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Métallurgie à l'azote : Nanoprecipitation amorphe et cristalline de nitrure de silicium dans le système Fe-Si-N

Abstract : In the context of ever more stringent environmental regulations, cost-effective weight savings have become a high-priority objective for car makers. Until today, the research effort deployed by steel makers has kept steel in a dominating position on this market against light alloys. However, the performances of current carbon steel grades start to show an asymptotic evolution. Nitrogen steel metallurgy represents a disruptive concept which is bound to outperform by far these current grades. Fe-Si-N alloys obtained through nitriding at 570 °C give rise to unexpected precipitation reactions. It has been shown that the species precipitating during nitriding is the stoichiometric nitride Si3N4. The precipitates display the unusual combination of an amorphous structure with a cuboidal morphology. This morphology can be explained by considering the minimization of the precipitation-induced elastic stress. This precipitation results in substantial hardening of the ferritic matrix and contributes to lowering the density of the final composite. The amorphous precipitates undergo a transition to a crystalline structure when the nitride alloys are subjected to annealing in a denitriding atmosphere. The occurring crystallites precipitate in the form of hexagonal prisms and exhibit a structure belonging to the P62c space group. Their composition however remains Si3N4. Finally, it has been demonstrated that such a transition can be modeled by the means of a kinetic model specifically developed in the present work to account for characteristic aspects of the nitriding treatment
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https://hal.univ-lorraine.fr/tel-01749459
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  • HAL Id : tel-01749459, version 1

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Hugo P. van Landeghem. Métallurgie à l'azote : Nanoprecipitation amorphe et cristalline de nitrure de silicium dans le système Fe-Si-N. Matériaux. Université de Lorraine, 2012. Français. ⟨NNT : 2012LORR0265⟩. ⟨tel-01749459⟩

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