Search

In this article, we investigate the benefit of including texture information in models of gloss perception of low-gloss textured achromatic plastic surfaces from the automotive industry. 4 models are compared: two gloss prediction models including texture information, one using data from reflectometry (M1) and one using data from goniophotometry (M2), and two models using data from reflectometry (M3) or goniophotometry (M4) alone. Both texture-corrected models (M1-M2) outclass the uncorrected intensity-based models, mainly because they are made texture invariant. Although the texture-corrected reflectometer-based prediction (M1) correlates rather well with sensory data, a more consistent fit is obtained by mixing textural to goniophotometric data (M2). This can be explained by the fact that contrast gloss is better than multiangle specular gloss at reflecting the observer's gloss evaluation strategy. © 2015 Wiley Periodicals, Inc. Col Res Appl, 2015

International audience

International audience; 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. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

To predict concentration changes in binary alloy due to preferential oxidation during high temperature oxidation, a model based on the flux balance of the oxidized element at the moving oxide/alloy interface has been developed. These changes are driven by the alloy oxidation rate k c . The model is numerically solved considering different initial alloy concentration profiles and a possible diffusion enhancement effect close to the alloy surface. After validating the model by comparison with Wagner’s analytical solution for Ni30wt%–Pt alloy oxidised at 850 °C, we show that the effects of an initial alloy depletion due to the presence of a passive oxide layer are cancelled after very short oxidation times. We also show that a diffusion acceleration due to work-hardening in the vicinity of the alloy surface induces an inflexion point in the depletion profile.

International audience; The amorphous structure of bulk metallic glasses results in outstanding properties, which are able to outshine the properties of common metallic biomaterials. In particular, Ti-based bulk metallic glasses are interesting candidates for the use as biomaterial or as structural material in lightweight applications. However, the usage of known Ti-based bulk metallic glasses with casting thicknesses of several millimeters is strongly limited due to the large content of rare, expensive and harmful elements. In this work, we design Ni-free Ti-based bulk metallic glasses with a low Cu-content, being suitable for the application as biomaterial. On the basis of the binary Ti-Ni system, novel glass forming alloys are developed by the addition of sulfur. In order to increase the glass forming ability and to reduce the Ni-content, Ti was substituted with Zr and Ni with Cu. Finally, it is possible to produce synchrotron X-ray amorphous samples with a diameter of 3 mm for the Ni-free alloy Ti40Zr35Cu17S8. Furthermore, it is shown that industrial grade raw materials can be used for the synthesis of amorphous samples of this alloy without diminishing the glass forming ability or the mechanical properties. Characterization by mechanical and electrochemical tests proves the superiority of the novel alloys over commercial amorphous and crystalline alloys. All in all, a new class of Ni-free Ti-based bulk metallic glasses with unprecedented properties is developed, being highly attractive for a widespread use.

International audience; The aim of this work is to study by numerical simulation a mathematical modelling technique describing charge trapping during initial charge injection in an insulator submitted to electron beam irradiation. A two-fluxes method described by a set of two stationary transport equations is used to split the electron current j(e)(z) into coupled forward j(e+)(z) and backward j(e-)(z) currents and such that j(e)(z) = j(e+)(z) - j(e-)(z). The sparse algebraic linear system, resulting from the vertex-centered finite-volume discretization scheme is solved by an iterative decoupled fixed point method which involves the direct inversion of a bi-diagonal matrix. The sensitivity of the initial secondary electron emission yield with respect to the energy of incident primary electrons beam, that is penetration depth of the incident beam, or electron cross sections (absorption and diffusion) is investigated by numerical simulations. (C) 2010 Elsevier Inc. All rights reserved.

International audience; A simple three-dimensional thermomechanical model for friction stir welding (FSW) is presented. It is developed from the model proposed by Heurtier et al. (2006) based on a combination of fluid mechanics numerical and analytical velocity fields. Those velocity fields are introduced in a steady state thermal calculation to compute the temperature field during welding. They allow partial sliding between the shoulder and the workpiece, the amount of which is provided as an additional result of the model. The thermal calculation accounts for conduction and convection effects by means of the particular derivative. The complete thermomechanical history of the material during the process can then be accessed by temperature and strain rate contours. The numerical results are compared with a set of experimental test cases carried out on an instrumented laboratory device. The choices for modeling assumptions, especially tribological aspects, are discussed according to agreements or deviations observed between experimental and numerical results. The amount of sliding appears to be significantly influenced by the welding conditions (welding and tool rotational velocities), and physical interpretations are proposed for its evolution.

