Conséquences de l'oxydation haute température sur l'injection de défauts et le comportement mécanique des matériaux métalliques

L'objet de ce travail est de rendre compte de l'effet de l'oxydation haute température de matériaux métalliques à la fois sur leur évolution microstructurale ainsi que sur leur comportement mécanique. Des matériaux modèles comme le nickel, le fer et l'alliage Ni-20Cr sont étudiés. L'analyse de la microstructure d'échantillons de nickel minces oxydés à 1000°C montre que les lacunes métalliques injectées au cours de la croissance cationique de l'oxyde (NiO) peuvent diffuser sur de grandes distances via les joints de grains du métal. Quand une oxydation symétrique du nickel est pratiquée, de nombreuses cavités intergranulaires sont en effet observées, résultat de la condensation de lacunes. La germination de ces cavités n'est cependant possible qu'en présence d'une teneur minimale en carbone dans le métal. Dans le cas où une des surfaces du métal est protégée de l'oxydation (oxydation non symétrique), les lacunes sont annihilées à l'interface métal/atmosphère réductrice qui constitue un puits parfait de lacunes. L'annihilation se manifeste alors par un creusement important des sillons formés par l'émergence des joints de grains à la surface du métal. La géométrie de la pièce métallique joue un rôle primordial sur la quantité de défauts diffusant dans le métal. Cette diffusion est accompagnée par celle d'oxygène si bien qu'un mécanisme de transport de l'oxygène par les lacunes est proposé. Les essais de traction menés à la température ambiante sur du nickel de structure bambou montrent que la couche d'oxyde, très adhérente, constitue une barrière à l'émergence des dislocations, conduisant à un durcissement du métal. L'alliage Ni-20Cr est quant à lui testé en traction et en fluage entre 25 et 825°C sous atmosphère contrôlée. ABSTRACT : The aim of this work is to account for the effects of the high temperature oxidation of metals on their microstructure and their mechanical properties. "Model" materials like pure nickel, pure iron and the Ni-20Cr alloy are studied. Nickel foils have been oxidised at 1000°C on one side only in laboratory air, the other side being protected from oxidation by a reducing atmosphere. After the oxidation treatment, the unoxidised face was carefully examined by using an Atomic Force Microscope (AFM). Grain boundaries grooves were characterised and their depth were compared to the ones obtained on the same sample heat treated in the reducing atmosphere during the same time. They are found to be much deeper in the case of the single side oxidised samples. It is shown that this additional grooving is directly linked to the growth of the oxide scale on the opposite side and that it can be explained by the diffusion of the vacancies produced at the oxide scale - metal interface, across the entire sample through grain boundaries. Moreover, the comparison between single side oxidised samples and samples oxidised on both sides points out that voids in grain boundaries are only observed in this latter case proving the vacancies condensation in the metal when the two faces are oxidised. The role of the carbon content and the sample's geometry on this phenomemon is examined in detail. The diffusion of vacancies is coupled with the transport of oxygen so that a mechanism of oxygen transport by vacancies is suggested. The tensile tests realised at room temperature on nickel foils (bamboo microstructure) show that the oxide scale can constitute a barrier to the emergence of dislocations at the metal surface. Finally, the Ni-20Cr alloy is tested in tensile and creep tests between 25 and 825°C in oxidising or reducing atmospheres

Investigations on the Diffusion of Oxygen in Nickel at 1000°C by SIMS Analysis

High-purity polycrystalline nickel foils have been oxidized at 1000°C in laboratory air before being analyzed in secondary ion mass spectrometry to locally measure the oxygen content in solid solution. The values obtained in metallic grains are surprisingly the same before and after the oxidation treatments (between 5 and 10 atom ppm) and they are much lower than the ones predicted from the literature solubility and diffusion coefficient data at 1000°C. It is shown that this discrepancy could have its origins in the purity level of the samples but also in the exclusive oxygen diffusion in nickel grain boundaries. This last assumption is supported by the occurrence of nickel oxide particles on the walls of voids located in grain boundaries

