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Design and structural optimization of a high entropy alloy (HEA) of the CoCrFeMnNi family with high mechanical resistance.

Abstract : High entropy alloys (HEAs) are a new class of materials obtained with a novel approach. They are made of at least 5 elements in similar proportions, which results in increased configurational entropy of the system and in some cases in higher stability of single solid solutions.In this work, a novel HEA based on equiatomic CoCrFeMnNi was optimized, characterized and patented. This alloy, referred to as A3S (austenitic super stainless steel), consists of 5 elements in non-equiatomic proportions (Co20Cr15Fe26Mn17Ni22). This composition was optimized with CALPHAD (Computer Coupling of Phase Diagrams and Thermochemistry) in Thermo-Calc software. The material possesses stable face-cubic centered solid solution structure and, unlike the equiatomic one, remains stable after annealing at 500oC up to 300 days. The facility of forming a nanostructure after hot forging is remarkable, and results in high resistance as well as ductility compared to stainless steels. These properties are improved at cryogenic temperatures where an additional mechanism of deformation by twinning is being activated. This compilation of properties is very promising for industry where new materials are required.A comparative study between the novel A3S and the equiatomic reference material has been conducted. First, the procedure of the alloy optimization by thermodynamic calculations has been presented. Then, microstructures and mechanical properties in different conditions (various annealing temperatures and times as well as types of deformation) have been examined. Finally, the last part reviews the mechanisms of recovery and recrystallization which differ significantly from classical materials.
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Michal Mroz. Design and structural optimization of a high entropy alloy (HEA) of the CoCrFeMnNi family with high mechanical resistance.. Other. Université de Lyon, 2018. English. ⟨NNT : 2018LYSEM003⟩. ⟨tel-02869758⟩

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