Introduction to Dislocations Hull, Derek; Bacon, D. J
Materials today (Kidlington, England),
2011., Letnik:
14, Številka:
10
eBook, Journal Article
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In materials science, dislocations are irregularities within the crystal structure or atomic scale of engineering materials, such as metals, semi- conductors, polymers, and composites. Discussing ...this specific aspect of materials science and engineering, Introduction to Dislocations is a key resource for students. The book provides students and practitioners with the fundamental principles required to understand dislocations. Comprised of 10 chapters, the text includes advanced computer modeling and very high- resolution electron microscopy to help readers better understand the structure of atoms close to the core of dislocations. It shows that atomic arrangement has a significant effect on the formation of dislocations and thereby on the properties of solids. The first two chapters of the book present an overview of dislocations. The crystal structures and the various defects and dislocations are discussed, and methods of observation and diagnosis of dislocations are covered. Chapters 3 to 5 discuss the behavior of dislocations and explain how changes in the structure and arrangement of atoms can affect the behavior of dislocations. The three chapters also discuss the mechanical properties of dislocations. The remaining chapters offer a detailed discussion of the mechanisms of dislocations and the mechanical strength of crystalline solids. The book is written for undergraduate- and graduate-level students in both materials science and mechanical engineering. Non-experts and novices working on mechanical properties, mechanisms of deformation and fracture, and properties of materials, as well as industrial and academic researchers, will find this book invaluable. * Long-established academic reference by an expert author team, highly regarded for their contributions to the field. * Uses minimal mathematics to present theory and applications in a detailed yet easy-to-read manner, making this an understandable introduction to a complex topic. * Unlike the main competition, this new edition includes recent developments in the subject and up-to-date references to further reading and research sources.
We investigate the structure of 1/3 <0bar.111> dislocations observed in Bi.sub.2Te.sub.3 nanowires. This particular type of dislocation is interesting because it has a large Burgers vector (b = 1.048 ...nm) with a component normal to the basal planes equal to the thickness of one full Bi.sub.2Te.sub.3 quintuple unit (i.e., c/3). Atomic-resolution high-angle annular dark-field scanning transmission electron microscopy observations show that the dislocations form with a complex dissociated core structure. This structure consists of two partial dislocations that separate a defected region consisting of a seven-plane-thick septuple unit, consistent with a local patch of Bi.sub.3Te.sub.4, rather than the normal Bi.sub.2Te.sub.3 quintuple layer structure. As we discuss, details of the core structure can be understood from an analysis of the crystallographic parameters of the observed partial dislocations. This analysis suggests a mechanism to accommodate the loss of tellurium through the heterogeneous nucleation and growth of seven-layer defects at 1/3 <0bar.111>--type dislocations.
This book is a practice-oriented manual teaching the successful examination technique developed and taught by the author known as "Graf's technique". The book is based on the author's experience of ...more than 20 years. It is easy to read and provides a real "hands-on" manual giving numerous practical tips. The book includes the fundamentals of hip sonography, static as well as dynamic techniques, anatomical identification of the echograms, typing, a measurement technique and usability check. The book also contains an atlas including a summary of the essential data and demonstrating correct and incorrect sonograms in different variations.
Posterior shoulder dislocations are rare injuries that are often missed on initial presentation. Cases left untreated for more than three weeks are considered chronic, cannot be reduced closely (they ...become locked) and are usually associated with a significant reverse Hill-Sachs defect. The aim of this study was to evaluate the outcomes of chronic locked posterior shoulder dislocations treated with the McLaughlin procedure (classic or modified).
This retrospective study included 12 patients with chronic locked posterior shoulder dislocation operated on between 2000 and 2021 by two surgeons in two institutions. Patients received a thorough clinical examination and radiological assessment before and after surgery. Shoulders were repaired with the McLaughlin or modified McLaughlin procedure. Outcomes were assessed by comparing pre- and postoperative values of clinical variables.
Most of the dislocations were of traumatic origin. The average delay between dislocation and surgical reduction was 13.5 ± 9.7 weeks. Postoperative clinical outcomes were favourable, with an average subjective shoulder value of 86.4 ± 11.1 and a normalized Constant -Murley score of 90 ± 8.3. None of the patients had a recurrence of shoulder dislocation, but one patient developed avascular necrosis of the humeral head and two patients developed glenohumeral osteoarthritis.
In this group of patients with chronic locked posterior shoulder dislocation, the clinical outcomes of McLaughlin and modified McLaughlin procedures were satisfactory, even when surgery was significantly delayed.
The modern understanding of metal plasticity and fracturing began about 100 years ago, with pioneering work; first, on crack-induced fracturing by Griffith and, second, with the invention of ...dislocation-enhanced crystal plasticity by Taylor, Orowan and Polanyi. The modern counterparts are fracture mechanics, as invented by Irwin, and dislocation mechanics, as initiated in pioneering work by Cottrell. No less important was the breakthrough development of optical characterization of sectioned polycrystalline metal microstructures started by Sorby in the late 19th century and leading eventually to modern optical, x-ray and electron microscopy methods for assessments of crystal fracture surfaces, via fractography, and particularly of x-ray and electron microscopy techniques applied to quantitative characterizations of internal dislocation behaviors. A major current effort is to match computational simulations of metal deformation/fracturing behaviors with experimental measurements made over extended ranges of microstructures and over varying external conditions of stress-state, temperature and loading rate. The relation of such simulations to the development of constitutive equations for a hoped-for predictive description of material deformation/fracturing behaviors is an active topic of research. The present collection of articles provides a broad sampling of research accomplishments on the two subjects.
