Various Types Of Dislocations Research Articles

Numerical investigation of internal damage in heterogeneous-structured laminates using a 3D multiple physical mechanisms based constitutive model

Heterogeneous-structured laminates (HSLs), which consist of multiple layers of metallic materials arranged in successive pairs of coarse-grained (CG) and nano-grained (NG) layers, have been recently reported to display excellent balance of strength and ductility. However, it is argued that accompanying state-of-art numerical models developed to simulate the deformation of this specific class of composite materials have limitations for investigating their underlying damage evolution. Addressing this issue is essential to support the rational design, optimisation and application of HSLs, especially when subjected to contact and dynamic processes. For this reason, a novel 3D numerical framework for HSLs is proposed and tested in this research considering published experimental findings and dislocation theories. This framework comprehensively considers the evolution of various types of dislocations and back stress while being coupled with the Johnson Cook damage criterion. The HSL specimens simulated here were made of alternating layers of CG and NG copper separated by interface affected zones. Following initial microhardness and uniaxial tensile simulations on homogenous copper with different grain sizes, simulations of HSLs were conducted to study the effect of different layer thickness and the volume fraction of the NG layer. Overall, a good correlation between numerical and experimental results was achieved. An important and distinguishing characteristic of this research is that the proposed model enables the evolution of internal damage and the synergetic effect between the CG and NG layers to be investigated. Through the evaluation of the damage accumulation factor in the NG layer, the simulations results yielded quantitative information which aligned with the following known experimental observations: 1) the smaller the layer thickness, then the smaller the internal damage and 2) the internal damage increases with the increase in volume content of the NG layer. In addition, for a set simulated strain of 10%, the developed model could be used to show that the damage accumulation factor in the NG layer was 10 times lower than that in its counterpart, i.e., a stand-alone NG layer not sandwiched between two CG layers.

Effect of accumulative hot rolling on the microstructure and mechanical properties of dual-phase titanium alloy consolidated via SPS sintering

The improvement of microstructure and mechanical properties of Ti alloys is of great significance for their industrial application. In this study, a developed multi-step hot deformation process was employed to regulate the grain boundaries (GBs), hetero-phase boundaries (PBs) and dislocation structure for dual-phase titanium alloy. The effects of accumulative thermal deformation ratios from 33% to 75% (2–8 cycles) on the microstructure and mechanical properties have been investigated. It is found that the sintered sample shows a typical Widmanstätten structure without obvious pores and micro-cracks. However, with the increase of the deformation ratio, for the hexagonal close packed (HCP) Ti, the crystal orientations of {011‾0} and {1‾21‾0} were gradually transformed into {0001}, with the change of high-angle grain boundaries (HAGBs) to low-angle grain boundaries (LAGBs). The density of stored geometrically necessary dislocations (GNDs) density raised dramatically from 1.95 × 1014 m−2 to 6.5 × 1014 m−2, and various types of dislocations have been characterized. For the β phase, in the thermal deformation process, high density α-precipitates with an average thickness below 20 nm had formed with Burgers relationship of {110}BCC//{0001}HCP and <1‾11 >BCC//<21‾1‾0 >HCP. Finally, the yield strength of the dual-phase titanium alloy increased dramatically from 881 MPa to 1178 MPa, and the ductility can still maintain at above 8.5%. The enhanced strength was mainly contributed by the increase of geometrically necessary boundaries (GNBs) and nano-lamellae boundaries (NLBs). Therefore, this strategy of combining hetero-phase boundaries (PBs) and dislocation engineering could open up new avenues to designing strong and ductile titanium matrix materials.

Framing Patellar Instability: From Diagnosis to the Treatment of the First Episode.

