1,720,963 research outputs found
Service induced fcc→hcp martensitic transformation in a Co-based superalloy
No full textCo-based superalloy, ECY768, applied on gas turbine vanes, shows the presence of cracks after service. EBSD studies revealed hcp transformation in the base material near cracked regions. This phase arises from a martensitic transformation of fcc matrix and bestows high fragility. The phase transformation is related to temperature and loading distribution that characterises components in service. However, at the service temperatures, the hcp transformation is not expected for ECY768. An in-house thermodynamic database was developed using the Calphad approach and thermodynamic calculations were applied to the complicated alloy composition for phase stability range evaluation. Moreover, a testing campaign was planned to artificially create this martensitic transformation and to comprehend the influence of plastic strain on fcc-hcp transformation. The transformation mechanisms were understood and some methods were developed for hcp-phase removal through refurbishment heat treatment
Effect of cooling rate on phase transformation in 6–8 wt% YSZ APS TBCs
No full textIn properly produced as-sprayed thermal barrier coatings of yttria partially stabilized zirconia (7YSZ) the only phase that can be found is the metastable tetragonal prime structure, t′. Even though t′is sometimes called “not transformable”, because it behaves as practically stable up to rather high temperature, long term exposure above 1200 ◦C produces its transformation in tetragonal, t, and cubic, c, phases. During cooling down to room tem-perature the t phase will transform in monoclinic, m, one. Although it is considered a martensitic transformation, high cooling rate, after prolonged high temperature exposure (over 1300 ◦C), can avoid or limit the evolution from t to m structure. The effect of the cooling rate on this transformation has been investigated in free standing TBCs both with porous microstructure and dense vertically cracked one, exposed at 1400 ◦C for 100 h. The samples have been analyzed by XRD and subsequent Rietveld refinement analysis to quantify the phase content: the results highlight that different cooling rates give different monoclinic contents, confirming the cooling rate effect on this transformation. Nevertheless, if the equilibrium, prevented by fast cooling, is restored, the trans-formation to m occurs; in fact it is sufficient to put in a furnace at low temperature for short duration the samples fast cooled down from 1400 ◦C in order to delete or weaken the cooling rate effect
Thermodynamic and Kinetic Simulation of the Brazing Process Applied to Ni-Base Superalloys
No full textA combination of thermodynamic and kinetic modeling, in the framework of the CALPHAD approach, has been used to simulate the brazing process applied to the joining of a René 80 superalloy by a Df4b filler. For thermodynamic equilibrium calculations the ThermoCalc software has been used, equipped with the TCNI8 database, while diffusion simulations have been obtained using the DICTRA software. As a result, elemental as well as phase precipitation profiles in the joining area have been obtained as a function of the distance and as a function of time, during the heat treatment adopted for the brazing process. Then, calculation results have been compared to experimental observations of a real joining obtained by applying the same heat treatment of the simulation. As a conclusion the simulation procedure was validated by experiments
Strain localizations in notches for a coarse-grained Ni-based superalloy: Simulations and experiments
Full text linkAlloys used for turbine blades have to safely sustain severe thermomechanical loadings during service such as, for example, centrifugal loadings, creep and high temperature gradients. For these applications, cast Ni-based superalloys characterized by a coarse-grained microstructure are widely adopted. This microstructure dictates a strong anisotropic mechanical behaviour and, concurrently, a large scatter in the fatigue properties is observed. In this work, Crystal Plasticity Finite Element (CPFE) simulations and strain measurements performed by means of Digital Image Correlations (DIC) were adopted to study the variability introduced by the coarse-grained microstructure. In particular, the CPFE simulations were calibrated and used to simulate the effect of the grain cluster orientations in proximity to notches, which reproduce the cooling air ducts of the turbine blades. The numerical simulations were experimentally validated by the DIC measurements. This study aims to predict the statistical variability of the strain concentration factors and support component design
Thermodynamic modeling and experimental investigation of the MgO-Y2O3-ZrO2 system
No full textSolid-state phase equilibria in the MgO-Y2O3-ZrO2 system as well as the equilibria including liquid were investigated in the whole-compositional range using high-temperature differential thermal analysis (DTA), X-ray diffraction (XRD), and scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM/EDX). Isothermal sections at 1493, 1573, 1693, and 1923 K were constructed based on experimental studies. The presence of tie line between MgO and Y4Zr3O12 in the temperature range between 1493 and 1573 K was confirmed. The eutectic melting in the MgO-Y2O3-ZrO2 system was established using DTA followed by SEM/EDX microstructure investigation. Based on the obtained experimental results, the thermodynamic database was derived
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
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
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