8,932 research outputs found

    Steigerung der thermischen Stabilität von warm- und kaltgewalztem Wolfram durch Kalium-Dotierung für die Fusionsenergietechnik

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    Fusionskraftwerke der Zukunft stellen enorme Materialanforderungen, beispielsweise in Bezug auf Hitzeresistenz an Plasmakontaktzonen der inneren Reaktorwand und des Divertors. Als abschirmendes Material stellt Wolfram nach aktueller Konzeption die mit Abstand beste Alternative dar. Nachteilig ist dabei jedoch dessen hohe Spröd-duktil-Übergangstemperatur (BDTT), welche aufgrund thermischer und mechanischer Belastungszyklen zu katastrophalem Risswachstum führen kann. Zwar kann die BDTT durch starkes Kaltwalzen drastisch abgesenkt werden, das dabei entstehende, feinkörnige Gefüge ist jedoch thermisch instabil, wodurch es bei relativ niedrigen Temperaturen wiederum zu einer Erhöhung der BDTT kommt. Neben einer Untersuchung der zu einer solchen Versprödung führenden mikrostrukturellen Restaurationsprozesse, wurde daher in der vorliegenden Arbeit das Potential zur Stabilisierung der Mikrostruktur durch eine Dotierung mit Kalium (K) umfassend analysiert, durch welche die Migration von Korngrenzen und Versetzungen mittels fein verteilter K-Blasen unterdrückt wird. Zwar wird K-Dotierung bereits seit mehr als 100 Jahren bei der industriellen Herstellung von Glühlampendrähten angewendet, die Kombination des Verfahrens mit hochgradiger Walzumformung von Wolfram stellt jedoch einen neuartigen Ansatz dar. Grundlage der Studie war zunächst die erfolgreiche Herstellung zweier äquivalent gewalzter Blechserien von technisch reinem Wolfram (Referenz) und K-dotiertem Wolfram mit bis zu sehr hohen logarithmischen Umformgraden von 4,7 bzw. 4,6 durch Warm- und Kaltwalzen. Anhand beider Materialserien wurde eine detaillierte Analyse der verformungsbedingten Evolution von Mikrostruktur (durch REM und EBSD), mechanischer Eigenschaften (durch Härteprüfung, Zugversuche und Untersuchungen der Risszähigkeit) und Verteilung von K-Blasen (durch REM) nach den Walzschritten erstellt. Als weiteres Kernstück der Arbeit wurde die Mikrostruktur nach isochronen, isothermen und aufheizratenkontrollierten Wärmebehandlungsreihen zwischen 600 °C und 2400 °C mittels Härteprüfung und REM-Analysen hinsichtlich aufgetretener Restaurationsprozesse in unterschiedlichen Temperaturregimen systematisch untersucht. Es ist herauszustellen, dass nach höchstem Umformgrad sowohl bei reinem als auch K-dotiertem Wolfram eine nochmals niedrigere BDTT unterhalb von −80 °C im Vergleich zu bisherigen Studien an reinen Wolframblechen vorgefunden wurde. Die Analyse ergab weiterhin, dass die verformungsbedingte Evolution der Mikrostruktur zwischen beiden Blechserien annähernd gleich verlief. Allerdings traten in den K dotierten Blechen mit geringerem Umformgrad mikrometerdicke Lagen auf, die jeweils einzelnen Texturkomponenten zuzuordnen sind und nahezu ausschließlich Kleinwinkelgrenzen enthalten. Da sich dieses Phänomen mit einer hohen BDTT der besagten Bleche in Verbindung bringen ließ, könnte dessen Vermeidung ein wichtiges Qualitätskriterium bei zukünftigen Materialproduktionen darstellen. Weitere mechanische Eigenschaften, wie Mikrohärte, Zugfestigkeit und Dehngrenze, ließen sich in Anlehnung an die Hall-Petch-Beziehung mit der Korngröße korrelieren, wobei in den kaltgewalzten Blechen ein signifikanter Einfluss durch Kleinwinkel-grenzen festgestellt wurde, welche oftmals in einer Hall-Petch-Beziehung nicht berücksichtigt werden. Die Mechanismen, welche die Dispersion der K-Blasen und damit die Zener-Kräfte gegenüber Erholung, Rekristallisation und Kornwachstum beeinflussen, wurden tiefgreifend analysiert. Maßgeblich ist dabei der Umformgrad der Bleche, der zu einer Streckung der Blasen führt, sowie die Parameter einer darauffolgenden Wärmebehandlung, die den Aufbruch der gestreckten Blasen ähnlich einer Plateau-Rayleigh-Instabilität bewirkt. Die theoretischen Grundlagen dieser Aufbruchskinetik wurden mit den experimentellen Ergebnissen in Einklang gebracht und das Modell für den Grenzfall von Blasen mit geringem Streckungsverhältnis weiterentwickelt. Eine nanoskalige Analyse der chemischen Zusammensetzung zeigte erstmals in-situ eine heterogene Elementverteilung innerhalb des Volumens von K-Blasen aus einer aluminiumreichen und einer volatilen, kaliumreichen Phase sowie erhöhte Gehalte von Silizium und Sauerstoff. Es konnte zudem eine Instabilität dieser chemischen Zusammensetzung nach einer Wärmebehandlung im Bereich von ca. 2400 °C nachgewiesen werden, welche bei Hochtemperaturanwendungen von Relevanz sein kann. Die Analyse wärmebehandelter Proben ergab, dass Bleche mit geringem Umformgrad in einem niedrigen Temperaturregime nur schwache mikrostrukturelle Änderungen durch Erholung zeigten. Bleche mit hohem Umformgrad offenbarten jedoch drastische Änderungen durch sogenannte erweiterte Erholung (auch als kontinuierliche Rekristallisation bezeichnet), welche einen maßgeblichen Grund für die Versprödung dieser Bleche darstellt. Durch K-Dotierung konnte eine nur geringfügig retardierende Wirkung gegenüber erweiterter Erholung nachgewiesen werden. Erst in einem mittleren Temperaturregime bewiesen K-dotierte Bleche eine deutlich retardierende Wirkung gegenüber der dabei stattfindenden Rekristallisation in Blechen mit geringem Umformgrad bzw. in Blechen mit hohem Umformgrad gegenüber der fortschreitenden erweiterten Erholung mit direktem Übergang zu Kornwachstum. Zudem ergaben sich durch die Dotierung deutliche Unterschiede in der Texturentwicklung, welche den weiteren Verlauf der Restauration durch Kornwachstum beeinflusst. In einem hohen Temperaturregime bewirkte die Dotierung eine nahezu vollständige Stabilisierung gegenüber normalem und abnormalem Kornwachstum bei geringem Umformgrad, bei hohem Umformgrad jedoch besonders starkes abnormales Kornwachstum. Auch etwaige Einflüsse durch unterschiedliche Aufheizraten von 1 K/s und 200 K/s unter Berücksichtigung einer thermischen Aktivierungsäquivalenz wurden an den dicksten Blechen untersucht, jedoch kein aufheizratenbedingter Effekt festgestellt. Dieses Ergebnis steht im Widerspruch zu bisherigen Studien, in denen eine thermische Aktivierungsäquivalenz nicht berücksichtigt wurde. Die komplexen Zusammenhänge von thermomechanischer Behandlung reiner und K-dotierter Wolframmaterialien und der daraus resultierenden mikrostrukturellen Evolution wurden abschließend einander gegenübergestellt. Aufgrund der gewonnenen Erkenntnisse scheint K-dotiertes Material mit geringem Umformgrad die meisten Vorteile für die Anwendung in Fusionsreaktorkomponenten aufzuweisen, insbesondere durch die enormen Auswirkungen der K-Dotierung auf Kornwachstum sowie der deutlichen Retardierung von Rekristallisation, aber auch durch einen geringeren Herstellungsaufwand im Vergleich zu hochgradig umgeformtem Wolfram