International audience; The applicability of three-axis plane-strain compression for grain refinement and improving the properties of iron aluminum alloy (Fe–16Al–5Cr–1Mo–0.1Zr at.%) has been studied. Particular attention was paid to evolution of the microstructure with respect to effective strain and mechanical properties under tensile load, as well as the factors that influence cracking. The obtained results showed that the multi-axis compression in the MaxStrain system, as a method of severe plastic deformation, is capable of refining the grain size in the Fe–16Al–5Cr–1Mo–0.1Zr (at.%) alloy to about 500 nm without affecting significantly the initial shape of the specimen with constrained ends. The deformation of the investigated alloy at RT with the strain to 2.7 is not enough to form an ultrafine-grained structure. This kind of homogeneous structure could only be obtained through the high temperature (600 °C) processing with a very large strain ( = 30.0). The ultrafine-grained structure was formed after deformation at 600 °C with the application of 67 passes ( = 30), and exhibited high ultimate tensile strength (UTS 880 MPa) with an elongation to failure of about 0.8%. In addition, it demonstrated a dimple fracture that was in contrast to its micro-sized grained counterparts with moderate UTS (550–750 MPa) and good elongation to failure (6–9%), but brittle fracture.

International audience; Single crystal foils of an FeAl alloy containing Ni and B are strained at elevated temperatures in an in situ dedicated transmission electron microscope (TEM). The observed characteristic "fork-shaped" dislocation configurations are shown to be a kind of "0 0 1 junction". This junction comes from a reaction between two 1 1 1 superdislocations gliding on different {1 1 2} planes; it results in the presence of a long and sessile edge 0 0 1 dislocation along the intersection of the glide planes. Several hypothetical core structures of such a junction are proposed and their possible effects on macroscopic mechanical properties of the alloy are discussed.

International audience; The average rotations of 157 individual grains measured in an Al-0.1%Mn polycrystal deformed in hot plane-strain compression to a strain of 1.2 have been compared to the predictions from several crystal plasticity models. The Taylor model successfully predicts the distributions of the rotation angles, but overestimates the rate of evolution of the rotation axes, which explains the overestimation of the final texture. The model also provides a first-order agreement for the rotation axes and the final components of the individual grains. The discrepancies are related to the influence of grain interaction. The main mechanisms of texture development at the level of the individual grains are identified through an analysis of their orientation trajectories. The introduction of local grain interactions, which produce a rotation variability at constant orientation, is shown to weaken texture development.

This paper investigates the mechanical deformation and the electrical contact resistance of an electroplated Ni micro-cylinder called micro-insert inserted in an Al thin film. A modified nanoindentation apparatus is used to perform the experiments with [email protected], [email protected] and [email protected] diameters micro-inserts having the same [email protected] height. Mechanical deformation of Ni micro-insert and Al film is described at different maximum loads corresponding to an equivalent stress of 0.8GPa, 1.6GPa and 3.2GPa. At equivalent stress less than 1.6GPa, Ni micro-insert exhibits an elastic deformation while at 3.2GPa it presents an elastic-plastic deformation with a large amount of compression and penetration into micro-insert foundation. Visco-plastic deformation of Al film is noticed during hold at all maximum loads. Beside, Al creep parameters are extracted using a combined Maxwell/Kelvin-Voigt phenomenological model. Mechanical results are coupled to electrical contact resistance measurement.

International audience; Dielectric breakdown constitutes an important limitation in the use of insulating materials under high-voltage since it can lead to the local fusion and sublimation of the insulator. The role of electrical charge transport and trapping in alumina ceramics on their resistance to this catastrophic phenomenon is studied in this work. In polycrystalline materials, the interfaces between the various phases play a main role because they constitute potential sites for the trapping of electrical charges. The density and the nature of these interfaces can be controlled by the way of the microstructure parameters. So, the aim of the present paper is to highlight the influence of average grain size and intergranular phase crystallization rate on the ability of polycrystalline alumina materials to resist to dielectric breakdown. Thus, it is shown that the control of the process conditions (sintering aids content, powder grain size and thermal cycle) makes it possible to change not only the density (by the average grain size) but also the nature (by the crystallization or not of anorthite) of the grain boundaries. On one hand, at room temperature a high density of interfaces, due to low grain size and highly crystallized intergranular phase, leads to a high dielectric strength. On the other hand, at higher temperature (250 degrees C), the presence of vitreous intergranular phase makes it possible to delay breakdown. That behaviour is explained thanks to charge transport and trapping characterizations. (C) 2009 Elsevier Ltd. All fights reserved.

A split sample of Al–0.1%Mn has been deformed by a series of compression tests in a channel-die at 400 °C to a final strain of 1.6. The orientations of 176 grains in a 4 × 4 mm 2 internal surface were followed by high-resolution electron backscatter diffraction at four different strains to compare with crystal plasticity models. Typically 3000 orientations per grain were used to quantify the average lattice rotations of each grain together with their orientation spreads (termed microtexture tracking). The average orientations tend towards the standard β -fibre plane-strain compression texture components, albeit with some variations. The in-grain orientation spreads develop strongly at first, then tend to saturate at high strains. Finally, the influence of grain environment on lattice rotation is examined by means of the rotation “variability at constant orientation”. On average and at the beginning of the deformation, two grains of the same initial orientation, but different neighbours, would rotate by angles that vary by 25% and axes separated by 37°; their orientations at e = 1.2 would vary by 12°.

