1,720,985 research outputs found
The effect of high temperature exposure on the dendritic segregation in a conventionally cast Ni based superalloy
No full textDendritic segregation in conventionally cast, commercially heat treated and exposed MarM002 turbine blades has been investigated using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). During casting Cr, Co and W segregate to the dendrites and Hf, Ti and Ta segregate to the eutectics. As a result of the dissolution of precipitates in the dendrites and coarsening of precipitates in the eutectics this segregation aggravates during exposure at 1000°C. At this high exposure temperature homogenisation of dendritic segregation does not occur because it involves long diffusion distances and a low driving force compared with dissolution and coarsening of precipitates
Modelling microstructural evolution in conventionally cast Ni-based superalloys during high temperature service
No full textModern conventionally cast (CC) Ni-based superalloys are highly alloyed metals that undergo a number of complex microstructural changes during in-service exposure to high temperatures. This paper focuses on modelling the kinetics of reactions involving carbides. An important element in various approaches for modelling of kinetics of reactions in Ni-based superalloys is so-called thermodynamic modelling (CALPHAD method). A detailed comparison of CALPHAD predictions made by one database with extensive long term experimental data for the MarM002 Ni-based superalloy for all phases is presented.A comparison of CALPHAD predictions made by the Thermotech database for Ni-based superalloys with long term experimental data shows that for the MarM002 Ni-based superalloy the volume fractions and compositions of the FCC gamma ?matrix, the L12 ordered gamma' and the M23C6 carbide phase are predicted well. Thermodynamic modelling is currently unable to predict the presence of M6C in this alloy. The reason for this is primarily due to the large number of carbides of different types and compositions within MarM002. There must be a balance of stability for all of the carbides and therefore slight perturbations to the Gibbs free energy polynomials describing each phase can limit the accuracy of the prediction for minor carbides. It is also shown that it is necessary to allow for a miscibility gap in the MC carbide phase to enable a more accurate prediction of its chemical composition in such a complex alloy system. A method for modelling the kinetics of reactions using an approximation of the diffusion fields around particles (i.e. akin to the well-known Johnson-Mehl-Avrami-Kolmogorov approach) is described. With this method the carbide reactions in during high temperature exposure of CC Ni-based superalloy can be accurately described.<br/
MC carbides in the Hf containing Ni based superalloy MarM002
No full textUnexposed MarM002 alloy turbine blades and blades exposed at 700-1000ºC for up to 250 days were studied by TEM, SEM, EDX and XRD. The shapes, distribution and compositions of the MC carbides do not vary significantly during exposure at 700 and 800ºC. All MC carbides contain significant amounts of Hf, Ta, Ti and W, and they are present with a very broad range of compositions. The latter is caused by a miscibility gap in the MC carbide phase which causes the formation of MC carbides to shift from Hf lean in the initial stages of solidification to Hf rich in the later stages. In the unexposed samples the Hf lean (Ti,Ta)C carbide is the most numerous one. The shape of the complex MC carbide diffraction peaks are explained semi-quantitatively on the basis of the compositional range of the MC carbides
Low temperature copper solubilities in Fe-Cu-Ni
No full textAn atom probe characterization of the copper concentration in the matrix of a model Fe–1.1at.%Cu–1.4% Ni alloy has been performed after isothermal ageing for various extended times at low temperatures. The matrix copper concentrations from material annealed at 500, 550 and 600°C are approximately 10% lower than the equilibrium values predicted from the Thermocalc™ program with the Kaufman database. Isothermal annealing for 4000 h at either 400 or 300°C was not sufficient to attain the equilibrium copper concentration. The kinetics of copper precipitation was investigated with the Starink-Zahra model derived for nucleation and growth reactions and a good fit with the atom probe data was found for an activation energy of 250 kJ mol?1
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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
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