dislocated state in a compact form. In addition to providing a convenient form, the correlations naturally identify dipoles, multipoles, and other higher order structures, such as cells, networks, and braids. As formulated, Kröner’s method would require high-resolution microscopy techniques, which limits its utility for experimental measurements. The current work presents a modification to Kröner’s method that would allow it to be used within the currently available resolution limits of bulk microscopy. Furthermore, in this work, newly developed microscopy techniques are employed to refine those resolution limits to more significant levels. The high-resolution bulk dislocation characterization is applied to a well-annealed nickel specimen and the results including visualizations of mesoscale structures are presented.

A self-consistent boundary element, parametric dislocation dynamics formulation of plastic flow in finite volumes

We present a self-consistent formulation of 3-D parametric dislocation dynamics (PDD) with the boundary element method (BEM) to describe dislocation motion, and hence microscopic plastic flow in finite volumes. We develop quantitative measures of the accuracy and convergence of the method by considering a comparison with known analytical solutions. It is shown that the method displays absolute convergence with increasing the number of quadrature points on the dislocation loop and the surface mesh density. The error in the image force on a screw dislocation approaching a free surface is shown to increase as the dislocation approaches the surface, but is nevertheless controllable. For example, at a distance of one lattice parameter from the surface, the relative error is less than 5% for a surface mesh with an element size of 1000 × 2000 (in units of lattice parameter), and 64 quadrature points. The Eshelby twist angle in a finite-length cylinder containing a coaxial screw dislocation is also used to benchmark the method. Finally, large scale 3-D simulation results of single slip behavior in cylindrical microcrystals are presented. Plastic flow characteristics and the stress–strain behavior of cylindrical microcrystals under compression are shown to be in agreement with experimental observations. It is shown that the mean length of dislocations trapped at the surface is the dominant factor in determining the size effects on hardening of single crystals. The influence of surface image fields on the flow stress is finally explored. It is shown that the flow stress is reduced by as much as 20% for small single crystals of size less than 0.15 μ m .

Fatigue Crack Growth Behaviors of AISI Type 347 Nuclear Piping Material

The effect of the temperature and microstructure on the fatigue crack growth behaviors of two microstucturally different AISI type 347 stainless steel(SS)s are studied at reactor operating temperature conditions. From the results, it was observed that the fatigue crack growth rate(FCGR) of type 347 SS increased as the test temperature was increased. However, its temperature sensitivity was reduced when the ΔK was large. The measured FCGRs for both TP347-LP and TP347-HP at 25oC and 345oC were lower than those presented in the ASME code. Especially near the threshold region, the trend curve in ASME code overestimated the FCGR of type 347 SS when compared with the obtained data. Even though the effect of the microstructure on the macro-crack growth rate was not significant during stage 2, the differences between threshold values for the specimen with a high precipitate density and the one with a low precipitate density were observed. It is assumed that these differences in the precipitate density, which resulted in a difference in mean free path length of dislocation movement, are the source of the difference in ΔKth of type 347 steels.

A theoretical model for the electromagnetic radiation emission during plastic deformation and crack propagation in metallic materials

This paper presents a theoretical model for the electromagnetic radiation (EMR) emissions during plastic deformation and crack propagation in metallic materials. It is shown that under an externally applied stress, edge dislocations within the plastic zone ahead of a crack tip form accelerated electric line dipoles which give rise to the EMR emissions. The dynamic motion of these dislocations becomes overdamped, underdamped or critically damped, depending upon the material/microstructural properties such as mass per unit length of dislocation, line tension, damping coefficient, and distance between the dislocation pinning points. The nature of the EMR signals, viz. exponential decay or damped sinusoidal, is decided essentially by these damping characteristics. The EMR emissions are followed by crack propagation in metallic materials. The EMR has a continuous frequency spectrum with a frequency bandwidth ranging from 108 to 1012 radians s−1, depending upon the properties of the metals. Screw dislocations do not contribute to the EMR emissions. The paper also presents some experimental results on the EMR emissions in ASTM B265 grade 2 titanium sheets. The nature (damped sinusoidal and exponential decay), amplitude and frequency of the observed EMR emissions are in conformity with the predictions of the theoretical model.