Article first published online: 17 DEC 2009; International audience; Aluminum sheet is currently used for body panels on a number of mass-produced vehicles, in particular for closure panels. AA5xxx alloys always contain coarse inter-metallic particles (Al x(Fe,Mn) ySi, Mg 2Si) after casting. In the present work inter-metallic particle break-up during hot reversible rolling of AA5182 alloy sheets has been analyzed. The sizes and shapes of inter-metallic particles in as-cast and industrially hot rolled AA5182 alloys sheets were characterized by 3D X-ray tomography observations. The relation between particle break-up and particle morphology was then analyzed statistically and by a micromechanical finite element (FE)-based model. The essential outcomes of the statistical approach may be summarized as follows. The inter-metallic particle population may be described by five morphological parameters. Secondly the comparison of the particle morphology in as cast and industrially rolled sheets leads to the definition of five classes. The evolution of each particle class as function of the rolling strain is provided. The statistical analysis shows which particles break-up. The stresses and strains in inter-metallic particles, embedded in an elasto-viscoplastic aluminum matrix submitted to plane strain compression, were analyzed by an FE model. A new failure criterion was proposed. The essential outcomes of the mechanical approach are as follows: a precise description of stress concentration mechanisms in nonconvex particles, a close description of the parameters controlling particle break-up, and finally a simplified classification of the failure behavior.

International audience; Different alumina materials were elaborated in order to vary microstructural parameters (grain size, densification, porosity, inter-granular phase). These ceramic materials were then characterized from the mechanical point of view (hardness, toughness, friction and wear) and dielectric breakdown. The comparison of these various results shows that, for all these properties, the grain size and also, the nature of the secondary phases and the microstructural parameters were the most significant. Moreover, from the tribological point of view, the dielectric characteristic of materials (breakdown strength) has a fundamental role in the creation of agglomerated wear debris (“third body”) and its properties: a finely agglomerated third body will be obtained for high breakdown strength. Such third body will be able to protect the substrate and thus to reduce later wear. In the same logic a correspondence between breakdown strength and toughness was established, thus confirming the existence of mechanical–electrical correlation for non-conductive materials.

International audience; Abstract: For heterogeneous materials assembly, the thermal expansion mismatch between the chip and the substrate is a roadblock for Hip chip bonding of ultrafine-pitch (= 20 mm). Residual strains in bumps and device warpage have been calculated to evaluate the thermomechanical limits of a conventional flip chip soldering process using micro bumping. As a solution to overcome these limits, this paper describes a new patented flip-chip technology representing a technological breakthrough compared to conventional methods such as soldering or bonding through conductive adhesives. Electrical connections are performed by the insertion of metallic micro-tips in a ductile material. As a low-temperature process and fluxless technology, this method is adapted to fine-pitch and large devices. As a proof of concept, we present the bonding results obtained on fine-pitch large arrays of daisy chains with 500 x 500 contacts and 30-mu m pitch. The electrical contact has been demonstrated and characterized in terms of resistance and yield.

International audience; A detailed accuracy analysis of electron backscatter diffraction (EBSD) elastic strain measurement has been carried out using both simulated and experimental patterns. Strains are determined by measuring shifts between two EBSD patterns (one being the reference) over regions of interest (ROI) using an iterative cross-correlation algorithm. An original minimization procedure over 20 regions of interests gives a unique solution for the eight independent components of the deviatoric displacement gradient tensor. It is shown that this method leads to strain measurements on simulated patterns with an accuracy better than 10(-4). The influence of the projection parameters is also investigated. The accuracy assessment is illustrated by two worked examples: (i) four-point bending of a silicon single crystal and (ii) Si(1 - x)Ge(x) layers on a Si substrate. Experimental results are compared with finite-element simulations.

International audience; Abstract Iron stories. The descriptions of the 18th century. Part III : Proposals for the physicochemical interpretation of cast-iron conversion in the finery hearth. The descriptions of iron metallurgy by the 18th-century scientists are exposed and discussed through cast iron production and refining into wrought iron or steel. In part III, we propose a synthesis of the descriptions exposed in the first two parts. Modern knowledge from thermochemistry and physical metallurgy is used to help and understand the signification given to the conversion of cast iron in the refining hearth. Résumé Les descriptions de la métallurgie du fer par les scientifiques du XVIIIe siècle sont exposées et discutées à travers la production de la fonte et l'affinage en fer ou acier. Cette partie III propose une étape de synthèse de l'ensemble de connaissances exposées dans les deux premières parties. Les outils actuels de thermochimie et de métallurgie microstructurale sont mis en œuvre pour aider à comprendre ce que signifiait l'affinage des différents type de fontes au bas foyer.

Intergranular penetration of liquid bismuth has been analysed in two pure metals, Cu at 500°C and Ni at 700°C, used either as polycrystals or as oriented bicrystals. At the liquid/solid interface, large grooves have developed in Cu-Bi, while micrometer-thick films were observed in Ni-Bi. The bismuth concentration measurements obtained by Auger electron spectroscopy indicate a zone of monolayer Bi segregation followed by a diffusion type profile over a distance of the order of 100 µm for Cu-Bi and a nanometer-thick film followed by similar diffusion type profile for Ni-Bi. In both cases the kinetics of intergranular penetration and embrittlement has been shown to be parabolic. It is concluded that no wetting occurs in Cu-Bi system at 500°C while Bi wets Ni at 700°C. It is postulated that the mechanism of intergranular penetration operates at a very tip of the penetration front, as opposed to the tip of liquid Bi film observed by scanning electron microscopy, and must be based on diffusion rather than wetting phenomena. Some suggestions are formulated for the future research in the area of intergranular penetration that can be split in two phenomena : grain boundary wetting above the wetting transition temperature and grain boundary diffusion below.

