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Analyse et modélisation de transformations de phase par précipitation dans des alliages de magnésium modèles.

Abstract : Kinetics of first stages of precipitation in Mg-RE alloys have been studied by means of Atom Probe and by Transmission electron microscopy techniques. Formation and development of ordered coherent β''-precipitates from the HCP solid solution of a binary Mg-2.6Nd and a ternary Mg-2.6Y-2.7Nd (wt.%) alloys were investigated at 150◦C. In the binary alloy, the β′'-precipitates form as plates on the prismatic planes. They grow in and directions, whereas the thickening of plates is insignificant or absent. Investigation of the ternary alloy showed that additions of Yttrium significantly slow down the kinetics. Only plate-like solute-rich atomic heterogeneities were observed at long aging times. The present work also makes a contribution to the determination of the binary Mg-Nd diagram ascertaining the high temperature solubility of Nd in Mg. In parallel, simulation of the kinetics of the formation of the D019 phase from the hcp solid solution has been undertaken in A-5.5%at B by Atomistic Kinetic Monte-Carlo method. Main attention has been paid to the investigation of the influence of the atom-vacancy mobility, controlled by the asymmetry parameter u, on the kinetic pathways. It was shown that u only influences short range ordering in the first shell during the very early stages. Overall kinetics is independent on the value of u. Coarsening of D019 precipitates proceeds by the evaporation-condensation mechanism whatever the value of u. The mean precipitate size increases according to the Lifshitz-lyozov-Wagner law.
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Viktor Kopp. Analyse et modélisation de transformations de phase par précipitation dans des alliages de magnésium modèles.. Matière Condensée [cond-mat]. Université de Rouen, 2010. Français. ⟨tel-00582337⟩

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