The patellofemoral joint (PFJ) is a complex articulation between the patella and the femur which is involved in the extensor mechanism of the knee. Patellofemoral disorders can be classified into objective patellar instability, potential patellar instability, and patellofemoral pain syndrome. Anatomical factors such as trochlear dysplasia, patella alta, and the tibial tuberosity-trochlear groove (TT-TG) distance contribute to instability. Patellofemoral instability can result in various types of dislocations, and the frequency of dislocation can be categorized as recurrent, habitual, or permanent. Primary patellar dislocation requires diagnostic framing, including physical examination and imaging. Magnetic resonance imaging (MRI) is essential for assessing the extent of damage, such as bone bruises, osteochondral fractures, and medial patellofemoral ligament (MPFL) rupture. Treatment options for primary dislocation include urgent surgery for osteochondral fragments or conservative treatment for cases without lesions. Follow-up after treatment involves imaging screening and assessing principal and secondary factors of instability. Detecting and addressing these factors is crucial for preventing recurrent dislocations and optimizing patient outcomes.

Streamwise vortices, cellular shedding and force coefficients on finite wing at low Reynolds number

Numerical simulations for flow past a finite rectangular wing with a NACA 0012 section at $Re=1000$ for various semi-aspect ratios ( $0.25\le sAR \le 7.5$ ) over a range of angles of attack ( $0^{\circ }\le \alpha \le 14^{\circ }$ ) reveal streamwise vortices, which increase in strength and number to occupy an increasing spanwise extent with increase in $\alpha$ . They result in non-monotonic spanwise variation of local force coefficients and increased strength of wing-tip vortex for $\alpha &gt;8^{\circ }$ . Viscous and pressure drag dominate for low and high sAR, respectively. The time-averaged drag coefficient first decreases and then increases with increase in $sAR$ . Vortex shedding for $\alpha =14^{\circ }$ is single cell and parallel for $sAR&lt;3$ . Shedding is in two cells with an oblique angle that varies with time, leading to large spanwise variation in the root mean square of local force coefficients for higher $sAR$ . Various types of dislocations, reported earlier in wakes of bluff bodies, are seen for different $\alpha$ and $sAR$ . Dislocations for $\alpha =14^{\circ }$ appear at the same spanwise location for $sAR=3$ and at different spanwise locations for $sAR\ge 4$ . Vortex shedding for $\alpha =12^{\circ }$ and $sAR=5$ exhibits one cell structure in the near wake and two cells in the far wake due to splitting and reconnection of vortices near the mid-span in the moderate wake. Linkages form between counter-rotating spanwise vortices for $sAR\ge 1$ . Additional linkages between shed- and wing-tip vortices are observed for lower $sAR$ . At each $\alpha$ , the strength of the wing-tip vortex and radius of its core, estimated using Rankine and Lamb–Oseen models, increases up to a certain $sAR$ beyond which it is approximately constant.

Эффективность рентгенологических методов диагностики нестабильности позвоночно-двигательного сегмента в шейном отделе

Introduction. Standard spine radiography is the basic method of radiodiagnosis, used for screening. X-ray imaging allows you to assess the anatomy of spinal motion segments in the cervical spine, determine the period of osteochondrosis and the tactics of further examination and treatment. Various types of dislocation of vertebrae are much more common than can be determined by spinal radiography in standard projections; as a result of dislocation, a pain syndrome occurs. Aim. To study the effectiveness of conventional methods for diagnosis of instability in the spinal motion segment (SMS) in the cervical spine. Materials and methods. We observed 50 patients (mean age 50.6 ± 2.19 years) who were admitted to the Clinic of the Irkutsk Scientific Center for Surgery and Traumatology (Neurosurgical Department) with a diagnosis Adult osteochondrosis of spine (ICD-10 code – M42.1). Conventional methods were used to diagnose SMS instability: plain radiography in anteroposterior and lateral projections, functional cervical spine radiography. Results. It was stated that the most common changes are characteristic of II and III periods of osteochondrosis: straightening of cervical lordosis – in 48 patients (96%), decrease in intervertebral disc height – in 46 patients (92%), anterior wedging of vertebral bodies – in 48 patients (96%), endplate sclerosis – in 37 patients (74%), presence of marginal osteophytes – in 45 patients (90%). In maximum flexion and extension, the CII–CVI vertebrae were involved in the formation of pathological mobility of the intervertebral segments, while the CI and CVII vertebrae remained stable. Instability in one cervical vertebra was revealed in 15 patients (30%). Most often, excessive mobility of 2 vertebrae was observed – in 22 people (44%), less often of 3 vertebrae – in 5 patients (10%). The dislocation of 4 vertebrae was quite rare – in 2 patients (4%). When analyzing the data of functional cervical spine radiography, we have found that the SMS instability in patients with intervertebral disc degeneration develops most often in the following segments: CII–CIII – 13 people (26%), СIII–СIV – 34 people (70%), CIV–CV – 29 people (58%), СV–СVI – 12 people (24%). Conclusion. The SMS instability still remains an urgent problem in neurology and neurosurgery. Plain radiography reveals only indirect signs of SMS instability. Diagnosis by functional spondylography made it possible to verify the SMS instability only in 36% of cases.