Refractory high-entropy alloys (RHEAs) are designed for high elevated-temperature strength, with both edge and screw dislocations playing an important role for plastic deformation. However, they can ...also display a significant energetic driving force for chemical short-range ordering (SRO). Here, we investigate mechanisms underlying the mobilities of screw and edge dislocations in the body-centered cubic MoNbTaW RHEA over a wide temperature range using extensive molecular dynamics simulations based on a highly-accurate machine-learning interatomic potential. Further, we specifically evaluate how these mechanisms are affected by the presence of SRO. The mobility of edge dislocations is found to be enhanced by the presence of SRO, whereas the rate of double-kink nucleation in the motion of screw dislocations is reduced, although this influence of SRO appears to be attenuated at increasing temperature. Independent of the presence of SRO, a cross-slip locking mechanism is observed for the motion of screws, which provides for extra strengthening for refractory high-entropy alloy system.
Double-endobutton technique, as a widely accepted strategy for the treatment of acromioclavicular joint dislocation, is undergoing constant improvement. This study aims to assess the clinical effect ...of a modified single-endobutton combined with the nice knot in the fixation of Rockwood type III or V acromioclavicular joint dislocation.
From January 2016 to June 2019, 16 adult patients (13 males and 3 females) with Rockwood type III or V acromioclavicular joint dislocation were treated with a modified single-endobutton technique combined with the nice knot in our department. The age ranged from 18 to 64 years old with an average of 32.8 years old. Operative time, intraoperative blood loss, post-operative clinical outcomes and radiographic results were recorded and analyzed. Preoperative and last follow-up scores in the Constant-Murley Scale, Neer score, Rating Scale of the American Shoulder and Elbow Surgeons and VAS scale and complications such as infection, re-dislocation, implant loosening, medical origin fracture and hardware pain were recorded and evaluated.
Sixteen patients were followed up for 6 to 18 months with an average of 10.3 months. The operative time was 50-90 min with an average of (62.5 ± 3.10) min. The intraoperative blood loss was 30-100 ml, with an average of (55.0 ± 4.28) ml. The complications, such as wound infection, internal fixation failure and fractures, were not found in these cases. According to Karlsson criteria, there were excellent in 14 cases, good in 2 cases at the final follow-up. The mean VAS score of the patients was 5.88 ± 0.26 preoperatively, compared with 0.19 ± 0.14 at the final follow-up evaluation. The difference was statistically significant (P < 0.05). The mean Constant score was 45.5 ± 2.0 preoperatively, compared to 94.0 ± 0.73 at the final follow-up evaluation. The difference was statistically significant (P < 0.05). Patients had statistically significant preoperative and postoperative AC (acromioclavicular distance) and CC (coracoclavicular distance) distances (P < 0.05); 6 months postoperatively the AC(P = 0.412) and CC(P = 0.324) distances were not statistically significant compared to the healthy side.
Nice knot provides a reliable fixation for the single-endobutton technique in the treatment of acromioclavicular dislocations. The modified single-endobutton technique combined with the nice knot can achieve good clinical outcomes in the treatment of Rockwood type III or V acromioclavicular joint dislocation.
Irradiation growth and creep are crucially dependent on the movement of dislocations under irradiation. However, despite being amongst the most commonly existing prior defects in polycrystals, the ...evolution of pre-existing dislocations and their impact on the formation of dislocation loops during irradiation have been relatively rarely investigated. In this study, the evolution of pre-existing dislocations and their impact on the formation of dislocation loops were investigated in magnesium by electron irradiation at different temperatures. The pre-existing dislocations have a profound influence on the loop size, density and Burgers vector of irradiation loops, and this influence is dependent on both the density of the pre-existing dislocations and irradiation temperature. Dislocation climb and glide were observed, but a significant change in pre-existing dislocation density was not seen.
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The nature of dislocation generation as a function of applied plastic strain in quasi-statically compressed polycrystalline pure nickel has been studied experimentally at ambient temperature. First, ...to ensure representative datasets of the geometrically-necessary dislocation densities (ρGND) associated with non-uniform plastic deformation, measurements over large (several millimeter square) areas were made using Hough-based EBSD methods. In addition, the total dislocation density (ρT) responsible for the overall work hardening is estimated from the measured flow stress based on Taylor's hardening model. Next, the statistically stored dislocation (SSD) density (ρSSD) is calculated by subtracting the GND density from the total dislocation density. The results demonstrate that in quasi-statically deformed nickel: i) the measured GND density varies linearly as a function of plastic strain in the range between 0.05 and 0.46; although Ashby's model predicts linearity for GND density evolution over entire range of strains, this study does not cover strains below 0.05; ii) the SSD density increases at a rate much faster than GND density; and iii) the SSD density exceeds the GND density at above 0.09 plastic strain. Both i) and ii) are in agreement with Ashby's prediction, while the magnitudes of GND density (iii) differ from Ashby's model prediction, particularly at large applied strains. Overall, this study enables the interplay of GNDs and SSDs in the hardening of nickel to be gleaned in a quantitative sense. This study illustrates that GNDs are the more important for the strength of polycrystalline metals in the early stages of work hardening, whereas SSDs contribute more to the strength at larger strains. Over the range of strain in this study, work hardening is predominantly through rapid multiplication of SSDs; whereas the GND density is initially higher than SSD density at 0.05 probably due to non-linear evolution of GNDs at low strains (<0.05), which will be the subject of future investigation.
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