    Ultra High Strength Steels for Roll Formed Automotive Body in White

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    One of the more recent steel developments is the quenching and partitioning process, first proposed by Speer et al. in 2003 on developing 3rd generation advanced high-strength steel (AHSS). The quenching and partitioning (Q&P) process set a new way of producing martensitic steels with enhanced austenite levels, realised through controlled thermal treatments. The main objective of the so-called 3rd generation steels was to realise comparable properties to the 2nd generation but without high alloying additions. Generally, Q&P steels have remained within lab-scale environments, with only a small number of Q&P steels produced industrially. Q&P steels are produced either by a one-step or two-step process, and the re-heating mechanism for the two-step adds additional complexities when heat treating the material industrially. The Q&P steels developed and tested throughout this thesis have been designed to achieve the desired microstructural evolution whilst fitting in with Tata’s continuous annealing processing line (CAPL) capabilities. The CALPHAD approach using a combination of thermodynamics, kinetics, and phase transformation theory with software packages ThermoCalc and JMatPro has been successfully deployed to find novel Q&P steels. The research undertaken throughout this thesis has led to two novel Q&P steels, which can be produced on CAPL without making any infrastructure changes to the line. The two novel Q&P steels show an apparent reduction in hardness mismatch, illustrated visually and numerically after nano-indentation experiments. The properties realised after Q&P heat treatments on the C-Mn-Si alloy with 0.2 Wt.% C and the C-Mn-Si alloy with the small Cr addition is superior to the commercially available QP980/1180 steels by BaoSteel. Both novel alloys had comparable levels of elongation and hole expansion ratio to QP1180 but are substantially stronger with a > 320MPa increase in tensile stress. The heat treatment is also less complex as there is no requirement to heat the steel back up after quenching due to one-step quenching and partitioning being employed on the novel alloys