In-situ observation of twinning and detwinning in AZ31 alloy

Twinning and detwinning behavior of a commercial AZ31 magnesium alloy during cyclic compression–tension deformation with a total strain amplitude of 4% (±2%) was evaluated using the complementary techniques of in-situ neutron diffraction, identical area electron backscatter diffraction, and transmission electron microscopy. In-situ neutron diffraction demonstrates that the compressive deformation was dominated by twin nucleation, twin growth, and basal slip, while detwinning dominated the unloading of compressive stresses and subsequent tension stage. With increasing number of cycles from one to eight: the volume fraction of twins at -2% strain gradually increased from 26.3% to 43.5%; the residual twins were present after 2% tension stage and their volume fraction increased from zero to 3.7% as well as a significant increase in their number; and the twinning spread from coarse grains to fine grains involving more grains for twinning. The increase in volume fraction and number of residual twins led to a transition from twin nucleation to twin growth, resulting in a decrease in yield strength of compression deformation with increasing cycles. A large number of < c >-component dislocations observed in twins and the detwinned regions were attributed to the dislocation transmutation during the twinning and detwinning. The accumulation of barriers including twin boundaries and various types of dislocations enhanced the interactions of migrating twin boundary with these barriers during twinning and detwinning, which is considered to be the origin for increasing the work hardening rate in cyclic deformation of the AZ31 alloy.

On the probability of formation of intermetallic compounds in dissimilar welded joints obtained by friction stir welding

The article describes the mechanisms and causes of the occurrence of intermetallic phases during friction stir welding of dissimilar joints. The nucleation and growth of intermetallic phases for a pair of dissimilar metals to be welded under comparatively favorable time and temperature conditions of the FSW is facilitated by the atomic-vacancy environment, which is responsible for the continuous atomic-structural bond and mass transfer of accumulated atoms in local regions of the welded joint with an equiaxial grain lamella-shear structure of the welded core. compounds with a concentration close to critical, combined with others in a superplastic state. In the process of forming a welded joint under the influence of a moving and rotating welding tool, the lamellae are subjected to bending and torsional stresses with simultaneous tension, causing them to generate point defects and especially a large number of various types of dislocations, triggering the formation of edge dislocations in the lamellae, which are lined up in the process into dislocation walls, dividing lamella grains into separate fragmentary subgrain boundaries, along which the processes of fragmentation and dispersion develop. This phenomenon is explained by the fact that the processes of fragmentation and dispersion of IMP lead to the composition of the nugget of the welded joint by fragments, often nano-sized fragments of various configurations, which act as hardeners of the weld nugget matrix.

Revelation of the dislocations in the C-face of 4H-SiC substrates using a microwave plasma etching treatment

A novel microwave plasma etching technique was used to reveal various types of dislocation on the C-face of 4H-SiC.

Size dependence of the dislocation segment vibration spectrum for various types of dislocations

The vibration spectra of the edge, mixed and screw dislocations, depending on the dislocation segment length, are considered. The value of Poisson’s ratio 1/3 is assumed. The normalized values of the eigenfrequencies and damping coefficients for different frequency numbers by equating the inverse matrix of dislocation oscillators generalized susceptibilities to zero were found. It is seen from the dependences obtained that in the case of a screw dislocation, the transformation of the dislocation oscillators generalized susceptibility poles occurs. For edge and mixed dislocations the transformation of the poles was not observed. This agrees with the results of the vibration spectrum orientation dependence study. The authors suggest that these differences are due to the presence of a screw dislocation quasilocal vibration branches.