Injection of vacancies at metal grain boundaries during the oxidation of nickel

Nickel foils have been oxidised at 1000 °C on one side only in laboratory air, the other side being protected from oxidation by a reducing atmosphere. After the oxidation treatment, the unoxidised face was carefully examined by using an atomic force microscope. Grain boundaries grooves were characterised and their depth were compared to the ones obtained on the same sample heat treated in the reducing atmosphere during the same time. Grain boundaries grooves are found to be much deeper in the case of the single side oxidised samples. It is shown that this additional grooving is directly linked to the growth of the oxide scale on the opposite side and that it can be explained by the diffusion of the vacancies produced at the oxide scale–metal interface, across the entire sample through grain boundaries. Moreover, the comparison between single side oxidised samples and samples oxidised on both sides points out that voids in grain boundaries are only observed in this latter case proving the vacancies condensation in the metal when the two faces are oxidised

Characterization of the mechanical behaviour of both fusion zone and base metal of electron beam welded TA6V titanium alloy

The fusion zone of an electron beam welded Ti-6Al-4V alloy presents a a' martensitic structure which leads to a change of mechanical properties. Starting from two manufacturing processing routes for the alloy (1) a b processing followed by the weld (which will be considered as the reference microstructures), (2) an a+b processing followed by welding and a post weld heat treatment (PWHT), the microstructure can be adjusted to avoid local difference of strength, fatigue properties and impact toughness. This results from the optimisation of the process and of the PWHT. The present work investigates the mechanical behaviour and the damage mechanism of both base metal and fusion zone in regards to the microstructure and to the heat treatment parameters

Recherche d’une alternative au TA6V - Relation entre microstructures et propriétés de deux alliages de titane αβ

To reduce the processing cost of aeronautical parts, ongoing research is conducted to optimize material’s performances in their environment. One of the research project at IRT Saint-Exupéry is dedicated to the resistance against fatigue crack propagation of two titanium alloys, Ti54M and Ti575, potential candidates to replace TA6V. In that goal, the two alloys are subjected to thermal treatments in order to generate lamellar microstructure which improves the fatigue crack propagation resistance. After these treatments, the microstructures are characterized and the mechanical properties assessed. By comparing the results, the mechanical properties of Ti54M are equivalent to the mechanical properties of TA6V. So Ti54M could be an alternative alloy of the TA6V provided that complementary analyses are made. Afin de réduire l'ensemble des coûts de fabrication des pièces aéronautiques, des recherches sont faites pour optimiser les performances des matériaux par rapport aux contraintes qu'ils supportent. Un des projets de recherche de l'IRT Saint-Exupéry vise évaluer la résistance à la propagation des fissures en fatigue de deux alliages de titane, le Ti54M et le Ti575, susceptibles de remplacer le TA6V. Pour cela les alliages ont subi des traitements thermiques afin de générer des microstructures lamellaires qui améliorent la résistance à la propagation des fissures en fatigue. Après ces traitements thermiques les microstructures sont caractérisées et les propriétés mécaniques statiques et en fatigue sont évaluées. La comparaison des résultats permet de montrer que le Ti54M, en plus d'offrir une usinabilité améliorée par rapport au TA6V, a des propriétés mécaniques équivalentes au TA6V et pourrait donc être une alternative sous réserve d'analyses complémentaires

Experimental Study of the Interactions Between Oxidation and Structural Defects

A multiscale study of the high temperature oxidation effects on high purity polycrystalline nickel behaviour is presented. In situ tensile tests have been carried out at room temperature on preoxidised thin foils. The results obtained lead to a better understanding of the strengthening effect of the oxide layer. Detailed examinations of the metal microstructure are also made after high temperature oxidation treatment. Atomic Force Microscope (AFM), Secondary Ion Mass Spectroscopy (SIMS), Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) are used to characterise the microstructure. Formation of voids at grain boundaries as well as vacancy injection are then discussed

Density functional calculations of the formation and migration enthalpies of monovacancies in Ni: Comparison of local and nonlocal approaches

We examine in this work the potential and the functional to be used in a density functional theory approach in order to describe correctly the formation and migration energies of monovacancies in nickel. As the formation enthalpy is not well-known experimentally at 0 K, we choose in a first step to determine some structural, magnetic, and elastic properties of the bulk, which are well-established experimentally. The comparison between both approaches, i.e., the local spin density approximation LSDA and the generalized gradient approximation GGA exchange-correlation functionals is analyzed. We conclude that the contribution of nonlocal GGA terms in order to describe correctly the electronic density is necessary to determine the formation and migration enthalpies and activation energy of monovacancy. The calculated formation Hv f and migration Hv m enthalpies differ significantly between both approaches. The overestimation of the LSDA approximation is of 0.25 eV for Hv f and of 0.23 eV for Hv m with respect to the GGA one, leading to a gap of 0.48 eV between both methods for the activation energy Q1. We show that the GGA results are comparable with experimental data if the thermal expansion contribution is taken into account through the lattice parameter variation. Finally, it is shown that the activation energy is nearly independent of the thermal expansion effects; thus we can expect that the curvature of the Arrhenius plot of the diffusion factor near the melting point is essentially due to the contribution of divacancies