    Fracture prediction of fully clamped circular brittle plates subjected to impulsive loadings

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    When a high explosive detonates at some distance from the structure, the generated hot gas with high magnitude pressure and temperature expand rapidly and force the surrounding air out of the volume it occupies. As a consequence, a high pressure shock discontinuity namely a shock wave is produced. As this shock propagates away from the charge, it may inflict widespread damage to any structure that it impacts on. It is the challenge of structural engineers to improve the blast-resistant performance of the vulnerable structures and design adequate and efficient protective engineering systems against such extreme loading. Most of studies on the response of plate subjected to blast loadings focus on the transient or permanent deformation without any failure occur. Available methods in the literature for predicting failure response only works for one specific load distribution while distribution of blast loadings could be significantly due to various scenarios. Predictive method are therefore required that can predict the failure response of plates under loading with arbitrary distribution and intensity that are fast to run and accurate. Conducting the experimental research dealing with blast loads needs to consider both cost and safety issues. Besides, numerical analysis requires a high computational time. Therefore, the given analytical approach provides an alternative for failure response investigation. This thesis proposes an analytical method to predict failure response of plates under more than one specific loading distribution. Besides, this thesis provides dimensionless I − K diagrams that could quickly determine the potential failure modes a plate will suffer under the given blast loading. The results of this thesis should be used to guide analytical approach development for the prediction of failure response of plates subjected to blast loadings. The simple method developed in this thesis can be employed for design purposes, especially, at the early stages requiring an understanding of the structural failure response and help to rapid evaluation of the likely damage a structure will sustain in the event of blast

    Innovation in Energy Security and Long-Term Energy Efficiency Ⅱ

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    The sustainable development of our planet depends on the use of energy. The increasing world population inevitably causes an increase in the demand for energy, which, on the one hand, threatens us with the potential to encounter a shortage of energy supply, and, on the other hand, causes the deterioration of the environment. Therefore, our task is to reduce this demand through different innovative solutions (i.e., both technological and social). Social marketing and economic policies can also play their role by affecting the behavior of households and companies and by causing behavioral change oriented to energy stewardship, with an overall switch to renewable energy resources. This reprint provides a platform for the exchange of a wide range of ideas, which, ultimately, would facilitate driving societies toward long-term energy efficiency

    Thermomechanical testing and modelling of railway wheel steel

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    Studies of thermal effects of tread braking on railway wheels show that the wheel temperatures may reach above 600 \ub0C, at which the mechanical properties of the wheel steel are significantly impaired. Computational models that simulate the thermomechanical behaviour of the wheels are commonly based on results from laboratory tests which do not reflect actual in-service scenarios. Anisothermal testing and modelling are omitted due to the difficulties in designing relevant experiments and implementation of the results. In this paper, a preexisting numerical material model is extended in order to implement fully anisothermal behaviour. This is done by performing several thermomechanical experiments mimicking real-world service and worst-case scenarios ranging from room temperature up to 650 \ub0C. The results from the laboratory testing are then used in combination with data from traditional isothermal tests to optimise the numerical material model by calibrating its material parameters. As part of this process it was found necessary to include a time- and temperature-dependent, non-recoverable (irreversible) mechanism for material softening and microstructural changes which occur above 400 \ub0C. Finite element simulations with the material model using the new parameters and the softening law show markedly improved adherence to anisothermal and strain-controlled experimental results compared to the preexisting model(s). The results demonstrate that anisothermal testing is a requirement for models that are intended to simulate material behaviour for thermomechanical loads and thermally induced microstructural changes

    High Throughput Discovery of Lightweight Corrosion-Resistant Compositionally Complex Alloys

