1,721,052 research outputs found
Grundlagen des Selektiven Elektronenstrahlschmelzens von Titanaluminiden
Full text linkTitanium aluminides have great potential for high temperature applications, for example as turbine blades in aero engines. The main advantage over conventional nickel based alloys is the high weight saving in combination with good creep resistance. In contrast, due to their high sensitivity to impurities and low ductility, various issues arise in the processing of this material. Selective electron beam melting (SEBM) as an additive manufacturing process offers significant advantages to face these challenges. Crucible free production in a vacuum chamber reduces contamination during production and a defined temperature control in the process prevents cracks during solidification and cooling of the component. The manufacturing of complex components, e.g. turbine blades with internal cooling channels, benefits from the geometric freedom of the process. However, the understanding of tailored melting strategies and the range of achievable properties are still incomplete and the holistic understanding of the processing of titanium aluminides in SEBM has not yet been achieved.
The present work develops the basis for the processing of titanium aluminides in selective electron beam melting and determines the relationships between the process parameters and the properties of the material. The titanium aluminide alloy Ti-48Al-2Cr-2Nb is used to define the processing window for dense samples and suitable melting strategies to tailor porosity and chemical properties. The microstructure results from a complex interaction of solidification, evaporation and in situ heat treatment. The main characteristics of the microstructure evolution in the process are presented and strategies for microstructure optimization are evaluated. A further step of property adjustment is implemented by subsequent heat treatments. Both approaches to tailor the microstructure are evaluated with regard to their static properties at room temperature and their creep properties. The mechanical properties show an direct dependency on the adjusted microstructure. Furthermore, the residual porosity significantly reduces the ductility of the brittle material, which is why suitable solutions for this challenge are addressed. The achieved mechanical properties are comparable to those of conventional casted material. The process is transferred to another titanium aluminide alloy, using the new understanding to considerably minimize the development effort. Finally, a feasibility study shows the successful application of the findings by manufacturing a turbine wheel for turbochargers with defined properties.Titanaluminide haben großes Potenzial für Anwendungen im Hochtemperaturbereich auf Grund der hohen Gewichtsersparnis und der gleichzeitig guten Kriechbeständigkeit gegenüber den überwiegend eingesetzten Nickelbasislegierungen, beispielsweise als Turbinenschaufeln in Triebwerken. Jedoch ergeben sich bei der Verarbeitung dieser Materialklasse angesichts ihrer hohen Sensibilität gegenüber Verunreinigungen und geringen Duktilität diverse Problemstellungen. Das Selektive Elektronenstrahlschmelzen (SEBM) als additives Fertigungsverfahren bietet hierbei wesentliche Vorteile, um diese Herausforderungen zu bewältigen. Durch die tiegelfreie Fertigung im Vakuum können Verunreinigungen während der Herstellung reduziert werden und durch eine definierte Temperaturführung im Prozess wird die Rissbildung beim Erstarren und Abkühlen des Bauteils vermieden. Die Fertigung von komplexen Bauteilen profitiert von der Geometriefreiheit des Prozesses, welche z.~B. innenliegende Kühlkanäle zulässt. Jedoch sind die Erkenntnisse hinsichtlich der optimalen Schmelzstrategie und des erreichbaren Eigenschaftsspektrums noch unvollständig und das gesamtheitliche Verständnis der Verarbeitung von Titanaluminiden im SEBM ist noch nicht geschaffen worden.
