Kinetics of vanadium carbonitride precipitation in steel: A computer model

We have constructed a computer model of the precipitation kinetics of vanadium carbonitride in steel that takes into account the composition evolution of the precipitates with time. The model takes advantage of the fast diffusion of nitrogen and carbon compared to niobium to derive the composition, size and rate of formation of the precipitates during their nucleation. A local equilibrium condition is used at the precipitate–matrix interface to derive the growth rate of each precipitate as a function of its size and the current matrix composition. Coarsening occurs naturally on account of the Gibbs–Thomson capillarity effect. For isothermal heat treatments, the calculations show that the precipitates nucleate as almost pure vanadium nitrides. They subsequently grow at the expense of solute nitrogen. When nitrogen is exhausted, the solute carbon precipitates and progressively transforms the nitrides into carbonitrides. The coarsening stage leads to a steady-state size distribution of niobium carbonitrides of the equilibrium composition

Atomic-scale study of low-temperature equilibria in iron-rich Al-C-Fe

The capability of the thermodynamic approach based on the independent point defect approximation to describe low-temperature phase equilibria is investigated and applied to the Al-C-Fe system. The method gives a reasonable description of the multicomponent and multisublattice Fe-rich corner and evidences numerous peculiarities concerning the ordered phases as well as the density-functional-theory (DFT) energy models. The study of Fe3Al(-C), revealing strong defect-induced instabilities, rules out the LDA, SLDA and GGA schemes and leaves (spin-polarized) SGGA as the only valid one. C stabilizes L12 Fe3Al with respect to D03, which justifies the fcc-type structure of the kappa Fe3AlC compound. The present work also helps in justifying the experimentally observed depletion of C in the kappa phase. Finally, a correct description of both Fe3C and kappa requires inclusion of interstitial carbon at low temperature, emphasizing the unexpected importance of interstitial defects in ordered phases

First principle calculations of the k-Fe3AlC perovskite and ironealuminium intermetallics

We present first principle calculations of the structural, electronic, magnetic, vibrational and elastic properties of the k-Fe3AlC perovskite, within the ab initio formalisms of the Density Functional Theory (DFT) and the linear response theory of the DFT. These properties are compared with those of the intermetallic Fe3Al-L12 isostructural phase of k, permitting to interpret the role of the carbon element. We also discuss the influence of the spin effects (GGA and SGGA approaches) on the vibrational properties of some FeeAl intermetallics

Precipitation of niobium carbonitrides in ferrite: chemical composition measurements and thermodynamic modelling

High-resolution transmission electron microscopy and electron-energy loss spectroscopy have been used to characterize the structure and chemical composition of niobium carbonitrides in the ferrite of a Fe–Nb–C–N model alloy at different precipitation stages. Experiments seem to indicate the coexistence of two types of precipitates: pure niobium nitrides and mixed substoichiometric niobium carbonitrides. In order to understand the chemical composition of these precipitates, a thermodynamic formalism has been developed to evaluate the nucleation and growth rates (classical nucleation theory) and the chemical composition of nuclei and existing precipitates. A model based on the numerical solution of thermodynamic and kinetic equations is used to compute the evolution of the precipitate size distribution at a given temperature. The predicted compositions are in very good agreement with experimental results

Modelling the precipitation of NbC on dislocations in a-Fe

A model has been developed for describing the precipitation of NbC on dislocations in ferrite in an Fe–C–Nb steel. This model is a continuous description of the classical laws for nucleation growth and coarsening, which are adapted to the specific case where precipitates only form on dislocations. This model is successfully applied on an extensive data set obtained by small-angle neutron scattering for a wide temperature range and two alloy contents. Using this model, it is possible to estimate the effects of process parameters on the final microstructure and, notably, it is shown that the initial dislocation density has a pronounced influence on the maximum precipitate density

Ab Initio Calculations of Phase Stabilities in the Fe-Al-C System and CALPHAD-Type Assessment of the Iron-Rich Corner