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    Compositionally complex alloys hold the promise of simultaneously attaining superior combinations of properties such as corrosion resistance, light-weighting, and strength. Achieving this goal is a challenge due in part to a large number of possible compositions and structures in the vast alloy design space. High throughput methods offer a path forward, but a strong connection between the synthesis of a given composition and structure with its properties has not been fully realized to date. Here we present the rapid identification of light weight highly corrosion-resistant alloys based on combinations of Al and Cr in a Cantor-like base alloy (Al-Co-Cr-Fe-Ni). Previously unstudied alloy stoichiometries were identified using a combination of high throughput experimental screening coupled with key metallurgical and electrochemical corrosion tests, identifying alloys with excellent passivation behavior. Importantly, the electrochemical impedance modulus of the exposure-modified, air-formed film at the corrosion potential was found as an accurate non-destructive predictor of corrosion and passivation characteristics. Multi-element EXAFS analyses connected more ordered type chemical short range order in the Ni-Al 1st nn shell to poorer corrosion. This report underscores the utility of high throughput exploration of compositionally complex alloys for the identification and rapid screening of vast stoichiometric space

    The influence of laser shock peening on corrosion-fatigue behaviour of wire arc additively manufactured components

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    The need for increased manufacturing efficiency of large engineering structures has led to development of wire arc additive manufacturing (WAAM), which is also known as direct energy deposition (DED) method. One of the main barriers for rapid adoption of the WAAM technology in wider range of industrial applications is the lack of sufficient performance data on the WAAM components for various materials and operational conditions. The present study addresses this essential need by exploring the effects of laser shock peening surface treatment on corrosion-fatigue crack growth (CFCG) life enhancement of WAAM components made of ER70S-6 and ER100S-1 steel wires. The experimental results obtained from this study were compared with the CFCG trends from nominally identical specimens without surface treatment and prove the efficiency of the examined surface treatment method for corrosion-fatigue life enhancement and crack growth retardation of WAAM built steel components, regardless of the material type and specimen orientation. Furthermore, the residual stresses in the WAAM built specimens with and without surface treatment were measured to validate the influence of beneficial residual stresses, arising from surface treatment, on subsequent CFCG behaviour of the material. The residual stress profiles show the beneficial compressive stress fields in the surface treated areas which result in CFCG life enhancement. The results from this study make significant contribution to knowledge by evaluating the suitability of WAAM built steel components for application in offshore environments

    Advances in Micro- and Nanomechanics

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    This book focuses on recent advances in both theoretical and experimental studies of material behaviour at the micro- and nano-scales. Special attention is given to experimental studies of nanofilms, nanoparticles and nanocomposites as well as tooth defects. Various experimental techniques were used. Magneto- and thermoelastic coupling were considered, as were nonlocal models of thin structures

    Preliminary investigation of slurry erosion behaviour of tantalum

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    Frequent premature failure of the ISIS spallation neutron source target prompted the investigation of previously unexplored aqueous slurry erosion response of pure tantalum (Ta) with an overarching aim to improve the service life of the target; hence, reducing the disposal of radioactive waste. Understanding such response of Ta is highly significant to many other applications such as nuclear and chemical processing. In this study, powder-metallurgically manufactured pure Ta was investigated with the help of an impinging jet aqueous slurry erosion apparatus using silicon carbide particles at a range of concentration, impact velocity, and incident angle. Results revealed a unique material removal mechanism consisting formation of extensive voids/cavities all over the eroded surface. These mechanisms are discussed considering the theories of solid particle erosion and the grain boundary sliding behaviour of Ta under localised indentation loading

    Numerical procedure to determine the performance and structural response of passive shock wave safety valves under blast loading

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    Traditional protective structures are usually equipped with ventilation systems. Main components of the latter are passive air blast safety valves. Their purpose in case of an explosive event outside the structure is to significantly reduce the blast pressure leakage into the structure in order to protect human individuals as well as technical installations. Until now, the performance determination of such valves is mostly realized by means of experimental tests in a shock tube. Considering industrial and modern civil protection applications with their practical implementation, additional methods are required to gain further insights into the behaviour of different valve closing mechanisms and to support novel developments as well as error analysis. For this reason, a practice-oriented procedure is presented, with the aim to extend the assessment of the closing behaviour and blast pressure leakage of passive air blast safety valves and the structural behaviour by numerical simulations. In a first preliminary step, potential software solutions have been evaluated based on literature research and expert knowledge. After evaluation of the obtained results, two different software pairs (fluid dynamic as well as structural dynamic tools) have been tested by carrying out indirectly coupled numerical simulations. The software pair APOLLO Blastsimulator & LS-DYNA achieved satisfactory results with the indirect coupling, so that direct fully coupled FSI simulations were additionally performed. To cover a broad range of blast safety valve applications, two different suitable test cases have been considered. In comparison to the experimental results, good agreement was achieved when analysing the pressure–time history of the blast pressure leakage and the closing time of the safety valve. Furthermore, the latter was confirmed by high-speed camera registrations during blast loading
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