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Relaxation des contraintes dans les couches de chromine développées sur alliages modèles (NiCr et Fe47Cr) : apport de la diffraction in situ à haute température sur rayonnement Synchrotron à l’étude du comportement viscoplastique : effets d’éléments réactifs

Abstract : Integrity of protective oxide scales developing at the metallic alloys surface at high temperature depends on the stress generation and their relaxation mechanisms. In this work, the behaviour of chromia scales formed on NiCr and Fe47Cr model alloys has been investigated. Raman spectroscopy was used to determine the residual stress level in chromia thin films after oxidation at different temperatures (700°C-1000°C) for 3 h and 18 h. A monotonous evolution of residual stresses with oxidation conditions was noted for the Ni30Cr/Cr2O3 system but not for the Fe47Cr/Cr2O3 one. The strain variations at microscopic scale was also determined by using atomic force microscopy. For the first studied system, non destructive relaxation by creep of the oxide and buckling can be more or less activated, in agreement with the residual stresses evolution. And a third additional stress release mode by intra film cracking could take place for the second system. In situ high temperature oxidation coupled with Synchrotron X-rays diffraction was also used to characterize the viscoplastic properties of chromia scales grown on Ni30Cr and Ni28Cr, with dissociating the effects related to thermal activation and grain size. The creep mechanism responsible of stress release in chromia scales has been evidenced by comparing experimental results with a diffusional creep model. Confrontation of the obtained activation energy (130 kJ.mol-1) with literature results has shown that this non destructive relaxation mode was likely governed by grain boundary transport of oxygen species. When a reactive element (Y or Zr) was added to the metallic substrate Ni28Cr, an important decrease of the oxidation rate was noted when increasing the amount of introduced element. No significant effect of this quantity on the residual stress level was however observed. At microscopic scale, a reduction of grain size has been also noted and two distinguished grains distribution appeared for the higher quantities. Results of in situ Synchrotron diffraction measurements and those of atomic force microscopy (grain boundary sliding) showed that the ability of chromia films to release stress thanks to their viscoplastic properties could be delayed and/or decreased with the presence of an active element. However, this mechanism could operate for smaller chromia films thicknesses.
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https://tel.archives-ouvertes.fr/tel-01661285
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Felaniaina Nirisoa Rakotovao. Relaxation des contraintes dans les couches de chromine développées sur alliages modèles (NiCr et Fe47Cr) : apport de la diffraction in situ à haute température sur rayonnement Synchrotron à l’étude du comportement viscoplastique : effets d’éléments réactifs. Matériaux. Université de La Rochelle, 2016. Français. ⟨NNT : 2016LAROS015⟩. ⟨tel-01661285⟩

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