Nanoindentation: A powerful tool to explore the wide chemical space of high entropy alloys

High entropy alloys (HEA) are multi-component alloys, without any minor or major elements (i.e. : all elements are very concentrated) and they form a unique solid solution. It was proven that, especially for the system Co-Cr-Fe-Mn-Ni, they exist for a very wide range of composition [1]. This opens the opportunity of multi-properties optimization, like cost, density and mechanical resistance. However, to take advantage of this opportunity, accelerated mechanical testing tools are required. Please click Additional Files below to see the full abstract

Interactive Impacts of Silver and Phosphorus on Autotrophic Biofilm Elemental and Biochemical Quality for a Macroinvertebrate Consumer

Autotrophic biofilms are complex and fundamental biological compartments of many aquatic ecosystems. In particular, these biofilms represent a major resource for many invertebrate consumers and the first ecological barrier against toxic metals. To date, very few studies have investigated the indirect effects of stressors on upper trophic levels through alterations of the quality of biofilms for their consumers. In a laboratory study, we investigated the single and combined effects of phosphorus (P) availability and silver, a re-emerging contaminant, on the elemental [carbon (C):nitrogen (N):P ratios] and biochemical (fatty acid profiles) compositions of a diatom-dominated biofilm initially collected in a shallow lake. We hypothesized that (1) P and silver, through the replacement of diatoms by more tolerant primary producer species, reduce the biochemical quality of biofilms for their consumers while (2) P enhances biofilm elemental quality and (3) silver contamination of biofilm has negative effects on consumers life history traits. The quality of biofilms for consumers was assessed for a common crustacean species, Gammarus fossarum, by measuring organisms’ survival and growth rates during a 42-days feeding experiment. Results mainly showed that species replacement induced by both stressors affected biofilm fatty acid compositions, and that P immobilization permitted to achieve low C:P biofilms, whatever the level of silver contamination. Gammarids growth and survival rates were not significantly impacted by the ingestion of silver-contaminated resource. On the contrary, we found a significant positive relationship between the biofilm P-content and gammarids growth. This study underlines the large indirect consequences stressors could play on the quality of microbial biomass for consumers, and, in turn, on the whole food web

Élaboration par solidification dirigée et comportement mécanique de céramiques eutectiques à base d’oxydes réfractaires : rôle de la microstructure sur la fissuration et la déformation plastique à haute température

Dans le contexte général lié aux économies d’énergie, l’amélioration sensible du rendement des turbines à gaz (aéronautiques ou terrestres), nécessitera d’augmenter notablement la température des gaz de combustion. Cela implique l’emploi de matériaux stables au-delà de 1 500°C. Les céramiques eutectiques préparées par solidification dirigée, à partir des systèmes Al2O3 - Ln2O3 (où Ln représente un élément lanthanide ou l’yttrium) sont une solution envisageable. En effet, leur microstructure, constituée d’un réseau interpénétré 3D de deux phases monocristallines, et exempte de pores, de colonies et de joints de grains, confère à ces systèmes eutectiques des propriétés mécaniques d’un bon niveau, et quasi-constantes jusqu’à des températures proches de leur température eutectique (> 1 700°C). Nos travaux ont consisté à élaborer plusieurs systèmes eutectiques binaires et ternaires, par ajout d’une phase ZrO2 renforçante. Les six systèmes présentant les microstructures les plus prometteuses (3 binaires : Al2O3 - Y3Al5O12, Al2O3 - Er3Al5O12, Al2O3 - GdAlO3, et 3 ternaires : Al2O3 - Y3Al5O12 - ZrO2, Al2O3 - Er3Al5O12 - ZrO2, Al2O3 - GdAlO3 - ZrO2) ont été retenus pour étudier certaines de leurs propriétés mécaniques. Plusieurs modes de fissuration, allant dans le sens de l’augmentation de ténacité détectée dans ces systèmes, ont été décelés après des essais de flexion biaxiale. Ces modes de fissuration ont été corrélés aux caractéristiques microstructurales et à la distribution des contraintes résiduelles, déterminées par le calcul et mesurées par une méthode piézo-spectroscopique. Enfin, l’étude du comportement en fluage à haute température a permis de mettre en évidence une évolution des mécanismes de déformation en fonction des conditions de sollicitation. L’étude MET post mortem a également souligné l’influence marquée du caractère interconnecté de la microstructure sur le comportement en fluage.In the general context of energy savings at a global scale, the improvement of the thermal efficiency of both terrestrial and aeronautical gas turbines will require to increase the turbine inlet gas temperature. The development of new materials, stable up to 1 500°C, is thus necessary. In this context, Directionally Solidified Eutectic Ceramics (DSEC), prepared from Al2O3 and Ln2O3-based systems, could be a potential solution. Their microstructure consists of two single-crystal phases continuously entangled in a threedimensional interpenetrating network without grain boundaries, pores or colonies. The outstanding stability of these microstructures gives rise to a high strength and creep resistance at high temperature. Our research consisted first in obtaining, by directional solidification, several eutectic systems, either binary or ternary (with addition of a toughening third ZrO2 phase). The six most promising DSEC (3 binary systems: Al2O3 - Y3Al5O12, Al2O3 - Er3Al5O12, Al2O3 - GdAlO3, and 3 ternary systems: Al2O3 - Y3Al5O12 - ZrO2, Al2O3 - Er3Al5O12 - ZrO2, Al2O3 - GdAlO3 - ZrO2) have then been selected to study some of their mechanical properties. Several crack propagation patterns have been detected after biaxial flexure testing, and partially explain the toughening which has been proven for DSEC. Attention has been paid to the possibility of crack deflection in the various phases and in the phase boundaries, a phenomenon which may markedly improve the toughness of these eutectic ceramics. These observations have been correlated to internal stress calculations and piezo-spectroscopic measurements. Finally, the study of the creep behavior showed that the deformation mechanisms evolve with the macroscopic solicitation (temperature and stress). microstructure. Moreover, post mortem TEM observations exhibited that creep mechanisms are strongly dependant on the entangled microstructure