Pattern formation and directional and spatial ordering of edge dislocations in bulk GaN: Microphotoluminescence spectra and continuum elastic calculations

We present a detailed microphotoluminescence study of the long-range strain fields surrounding threading dislocations as well as their interaction and pattern formation in GaN bulk crystals. The stress patterns are detected by tiny energy shifts of the near-band-edge spectral lines and show a dipolelike stress state around the dislocation core of edge- or mixed-type dislocations, with an angular orientation coinciding with high-symmetry crystal directions. We detect tens of micrometers long linear lineups of edge dislocations with sizable effects on the local strain states and photoluminescence peak positions. With continuum elastic strain simulations, we demonstrate that the observed patterns of threading dislocations are energetically favorable. We calculated a binding energy per threading dislocation length of $50\phantom{\rule{0.3em}{0ex}}\mathrm{meV}∕\ensuremath{\mu}\mathrm{m}$ against gliding of an edge dislocation within a lineup of edge dislocations of parallel Burgers vector orientation.

Interaction of a surface wave with a dislocation

The scattering of a surface wave by a pinned edge dislocation in a semi-infinite, homogeneous, isotropic, three-dimensional elastic solid is investigated analytically and numerically. An incident wave excites the dislocation that responds by oscillating as a string endowed with mass, line tension, and damping. The oscillations of the stringlike dislocation generate secondary (``scattered'') elastic waves that are the primary object of interest in this study. The back reaction of the re-emitted waves on the dislocation dynamics is neglected, but the wavelength of the radiation is allowed to be large, comparable, or small compared to the length of the dislocation. In view of recent experimental visualizations of these phenomena, we focus particularly on the field behavior at the free surface near the dislocation, and not just on the far field. For the same reason, it is the vertical component of displacement at the free surface that is studied in detail. An efficient numerical procedure for the computation of the appropriate components of the Green's function, using a Filon quadrature for the integration of rapidly oscillating functions, is developed. The numerics is validated with known analytical expressions. The secondary radiation generated by the response of the dislocation to the incident wave is also calculated numerically, and the results are also validated by comparing them with the analytical expressions that can be obtained when the radiation wavelength is very long compared to dislocation length. The interference pattern between incident wave and secondary wave that is generated at the free surface is studied in detail and found to depend strongly not only on wavelength and dislocation geometry (length and orientation) but also on dislocation depth, with the response of the dislocation being a particularly sensitive function of such depth. Results are compared with recent experiments of Shilo and Zolotoyabko [Phys. Rev. Lett. 91, 115506 (2003)] that report visualizations of the surface-wave--dislocation interaction using stroboscopic x-ray imaging. A satisfactory agreement is found. Dislocation velocities of a few percent of the speed of sound and viscosity coefficients of about ${10}^{\ensuremath{-}5}\phantom{\rule{0.3em}{0ex}}\mathrm{Pa}\phantom{\rule{0.2em}{0ex}}\mathrm{s}$ are inferred.

Effect of sulphur on the strengthening of a Zr–Nb alloy

The effect of sulphur on the strengthening and the thermally activated deformation of cold-worked Zr–1 Nb alloy was investigated. In the present study, the sulphur strengthening was observed even at room temperature unlike the previous study of Ferrer et al. The flow stress increased by 65 MPa at room temperature with the addition of sulphur as little as 20 ppm. With further increase of sulphur content up to 300 ppm, negligible change of the flow stress was observed. The additive strengthening behavior in which the entire stress–strain curve shift upward by the friction stress due to the addition of sulphur was observed in the Zr–Nb alloy of the present study. The activation volume decreased slightly (from 110b 3 to 80b 3) with the addition of 300 ppm sulphur at room temperature. The rate-controlling mechanism of the deformation can best be explained by the dislocation interaction mechanism in which the segregation of alloying elements such as oxygen and sulphur atoms affects the activation length of dislocations.

The mean free path of dislocations in nanoparticle and nanorod reinforced metal composites and implication for strengthening mechanisms

The mean free path length of dislocations plays an important role in the plastic behavior of metals, which may be significantly enhanced by the addition of nanoparticles. The effects of particle distribution pattern, particle size, shape and volume fraction on the dislocation mean free path length and average obstacle distance are analyzed in two- and three-dimensional models. As the particle volume fraction increases, the dislocation mean free path length and average distance between dislocations obstacles can be significantly reduced, implying enhancement of strength. The random particle distribution exhibits the best combination of dislocation mean free path length and average obstacle distance. In addition, both dislocation mean free path and average obstacle distance can be significantly reduced by changing the particle shape from nanosphere to nanorod. The present analysis may provide useful information for designing the particle enforced composite materials.