Publié suite au congrès High Temperature Corrosion and Protection of Materials 7; International audience; Due 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.%).

Proceedings on Advanced Materials and Concepts for Photovoltaics EMRS 2007 Conference, Strasbourg, France; International audience; Among the methods for enlarging the grain size of polycrystalline silicon (poly-Si) thin films, aluminium induced crystallization (AIC) of amorphous silicon is considered to be a very promising approach. In the AIC process, a thin a-Si layer on top of an aluminium layer crystallizes at temperatures well below the eutectic temperature of the Al/Si system (Teu = 577 °C). By means of electron backscattering diffraction (EBSD), we have mainly studied the effect of the aluminium layer quality varying the deposition system on the grain size, the defects and the preferential crystallographic orientation. We have found a strong correlation between the mean grain size and the size distribution with the Al deposition system and the surface quality. Furthermore, we show for the first time that more than 50% of the surface of the AIC films grown on alumina substrates are (103) preferentially oriented, instead of the commonly observed (100) preferential orientation. This may have important consequences for epitaxial thickening of the AIC layer into polysilicon absorber layers for solar cells.

The growth and coalescence of voids nucleated by decohesion or cracking of second phase particles is a common damage process for many metallic alloys. Classical damage models, based on void growth and coalescence, predict a ductility increase if the stress triaxiality is decreased. But experiments show that the material ductility decreases at very low stress triaxialities typical of sheet metal forming operations. At very low stress triaxiality no void growth is observed in metals containing second phase particles. In the present work, a new damage model for metals containing second phase particles submitted to low stress triaxiality loading is proposed. The new model is based on the observed physical damage mechanism, i.e. strain localization by reducing the inter-particle spacing during large material rotations. A two step modelling strategy has been followed to determine the ductility at low stress triaxiality. In the first step Thomason's void coalescence model is extended to large material rotations and shearing. In the second step the principles of applying this model to damage nucleated at second phase particles are described. The large material rotations observed under low stress triaxiality loading lead to large changes in the microstructure. Thus, in the second step, first appropriate representative volume and material elements are determined and then the critical damage parameters. Finally, as an example, the trimming behaviour of two aluminium sheet alloys is analyzed by the new model and the model predictions shown to be in good agreement with the experimental data, in particular for the blade displacement to crack initiation. The main outcomes of this work are: (1) a void coalescence model valid at low stress triaxiality, (2) a damage criterion valid at small stress triaxiality and large material rotations, (3) a damage variable expressed in a simple closed form for materials containing second phase particles. The damage analysis in a small fixed volume with a representative microstructure (Eulerian approach) and the damage analysis in all the material elements of the considered structure are compared in detail. In trimming (or similar processes), the major contribution to damage the material movement bringing second phase particles closer together and the void growth may be neglected. This simplifies considerable the analysis.

International audience; The study was carried out to understand the mechanisms occurring during dynamic recrystallization of hot deformed 11 % chromium stabilized ferritic stainless steels and to compare the behaviour induced by various types of stabilization. The experimental temperatures ranged from 800 to 1150 °C and strain rates from 10-2 to 15 s-1. The development of the textures and microstructures was analysed using EBSD maps. It was observed that continuous dynamic recrystallization occurs in all materials starting at the onset of straining. Niobium has a more pronounced influence on hardening than titanium during hot deformation. The D2 component was found as the major texture component at the steady state for the torsion tests conducted along the negative shear direction. It was likely to be formed by the combination of straining and growth of the grains exhibiting both low stored energy and low rotation rate of the crystallographic axes. The texture evolution was the main reason for the flow stress behavior during the hot torsion tests. A complementary study was carried out to understand the texture formation occurring during the direct (negative shear direction) and reversed (positive shear direction) hot torsion. After reversion of the shear direction, i.e. during positive shear, the above major texture component is gradually changed into the D1 component. Using the method of Continuum Mechanics of Textured Polycrystals, the stress evolution is explained by the volume fraction changes of each component at various strains, associated with their respective Taylor factors. This simplified approach leads to a good agreement with experimental results.

International audience; Chromium-rich nickel base alloys 617 and 230 are promising candidate materials for very high temperature gas-cooled reactors (VHTR) but they must resist corrosion in the impure primary cooling helium over very long times. The impurities of the hot helium can promote the development of chromium-rich surface oxides that appear to protect the alloys against intensive corrosion processes. However above a critical temperature (typically in the range 1173-1273 K), chromium oxide is reduced by carbon from the alloy and the surface layer is not stable anymore. Depending oil the gas composition, the unprotected material rapidly either gains or loses carbon with a dramatic impact on its mechanical properties. The deleterious reaction of chromia and carbon thus fixes all ultimate reactor operating temperature. Critical temperature measurements are presented for alloys 617 and 230 and the influence of carbon monoxide partial pressure in helium is discussed.

