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Etude thermodynamique et mécanique d'alliages à haute entropie

Abstract : For a long time, development of alloys was restricted to one principal element, or two, with minor elements added for performance optimization. In 2004, a new concept of materials was born : multi-component alloys forming a solid-solution and in which all components are very concentrated. These new alloys, named high entropy alloys, can combine high mechanical resistance and large ductility. By de nition this new material concept should make it possible to explore an almost in nite eld of chemical compositions. But in the meantime, the thermodynamic stability of these systems was poorly known and severely limits the choice of alloy compositions. In this context, the rst objective of this study was to fully determine the composition range of existence of a unique face centered cubic (fcc) solid solution within the multi-component Co-Cr-Fe-Mn-Ni system. To address this problem, the phase stability was theoretically and experimentally investigated. Using the Calphad approach and a new database (TCHEA-1), the stable phases of 10 626 compositions could be calculated, at several temperatures. The comparison between calculation and experimental results indicates that the fcc solid solution is accurately described by this database. Finally, it was shown that the fcc phase is stable over a wide range of composition, which was completely described. Now, it is possible to choose a priori a composition which will form a solid solution within this system. The heat of mixing of the fcc phase was compared between density functional theory (DFT) and Calphad calculations for binaries, ternaries, quaternaries and quinary systems. Signi cant differences were found with the predictions made by the TCHEA-1 database. In addition, these calculations have highlighted the absence of ternary and quaternary interaction for the Co-Cr-Fe-Mn-Ni system. However, the infuence of the composition on the fcc solid solution strengthening was not fully understood, which limits mechanical optimization. So, the evolution of structural and mechanical properties of multi-components alloys was experimentally investigated. Twenty ve alloys from the Co-Cr-Fe-Mn-Ni system forming a unique fcc solid solution were processed. The lattice parameter was measured by X-ray diffraction while the hardness and elastic modulus were measured by nano-indentation. The role of each element on the mechanical behaviour is presented. Finally, a model to assess the solid solution strengthening for this system is studied.
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Contributor : Guillaume Bracq <>
Submitted on : Wednesday, November 14, 2018 - 9:39:58 PM
Last modification on : Wednesday, December 4, 2019 - 4:39:00 PM
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Guillaume Bracq. Etude thermodynamique et mécanique d'alliages à haute entropie. Matériaux. Paris Est, 2018. Français. ⟨tel-01923075⟩



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