1,721,166 research outputs found
Short fatigue crack propagation during low-cycle, high cycle andvery-high-cycle fatigue of duplex steel – An unified approach
Full text linkThe present paper reviews experimental results on the fatigue damage of austenitic–ferritic duplex steel under various load levels ranging from LCF to VHCF, placing the focus towards the relationship between the crystallographic orientation of individual grains and grain patches that exhibit slip band formation, fatigue crack initiation and growth. A combination between fatigue testing of electropolished specimens and analytical electron microscopy (SEM/EBSD, TEM) revealed that under LCF loading conditions almost all the ferrite and the austenite grains showed plasticity, while under HCF and VHCF loading conditions, slip band formation was limited to the softer austenite grains and a low plastic activity is observed in the ferrite. Once being formed, the bands generate high stress concentrations, where they impinge the α–γ phase boundaries, eventually, leading to the crack initiation. This is discussed by applying a numerical simulation approach based on the finite-element (FEM) and the boundary-element (BEM) method.Fil: Krupp, Ulrich. University Of Applied Sciences; AlemaniaFil: Alvarez, Iris. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Rosario. Instituto de Física de Rosario (i); Argentin
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
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
Entwicklung und Prozessierung aluminiumlegierter ausscheidungshärtender Schmiedestähle
No full textTo cope with the energy transition, the internal combustion engine, whether operated with diesel fuel in combination with exhaust gas treatment or with synthetic fuels, will continue to play an important role, at least in the transportation sector. In times of energy scarcity and the continuing stagnation in the expansion of renewable energies, as well as the supply network for electrically powered vehicles, it is questionable whether electrification of the transport sector can be achieved without maintaining the internal combustion engine, at least in the medium term. For this reason, it is of great importance to continuously develop existing engine concepts with regard to efficiency and emissions. In this research project, precipitation-hardening aluminium-alloyed steels for diesel pistons were therefore developed which, compared with competitor materials - particularly cast aluminium alloys and quenching and tempering steels - are characterized by significantly higher strength at high temperatures and higher oxidation resistance. In addition, with the help of the research work carried out, it was possible to show that, through an ably combination of forging parameters, despite the bcc crystal structure of the investigated materials, significant grain refinement can be achieved as a result of the superposition of several recrystallization mechanisms. For the alloy development, thermodynamic simulations were first carried out, which aimed at the effect of various elements on the precipitation of the intermetallic Laves phase, which can significantly improve the material strength even at high temperatures. Based on the simulations performed, five steels could be produced by melting metallurgy. The process parameters for the thermally induced precipitation of the Laves phase were narrowed down by simulating the precipitation process and investigating the influence of temperature and holding time on the material hardness. Thus, the temperature range for the thermally induced precipitation of the Laves phase could be determined to 550-650 °C. The precipitates were characterized by means of synchrotron X-ray diffraction, transmission electron microscopy and scanning electron microscopy. It was shown that a combination of niobium and molybdenum in particular leads to a significant acceleration of precipitation. Furthermore, it was demonstrated that both chromium and silicon also have an accelerating effect. The precipitation of the Laves phase can significantly increase the strength measured by tensile tests. Even at test temperatures of 500 °C, the influence of the Laves phase on the strength is not weakened. Moreover, by using the above-mentioned elements, significant precipitation hardening can be achieved already after 0.5 or 1 h, respectively. The optimum forging parameters were determined with the aid of a deformation dilatometer and a specially constructed test rig for carrying out hot deformation tests with very high strains and strain rates. It was shown that high temperatures of 1250 °C are necessary to trigger dynamic recrystallization. At low strains and strain rates, only continuous dynamic recrystallization occurs at a few locations in the microstructure. By increasing both mentioned parameters, discontinuous dynamic recrystallization and geometric dynamic recrystallization can also be activated, greatly increasing the recrystallized fraction of the microstructure and massively lowering the grain size. As a result of grain refinement, toughness can be significantly increased
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
Anwendbarkeit unterschiedlicher Wärmebehandlungsrouten beim Presshärten von Mittelmanganstahl
No full textPress-hardening of manganese-boron steels is one of the most efficient production processes for high strength automotive components. However, the residual formability of these sheet components is greatly limited by the formation of a fully martensitic microstructure. To extend the application of press-hardened components also to impact energy-absorptive parts of the vehicle body, the use of third-generation advanced high strength steels, especially medium manganese steels, is gaining increasing attention. The alloying concept of these steels allows the critical cooling rate and the Ac3-temperature to be significantly lowered compared to manganese-boron steels, while introducing a certain amount of retained austenite improves the ductility of the material. Therefore, the present research analyzes the application potential of press-hardening of a medium manganese steel (Fe-0,3C-5Mn-1,5Si) in combination with an intercritical annealing or quenching & partitioning treatment. These heat treatments lead in general to ultrafine-grained multiphase microstructures containing retained austenite and different body-centered cubic phases, such as fresh martensite, tempered martensite, and ferrite, with different carbon content each. Since, depending on the phase fractions, strongly different mechanical properties are expected, the qualitative distinction but also the quantitative determination of the body-centered cubic phases are of extraordinary importance to be able to understand the correlation between microstructure and mechanical properties. In the present