Correlations between dislocation density evolution and spall strengths of Cu/Ta multilayered systems at the atomic scales: The role of spacing of KS interfaces

The deformation and failure (spallation) behavior of Cu/Ta multilayered systems with Kurdjumov–Sachs (KS) orientation relationship is investigated at the atomic scales under shock loading conditions. Molecular dynamics (MD) simulations investigate the role of spacing between KS interfaces on the nucleation, evolution and interaction of defect structures (dislocations) in the Cu/Ta multilayered microstructures with layer thicknesses ranging from 3 nm to 47 nm. The shock compression response and failure response is investigated using the computed values of the Hugoniot elastic limit (HEL) and the spall strengths, respectively. KS interfaces serve as strong barriers to dislocation propagation and transmission across the interface and the spacing of the interfaces is observed to influence the spall behavior. The variation of the spall strength values suggests a critical interface spacing of 6 nm, below which the spall strength of the multilayered microstructure is observed to be lower than that for single-crystal Cu for the same loading conditions. The correlations between the temporal evolution of densities of various types of dislocation at the spall planes for the various microstructures and the resulting values of the spall strengths provide a clear rationale for why a microstructure results in increased/decreased spall strength values for the multiphase system.

Thermodynamic Aspects in Non-Ideal Metal Membranes for Hydrogen Purification.

In this paper, an overview on thermodynamic aspects related to hydrogen-metal systems in non-ideal conditions is provided, aiming at systematically merging and analyzing information achieved from several different studies present in the open literature. In particular, the relationships among inner morphology, dissolved hydrogen and internal stresses are discussed in detail, putting in evidence the conformation complexity and the various types of dislocations induced by the presence of H-atoms in the lattice. Specifically, it is highlighted that the octahedral sites are preferentially occupied in the FCC metals (such as palladium), whereas tetrahedral sites are more energetically favored in BCC-structured ones (such as vanadium). These characteristics are shown to lead to a different macroscopic behavior of the two classes of metals, especially in terms of solubility and mechanical failure due to the consequent induced stresses. Furthermore, starting from the expression of the chemical potential generally presented in the literature, a new convenient expression of the activity of the H-atoms dissolved into the metal lattice as a function of the H-concentration is achieved. Such an activity expression is then used in the dissolution equilibrium relationship, which is shown to be the overall result of two different phenomena: (i) dissociative adsorption of molecular hydrogen onto the surface; and (ii) atomic hydrogen dissolution from the surface to the metal bulk. In this way, the obtained expression for equilibrium allows a method to calculate the equilibrium composition in non-ideal conditions (high pressure), which are of interest for real industrial applications.

Defect and damage evolution during spallation of single crystal Al: Comparison between molecular dynamics and quasi-coarse-grained dynamics simulations

The links between loading orientation of single crystal Al and the dynamic evolution of defects (dislocations, twins, stacking faults etc.) during spallation are investigated using molecular dynamics (MD) simulations. The microstructural evolution during the shock compression of single crystal Al is observed to be primarily guided by the nucleation and evolution of Shockley partials and twin partials. The shock response and spallation behavior of single crystal Al is observed to be anisotropic and is influenced by the density of various types of dislocations during shock compression and void nucleation at the spall plane. The capability of the “quasi-coarse-grained dynamics” (QCGD) method to reproduce the MD predicted nucleation, interaction and evolution of dislocations during shock compression and spallation of single crystal Al is discussed. The QCGD method retains the relative contribution of different types of dislocations during propagation of shock compression wave, interactions of the wave and spallation of single crystal Al as predicted by MD using a fraction of representative atoms allowing the modeling of of larger sized systems for larger times.

Traumatic Hip Dislocation: What the Orthopedic Surgeon Wants to Know.

