Resampling technique applied to statistics of microsegregation characterization

Characterization of chemical heterogeneities at the dendrite scale is of practical importance for understanding phase transformation either during solidification or during subsequent solid-state treatment. Spot analysis with electron probe is definitely well-suited to investigate such heterogeneities at the micron scale that is relevant for most solidified products. However, very few has been done about the statistics of experimental solute distributions gained from such analyses when they are now more and more used for validating simulation data. There are two main sources generating discrepancies between estimated and actual solute distributions in an alloy: i) data sampling with a limited number of measurements to keep analysis within a reasonable time length; and ii) uncertainty linked to the measurement process, namely the physical noise that accompanies X-ray emission. Focusing on the first of these sources, a few 2-D composition images have been generated by phase field modelling of a Mg-Al alloy. These images were then used to obtain "true" solute distributions to which to compare coarse grid analyses as generally performed with a microanalyser. Resampling, i.e. generating several distributions by grid analyses with limited number of picked-up values, was then used to get statistics of estimates of solute distribution. The discussion of the present results deals first with estimating the average solute content and then focuses on the distribution in the primary phase

Transformations et contraintes de cohérence dans les superalliages et les intermétalliques de base TiAl

Après avoir rappelé que toute transformation à l’état solide génère des distorsions élastiques et des contraintes mécaniques – dites « de cohérence » ou « de compatibilité » – cet article identifie quatre mécanismes de relaxation totale ou partielle de ces contraintes : (a) par déformation plastique; (b) par adaptation de la morphologie des constituants; (c) par sélection de variants cristallins et (d) par modification des équilibres de phases. Des exemples en sont ensuite donnés dans le cas de la précipitation, de la dissolution et de la « mise en radeaux » de la phase γ′ des superalliages monocristallins à base de nickel (mécanismes b et d) ainsi que dans le cas de la transformation α → α2 − γ lamellaire des alliages intermétalliques de base TiAl (mécanisme c)

Chemical composition around γ/γ' interface in single crystal nickel-based superalloys in the early stage of creep

The aim of this work was to examine if a chemical anisotropy around γ' precipitates could be induced by the applied stress at the beginning of a compressive creep test. This question was solved in terms of concentration profiles around γ' in order to see which alloying elements diffuse and if they do, in which direction. The results were discussed in the light of finite element simulations where the local stress states were determined in terms of equivalent Von Mises stress and hydrostatic pressure

Internal Mismatch Stresses in Nickel-Based Superalloys : A Finite Element Approach

A three-dimensional finite element (F.E.) model is used to estimate the mismatch stresses at a microscopic scale in γ-γ' single-crystal nickel-based superalloys. F.E. calculations point out the specificity of stress/strain distributions in these alloys, in particular the fact that internal stresses concentrate in the connected γ matrix. The analysis of stress/strain fields and elastic energy evolution as a function of precipitate morphology and volume fraction allows to explain some atypical precipitate shape changes such as "splitting" during precipitation treatments or "raftening" during high temperature creep tests

High temperature thermal diffusivity of nickel-based superalloys and intermetallic compounds

By means of an installation developed in our laboratory, we measured the thermal diffusivity (α) as a function of temperature for several single and polycrystal nickel-based superalloys as well as for different intermetallic compounds with a L12 (Ni3Al, Ni3Si, Ni3Ge, Ni3Fe, Zr3Al, Co3Ti), L10 (TiAl) or B2 (NiAl) structure. In the case of nickel-based superalloys, the experiments pointed out an unexpected but reproductible slope change in the α=f(T) curves at about 750°C, which is not explained at this time. It was shown that the initial microstructure, the crystal orientation and the grain size seem to have no noticeable influence on the thermal behaviour. Slight variations between different alloys were rather attributed to a chemical composition effect. In the case of intermetallic compounds the temperature dependence of the thermal diffusivity appeared to drastically change from an alloy to an other. Thus, no clear relation could be found between thermal behaviour, crystallographic structure and strength temperature dependence of these alloys

Microbial mobilization of cesium from illite: Role of organic acids and siderophores

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