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Etude de la migration des joints de grain mixtes flexion-torsion dans le Ni par dynamique moléculaire

Abstract : Recent innovations in materials rely heavily on the addition and control of interfaces.This is true for massive crystalline materials that are essential for structural applications, but also for functional materials, micro-objects, or nanostructured materials. Among these interfaces are the grain boundaries (GBs) which are defined as the boundary delimiting two grains with different crystallographic orientations.Grain boundaries strongly contribute to the macroscopic properties of materials. The work done is a contribution to the study of the behavior of GBs during thermomechanical treatments inducing their mobility. We are talking about migration. Indeed, during a thermomechanical loading, grain boundaries (GB) generally migrate through polycrystalline samples. Despite an extensive literature on the subject, understanding the collective behavior of dislocations and GB remains partial, at least on the macroscopic scale. This is due, firstly, to the fact that several driving forces are active simultaneously during the experiments on polycrystals : (i) the surface tension of a curved boundary (ii) the differential elastic energy for elastically anisotropic materials, and finally (iii) the coupling between plasticity and migration. On the other hand, the mobility of such interfaces is closely related to their atomic structure.Simulations of GBs migration by Molecular dynamics, under the action of asynthetic driving force, are carried out. A wide variety of GBs, represented by nickel bicristals, is studied. GB mobility is first discussed for various GB planes and temperatures. Through this study, three characteristics with respect to temperature were observed : thermally activated character, athermic character, and antithermal character. To these characters is added other behaviors varying with the temperature.The simulations also allowed the identification of some elementary migration mechanisms for basic GBs. GBs with step migration or disconnection migration were studied by the Nudged Elastic Band method. This method allowed determiningt he path of minimum energy (MEP) during the migration. The evolution of the grain boundary during its migration shows the nucleation and the displacement of the steps, identified as disconnections.
Keywords : Migration
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Zakaria El Omari. Etude de la migration des joints de grain mixtes flexion-torsion dans le Ni par dynamique moléculaire. Science des matériaux [cond-mat.mtrl-sci]. Université Sorbonne Paris Cité, 2018. Français. ⟨NNT : 2018USPCD021⟩. ⟨tel-02464073⟩

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