The goal of this article is to improve the description of the Fe-Al-C phase diagram through the density-functional theory (DFT) calculation of the Fe3AlC compound. Several binary stoichiometric compounds in the iron-rich part of the ternary Fe-Al-C system and the ternary stoichiometric carbide Fe3AlC are studied ab initio. The results in terms of lattice parameter, bulk modulus, magnetization, and enthalpy of formation are compared with the calculated values available in the literature. Via a fitted entropy of formation of the ternary carbide, the Gibbs energy of formation of this compound is evaluated as a function of temperature. A CALPHAD-type procedure is then used to calculate ternary Gibbs isotherms at 800 °C, 1000 °C, and 1200 °C. The isotherms are compared with the experimental data of the literature

Imaging the stick-slip peeling of an adhesive tape under a constant load

Using a high speed camera, we study the peeling dynamics of an adhesive tape under a constant load with a special focus on the so-called stick-slip regime of the peeling. It is the first time that the very fast motion of the peeling point is imaged. The speed of the camera, up to 16000 fps, allows us to observe and quantify the details of the peeling point motion during the stick and slip phases: stick and slip velocities, durations and amplitudes. First, in contrast with previous observations, the stick-slip regime appears to be only transient in the force controlled peeling. Additionally, we discover that the stick and slip phases have similar durations and that at high mean peeling velocity, the slip phase actually lasts longer than the stick phase. Depending on the mean peeling velocity, we also observe that the velocity change between stick and slip phase ranges from a rather sudden to a smooth transition. These new observations can help to discriminate between the various assumptions used in theoretical models for describing the complex peeling of an adhesive tape. The present imaging technique opens the door for an extensive study of the velocity controlled stick-slip peeling of an adhesive tape that will allow to understand the statistical complexity of the stick-slip in a stationary case

Ferrite recrystallization and austenite formation during annealing of cold-rolled advanced high-strength steels: In situ synchrotron X-ray diffraction and modeling

Ferrite recrystallization and austenite formation occurring during annealing of cold-rolled advanced high-strength steels are key mechanisms as they largely determine the final microstructure and mechanical properties. However, the influence of processing parameters on these mechanisms and their interactions is still not fully understood. This is particularly the case for Dual-Phase steels having an initial cold-rolled microstructure con-sisting of ferrite and martensite before annealing, which were scarcely investigated compared to ferrite-pearlite initial microstructures. In situ synchrotron X-ray diffraction experiments together with post-mortem metallo-graphic analysis allowed clarifying both ferrite recrystallization and austenite formation during annealing of a ferrite-martensite initial microstructure depending on the process parameters of the annealing cycle. Results showed a major influence of recrystallization state on austenite formation, leading to an unexpected effect of heating rate on austenite formation kinetics. A modeling approach was undertaken to rationalize the influence of heating rate on austenite formation by taking into account the bi-phased ferrite-martensite initial microstructure and the effect of ferrite recrystallization state

What impacts of climate change on surface water in France by 2070? Results of the Explore2070 project in metropolitan France and overseas departments

