David Delafosse

The present paper concerns the oxide layer thickness determination of oxidized metals in the case where an optical interference phenomenon occurs due to multiple reflections inside the oxide layer. The paper focuses on anodized titanium but can be extended to the layer thickness determination of any material composed of a non-absorbing layer over an absorbing substrate. It establishes theoretical formulae to compute the oxide layer thickness from the positions of the local extrema of the material reflectance spectra. In contrast with many publications these formulae take into account the air/oxide and oxide/metal interfaces’ electromagnetic phase-shift. They make also the distinction between TE-, TM- and non-polarized light and are valid for all incidence angles. By applying these formulae to simulated reflectance spectra with known oxide thicknesses, we show that neglecting the interface phase-shift is not appropriate for determining the oxide thickness of samples with thin oxide t...

Methodologie, donnees collectees et traitements des specialistes Approche analyse sensorielle ► Est-ce que presenter un objet fini (coque de telephone) ou un echantillon de matiere change la perception ? Si oui, comment ? ► Experiences-selon un protocole precis :-presenter a 23 utilisateurs 12 echantillons ou 12 objets-les faire placer sur la Nappomatic ®-demander a caracteriser les groupes formes avec des mots libres. ► Traitement des donnees avec des outils standards (a l'exception des mots, donc base uniquement sur les coordonnees x et y)-Clustering assez ressemblant entre les deux types : echantillons versus objets Approche complementaire en analyse sensorielle Croissement des competences Collaboration interdisciplinaire : analyse sensorielle et data science Validation de l'experience ► nombre d'utilisateurs suffisants et resultats fiables ► pas de differences entre les testeurs Analyse du texte produit ► trouver une equivalence via une fonction de similarite entre l...

Abstract: Many mechanisms have been proposed to describe the different hydrogen embrittlement processes. The materials' variability and their properties, their conditions of use and the nature of the surrounding environment make it difficult to establish a single fundamental theory or approach to describe hydrogen embrittlement (HE). Experimental validations of these mechanisms are often based on a set of conditions (metallurgy, mechanical loading, surface hydrogen activity, volume concentration, etc.) which are favorable to the emergence of a particular mechanism, making it difficult to get an overview of the various interaction modes between the adsorbed hydrogen or hydrogen in solid solution and the crystal defects. Kirchheim points out that from a thermodynamic point of view, all the approaches proposed in the literature are based on a decrease of the defect formation or emission energy (dislocations, vacancies, microcavities), of the cohesion and surface energies, or the energy associated with the displacement of defects such as dislocations, in the presence of adsorbed or absorbed hydrogen. The brittle fracture of deformable crystal materials subjected to quasi-static monotonic loading arises from a competition between, on the one hand, the accumulation rate of elastic energy at the crack tip until a critical value intrinsic to the material is reached (the critical elastic energy release rate, related to the fracture toughness and to Young's modulus) and, on the other hand, the dynamics of crystal defect creation/multiplication that blunt the existing cracks or their nuclei, or screen the loading applied to them. Understanding the hydrogen-induced damage and fracture requires taking into account the effects of hydrogen on the crystal cohesion as well as the plasticity or the phase transformations at the crack tip. In the presence of intrinsically brittle interfaces, heterogeneities or phases, hydrogen can affect the material's cohesion through (co-)segregation effects, through the accumulation of deformation incompatibilities or induced precipitation. If, from a formal point of view, these mechanisms belong to the two preceding categories (intrinsic embrittlement or plasticity modification), from a practical point of view, their study and modeling have led to specific developments. We thus find four major classes of hydrogen-assisted damage mechanisms in the literature: brittle fracture through the reduction of the material's cohesion, damage due to the formation of vacancies and their condensation, fracture due to the local increase of plasticity (“direct”, through localized shear; or “indirect”, through the accumulation of internal stresses near the interfaces) and fracture through the formation of a brittle phase (hydride). The first model is based on the idea of the decrease in the interfaces' cohesion energy (lattice, grain boundaries, inter-phase, etc.) caused by the segregation of hydrogen, which promotes the formation and propagation of cracks. The second model is based on the formation of new defects such as vacancies followed by microcavities in the presence of hydrogen. The multiplication and localization of these defects can lead to the initiation and propagation of cracks. The third approach of HE is based on the emission of dislocations favored by the presence of hydrogen (decrease of elastic interactions and line tension) which can induce the initiation of a crack and the localization of the plastic deformation in front of the crack tip. Finally, the fourth model is based on the formation of a brittle hydrogen-rich hydride phase. In order to better understand the different aspects of the hydrogen-material interaction as well as its harmful effects, we will describe in more detail the four HE models and the various associated mechanisms.