Finite-size effects in dislocation glide through random arrays of obstacles: Line tension simulations

The stress required to force a dislocation to glide through a random environment of obstacles depends on both the dislocation length and the glide distance because of the statistical nature of the obstacle configurations. In order to study this finite size effect, we employ a line tension model in conjunction with an evolution algorithm inspired from larger scale dislocation dynamics simulations. We show that with finite arrays, the estimated critical resolved shear stress is larger than its infinite array limit. Moreover, the lower the resistance and the density of the obstacles, the larger the overestimation. The controlling parameters, estimated from Friedel's law, are the average number of obstacles along the dislocation line and the average number of obstacle configurations met by the dislocation in its glide. We analyze this effect through a model based on an analogy with branching processes.

Dislocation–solute cluster interaction in Al–Mg binary alloys

The close-range interaction of dislocations and solute clusters in the Al–Mg binary system is studied by means of atomistic simulations. We evaluate the binding energy per unit length of dislocations to the thermodynamically stable solute atmospheres that form around their cores, at various temperatures and average solid solution concentrations. A measure of the cluster size that renders linear the relationship between the binding energy per unit length and the cluster size is identified. The variation of the interaction energy between a dislocation and a cluster residing at a finite distance from its core is evaluated and it is shown that the interaction is negligible once the separation is larger than approximately 15 Burgers vectors. The data are relevant for the dynamics of dislocation pinning during dynamic strain ageing in solid solution alloys and for static ageing.

Electrical activity of dislocations in epitaxial ZnO- and GaN-layers analyzed by holography in a transmission electron microscope

Transmission electron microscopy was used to characterize dislocations in n-ZnO and n-GaN epitaxial layers grown on Al 2O 3(0 0 0 1) substrates. Threading segments of misfit dislocations with screw, edge and mixed character are observed with varying fractions depending on the epitaxial growth procedure. The electrical activity of single dislocations is studied by off-axis electron holography in a transmission electron microscope. An electrostatic potential in the vicinity of the dislocations is detected for ZnO and GaN, which clearly demonstrates that charges are present close to the dislocation core. Applying Read′s model for the potential of a screened line charge, high charge densities of approximately 2 elementary charges per nm dislocation length are derived.

Heterostructures Ge xSi 1−x/Si(0 0 1) grown by low-temperature (300–400 °C) molecular beam epitaxy: Misfit dislocation propagation

In previous publications [ Tuppen and Gibbings, J. Appl. Phys. 68 (1990) 1526; Hull et al., J. Appl. Phys. 70 (1991) 2052; Houghton, J. Appl. Phys. 70 (1991) 2136], the glide velocity of dislocations was analyzed and estimated in GeSi/Si (0 0 1) heterostructures grown at a temperature of 550 °C. The method of growing GeSi films with the use of a low-temperature Si buffer has been developed recently and was not involved in these studies. The present work deals with a more detailed analysis of dislocation propagation velocities in GeSi films grown with the use of the low-temperature Si buffer and those grown at low-temperatures. The dislocation velocity is estimated by the measuring of the length of misfit dislocations observed in annealed films. Additionally a method for estimating the mean threading dislocation glide velocity is used, which involves the measured threading dislocation density in a particular film and degree of its plastic relaxation directly related to the number of threading dislocations and their glide velocity. It is shown that dislocation glide velocities in heterostructures grown with the use of the low-temperature Si buffer and at low-temperatures are higher than the values predicted by the classical calculations by an order of magnitude.

Oxygen Segregation to Nanopipes in Gallium Nitride

AbstractThis paper examines the core structure and composition of threading dislocations in GaN grown by hydride vapour phase epitaxy. Transmission electron microscopy showed that screw dislocations have widely varying core structures from open cores (“nanopipes”) to closed cores, with irregular variations between the two observed along the length of many dislocations. New evidence was found demonstrating that the equilibrium structure of screw dislocations is a closed core configuration. Electron energy loss spectroscopy combined with high resolution imaging showed that 1.7±0.3 monolayers of nitrogen were substituted by oxygen at the surfaces of nanopipes. In contrast, closed core dislocations showed little evidence of oxygen segregation. It is argued that these results support a model where nanopipe formation is controlled by the segregation of oxygen to pits predominately associated with, albeit not exclusively, dislocations. The implication of the results in understanding the electronic behavior of dislocations in GaN is also discussed.