Insights into the phase diagram of the CrMnFeCoNi high entropy alloy using electromagnetic melting and casting

International audienceHigh entropy alloys are single phase metallic materials, composed of more than 4 elements in equi-atomic proportionwhich is the maximum in the configurational entropy. In order to study the thermodynamics of those alloys, anequimolar CrMnFeCoNi HEA was processed by electromagnetic melting, casting and annealing under varyingconditions (cooling rate and annealing duration) and the evolution of the structure and microstructure was studied. Aftercasting, a dendritic microstructure was formed whose characteristic length depends on the cooling rate. Annealing leadsto a chemical homogenization of the microstructure. Indeed a true solid-solution down to the atomic scale wasevidenced in an HEA by atom probe tomography. It was shown that this face-centered cubic single-phase solid solutionis the high temperature stable state of the equimolar CrMnFeCoNi alloy. By analogy with completely miscible binaryalloy, the CrMnFeCoNi HEA can be described by a schematic phase diagram of the CrFeCo - MnNi system consistingof a liquidus and a solidus

Insights into the phase diagram of the CrMnFeCoNi high entropy alloy using electromagnetic melting and casting

International audienceHigh entropy alloys are single phase metallic materials, composed of more than 4 elements in equi-atomic proportionwhich is the maximum in the configurational entropy. In order to study the thermodynamics of those alloys, anequimolar CrMnFeCoNi HEA was processed by electromagnetic melting, casting and annealing under varyingconditions (cooling rate and annealing duration) and the evolution of the structure and microstructure was studied. Aftercasting, a dendritic microstructure was formed whose characteristic length depends on the cooling rate. Annealing leadsto a chemical homogenization of the microstructure. Indeed a true solid-solution down to the atomic scale wasevidenced in an HEA by atom probe tomography. It was shown that this face-centered cubic single-phase solid solutionis the high temperature stable state of the equimolar CrMnFeCoNi alloy. By analogy with completely miscible binaryalloy, the CrMnFeCoNi HEA can be described by a schematic phase diagram of the CrFeCo - MnNi system consistingof a liquidus and a solidus

Insights into the phase diagram of the CrMnFeCoNi high entropy alloy using electromagnetic melting and casting

International audienceHigh entropy alloys are single phase metallic materials, composed of more than 4 elements in equi-atomic proportionwhich is the maximum in the configurational entropy. In order to study the thermodynamics of those alloys, anequimolar CrMnFeCoNi HEA was processed by electromagnetic melting, casting and annealing under varyingconditions (cooling rate and annealing duration) and the evolution of the structure and microstructure was studied. Aftercasting, a dendritic microstructure was formed whose characteristic length depends on the cooling rate. Annealing leadsto a chemical homogenization of the microstructure. Indeed a true solid-solution down to the atomic scale wasevidenced in an HEA by atom probe tomography. It was shown that this face-centered cubic single-phase solid solutionis the high temperature stable state of the equimolar CrMnFeCoNi alloy. By analogy with completely miscible binaryalloy, the CrMnFeCoNi HEA can be described by a schematic phase diagram of the CrFeCo - MnNi system consistingof a liquidus and a solidus

Synthesis, sintering, transport and thermal properties of Na 2 Fe 2 Ti 6 O 16 freudenbergite

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Chemically architectured alloys: How interphase width influences the strengthening

International audienceChemically architectured alloys are a new concept of microstructure in which two phases are separated by a 3D network of fluctuations of composition, which is called interphase, and which induces a strengthening. Chemically architectured alloys were processed by spark plasma sintering of a mixture of pure Ni and CoCrFeMnNi high entropy alloy with varying conditions. They were characterized by scanning electron microscopy coupled with energy dispersive spectroscopy and electron backscattered diffraction, microhardness and compression tests. It was shown that the width of the interphase can be decreased by decreasing the sintering temperature and increasing the applied pressure. The strengthening effect of the interphase increases when its width decreases and its volume fraction increases. The microstructure of the chemically architectured alloys can be finely controlled by the processing parameters which will permit to maximize the strengthening. Chemical architecturation is thus an efficient and tunable strengthening mechanism