International audience; A new high temperature channel-die compression equipment is introduced. It is designed to operate under inert atmosphere at constant strain rate. Its main characteristics are described and compared to those of a previous appliance based on different principles, and designed for tests up to 600 ° C [C. Maurice, J.H. Driver, Acta Metall. Mater. 41 (1993) 1653–1664]. Tests have been performed in the same conditions with both appliances. The stress–strain curves show good agreement, but the deformed samples have different shapes. This difference is explained with the help of a finite element analysis which indicates that the deformation is more homogeneous in the new appliance, where the walls are unconstrained in the vertical direction. The new equipment is currently used to investigate the microstructural evolution of beta quenched Zircaloy-4 up to 750 °C. Some results regarding stress–strain relationships of Zircaloy-4 and deformation heterogeneities of lamellar microstructures are provided; they illustrate the potentialities of this equipment for the simulation of industrial processes.

Publication suite au congrès : Proc. Rex & GG 3 (Corée du Sud 2006); International audience; This paper described new characterization methods and data to quantify the influence of solute atoms on grain boundary and sub-grain boundary mobilities in Al-Mn alloys with a view to their integration into recovery and recrystallization modelling. Detailed SEM measurements of grain boundary mobilities during recrystallization have been made by in-situ annealing experiments on cold deformed Al – 0.1 and 0.3wt.% Mn binary alloys. Stored energies are estimated from the sub-grain sizes and misorientations and the boundary velocities directly measured in the temperature range 200-450°C. It is shown that in many cases good agreement with the Cahn, Lücke, Stüwe model for solute drag is obtained, e.g. the activation energies are intermediate between those of boundary and volume solute diffusion. Some particular cases of rapid growth occur in Al-0.1%Mn indicating boundary breakaway from solute clouds. A complementary study of sub-grain boundary mobilities has started on the same alloys; in this case the average mobilities are estimated from FEG-SEM growth data for the average sub-grain size for temperatures in the range 150-300°C. The results are compared with some previous data on Al- Si and show similar rates.

International audience; The evolution of the French word fonte (cast-iron or smelting) is described, taking into account the different significations of this word and the attempts for rationalisation of it use, in parallel with the word fusion (melting). A comparison is done with the corresponding words in Latin and English. The sequence fer de fonte, fonte de fer and finally fonte cast iron is analysed as a function of time, and compared with the old notion of fonte du minerai (smelting) and the still-used lit de fusion (burden). L'évolution du mot « fonte » est décrite, du point de vue des sens successifs du mot et des tentatives de rationalisation de son emploi, en parallèle avec le terme « fusion ». Une comparaison est faite avec les expressions correspondantes en bas latin et en anglais. La séquence « fer de fonte, fonte de fer » et enfin « fonte » est analysée au cours du temps, par rapport à la notion ancienne de « fonte du minerai » et à celle encore actuelle de « lit de fusion ».

International audience; This article presents an application of identification via finite elements model update (FEMU) to experimental fields of a plate with a hole using stereo correlation. The presentation includes the evolution of material properties during a tension test, and a first approach in estimating statistical confidence intervals from a single experimental field. These are accomplished by characterizing the material at different load cases, and by repeating the identification multiple times for random samplings of the full field and for random redistributions of noise.

International audience; Geometric aspects are a major issue in models of recrystallization that rely on statistical grain-population descriptions, because the growth of recrystallized grains has to be compensated by the loss of volume of deformed grains, thus leading to concomitant variations in all geometric variables. A geometric framework for such models was thus designed. It is based on mesostructure units (MSUs), each of which represents an aggregate of similar grains. The evolution of MSUs is controlled by two kinds of inputs, nucleation rates and grain-boundary velocities, from which the evolution of microstructure is described in an internally consistent fashion. The geometric framework was applied initially to the necklace recrystallization of fine-grain microstructures, viz., the usual form of recrystallization comprising nucleation on initial grain boundaries. It was extended to describe particle-stimulated nucleation (PSN) in order to treat geometric effects related to intragranular recrystallization, as is found in coarse ingot microstructures. For both the necklace-only and necklace-and-PSN cases, test cases using simple inputs were performed to validate the behavior of the geometric framework.

International audience; One of the most popular approaches to ultrafine grain formation within bulk samples is severe plastic deformation (SPD). In this paper, grain refinement by two SPD techniques involving different strain paths, namely multiaxial compression (MAC) and torsion, has been investigated. A coarse-grained, high-purity iron-carbon alloy was deformed within the ferritic domain under cold and warm working conditions to large strains (>>1). Subsequent analyses by orientation imaging microscopy revealed that the nature and extent of refinement depend upon the processing route. In MAC, fragmentation occurs mainly by means of a dense, homogeneous network of low angle boundaries (LAB, 2 ≤ θ ≤ 15°) delimiting subgrains, and only a few strain-induced high angle boundaries (HAB, θ ≥ 15°) are observed in localised regions of cold worked samples. In torsion, however, the proportion of HAB is significantly higher (37-54%), giving rise to a fragmented microstructure comprising ultrafine crystallites (1-2 μm).