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.

Ni single crystals oriented for single slip, exhibit well defined cyclic stress-strain curves at room temperature, corresponding, partly to the formation of persistent slip bands in a given plastic strain range. The aim of the paper is to study the influence of hydrogen on the cyclic plastic behaviour of such crystals. Tests on pre-charged specimens show a decrease of the saturation stress, in comparison with pure nickel samples. Such effects are discussed in terms of hydrogen-dislocation interactions, and reduced cross slip ability in presence of hydrogen.

Selected topics in field of the study of the mechanisms of corrosion and of oxidation of metals or alloys are presented. The first part reports a new model for the mechanism of the breakaway oxidation of ferritic stainless steels in water vapour. The second part is devoted to the ...

ABSTRACT The dynamic interaction between diffusing hydrogen and mobile dislocations in nickel and binary nickel–16wt.% chromium is analysed in terms of the solute drag phenomenon. Static strain ageing experiments are used to measure the saturated dislocation pinning force as a function of the hydrogen concentration. Hydrogen-dislocation interactions cause a shielding of the pair interactions between edge dislocations. The influence of this screening effect is analytically evaluated on the line tension of curved dislocations, and we assess its consequences on the critical force for the expansion of a dislocation loop and on 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.

304L and 316L SS samples are SCC-tested in PWR water for various conditions: strain level, duration, pH, surface finish. The passive Chromium-rich oxide layer, the consistent oxidation feature, is characterized using Transmission Electron Microscope analysis. EDX analysis and Energy-filtered images are used to map the oxide penetration at the surface of the sample and at contiguous grain boundaries. Statistical data obtained by these characterizations are exploited to considerate the effects of deformation, pH and duration on oxidation. Complementary analyses of intergranular crack initiations allow to invetigate the oxidation phenomenon at grain boundaries and its influence on intergranular cracking.

Holistic approach for intelligent innovation is obtained in integrated design teams by mixing up backgrounds and skills into multidiscipline teams. However, is mixing enough? To go further than a simple addition of skills and create a synergy, the key ingredient is to favour communication and understanding between individuals in the team. Our aim is to impulse the establishment of commons languages and favour constructive interactions between future engineers and future designers. It aims at preparing future engineers for team-working with people with various backgrounds, different methodologies and skills. A specific educational program has been tailored for engineering students to favour mind opening through variety of lectures and hand-on activities. The focus point is a multidiscipline international workshop in design and engineering, gathering students from both areas. The workshop process has been studied and it was demonstrated that significant knowledge transfer and efficien...

ABSTRACT An innovative hydrogenation method to investigate the hydrogen embrittlement of metals and alloys is hereby presented. The benefits of electroplating samples with copper and nickel prior to gaseous hydrogenation at mid-range temperatures are quantified. It is showed that these electrodeposited layers allow to control the hydrogen desorption rate occurring after hydrogenation, during the cooling of the hydrogenated specimen. The present study demonstrates the capability of the method to control the introduced total hydrogen concentration within a margin of 0.2 wt.ppm. The applicability of the described method to further investigations into hydrogen concentrations effects on hydrogen embrittlement of ferritic alloys by the means of mechanical tests is evaluated.

ABSTRACT Les interactions entre hydrogène en solution solide et plasticité sont étudiées et modélisées du point de vue de la métallurgie physique et de la théorie élastique des défauts discrets dans les cristaux cubiques à faces centrées. Des essais de vieillissement statique permettent d'évaluer quantitativement l'interaction élastique entre dislocations mobiles et atmosphères de solutés dans le nickel et dans un alliage binaire nickel-chrome. Les principes du couplage élastique entre solutés et dislocations sont rappelés et la notion d'indice d'écrantage des interactions de paires est introduite. Cette notion permet d'évaluer les effets de l'hydrogène sur l'énergie de ligne des dislocations, la tension de ligne, la contrainte critique d'activation de sources de Franck-Read, la stabilité des jonctions et la distance de séparation entre partielles qui gouverne le glissement dévié. Des essais de traction sur des monocristaux orientés pour le glissement simple permet d'évaluer les effets macroscopiques du couplage hydrogène – plasticité. L'observation par microscopie électronique à transmission des structures de déformation permet de mettre en évidence le glissement dévié comme étant le mécanisme élémentaire de plasticité le plus fortement affecté par la présence d'hydrogène en solution. Ces résultats sont discutés sous l'angle des mécanismes de rupture par effets de l'hydrogène.