High temperature oxidation of metals: vacancy injection and consequences on the mechanical properties

The aim of this work is to account for the effects of the high temperature oxidation of metals on their microstructure and their mechanical properties. "Model" materials like pure nickel, pure iron and the Ni-20Cr alloy are studied. Nickel foils have been oxidised at 1000ʿC on one side only in laboratory air, the other side being protected from oxidation by a reducing atmosphere. After the oxidation treatment, the unoxidised face was carefully examined by using an Atomic Force Microscope (AFM). Grain boundaries grooves were characterised and their depth were compared to the ones obtained on the same sample heat treated in the reducing atmosphere during the same time. They are found to be much deeper in the case of the single side oxidised samples. It is shown that this additional grooving is directly linked to the growth of the oxide scale on the opposite side and that it can be explained by the diffusion of the vacancies produced at the oxide scale - metal interface, across the entire sample through grain boundaries. Moreover, the comparison between single side oxidised samples and samples oxidised on both sides points out that voids in grain boundaries are only observed in this latter case proving the vacancies condensation in the metal when the two faces are oxidised. The role of the carbon content and the sample's geometry on this phenomemon is examined in detail. The diffusion of vacancies is coupled with the transport of oxygen so that a mechanism of oxygen transport by vacancies is suggested. The tensile tests realised at room temperature on nickel foils (bamboo microstructure) show that the oxide scale can constitute a barrier to the emergence of dislocations at the metal surface. Finally, the Ni-20Cr alloy is tested in tensile and creep tests between 25 and 825ʿC in oxidising or reducing atmospheres.L'objet de ce travail est de rendre compte de l'effet de l'oxydation haute température de matériaux métalliques à la fois sur leur évolution microstructurale ainsi que sur leur comportement mécanique. Des matériaux modèles comme le nickel, le fer et l'alliage Ni-20Cr sont étudiés. L'analyse de la microstructure d'échantillons de nickel minces oxydés à 1000°C montre que les lacunes métalliques injectées au cours de la croissance cationique de l'oxyde (NiO) peuvent diffuser sur de grandes distances via les joints de grains du métal. Quand une oxydation symétrique du nickel est pratiquée, de nombreuses cavités intergranulaires sont en effet observées, résultat de la condensation de lacunes. La germination de ces cavités n'est cependant possible qu'en présence d'une teneur minimale en carbone dans le métal. Dans le cas où une des surfaces du métal est protégée de l'oxydation (oxydation non symétrique), les lacunes sont annihilées à l'interface métal/atmosphère réductrice qui constitue un puits parfait de lacunes. L'annihilation se manifeste alors par un creusement important des sillons formés par l'émergence des joints de grains à la surface du métal. La géométrie de la pièce métallique joue un rôle primordial sur la quantité de défauts diffusant dans le métal. Cette diffusion est accompagnée par celle d'oxygène si bien qu'un mécanisme de transport de l'oxygène par les lacunes est proposé. Les essais de traction menés à la température ambiante sur du nickel de structure bambou montrent que la couche d'oxyde, très adhérente, constitue une barrière à l'émergence des dislocations, conduisant à un durcissement du métal. L'alliage Ni-20Cr est quant à lui testé en traction et en fluage entre 25 et 825°C sous atmosphère contrôlée

Effects of an oxyde layer on the tensile behavior of pure nickel foils

We present in this paper the consequences of an oxide scale grown on pure nickel on the mechanical behaviour. The presence of the oxide layer hardens the metal for low strain level, and reduces the ductility. Activation volumes as measured during stress relaxation experiments show that the dislocation motion mechanism is not affected by the presence of the oxide and it relies on forest interactions. It is shown that the main contribution of the oxide is to act as a barrier for dislocations and accelerates the storage of dislocations thus increasing the total dislocation density which causes hardening of the metal

Microstructure Changes Due to Heat Treatment and Their Effects on the Plasticity of Aluminium Alloys

International audienc