Hydrogen - Plasticity interactions : modelling and experiments in hydrogenated nickel alloys

International audienceThe effect of hydrogen on tensile tests of nickel and binary nickel - 16 wt. % chromium is analysed in terms of solute drag phenomenon. Static Strain Ageing experiments are used to measure the saturated dislocation pinning force as a function of the H concentration. First order hydrogen - dislocation interactions causes a shielding of the pair interactions between edge dislocations. The influence of this screening effect is analytically evaluated on the selfenergy and line tension of curved dislocations, the critical force for the expansion of a dislocation loop and the dissociation mechanism. These results are used to interpret experimental results on the plastic flow of hydrogen-charged nickel single crystals oriented for easy glide. This study illustrates the mechanisms of H - dislocation interactions and their consequences on the different contributions of hydrogen to the flow stress of nickel

Chemical Evolution in the Substrate due to oxidation: A Numerical Model with Explicit Treatment of Vacancy Fluxes

To get a better understanding of oxidation behavior of Ni-base alloys in PWR primary water, a numerical model for oxide scale growth has been developed. The final aim of the model is to estimate the effects of possible changes of experimental conditions. Hence, our model has not been restricted by the classical hypothesis of quasi-steady state and can consider transient stages. The model calculates the chemical species concentration profiles, but also the vacancy concentration profiles evolution in the oxide and in the metal as a function of time. It treats the elimination of the possible supersaturated vacancies formed at the metal/oxide interface by introducing a dislocation density at the interface and in the metal bulk. This latter density can be related to the cold-working state. Its effect on the vacancy profile evolution is studied in the case of a pure metal. Eventually an extension of the present model to the oxidation of Ni-base alloys is discussed regarding a recent vacancy diffusion model adjusted on Ni-base alloys

Démarche utilisée par Creusot Forge pour l'amélioration des gammes de fabrication de ses pièces

Creusot Forge s'est tourné vers la simulation numérique afin d'améliorer et de développer ses gammes de forgeage. Les récentes évolutions du logiciel FORGE ont permis à Creusot Forge de développer une approche originale de simulation de ses opérations de forgeage et de traitement thermique visant à garantir la qualité de ses produits. Face au renouveau des programmes nucléaires civils et au développement de centrales de nouvelle génération, cette démarche a été largement utilisée en s'appuyant sur 40 années de savoir faire ainsi que sur une parfaite connaissance métallurgique de ses lingots

Interactions Hydrogène - Plasticité dans le Nickel et ses Alliages

We evaluate the different contributions of the hydrogen-dislocation interactions to the plasticity of fcc materials in order to feed predictive models of stress corrosion cracking. Static strain ageing experiments are used to quantify the hardening contribution of solute drag by dislocations to the flow stress. We demonstrate the role of hydrogen transport by dislocations on the fracture mechanism. We model the influence of the screening of the elastic field of dislocations by hydrogen on elementary plasticity mechanisms and we conclude that the decrease of the cross slip ability arises from the combined action of elastic and core effects. The testing of single crystals shows that the major effect is on the cross slip mechanism. Tensile tests on polycrystals enlighten the diversity of macroscopic responses observed in alloys.Nous évaluons les différentes contributions des interactions hydrogène-dislocations sur la plasticité de matériaux cfc hydrogènes afin d'alimenter des modèles prédictifs d'endommagement par corrosion sous contrainte. Par des essais de vieillissement, nous quantifions la contribution positive a la contrainte d'écoulement du traînage des atmosphères de solutés par les dislocations. Nous démontrons le rôle du transport de l'hydrogène par les dislocations dans le mécanisme de fragilisation. Nous modélisons l'influence de l'écrantage du champ élastique des dislocations par l'hydrogène sur les mécanismes élémentaires de la plasticité et montrons que la diminution de la probabilité de glissement dévié résulte de l'action conjuguée d'effets élastiques et de cœur. Les essais sur monocristaux démontrent la contribution prépondérante des effets sur le glissement dévié. Les essais de traction sur des polycristaux illustrent la diversité des réponses macroscopiques observées dans les alliages

Interactions hydrogène-plasticité dans le nickel et ses alliages

Nous évaluons les différentes contributions des interactions hydrogène-dislocations sur la plasticité de matériaux CFC hydrogènes afin d'alimenter des modèles predictifs d'endommagement par corrosion sous contrainte par des essais de vieillissement, nous quantifions la contribution positive à la contrainte d'écoulement du trainage des atmosphères de solutes par les dislocations. Nous démontrons le rôle du transport de l'hydrogène par les dislocations dans le mécanisme de fragilisation. Nous modélisons l'influence de l'écrantage du champ élastique des dislocations par l'hydrogène sur les me canismes élémentaires de la plasticité et montrons que la diminution de la probabilité de glissement dévié résulte de l'action conjuguée d'effets élastiques et de coeur. Les essais sur monocristaux démontrent la contribution prépondérante des effets sur le glissement de vie. Les essais de traction sur des polycristaux illustrent la diversité des réponses macroscopiques observées dans les alliages.ST ETIENNE-ENS des Mines (422182304) / SudocSudocFranceF

