Comparison of local electrochemical impedance measurements derived frombi-electrode and microcapillary techniques

In the present paper, local electrochemical impedance spectrawere obtained on a 316L stainless steel from two configurations: a dual microelectrode (bi-electrode) and microcapillaries. With the bi-electrode, the local impedance measurements were made from the ratio of the applied voltage to the local current density calculated from the application of the ohm’s law. With the use of microelectrochemical cells, the specimen surface area in contact with the electrolyte is limited by the use of glass microcapillaries and the local impedance was defined fromthe ratio of the local potential to the local current restricted to the analysed surface area. Differences and similarities observed in local impedance spectra obtained with the two configurations were describe

Influence of cutting process mechanics on surface integrity and electrochemical behavior of OFHC copper

The authors gratefully acknowledge the support received from IC ARTS and CEA ValducSuperfinishing machining has a particular impact on cutting mechanics, surface integrity and local electrochemical behavior. In fact, material removal during this process induces geometrical, mechanical and micro-structural modifications in the machined surface and sub-surface. However, a conventional 3D cutting process is still complex to study in terms of analytical/numerical modeling and experimental process monitoring. So, researchers are wondering if a less intricate configuration such as orthogonal cutting would be able to provide information about surface integrity as close as possible to that one generated by a 3D cutting process. For that reason, in the present paper, two different machining configurations were compared: face turning and orthogonal cutting. The work material is oxygen free high conductivity copper (OFHC) and the cutting tools are uncoated cemented carbide. The research work was performed in three steps. In the first step, the process mechanics of superfinishing machining of OFHC copper was performed. In the second step, the surface integrity and the chemical behavior of the machined samples were analyzed. Finally, in the third step, correlations between input parameters and output measures were conducted using statistical techniques. Results show that when applying low ratios between the uncut chip thickness and the cutting edge radius, the surface integrity and cutting energy are highly affected by the ploughing phenomenon. Otherwise, the most relevant cutting parameter is the feed. In order to compare face turning with orthogonal cutting, a new geometrical parameter was introduced, which has a strong effect in the electrochemical behavior of the machined surface

Finite element analysis of laser shock peening of 2050-T8 aluminum alloy

AbstractLaser shock processing is a recently developed surface treatment designed to improve the mechanical properties and fatigue performance of materials, by inducing a deep compressive residual stress field. The purpose of this work is to investigate the residual stress distribution induced by laser shock processing in a 2050-T8 aeronautical aluminium alloy with both X-ray diffraction measurements and 3D finite element simulation. The method of X-ray diffraction is extensively used to characterize the crystallographic texture and the residual stress crystalline materials at different scales (macroscopic, mesoscopic and microscopic).Shock loading and materials’ dynamic response are experimentally analysed using Doppler velocimetry in order to use adequate data for the simulation. Then systematic experience versus simulation comparisons are addressed, considering first a single impact loading, and in a second step the laser shock processing treatment of an extended area, with a specific focus on impact overlap. Experimental and numerical results indicate a residual stress anisotropy, and a better surface stress homogeneity with an increase of impact overlap.A correct agreement is globally shown between experimental and simulated residual stress values, even if simulations provide us with local stress values whereas X-ray diffraction determinations give averaged residual stresses

High spatial resolution strain measurements at the surface of duplex stainless steels

International audienceThe determination of local strain fields at the surface of materials is of major importance for understanding their reactivity. In the present paper, lithography is used to fabricate grid points at the microscale and to map strain gradients within grains and between grains. This method was applied to duplex stainless steels which exhibit heterogeneous strain distributions under straining conditions. The influence of various parameters (the specimen microstructure, the density of slip bands, the number of systems activated and the grid geometry) on the strain value was discussed

Etude des processus de corrosion du nickel par dynamique moléculaire avec un potentiel réactif ReaxFF

La compréhension des processus de corrosion est un élément majeur de l étude de la durabilité des matériaux en environnements agressifs. C est dans ce cadre que ce travail de thèse se situe. L objectif est d apporter un nouvel éclairage sur les processus de corrosion du nickel en milieu aqueux en utilisant la simulation par dynamique moléculaire avec un champ de force réactif ReaxFF .Nous nous sommes particulièrement intéressé aux réactions entres les molécules d eau et les surfaces monocristallines de nickel. Ces résultats ont montré aucune dissociation de l eau sur les surfaces. Par ailleurs, une adsorption des molécules en bicouche a été constatée, traduisant la polarisation mutuelle entre l eau et le nickel, ce que confirme le calcul de charges. La surface du métal s est chargée positivement alors que la première couche d eau se charge négativement, reproduisant la double couche électrique. L introduction d un champ électrique extérieur a conditionné la réaction entre l eau et le nickel. La présence du champ a permis d observer les différentes étapes d oxydation du nickel : l adsorption des molécules d eau puis leurs dissociations en OH- et H3O+. La dissolution des atomes de nickel à la surface puis la pénétration de O dans Ni. Ces processus d oxydation sont répétés au cours du temps pour obtenir le film d oxyde. L épaisseur du film d oxyde dépend linéairement de l intensité du champ électrique. La cinétique d oxydation est améliorée pour la surface (110) par rapport aux autres surfaces étudiées. L étude de l oxydation d un bicristal de nickel a montré des processus identique. Par ailleurs, nous avons constaté que les premiers germes d oxydation se sont produits au niveau des joints de grains et se sont propagés sur toute la surface. L ensemble de ces résultats est en bon accord avec les observations expérimentalesUnderstanding of corrosion processes is important for the study of the durability of materials in aggressive environment. The objective is to provide new lights on the corrosion processes of nickel in aqueous condition with molecular dynamics simulation using a reactive force field "ReaxFF."We are particularly interested by the reaction between water molecules and mono-crystalline surfaces of nickel. No dissociation of water was showed on the surfaces. Moreover, an adsorption of molecules in bilayer was observed, reflecting the mutual polarization between water and nickel, which confirms the charge calculation. The metal surface is positively charged and the first water layer charged negatively, reproducing the electrical double layer. The introduction of an external electric field has conditioned the reaction between water and nickel. The presence of the field permits to observe the different stages of nickel corrosion: adsorption of water molecules, their dissociation into OH- and H3O+, the dissolution of the nickel atoms on the surface and the penetration of O in Ni. These oxidation processes are repeated over time to obtain the oxide film. The thickness of the oxide film depends linearly on the electric field intensity. The oxidation kinetics is improved to the surface (110) compared with other orientations studied. All these results are in good agreement with experimental observationsDIJON-BU Doc.électronique (212319901) / SudocSudocFranceF