Hip dislocation is an important orthopedic emergency usually seen in young patients who have experienced high-energy trauma, often resulting in significant long-term morbidity. Rapid identification and reduction is critical, as prolonged dislocation increases the risk of developing avascular necrosis of the femoral head, and posttraumatic osteoarthritis is a common complication, even in the absence of associated fractures. Identification and timely management of hip dislocation are highly dependent on imaging, both at presentation and after attempted reduction. It is imperative for the radiologist to understand imaging features that guide management of hip dislocation to ensure timely identification, characterization, and communication of clinically relevant results. Although the importance of prompt identification of hip dislocation is universally recognized, the significance of imaging features that guide correct management and are thought to prevent complications is less emphasized in the radiology literature. In this article, the authors review the anatomy of the hip, common injury mechanisms for various types of dislocations, and imaging findings for associated injuries. They review the most commonly used classification systems and propose a simplified checklist approach to hip dislocation to aid rapid interpretation and communication of the most clinically relevant imaging features to the treating orthopedic surgeon. ©RSNA, 2017.

Scratching of hcp metals: A molecular-dynamics study

Using molecular-dynamics simulation, we study the scratching of Ti and Mg crystals by a hard tip. Both the basal and a prismatic surface orientations are considered. We focus on the characterization of defects and plasticity in the crystal. Among the various types of dislocations generated the partials b=13〈1¯100〉 are most prominent. During scratch dislocation reactions constantly reorganize the dislocation network adherent to the scratch groove. Plastic activity is more pronounced for the prismatic surface, where dislocation loops – mainly consisting of b=13〈2¯110〉 and 13〈1¯100〉 dislocations – are emitted into the substrate. Under the basal surface I2 intrinsic stacking faults, corresponding to b=13〈1¯100〉 partials are formed parallel to the scratch groove bottom, while they are oriented perpendicular to the surface under the prismatic surface. We find that mainly the {011¯0}〈2¯110〉 slip system is activated, and relate the form of the pile-up generated to this slip. The tangential hardness is systematically smaller than the normal hardness under scratching. The basal surface has smaller hardness than the prismatic surface.

Etiopathogenesis of Anterior Glenohumeral Dislocation in Professional Rugby Players

Introduction: The glenohumeral joint is the most frequently dislocated joint in the body. There are various types of dislocations, but the most common is anterior glenohumeral joint dislocation (95%). The injury mechanism is often forced abduction accompanied by external rotation of the arm. There are a series of injuries that make the shoulder more predisposed to the occurrence of this type of injury. Objective: Review the existing literature on the etiopathogenesis of anterior glenohumeral luxation/dislocation and the mechanisms that cause such injuries in Professional Rugby. Material and Methods: Retrieval and re-analysis of data was carried out performing a critical review of existing literature published over the last 5 years and available on PubMed using the following key-word search criteria: ﾓanterior shoulder dislocationﾔ, ﾓshoulder injuries in rugbyﾔ and ﾓshoulder injuriesﾔ. Development and literature review: Shoulder injuries are common in rugby players, but the mechanisms of injury are less understood. Glenohumeral luxation represents 14% of all shoulder injuries and 62% of the disorders occur during blocking the ball. In the majority of cases (71%), injury is sustained during a tackle. Horsley et al defines the primary mechanism of injury as direct tackling (56%), followed by falling onto the arm (10%). Acromioclavicular joint injuries were the most common, with shoulder dislocations and instability accounting for the most severe injuries and greatest periods of absence from matches and scheduled training. James Crichton et al found that dislocations of the glenohumeral joint were the most common injury in 67% of rugby players. These dislocations were accompanied by associated injuries. Contact was responsible for 97% of injuries, with tackling or being tackled accounting for 65%. Longo et al describe three mechanisms: abduction and external rotation, flexion and internal rotation and landing heavily on a flexed elbow. Heady et al describe the ﾓTry Scorerﾔ, ﾓTacklerﾔ and ﾓDirect Impactﾔ mechanisms. Conclusions: (1) Repeated direct impact (blocking of the ball and the tackle) is the leading cause of most shoulder injuries; (2) There are discrepancies in the existing literature on the order and frequency shoulder dislocations occur in these athletes, but there is consensus that it is a common injury, a severe injury and can be the cause of great instability; (3) In some instances, glenohumeral dislocations can be accompanied by other injuries; (4) There is similarity in the mechanisms of injury, but all do not completely coincide.