Experimental Study of Size Effect in Quasi-Simple Shear Plastic Deformation of Pure Aluminium Single Crystal

The size effects on yield stress and hardening evolution in the crystal plastic deformation processes has been recognized through experimental observations. The quasi-simple shear experiments to evaluate the initial yield stress and the hardening evolution by employing different sizes of square plane specimen was executed. Consequently, the following results were obtained. As a first experimental result, the resolved shear stress depends on the initial mean free pass length of dislocations. As a second one, the process, in which dislocations reach and pass through the surface of a specimen, decreases the hardening evolution. As a third one, it was confirmed that the smaller the specimen, the higher hardening evolution because of accumulation of dislocation. Further, it was concluded that initial mean free pass length and geometrically necessary dislocation effect on hardening evolution should be introduced in the crystalline plastic constitutive equation.

Acoustic study of kinetics of vacancy diffusion toward dislocations in aluminum

Using ultrasonic shear waves, we study the kinetics of migration of vacancy to dislocations in aluminum. Ultrasonic attenuation is sensitive to the effective dislocation length, which is shortened by the segregation of point defects. Two principal measurements were done: (i) measurement of temperature dependence of decay behavior of attenuation after elastic deformation; (ii) continuous measurement of attenuation of a specimen quenched from 573 K to cold-water and then to liquid-nitrogen temperatures. The specimens were polycrystalline and monocrystal aluminum with 99.995% purity. The former measurement provided us with the activation energy 0.28 eV for migration of dominant point defects through the Granato–Hikata–Lücke theory. The latter measurement informed us of the kinetics of vacancy: the attenuation coefficient rapidly decreased twice at 125 and 250 K, corresponding to activation energies 0.28 and 0.61 eV for migration, respectively. Thus, the same mechanism occurs for the attenuation change after elastic deformation and at 125 K after quenching. We attributed the 0.28-eV migration as fast diffusion of vacancy along dislocations. The 0.61-eV migration is consistent with the bulk diffusion of vacancy in aluminum. Throughout this study, the electromagnetic acoustic resonance method was used for making noncontacting and highly accurate measurement of attenuation.

Amplitude-Dependent Internal Friction of Dislocation Interactions in Dual-Phase Steel

Abstract Dislocation interactions in the ferrite matrix of a dual-phase steel have been studied as a function of martensite volume fraction (11–25%) and tensile straining (0–5%) by strain-induced amplitude-dependent (strain amplitude varying from 10−7 to 10−4) internal friction measurements at room temperature, using a high-frequency composite oscillator (40 kHz). Results showed that a higher volume fraction of martensite increases the internal stress in the softer ferrite matrix and restricted the dislocation motions, giving rise to a lower amplitude-dependent internal friction. Application of tensile straining further shortened the effective mobile dislocation length which was revealed by the internal friction measurements and this resulted in a high initial strain hardening rate of the high-martensite samples. Reduction in the martensite volume fraction and the mobile dislocations in the ferrite matrix through process control resulted in an increase in the amplitude-dependent internal friction, indicat...

Effects of doping and preliminary processing on the magnetically stimulated mobility of dislocations in InSb single crystals

The effect of the type of conductivity and the doping level of InSb single crystals on the mobility of fast 60° dislocations in a magnetic field is discovered. It is found that doping of a pure InSb crystal with tellurium (n-type impurity) to 1018 cm−3 reduces the mobility of dislocations to the background level. At the same time, in p-type InSb crystals doped with Ge with the same carrier concentration (1018 cm−3), the magnetoplastic effect is manifested clearly. It is shown that preliminary mechanical loading and, hence, internal stresses in the crystal affect not only the mean path length of dislocations in a magnetic field but also the magnitude of the threshold magnetic field below which the magnetoplastic effect is not observed. Possible reasons for these phenomena are discussed.

Influence of contamination on the dislocation-related deep level C1 line observed in deep-level-transient spectroscopy of n-type silicon: A comparison with the technique of electron-beam-induced current

Misfit dislocations containing different amounts of contamination were analyzed by deep-level-transient spectroscopy (DLTS). The amount of dislocation contamination was determined from the temperature dependence of the dislocation contrast, c(T), measured by electron-beam-induced current (EBIC). The C1 line was not observed for clean 60° dislocations in n-type Si, but appeared upon contamination of dislocations by gold. The density of levels related to C1 measured by DLTS is in good agreement with the density of impurities determined from the EBIC c(T) behavior. The width of the distribution of energy levels that form the C1 line increases with the amount of contamination. It is rather sharp for dislocations contaminated with less than 105 impurities/cm dislocation length, and becomes broadened for dislocations with about 106 impurities/cm length (ΔE=50 meV). The experimental observations are explained by accommodation of impurities in both the core and in the strain field of the dislocations.