Local electrochemical impedance spectroscopy study of the influence of ageing in air and laser shock processing on the micro-electrochemical behaviour of AA2050-T8 aluminium alloy

In the present paper, the micro-electrochemical behaviour of AA2050-T8 in 0.1 M NaCl was first studied after polishing by means of local electrochemical impedance spectroscopy and field-emission scanning electron microscopy/electron dispersive spectroscopy (FE-SEM/EDS). The influence of ageing in air and LSP was then investigated. Ageing in air for three months led to an increase in the charge transfer resistance. This was clearly observed in sites surrounding constituent particles where the charge transfer resistance was in the order of 1 MΩ cm2 (instead of 10-3 MΩ cm2 after polishing). Increased passivity around particles was confirmed from FE-SEM/EDS analysis. On the other hand, the effect of LSP on the behaviour of the matrix was found to be extremely beneficial. Both charge transfer and oxide film resistances were sharply increased to approximately 0.35 MΩ cm 2 and 1 MΩ cm2, respectively, as opposed to approximately 1.9 × 10-3 MΩ cm2 and 4.2 × 10-3 MΩ cm2, respectively, after ageing in air. This was attributed to the presence of compressive stresses at the specimen surface after LSP treatment. Stress values were determined from XRD measurements

Ta2C precipitation after low pressure carburizing of tantalum

Low pressure carburizing cycles were carried out on tantalum samples. A Ta2C precipitates layer is present under the TaC and Ta2C surface layers. As the TaC and Ta2C layers, the Ta2C precipitates layer (TPL) grows by carbon diffusion in tantalum matrix. Specific zones with low precipitates content were observed at the bulk surface and in the tantalum grain boundaries. Other zones with lower precipitates content are also present in carburized and annealed samples. All the zones with low precipitates content are induced by the Ta2C precipitation mechanism described by Dahmen et al. [1,2]. Indeed, Ta2C needs vacancies to precipitate. If vacancy sinks are present at the bulk surface, at a grain boundary or in a dislocation, then Ta2C precipitation is highly reduced. Another effect of the annealing treatment is an increase of the Ta2C precipitates layers growth rate

Downregulation of the Drosophila Immune Response by Peptidoglycan-Recognition Proteins SC1 and SC2

Peptidoglycan-recognition proteins (PGRPs) are evolutionarily conserved molecules that are structurally related to bacterial amidases. Several Drosophila PGRPs have lost this enzymatic activity and serve as microbe sensors through peptidoglycan recognition. Other PGRP family members, such as Drosophila PGRP-SC1 or mammalian PGRP-L, have conserved the amidase function and are able to cleave peptidoglycan in vitro. However, the contribution of these amidase PGRPs to host defense in vivo has remained elusive so far. Using an RNA-interference approach, we addressed the function of two PGRPs with amidase activity in the Drosophila immune response. We observed that PGRP-SC1/2–depleted flies present a specific over-activation of the IMD (immune deficiency) signaling pathway after bacterial challenge. Our data suggest that these proteins act in the larval gut to prevent activation of this pathway following bacterial ingestion. We further show that a strict control of IMD-pathway activation is essential to prevent bacteria-induced developmental defects and larval death

Impact des conditions d'usinage sur la zone du matériau affectée par le procédé

Les procédés d'usinage peuvent créer des contraintes résiduelles à la surface des pièces usinées et ainsi modifier la microstructure et la texture de la matière proche de la surface. De tels changements sont importants pour l'étude du comportement des pièces au cours du temps. Il est alors nécessaire de quantifier les contraintes résiduelles, les composantes de texture et de déterminer la microstructure induite par l'usinage pour comprendre et prédire le comportement des surfaces considérées. Dans cette étude, le matériau cuivre pur a été choisi et considéré comme un "système modèle". En tournage, l'influence du rayon de bec, de la vitesse de coupe et de la présence de lubrifiant sur le champ de contraintes et la texture a été quantifiée à l'aide de techniques DRX. Les modifications micro-structurales ont été évaluées à l'aide de la microscopie électronique à balayage. Enfin, le comportement électrochimique de la surface a été étudié en utilisant la technique de la micro-cellule électrochimique

On the optimization of the cutting conditions for an improved corrosion resistance of OFHC copper

Machining has a particular impact on the surface integrity and on corrosion resistance of components. In fact, material removal induces geometrical, mechanical and micro-structural modifications in the machined surface and sub-surface that alter the electrochemical behavior of the material, and so the aging process. In this study, oxygen free high conductivity copper (OFHC) has machined under orthogonal cutting conditions using uncoated cemented carbide tools. Then, the corrosion resistance in 0.1 M NaCl salt fog atmosphere of the machined samples is analyzed. Finally, the optimal cutting conditions, including the tool geometry, for an improved corrosion resistance are identified. Their influence on the surface integrity, and then their consequences on the surface aging response is discussed