study, it is shown that using phase maps combined with grain average band slope of electron backscatter diffraction measurements is a suitable method to distinguish quantitatively fresh and tempered martensite as well as martensite and ferrite which was validated by electron probe micro analysis. Thermodynamic simulation and dilatometer experiments were performed and analyzed to understand the phase transformation kinetics of Fe-0,3C-5Mn-1,5Si. Based on dilatation results and electron backscatter diffraction analysis, the Koistinen-Marburger equation was adapted to fit the investigated medium manganese steel. According to the findings, suitable heat treatment process windows for intercritical annealing and quenching & partitioning were determined and reproduced in both the dilatometer and salt baths. Regarding the quenching & partitioning conditions, quenching temperature has turned out to be an important influencing parameter determining the phase fractions. It was shown that an exceptional property combination of high strength and ductility could be achieved when the phase fraction of fresh martensite was less than 20%, which was reached with a quenching temperature ≤150 °C. Higher amounts of fresh martensite lead, due to the difference in the plasticity of the different phases, to increased stresses at the interfaces and brittle intergranular fracture.Unlike the quenching & partitioning conditions, the intercritically annealed samples show discontinuous yielding behavior with Lüders-plateau and serrations. The Lüders-plateau could only be attenuated in the condition where also fresh martensite, due to insufficient thermal stability of the austenite, existed. Consequently, more mobile dislocations were present in the microstructure, which carry the plastic deformation. Selected heat treatments which reached promising mechanical properties were reproduced in a laboratory-scale press-hardening system equipped with a heated hat-shaped tool. Even though the optimum temperature range of quenching temperature could not yet be precisely set in the press hardening tests carried out, initial experiments show that a multiphase microstructure with retained austenite fractions of more than 10% can be achieved by combining press hardening with quenching & partitioning. To match the optimum time for component removal and thus achieve better mechanical properties, the integration of improved sensor technology, is indispensable. The evaluation of sensor data in combination with approaches of integrated computational materials engineering would also allow to predict properties based on process control and to identify production scrap or to avoid it by an adapted heat treatment
Oxidation behavior of wrought, cast and additive manufactured Co-base alloys for high temperature applications
No full textVarious high temperature industrial processes require construction materials that possess an optimal balance of mechanical strength, corrosion resistance and fabricability. Among the main candidate materials considered for the long-term service at temperatures exceeding 600°C are metallic Fe-, Co- and Ni-base alloys. The present study followed the oxidation mechanisms of wrought, cast and additive manufactured (AM) Co-base alloys during air exposures at 950-1050°C to address the effects of alloy microstructure, minor alloying elements and specimen thickness on oxidation kinetics and subscale depletion processes. Substantial differences in oxidation behavior between cast and AM (produced by Selective Laser Melting) versions of alloy MAR-M-509 were correlated with the alloy grain morphology and size as well as distribution of the strengthening precipitates. Faster oxidation of the cast alloy was shown to be mainly induced by: (1) oxidation of coarse primary Ta-rich carbides, that oxidized rapidly forming thick oxide nodules and promoting development of cracks/pores in the adjacent Cr2O3. Cracking of the scale and its spallation provided direct access of nitrogen to the Cr-depleted base metal causing internal nitridation. (2) Incorporation of other alloy constituents (Co, Ni, W) into the scale was observed from Cr-depleted alloy thus decreasing the scale grain size and increasing its growth kinetics. In contrast, the microstructure of the AM MAR-M-509 featured fine carbide precipitates that were found to dissolve in the subscale region forming internal oxides and resulting in more homogeneous oxide scale compared to the cast alloy. EDX analyses in the subscale zone of AM specimens further revealed virtually flat Cr concentration profiles, contrary to the cast alloy which exhibited strong Cr depletion. Such behavior was proposed to be the result of enhanced Cr diffusion due to contribution of grain-boundary transport in the fine-grained AM alloy. High temperature exposures of the wrought Co-Cr alloys Haynes 25 and Haynes 188 showed significantly faster oxidation of the former material. Relatively high Mn content in Haynes 25 facilitated formation of Cr/Mn-spinel and increased the scaling rate of Cr2O3, which was found to exhibit a significant part of inward growth. TEM and APT studies of the chromia scale revealed a presence of intragranular Cr/Mn-particles possibly nucleating simultaneously with Cr2O3. Substantially better performance of Haynes 188 was interpreted in terms of differences in composition of Cr, Ni, Mn and especially La. The chromia scale formed on the alloy surface featured a distinct duplex morphology with coarse columnar oxide grains in the outer part and fine equiaxed grains in the inner layer. Observed sub-parabolic oxidation of Haynes 188 was attributed to the slow inward growth of the inner chromia layer as a result of reactive element incorporation into the scale, which could suppress outward cation transport. Dependence of oxidation kinetics on specimen thickness was analysed by oxidizing 0.15-1mm sections of wrought alloy Haynes 188. Exposures at 950 and 1000°C showed higher oxidation kinetics of thinner specimens with more pronounced Cr depletion in agreement with the common observations reported for Fe-Cr and Ni-Cr alloys. Higher oxidation rate of thin Haynes 188 specimens could be clearly observed already after initial hours of isothermal exposures in a thermobalance. The possible explanation of this effect was referred to low intrinsic creep strength of thin sections resulting in more pronounced response to oxide growth stresses. In contrast, during the testing at 1050°C the increase of oxidation-induced Cr loss with decreasing section thickness was not monotonous and showed a maximum in the 0.5 mm thick specimen. This result was correlated with rapid exhaustion of Mn reservoir in thinner specimens that appeared to reduce Cr depletion kinetics due to less pronounced Mn incorporation into the scale
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