Dislocation Configurations in an Extruded ZK60 Magnesium Alloy

Transmission electron microscopy (TEM) was performed to study the types of dislocations and their configurations in a commercially extruded ZK60 magnesium (Mg) alloy. We mainly employed the weak-beam dark-field (WBDF) technique with \( \left\langle {01\overline 1 0} \right\rangle \) zone axis to unambiguously identify Burgers vectors of various types of dislocations in the microstructure. It was found that nonbasal 〈a〉 dislocations and 〈c〉 dislocations were predominant. No 〈c + a〉 was observed. Rearrangement of these dislocations, due to dynamic recovery during extrusion, led to cell walls and low-angle grain boundaries with relatively high density dislocations. Dissociation of the 〈c〉 dislocations with a relatively narrow split distance was observed. The dissociated 〈c〉 dislocations could form a nodular structure. Fringes of stacking faults from the dislocation dissociation were observed.

Hardness, Yield Strength, and Dislocation Velocity in Elemental and Compound Semiconductors

Current knowledge on macroscopic plasticity indications, i.e., hardness and yield strength, and on microscopic indication, i.e., velocity of individual dislocations, in elemental and IV-IV, III-V, and II-VI compound semiconductors including GaN and ZnSe are reported and discussed on their mutual correlations. The Vickers hardness of the semiconductors can provide conventional information on the material plasticity in a wide temperature range up to their inciting points over a wide range of size scales in various material forms. Hardness H y in diamond- and sphalerite-type semiconductors has a universal relationship on their temperature dependence similar to the yield strength t y with a relation H v = (70-100) τ y in the low temperature region. Yield strength obtained by normal tensile or compression tests are expressed by an experimental equation as a function of the strain rate and temperature. The velocities of various types of dislocations measured directly in several semiconductors are described with an empirical equation as a function of the stress and the temperature. Through the analysis of yield strength data in terms of the collective motif in in of dislocations during the plastic deformation, the dislocation motion. rate-controlling plastic deformation, are deduced. The activation energy for dislocation motion has a linear relation to the band gap energy, depending on the types of semiconductors, elemental. III-V compounds, and II-VI compounds.

Condition of Vertebral Arteries in Experimental Modelling of Low Cervical Vertebrae Dislocation

Condition of vertebral arteries in experimental modeling of cervical vertebrae dislocations at C3-C6 level was studied on 20 section-blocks of cervical spine. Vertebral arteries were contrasted and cervical block was fixed using external fixation device. Various types of dislocation and subluxation under X-ray control were produced. Thus, it was possible to observe dynamic picture of vertebral arteries deformation in traumatic cervical vertebrae dislocation. To confirm the results of angiography the cut up of frozen section-blocks in sagittal plane and in vertebral arteries projection as well as visual control (photography of vertebrae dislocation steps) were performed. Results of experimental study allowed to detect peculiarities of vertebral arteries deformations in cervical vertebrae dislocation and to determine the correlation between the degree of vertebrae dislocation and severity of extracranial vertebral artery deformation. It enables to predict the degree of hemodynamic disorders in the system of vertebral arteries, to determine the role of vascular factor in cervical spine injury and to use the differentional approach to the choice of treatment tactics using routine roentgenography.

Latent Hardening of Germanium Crystals

The latent hardening of six secondary glide systems of germanium crystals has been measured at the begin of easy glide. The results can be explained in case of intersecting systems by adding a term for forest interaction to the flow stress of single slip. This term amounts to (αGb √Nf with α ≈︂ 0.35 for various types of dislocation cutting, and α = 0.64 for the cross glide system. (G being the appropriate shear modulus, b the length of the Burgers vector and Nf the density of forest dislocations). The Bauschinger effect seems to confirm the anisotropy of dislocation mobility detected by Farber and Iunin.

Interaction of dislocations and deformation hardening of the ordered CuAu alloy

The critical configurations of combinations of various types of dislocations in ordered alloys with L10 superstructure are analyzed numerically. The characteristics of the dislocation combinations, as well as the partial resistance to motion of dislocations, owing to interaction of dislocations, are calculated for polycrystals and single crystals with different orientations of the ordered alloy CuAu.