The use of hydrogenated nickel as a model system for studying solute-dislocation interactions

The effect of hydrogen on tensile tests of nickel and binary nickel – 16 wt. % chromium alloy is analysed in terms of solute drag phenomenon. Static Strain Ageing experiments are used to measure the saturated dislocation pinning force as a function of the H concentration. First order hydrogen-dislocation interactions cause a shielding of the pair interactions between edge dislocations. The influence of this screening effect is analytically evaluated on the self-energy and line tension of curved dislocations, the critical force for the expansion of a dislocation loop and the dissociation mechanism. These results are used to interpret experimental results on the plastic flow of hydrogen-charged nickel single crystals oriented for easy glide.This study illustrates the mechanisms of H-dislocation interactions and their consequences on the different contributions of hydrogen to the flow stress of nickel

Correlation between dislocation organization and slip bands: TEM and AFM investigations in hydrogen-containing nickel and nickel-chromium

International audienceVarious forms of the plastic deformation in single crystals are studied in pure and hydrogen-containing nickel and nickel alloys oriented for single slip [1 3 5] and strained in the stage III regime (shear strain, gamma = 0.8). The heterogeneity of deformation is investigated at two distinct scales: slip bands and dislocation structures, using atomic force microscopy (AFM) and transmission electronic microscopy (TEM). Size and distribution of slip band thicknesses and geometrically necessary boundary (GNB) spacing are comparable. GNB structures both screen the long-range stress fields and decrease the mean free path of mobile dislocations, whereas equiaxed cells only impede dislocation motion through their role as obstacles. Consequently, GNB formation localizes deformation in specific slip bands. Additionally, the observed similarity between GNB spacing and equiaxed cell size suggests a correlation between these microstructural features. The impact of solid solution atoms on the inter-wall spacing is established for chromium and hydrogen. Both decrease the GNB spacing because of a decrease of the cross-slip probability and stacking fault energy, combined to a shielding effect for the later. The effect of GNB spacing on strain hardening is discussed in terms of the length scale associated with GNBs and the effect of solute content. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Influence of Al on the High Temperature corrosion behaviour of Inconel 617 in VHTR primary coolant atmosphere

Publié suite au congrès High Temperature Corrosion and Protection of Materials 7International audienceDue to the specific in service VHTR conditions, the corrosion behaviour of Inconel 617, candidate alloy for the IHX design, has been investigated at elevated temperatures in representative helium containing impurities (CO, H2O, H-2 and CH4) in the range of mu bar. The role of Al in the corrosion behaviour of IN617 is investigated using Ni-22Cr-9Mo base model alloys containing different Al levels (from 0 to 2wt.%)

Comparison of the high temperature surface reactivity in impure helium of two materials for gas cooled reactors

International audienceNickel base alloys Haynes 230 and Inconel 617 are of interest for gas cooled reactors. At high temperature in impure helium, they generally form surface chromium-rich oxides. However above a critical temperature called T-A, the scales are not stable anymore and the chromia destruction comes with a production of carbon monoxide. Reactivity tests on model alloys, with and without carbon, prove that chromia is reduced by the carbon from the alloy. TA VS P(CO) curves were also plotted for the two commercial alloys based on the experimental determination of T-A in various atmospheres with increasing partial pressures of carbon monoxide. Unexpectedly, both materials exhibit an almost ideatical behavior although a basic equilibrium approach suggests that the chromia scale would be reduced in different conditions due to the thermodynamic particularity of the interfacial alloy/scale system

Chemical Evolution in the Substrate due to oxidation: A Numerical Model with Explicit Treatment of Vacancy Fluxes

International audienceTo get a better understanding of oxidation behavior of Ni-base alloys in PWR primary water, a numerical model for oxide scale growth has been developed. The final aim of the model is to estimate the effects of possible changes of experimental conditions. Hence, our model has not been restricted by the classical hypothesis of quasi-steady state and can consider transient stages. The model calculates the chemical species concentration profiles, but also the vacancy concentration profiles evolution in the oxide and in the metal as a function of time. It treats the elimination of the possible supersaturated vacancies formed at the metal/oxide interface by introducing a dislocation density at the interface and in the metal bulk. This latter density can be related to the cold-working state. Its effect on the vacancy profile evolution is studied in the case of a pure metal. Eventually an extension of the present model to the oxidation of Ni-base alloys is discussed regarding a recent vacancy diffusion model adjusted on Ni-base alloys