Daher werden in der vorliegenden Arbeit die Grundlagen zur Verarbeitung von Titanaluminiden im Selektiven Elektronenstrahlschmelzen entwickelt und die Zusammenhänge zwischen Prozessbedingungen und dem Eigenschaftsprofil des Materials ermittelt. Dazu werden anhand der Titanaluminidlegierung Ti-48Al-2Cr-2Nb die Prozessfenster für dichte Proben abgesteckt und mögliche Schmelzstrategien zur Optimierung der Porosität und der chemischen Eigenschaften aufgezeigt. Die Mikrostruktur entsteht aus einem komplexen Zusammenspiel von Erstarrung, Verdampfung und \textit{in situ} Wärmebehandlung. Die Grundzüge der Mikrostrukturentwicklung im Prozess werden dargelegt und Strategien zur Mikrostrukturoptimierung evaluiert. Mittels nachgeschalteter Wärmebehandlungen wird eine weitere Stellschraube zur Anpassung des Eigenschaftsprofils untersucht. Die Mikrostrukturen aus beiden Ansätzen werden hinsichtlich ihrer statischen Eigenschaften bei Raumtemperatur und ihren Kriecheigenschaften bewertet. Aus den Ergebnissen lässt sich eine direkte Abhängigkeit der mechanischen Eigenschaften von der eingestellten Gefügemorphologie ableiten. Die Restporosität reduziert zudem deutlich die Duktilität des spröden Werkstoffs, weshalb hier geeignete Lösungsansätze diskutiert werden. Die erreichten mechanischen Eigenschaften sind vergleichbar mit konventionellen Gussbauteilen. Zuletzt wird der Übertrag des Kenntnisstands auf eine weitere Titanaluminidlegierung umgesetzt und dadurch Entwicklungsaufwand stark minimiert. Zusätzlich zeigt eine Machbarkeitsstudie die erfolgreiche Anwendung der Erkenntnisse auf die Fertigung eines Turbinenrads für Turbolader mit definierten Eigenschaften
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Neuartige CoNiCr-basierte Superlegierungen: Mikrostruktur, mechanische Eigenschaften und Oxidationswiderstand
Full text linkRecent research on high and medium entropy alloys revealed that the CoNiCr-system provides interesting properties, however, the high temperature strength of such single-phase alloys is not sufficient. Therefore, new high temperature superalloys based on the CoNiCr-system and strengthened by γ′ -(Ni,Co)3(Cr,Al,Ti) precipitates have been developed within this work and the effect of different refractory element additions (Nb, Mo, Ta, W) on microstructure, mechanical properties and oxidation behavior was investigated.
The newly designed alloy compositions were determined based on literature research and guided by CALPHAD calculations. The elemental partitioning behavior and the lattice misfit between the γ and γ′ phases were determined by STEM-EDS and HEXRD. Ta and Nb strongly concentrate within the γ′ phase, whereas Mo weakly partitions to the γ phase. W distributes equally between the γ and γ′ phases. These new superalloys have an unexpectedly high positive misfit compared with some conventional Ni-based superalloys and Co-based superalloys. Nb and Ta additions increase the lattice misfit further, while Mo and W decrease the lattice misfit. The effect of refractory elements alloying on the yield stress at different temperatures was evaluated. Alloying with Nb or Ta significantly improves the high temperature mechanical properties. By modelling it was found that additions of Ta or Nb strengthen the γ′ phase more than Mo and W do. However, Mo has the highest solid solution strengthening effect in the γ phase, followed by W. Creep experiments were done under compression at 750 °C and 850 °C, respectively. Additions of W and Ta can efficiently improve the creep resistance of the alloys in the intermediate temperature range. The high temperature oxidation in novel CoNiCr-based superalloys was investigated. During high temperature oxidation, static recrystallization was triggered at the specimens’ surface. Nb, W, Ta and Mo alloying cannot avoid the formation of recrystallization at the surface completely. However, Ta addition decreases the depth of recrystallization at the surface.
Compared with some conventional Ni-based superalloys, the investigated new CoNiCr-based superalloys exhibited better mechanical properties at high temperature, which indicates that these alloys with their complex compositions are possible candidates for high temperature applications. However, there are some critical problems, such as discontinuous reactions at grain boundaries and Portevin-le-Chatelier phenomenon during compressive deformation, that still need to be solved in the future
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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