The corrosion behavior in impure helium of Haynes 230, a nickel base alloy candidate for heat exchangers in Very High Temperature Reactors (VHTR), has been investigated. The study focused on the formation and the subsequent destruction of the surface oxide layer at 900 °C and 980 °C. In-situ gas-phase analysis coupled to post-exposure surface analyses showed that a chromium-rich surface oxide formed on Haynes 230 at 900 °C but was unstable above a critical temperature T A : the chromium-rich oxide reacted with carbon in solution in the alloy to produce chromium and CO(g). The effect of carbon monoxide partial pressure in the gas phase as well as the influence of chromium and carbon activities in the alloy on T A are discussed taking thermodynamics and kinetics aspects into account.

International audience; Energy transfer distributions dE/dx and respective excitation depth functions Φ of electron beams in heterogeneous layered samples are described by a successive Effective Layer method. This method is based on electron transmission rates through the top multilayer system replaced by an “effective layer” of the following bottom material. Thus energy depositions in heterogeneous layered metal samples (Al, Ag and Au) and dielectric insulating samples SiO2–Al2O3 are given. For the latter ones special energy-range relations R(E0) have been deduced for the common energy regions (1–30) keV of scanning electron microscopy (SEM). Application are given by cathodoluminescence depth profiling and electron beam charging of non-conductive samples.

Publication suite au congrès : Progress in Light Metals, Aerospace Materials and Superconductors, Beijing, 2006; International audience; An Al-3%Mg-0.25%Sc-0.12%Zr alloy was deformed by triaxial forging at 20-400 °C up to strains of about 3. A study of its textural evolution reveals the tendency towards three symmetrical variants of a component. This experimental observation is supported by a 3D spatially resolved crystal plasticity analysis. Samples strained at room temperature undergo grain fragmentation in the form of fine substructures and relatively weak textures. Conversely, at 300 °C and above, more homogeneous intergranular deformation and rotations give rise to stronger textures. This eventually encourages grain coalescence and thus the development of interpenetrating "orientation chains", creating a new type of microstructure. The influence of this texture development on the specific work hardening behaviour is discussed.

International audience; Dielectric breakdown is the main cause of insulator degradation. Breakdown strength strongly depends on materials microstructure (grain size, grain boundaries nature,...) [Liebault, J., Vallayer, J., Goeuriot, D., Tréheux D., Thévenot, F., How the trapping of charges can explain the dielectrics breakdown performance of alumina ceramics, J. Eur. Ceram. Soc., 2001, 21, 389–397; Si Ahmed, A., Kansy, J., Zarbout, K., Moya, G., Liebault, J., Goeuriot, D., Microstructural origin of the dielectric breakdown strength in alumina: a study by positron lifetime spectroscopy, J. Eur. Ceram. Soc., 2005, 25, 2813–2816]. The experimental study of these materials behaviour towards charge injection was performed by the scanning electron microscopy mirror effect (SEMME) method. It allows to measure the amount of injected charges finally trapped in the insulator. In order to explain the experimental results, we developed an iterative computer simulation of the self-consistent charge transport in bulk alumina samples during electron beam irradiation, based on a new flight-drift model (FDM). Ballistic and drift electron and hole transport as well as their recombination, trapping and detrapping (due to temperature and electric field) are taken into account. As a main result the time dependent secondary electron emission rate and the spatial distributions of currents, charges, field and potential are obtained. The analysis of these two kinds of results allowed us to identify the effect of the microstructure on the behaviour of the injected charges in the insulator and then to propose some mechanisms depending on the temperature leading to a good dielectric breakdown resistance. Indeed, at room temperature a huge localisation of charges limits further injection into the sample that permits to delay breakdown. On the other hand, when the temperature increases, the efficiency of the charge spreading behaviour is improved, leading to a higher dielectric breakdown strength.

International audience; Various 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.

International audience; 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.

International audience; Anodizing of aluminum generates a nanoporous alumina layer comprising cylindrical nanopores extending essentially perpendicular to the substrate. Visual observation of such nanoporous layers shows a colored specular reflectance consistent with reflectance spectrum measurements. Such structural colors raise interest in particular for producing non-fading colors. However the optical behavior is complex and requires specific characterization setups and modelling approaches. A morphological and optical characterization of nanoporous anodic alumina has been performed and enabled to draw the chromatic paths. Moreover, the optical behavior has been compared to numerical simulations of optical properties using the Fourier Modal Method, thus demonstrating the impact of short-range order and long-range disorder on the formation of structural colors in these partially ordered nanostructures.