This work is concerned with plastic deformation at the tip of a ductile tearing crack during propagation. Two kinds of effects are investigated: the thermomechanical coupling at the tip of a mobile ductile crack, and the influence of Dynamic Strain Aging (DSA) on ductile fracture. Three alloys are studied: a nickel based superalloy (N18), a soft carbon steel, and an Al-Li light alloy (2091). The experimental study of the thermo mechanical coupling effects by means of infrared thermography stresses the importance of plastic dissipation in the energy balance of ductile fracture. Numerical simulations involving plastic deformation as the only dissipation mechanism account for the main part of the measured heating. The effects of DSA on ductile tearing are investigated in the 2091 Al-Li alloy. Based on the strain rate/temperature dependence predicted by the standard model of DSA, an experimental procedure is set up for this purpose. Three main effects are evidenced. A maximum in tearing resistance is shown to be associated with the minimum of strain rate sensitivity. Through a simple model, this peak in tearing resistance is attributed to an increase in plastic dissipation as the strain rate sensitivity is decreased. Heterogenous plastic deformation is observed in the crack tip plastic zone. Comparison with uniaxial testing allows us to identify the observed strain heterogeneities as Portevin-Le Chatelier instabilities in the crack tip plastic zone. We perform a simplified numerical analysis of the effect of strain localization on crack tip screening. Finally, small crack propagation instabilities appear at temperatures slightly above that of the tearing resistance peak. These are interpreted as resulting from a positive feed-back between the local heating at the tip of a moving crack and the decrease in tearing resistance with increasing temperature.

ABSTRACT Dynamic strain aging (DSA) phenomena, which stem from the dynamic interaction of mobile dislocations and diffusing solute atoms, are observed in many dilute alloys at intermediate temperatures. In uniaxial deformation, DSA may lead to a negative strain rate sensitivity (SRS) of the flow stress and to the occurrence of jerky flow, alias the Portevin-Le Chaetelier (PLC) effect. As little is known to date about the influence of DSA and the PLC effect on fracture-related properties, the aim of the present study is to investigate ductile fracture in a commercial Al-Li alloy, the 2091 alloy. In the standard treatment conditions used all through this work, this alloy exhibits serrated yielding in uniaxial deformation, associated with a depletion of the SRS. The present study is, therefore, focused on properties related to one single heat treatment, for which fracture is purely intragranular. Experimental results are reported and discussed on the influence of strain rate and temperature on both uniaxial deformation and the tearing behavior of the 2091 alloy. Evidence is found for an anomalous behavior of the tearing resistance in the temperature domain where DSA occurs. Further, heterogeneous deformation is shown to take place in such conditions in the plastic zone ahead of the crack tip region.

ABSTRACT The role of interfaces in stress corrosion cracking and corrosion fatigue is fat reviewed through a study ia duplex austentic - ferritic stainless steels. The preculiar behaviour of these alloys in low cycle fatigur and in corrosion fatigue is explained by mechanical and electrochemical coupling mechanisms at β-γ interface. The cyclic behaviour of the individual phases is described in terms of deformation mechanism, cyclic hardening and crack nucleation processes. The continuity of slip at coherent interfaces is pointed out as well as the role of nitrogen additions. Elementary background from electrochemical corrosion is given. It illustrates the coupling mechanisms in corrosion fatigue. The mechanisms of fatigue crack propagation in the duplex microstructure are described, along with the effect of hydrogen discharge at the crack tip. These effects are compared to the stress corrosion cracking resistance of ductile austenitic stainless steels, and the role of grain boundries in f.c.c. polycrystals is underlined. Conclusions are drawn concering the role of interfaces. The important issue of nitrogen doping is addressed with respect to the mechanical / electrochemical coupling mechanisms in these alloys.

ABSTRACT Surface finishes based on structural colors raise interest in product design and architecture in that long lasting colored surfaces can be produced with low risks if color fading. In this paper, aluminum surface are patterned through anodization with additional chemical post-treatment. This electrochemical technique is advantageous in that it is already industrially mature and enables the treatment of large surfaces. The resulting surface shows self-organized nanopatterns that lead to specific colored effects. In particular, colored effects similar to rainbow colors are observed in backscattering while the sample preserves its typical grayish metallic color when observed in direct reflection. Optical characterization of the sample have been performed and compared to optical simulations using Fourier modal method (FMM) method. A strong consistency between the simulation results and the experimental measurements has been obtained. It has been demonstrated that the color effects observed in backscattering are due to first order diffraction phenomena.

The mechanical behaviour of nickel base superalloy 718 was investigated by means of constant extension rate tensile (CERT) tests performed at 5×10−7 s−1, in air and under secondary vacuum, in the temperature range 400–600°C. Below 470°C, the Portevin–Le Chatelier (PLC) effect occurs and leads to shear fracture. Above 500°C, the PLC effect disappears and oxidation-induced intergranular cracking takes place. The