Palladium coating on quenched-tempered martensitic steel for hydrogen electrochemical permeation tests

A coating process is proposed to obtain a thin palladium layer on high-strength steel. The quality of the layer is investigated by SEM observation. Potentiokinetic curves are performed to ensure no hydrogen formation during the coating process. Coated samples are tested by hydrogen permeation test and compared to non-coated samples. Permeation data analysis and boundary conditions are discussed based on permeation results and 1D simulations

Mobility and trapping of hydrogen in high-strength steel

6 pagesInternational audienceElectrochemical permeation and thermo-desorption tests are performed to evaluate hydrogen mobility in high strength steel. Experimental parameters are used in a Krom like phenomenological diffusion model. This model is developed to simulate hydrogen diffusion and trapping in processing zones of specimens subjected to fatigue loadings

Shear on the Flow Surface of Metallic Crystals

11p. Publié suite au congrès SHEAR07, Nancy 4-7 septembre 2007, dans n° spécial SHEAR BEHAVIOR AND RELATED MECHANISMS IN MATERIALS PLASTICITYInternational audienceThe present article addresses the following question: How is it that shears are so common in the plastic deformation of metallic alloys? An answer is sought in a geometric description of the shear flow when the deformation is produced by slip systems gliding according to the Schmid law. Such flows are represented schematically by what is called “simple shear” and a kinematic study is done of the way these shears can be produced by the joint activity of various slip systems. This implies specific conditions on the glide rates, which can be known analytically thanks to adequate parametrizations. All the possible shears have been calculated in the case of cubic metals deforming with identical critical resolved shear stresses (Bishop and Hill polyhedron). Three dimensional representations are given in the space of the Bunge angles associated with the principal directions of the shears. A special attention has been given to the number of slip systems involved. Most of the shears are not far from some combination of two or three systems. This is quantified by defining the misorientation omega between a shear taken at random and the set of shears produced by the glide on two or three octahedral slip systems. It is found that in most cases, omega<15 deg. The maximum value of omega (30.5 deg) is found for the orientations called Cube and U in rolled metals

1D numerical simulation of charge trapping in an insulator submitted to an electron beam irradiation. Part I: Computation of the initial secondary electron emission yield

International audienceThe 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

Modélisation mathématique et simulation numérique par un schéma volumes-finis de la synthèse du carbure de titane par procédé d'autocombustion SHS

Dans ce travail, nous considérons le problème de la synthèse du carbure de titane par procédé SHS d'un point de vue mathématique et numérique. Après avoir modélisé ce procédé de combustion par une équation parabolique nonlinéaire couplée à une équation différentielle, nous prouvons l'existence et l'unicité de la solution du problème faible couplé linéarisé et montrons qu'elle satisfait à un principe du maximum. Une discrétisation volumes-finis du problème couplé est établie ainsi qu'une estimation d'erreur, des propriétés de stabilité et de principe du maximum relatif à la solution discrète. Le schéma numérique est implémenté dans notre code de calcul Héphaïstos écrit en C++. Une analyse de sensibilité de la solution discrète au raffinement de maillage est conduite. Nous étudions en une dimension d'espace la sensibilité du temps d'induction aux paramètres thermophysiques et cinétiques ainsi que la nature de la propagation du front de combustion en une et deux dimensions d'espaceIn this work, we consider the titanium carbide synthesis by SHS process from a mathematical and numerical point of view. The modelling of this combustion process is based upon a coupling between a nonlinear parabolic equation and a differential equation. We prove the existence and uniqueness of the solution of the weak, linearised, coupled problem and show that the solution satisfy a maximum principle property. A finite-volume discretization of the coupled problem is established with an error estimate, stability and discrete maximum principle properties of the discrete solution. The numerical scheme is implemented into our C++ software Héphaïstos. A sensibility analysis of the discrete solution with respect to mesh refinement is conducted. We investigate in one spatial dimension the sensibility of the induction time with respect to thermophysical and kinetics parameters and analyse the nature of the propagation of the combustion front in one and two spatial dimensionsST ETIENNE-BU Sciences (422182103) / SudocSudocFranceF

An implicit finite volume discretization for the multidimensional numerical simulation of TiC combustion synthesis

International audienceA reaction–diffusion mathematical modelling of titanium–carbide combustion synthesis from a mixture of titanium and carbide reactive powders by self-propagating high temperature synthesis process is presented. This modelling results in the coupling between a nonlinear parabolic equation expressing the enthalpy balance of the system with nonhomogeneous Neumann boundary conditions and a nonlinear differential equation describing the exothermic chemical reaction in the system. In order to analyze by numerical simulation the influence of the boundary conditions over the ignition and propagation of the combustion front in the reactive mixture, an implicit finite volume scheme is constructed for the discretization of the governing equations of the process. An error estimate for the discrete approximation of the parabolic equation is established. A positivity property of both the sparse matrix arising in the discretization of the reaction–diffusion equation and the discrete approximation of the differential equation is presented. Detailed unsteady numerical simulations in one and two spatial dimensions of the influence of radiative boundary conditions over induction time and ignition transients are reported and discussed

X-ray methods for strain energy evaluation of dislocated crystals

International audienceTwo X-ray methods are applied to estimate the strain energy of crystals containing dislocations, a simpler method based on the full width at half-maximum (FWHM) of the diffraction peaks, and asymptotic line profile analysis (LPA), which exploits the functional form of the Fourier transform corresponding to small Fourier parameters. It is shown analytically that, in the single-defect approximation, the modified Williamson–Hall (mWH) plot of the FWHMs is linear and the slope of the line is directly related to the strain energy of the dislocation system. Evaluation of the numerically generated peaks for randomly arranged edge dislocation dipoles shows that the method based on the mWH plot gives accurate strain energy, while asymptotic LPA overestimates it by about 50%. The accurate result given by the mWH plot is explained by the long correlation distance associated with the FWHM, which better captures the dislocation arrangement over large distances. By contrast, asymptotic LPA is related to atomic correlations over distances smaller than the mean dislocation–dislocation spacing, where the displacement gradient is mainly determined by the field of single dislocations. Therefore, asymptotic LPA leads to a very accurate dislocation density (with error less than 1%) and the result is independent of the dislocation arrangement. However, these short-range correlations overestimate the outer cut-off radius by one order of magnitude

Testing metals in tension and relaxation at elevated temperatures

International audienceThis article describes an apparatus for testing viscoplastic metallic alloys in tension at temperatures up to 400◦C. Its distinctive feature is a two-shelled furnace which encompasses the test-piece. The extensometer is attached to the shoulders of the specimen and remains outside the oven, so that it works at room temperature. The strain εrs in the reduced section inside the tight fitting oven is calculatedwith the help of a finite element software from the strain εext given by the extensometer. In the elastic range, the set-up was used for the measurement of Young’s moduli. In the plastic and viscoplastic ranges, it was used to draw work-hardening curves and to perform relaxation tests representative of in-service conditions. In this later case, a method to derive the strain rate sensitivity from the decrease with time of the registered stress is presented. The furnace can be easily machined in a mechanical workshop for all shapes and dimensions of test-pieces, so that it can be adapted to various studies of the workability of metallic alloys, especially those which necessitate a rapid rise and precise maintenance in temperature