Publication suite au congrès : Advances in Laser and Lamp Processing of Functional Materials. E-MRS Symposium J (Strasbourg 2006); International audience; Surface marking of aluminum single crystal is performed with femtosecond laser pulses. X-ray analysis allows to measure thermal and mechanical effects induced by the femtosecond laser pulses. These effects are estimated by comparing the pole figures (crystallinity) and the broadening of the diffraction peaks (mechanical contribution) before and after the laser irradiation. The results show that the femtosecond laser treatment ensures a re-crystallization of the structure and the presence of mechanical residual stresses. The analysis of the pole figures provides the sign of a re-crystallization on smaller volumes compared to initial ones. After the laser irradiation, the crystallization is perfectly oriented like the (1 1 0) orientation of the massive sample. Moreover, following the laser treatment, we show that the crystallographic structure is purer than the initial one. We also prove that the laser effect is persistent on a typical scale of 10 μm beyond the surface.

International audience; We investigated the role of second phases in the stress corrosion cracking of a complex copper–aluminium alloy CuAl9Ni3Fe2. We conducted slow strain rate tensile tests in synthetic sea water. Cracking is mainly intergranular. The crack path is determined by the electrochemical interactions between the α matrix and the second phases. We observe the selective dissolution of the aluminium rich areas that are anodic with respect to the α matrix. Under cathodic polarisation, the detrimental effect on mechanical properties is enhanced. The number of cracks is reduced. Crack blunting by dissolution is impeded.

International audience; An original model to simulate single grain boundary motion and its interaction with particles (Couturier G, Maurice C, Fortunier R. Phil Mag 2003;83:3387) is applied to model curvature driven grain growth. For single phase material, the single grain boundary model closely matches the grain coarsening kinetics of a 3D multi boundary vertex model. In the presence of spherical incoherent particles the growth rate slows down to give a growth exponent of 2.5. When the boundary is anchored there is a significantly higher density, four times higher, of particles on the boundary than the density predicted by the classic Zener analysis, but particles exert, on average, a drag force about half of the maximum value assumed by Zener. As a result the Zener drag is increased by a factor of 2.2. The limiting grain radius is compared with some experimental results.

Publié suite au congrès Association Française de Mécanique (AFM) : Identification des propriétés mécaniques à partir de mesures de champs, Clermont-Ferrand : France (2002); National audience; La variation de viscosité de l'huile en fonction de la pression conditionne le comportement des systèmes mécaniques lubrifiés. Une stratégie de caractérisation de la piézoviscosité basée sur des techniques d'analyse d'images et une identification numérique fine est développée en deux temps. Tout d'abord, l'identifiabilité de la piézoviscosité est analysée, dans un cadre de moindres carrés, à partir de données viscosité-pression pour trois modèles rhéologiques (Barus, Roelands et WLF). Des difficultés numériques d'identification dues à la grande variation des ordres de grandeurs des paramètres et des viscosités sont constatées. La solution apportée est un algorithme de Levenberg-Marquardt amélioré par des techniques de mises à l'échelle et une prise en compte de bornes sur les paramètres. La non-identifiabilité du modèle WLF à température constante est prouvée. Puis, la procédure d'identification de piézoviscosité est appliquée à des champs de hauteur du lubrifiant et de pression simulés et bruités. L'identification procède par minimisation du résidu global des équations de Reynolds. Une technique de lissage et dérivation des champs de mesures, basée sur une approximation polynomiale locale est présentée. Les paramètres estimés à partir des champs bruités ont une précision de l'ordre de 5 %. -------------------------------------------------------------------------- In order to design better mechanisms, the piezoviscosity, i.e., the sensitivity of lubricant viscosity to pressure changes, needs to be accounted for. A strategy for identifying piezoviscosity based on image analysis and numerical identification is devised in two steps. Firstly, the identifiability of three piezoviscosity models (Barus, Roelands and WLF) is analyzed by minimizing a least squares distance between pressure-viscosity responses. Numerical difficulties stem from magnitude differences between parameters and from the wide range of viscosity/pressure values. A strategy based on an improved Levenberg-Marquardt algorithm is proposed. Improvements concern respect of parameter bounds and scaling. The non-identifiability of the WLF piezoviscosity model at constant temperature is proved. Secondly, the piezoviscosity identification strategy is applied to data for noisy lubricant thickness and pressure fields. In this case, piezoviscosity can be recovered by minimizing the Reynolds equations residual globally in the field. A method for smoothing and numerical differentiation of the measurements is described. It is based on building a local polynomial approximation to the field. Using noisy field measurements, piezoviscosity parameters are finally estimated within 5% accuracy.

The thermoluminescence (TL) and fluorescence spectra of magnesium aluminium oxynitride “MgAlON” and magnesium aluminate MgAl 2 O 4 spinel powders have been analysed. Two broad peaks at about 100 and 300 °C appear in the TL spectra of MgAl 2 O 4 and seem to be related to electron and hole release from [Al Mg ] and [Mg Al ′] traps, respectively. The intensity of each peak strongly depends on the nature of the annealing atmosphere (neutral, oxidizing or reducing). TL spectral analyses have shown the presence of two intense emissions at 520 and 690 nm attributed to the Mn 2+ and Cr 3+ emitting centers, respectively. A study on MgAl 2 O 4 is used as a reference for MgAlON powders, which can be considered as a solid solution derived from magnesium aluminate. Disturbance in the emission spectrum of Cr 3+ is clearly observed with the nitrogen/oxygen substitution.