International audienceLes questions relatives à la disponibilité et à la gestion de l'eau concentreront l'essentiel des mesures d'adaptation qui seront prises dans les décennies à venir pour faire face aux conséquences des changements climatiques. Ces mesures devront également prendre en compte les évolutions socio-économiques, en termes de démographie, d'aménagement du territoire et de politiques publiques (agriculture, énergie, transports, etc.). Il est dès lors primordial pour les décideurs et gestionnaires de quantifier les évolutions socio-économiques possibles, ainsi que le devenir de la ressource en eau et de sa variabilité temporelle et spatiale. C'est dans ce contexte que le projet Explore2070 avait pour objectif d'évaluer les impacts possibles des changements climatiques et socio-économiques futurs sur les grandes masses d'eau (surface, souterrain, littoral) et la biodiversité, en France métropolitaine et sur les départements d'Outre-mer (Guadeloupe, Guyane, Martinique et Réunion). Piloté par le Ministère de l'Ecologie (MEDDTL), ce projet a rassemblé de nombreux bureaux d'études et instituts de recherche pour dresser un panorama général des évolutions à attendre à l'horizon 2070. Par son ampleur et la variété de ses objectifs, ce projet doit permettre de répondre à de nombreuses questions des gestionnaires et de mieux apprécier les enjeux de ces évolutions. Dans la suite, nous présentons les travaux réalisés spécifiquement sur la quantification des évolutions des eaux de surface. Nous détaillerons dans ce qui suit la démarche générale de modélisation proposée, la nature des résultats obtenus, ainsi que la façon dont les incertitudes ont été quantifiées. Ce dernier point est indispensable pour aider à la prise de décision dans un avenir incertain. / Water availability and water management will be the focus of most of the adaptation measures that will be taken in the next decades to face the consequences of climate change. These measures will have to account for the socio-economic evolutions, in terms of population size, town and country planning, as well as public policies (agriculture, energy, transports, etc.). It is therefore essential for decision makers and managers to be able to quantify the possible socio-economic evolutions together with the evolution of water resources and their temporal and spatial variability. In this context, the Explore2070 project aimed at evaluating the possible impacts of future climate and socio-economic changes on water bodies (surface water, groundwater and coastal water) and biodiversity, in metropolitan France and overseas departments (Guadeloupe, Martinique, French Guyana, and Réunion Island). The project was managed by the French Ministry of Ecology (MEDDTL) and gathered several consultancies and research institutes to establish a general overview of the expected evolutions by 2070. Through both the extent and the variety of the project’s objectives, Explore2070 will provide answers to many questions raised by managers and better evaluate the stakes related to these evolutions. In the following, we present the work done specifically to quantify the evolution of surface water. The general modelling approach, the type of results and the way uncertainties were quantified are detailed. Uncertainty quantification is essential to help decision making in an uncertain future

Sélection de variables pour la classification par mélanges gaussiens pour prédire la fonction des gènes orphelins

Biologists are interested in predicting the gene functions of sequenced genome organisms according to microarray transcriptome data. The microarray technology development allows one to study the whole genome in different experimental conditions. The information abundance may seem to be an advantage for the gene clustering. However, the structure of interest can often be contained in a subset of the available variables. The currently available variable selection procedures in model-based clustering assume that the irrelevant clustering variables are all independent or are all linked with the relevant clustering variables. A more versatile variable selection model is proposed, taking into account three possible roles for each variable: The relevant clustering variables, the redundant variables and the independent variables. A model selection criterion and a variable selection algorithm are derived for this new variable role modelling. The interest of this new modelling for discovering the function of orphan genes is highlighted on a transcriptome dataset for the arabidopsis thaliana plant.Les biologistes s’attachent actuellement à prédire la fonction des gènes d’organismes de génome séquence à partir de données transcriptomes, issues de l’utilisation des puces à ADN. Le d´développement de cette technologie permet de tester l’expression de l’ensemble du génome dans de nombreuses conditions expérimentales. Cette quantité d’information peut alors sembler être un atout pour la classification des gènes. Pourtant il est courant que seul un sous-ensemble contienne l’information pertinente pour la classification. Les procédures de sélection des variables en classification non supervisée par mélanges gaussiens supposent généralement que les variables non informatives pour la classification sont soit toutes indépendantes, soit liées à des variables informatives. Nous proposons une nouvelle modélisation du rôle des variables plus polyvalente : les variables sont soit informatives pour la classification, soit redondantes, soit totalement indépendantes. Nous proposons un critère de sélection des variables et un algorithme pour cette nouvelle modélisation. L’intérêt de cette nouvelle modélisation pour la prédiction de la fonction des gènes orphelins est illustrée sur un ensemble de données transcriptomes obtenues chez Arabidopsis thaliana