International audience

An analytical model is first proposed for the determination of a friction-corrected flow stress in channel-die compression. A variational approach is then used for estimating the strain and strain-rate heterogeneities as well as the shape changes during deformation for two specimen geometries. The results are compared with finite element calculations. Results obtained from the two methods are in good agreement, except for the outer profiles of the deformed specimens. Finally, the various components of the powers involved in the deformation process are compared.

International audience; The importance of sleep analysis lies in the preventive detection of health problems. Several types of research and scientific studies have shown that memory problems and cognitive problems are the direct results of poor quality sleep. Cognitive frailty is not the only frailty influenced by the lack or poor quality of sleep. Frailty can be defined as a lack of physiological reserves that no longer allow us to respond to needs or too stressful situations (illness, hospitalization, fall, etc.). In this article, we will present a part of our research work that aims to have a collaborative data analysis approach for the detection of entry into a state of fragility that will lead to becoming a resident in a nursing home, by using two different machine learning methods; supervised and non-supervised classification. We will use as inputs several sleep data.

International audience; Low cycle fatigue tests are performed on a high strength tempered martensitic steel at different plastic strain amplitudes at room temperature. Internal and effective components of the flow stress are analyzed using Handfield and Dickson's method. The internal stress is affected by the plastic strain amplitude. Conversely, the evolution of the athermal component of the effective stress with the number of cycles is independent of the plastic strain amplitude. The thermal part of the effective stress increases with the plastic strain amplitude, but remains constant with plastic strain accumulation. Microstructural changes in the cyclically deformed material are investigated by means of transmission electronic mycroscopy and X-Ray characterizations. Internal and effective stress evolutions are discussed based on these observations

International audience; Two major types of Cube bands/segments have been observed in heavily (90%) cold rolled A1-0.1wt %Mn using the EBSD technique with a FEG SEM: i) intergranular "transition bands" as thin cube segments aligned along RD between S and Cu oriented grains and ii) as transgranular "strain localized" bands situated in some particular grains. Their evolution is studied by light annealing at 275 degrees C and 300 degrees C and EBSD observations of exactly the same areas to directly correlate local deformation substructure with recrystallization. Only the intergranular cube "transition bands" give rapid recrystallization nucleation to cube grains of dimension >10 mu m. In particular the fastest growing cube grains have a near 40 degrees < 111 > relation with part of their surroundings.

International audience; Strain-induced surface roughness was quantitatively correlated with microtexture spatial distributions from the surface to the midthickness of a strongly roping AA6016 sheet. The microtexture variations through the sheet were measured by acquiring a series of large area electron backscatter diffraction (EBSD) scans at intervals of about the grain size. The orientations of the different layers were translated into out-of-plane strains using a crystal plasticity model for comparison with the surface roping. The spatial distributions (particularly the characteristic wavelengths) of the microtexture layers were determined by areal auto-correlation functions, and quantitatively compared to the surface appearance by correlation coefficients. The results show that texture banding of Cube/Goss components along the rolling direction is present from the surface to midthickness but with significant variations from one grain layer to another. The wavelength correlation coefficient exhibits a maximum at about 55 mu m, or 2 grain thicknesses, below the surface. It is shown that the spatial distribution of Cube/Goss components of the first three or four surface grain layers plays an important role in strong roping of AA6016 sheets.

International audience; Within the context of the sintering process simulation, this paper proposes a numerical strategy for the direct simulation of the matter transport by surface diffusion, in two and three dimensions. The level set formulation of the surface diffusion problem is first established. The resulting equations are solved by using a finite element method. A stabilization technique is then introduced, in order to avoid the spurious oscillations of the grain boundary that are a consequence of the dependence of the surface velocity on the fourth-order derivative of the level set function. The convergence and the accuracy of this approach are proved by investigating the change in an elliptic interface under surface diffusion. Cases in direct relation with the sintering process are analyzed besides: sintering between two grains of the same size or of two different sizes. Finally, 3D simulations involving a small number of particles show the ability of the proposed strategy to deal with strong deformations of the grain surface (formation of necks) and to access directly important parameters such as the closed porosity rate. Copyright (C) 2010 John Wiley & Sons, Ltd.

International audience; The moving screen technique for pattern centre localisation is revisited. A cross-correlation based iterative procedure is developed to find both the zoom factor and the zoom centre (which is also the pattern centre) between two EBSD diffraction patterns acquired at two camera positions. The procedure involves two steps: first, a rough estimate of the pattern centre position and zoom factor (the ratio of the two detector distances) is obtained by cross-correlating the entire images. Then, based on this first estimate, cross-correlation of smaller regions of interest (ROIs) gives the displacement field which is interpreted as a zoom factor misfit coupled with a zoom centre position misfit. These misfits are iteratively decreased until the displacement field is reduced to the noise level. The procedure is first applied to simulated patterns and it is shown that the iterative procedure converges very rapidly to the exact solution with an accuracy better than 1/100th of pixel. The potential of this technique for experimental patterns is discussed and recommendations for new EBSD detectors are proposed.