Search

International audience; Small TiO2 contents in the slag-glass significantly reduce their cementitious reactivity. We investigated the role of Ti in the glass network and its influence on slag-glass dissolution. XANES and EPR analysis showed that 67% of Ti as Ti(IV) with about 70% in [5] Ti and 30% in [4] Ti coordination, and 33% as Ti(III). The presence of Ti had only a minor impact on initial dissolution rates at pH 11 (R ≈ 10-6 molglass.s-1 .m-2). During dissolution, Ti accumulated in an amorphous layer at the glass-surface. Ti-coordination in this layer was 50% [5] Ti and 50% [6] Ti. At pH 11, the layer was mainly composed of TiO2 (66 wt%), but intermixed with a hydrotalcitelike phase at pH 13. The loss of cementitious reactivity in the presence of Ti appears to be not only due to stabilization of the glass structure but also to the formation of a Ti-rich surfacelayer, that may become passivating.

International audience; The effect of additives on metal/oxide interfaces is explored in situ and in real time on evaporated films by a combination of surface science techniques, among which a very flexible optical method shows a unique ability to scrutinize the growth and wetting properties of supported clusters that involve several elements. The study focuses on Cr at the Zn/α-Al2O3(0001) interface at 300 K. A particular interest of the present interface is that Zn does not stick at all on bare alumina. The sticking and morphology of both Cr and Zn films during their growth are analyzed from sub-monolayer to multilayer thicknesses. After an initial oxidation reaction with residual OH groups, shown to be detrimental to Zn adhesion, Cr growth proceeds through the formation of high aspect ratio particles that percolate around an average thickness of 10 Å. With regard to Zn growth on a Cr deposit, two very distinct stages can be distinguished. In the sub-monolayer thickness range, Cr forms a seed layer that drastically increases the Zn sticking coefficient from 0 to nearly 1 due to a diffusion length of physisorbed Zn adatoms before desorption larger than Cr island separation; Zn clusters are anchored on the Cr seeds that they encapsulate, but their wetting behavior is dictated by the interaction with alumina. In a second stage, as soon as the Cr film percolates, it forms an adhesion layer on which Zn grows in a nearly two-dimensional mode. In all cases, Cr films are stable upon annealing. On Cr-covered alumina, the Zn desorption energy is enhanced as compared to bare surfaces, which, in line with atomistic simulations, is assigned to the formation of more favorable Cr–Al2O3 and Cr–Zn than Zn–Al2O3 bonds. Generally speaking, the ability demonstrated herein of small amounts of additives to dramatically increase the adhesion of films is of great practical interest. It shows that noncontinuous and partially oxidized films of additives, closer to realistic cases of application, can strongly enhance the sticking of films. Also, anchoring a functional film by discrete predeposited seeds can keep its properties intact.

Published in IFAC PapersOnLine, 54(11):61:66, 2021; International audience; In order to optimize the maintenance operations and limit the impact of material wearing on the galvanizing coating quality and defects, this paper proposes a data-driven model to estimate the remaining useful life of a roller system immersed in a galvanizing bath. The proposed model comes from the Hidden Markov Model family and is trained in non-supervised mode. The pros and drawbacks are given, in conclusion, for a possible use in a control loop.

International audience

Controlling austenite formation kinetics and its mean chemistry during annealing are two ways to act on the in-use and mechanical properties of high-carbon high-formability steels. Indeed, these characteristics influence austenite stability at room temperature after cooling and are therefore at the origin of potential TRIP effects under mechanical loading. Atom probe tomography analyses and Thermo-Calc/DICTRA calculations were used to understand how austenite forms during 1 °C/s heating of a 0.2 wt% C high-formability steel. The proposed analysis is conducted in the quaternary FeCMnCr system. It is shown that the transformation kinetics can be rationalized from the evolution of interfacial compositions, which are imposed by the operative tie-lines positions in the relevant phase diagrams. This analysis method helps to gain insight on the evolution of phases chemistries upon heating. Good agreement is found between experimental and calculated kinetics, but also between APT-measured and calculated phases chemistries. Mn and Cr partitioning between ferrite and austenite is confirmed during heating. APT also allows for the characterization of segregation on $$\alpha /\gamma$$ interphase boundary, showing that Mn segregation already occurred at 750 °C.

International audience; In situ X-ray Micro-Tomography (XMT) analyses with a pixel size of 0.325 µm were conducted on slag-based blended cements containing 75% of Ground-Granulated Blast-Furnace Slags (GGBS) and 25% of Ordinary Portland Cement (OPC), with or without CaCl2 acceleration. Results show the identification of the main cementitious phases and the need to perform data repeatability tests in order to allow their quantification. Investigation of the early hydration during the first 31 h of hydration by image subtraction showed (i) the dissolution of OPC/GGBS, (ii) the precipitation of hydrates including C-S-H, (iii) the accelerating effect of CaCl2, and (iv) phase-identification based on their specific grey level.

International audience; Small TiO 2 contents in the slag-glass significantly reduce their cementitious reactivity. We investigated the role of Ti in the glass network and its influence on slag-glass dissolution. XANES and EPR analysis showed that 67% of Ti was present as Ti(IV) with about 70% in [5] Ti and 30% in [4] Ti coordination, and 33% as Ti(III). The presence of Ti had only a minor impact on initial dissolution rates at pH 11 (R ≈ 10 − 6 mol glass .s − 1 .m − 2). During dissolution, Ti accumulated in an amorphous layer at the glass-surface. Ti-coordination in this layer was 50% [5] Ti and 50% [6] Ti. At pH 11, the layer was mainly composed of TiO 2 (66 wt%), but intermixed with a hydrotalcite-like phase at pH 13. The loss of cementitious reactivity in the presence of Ti appears to be not only due to stabilization of the glass structure but also to the formation of a Ti-rich surface-layer, that may become passivating.

International audience; The effects of boron segregation and austenite grain size on the bainitic isothermal transformation of a high-strength steel were studied independently. Dilatometry, microstructural observations and previous atom probe tomography analyses showed that bainitic transformation rate decreases with increasing boron excess at austenite grain boundaries. We also found that boron segregation causes an unexpected grain size effect: large austenite grains transform faster than small ones. A kinetic model assuming slow nucleation at austenite triple junctions and rapid growth of bainitic nuclei successfully described our experimental data. These results confirm that bainite nucleation is inhibited by segregated boron at grain boundaries. On this basis, austenitic grain size could be optimized to make the most of boron addition in advanced high-strength steels.

International audience; Electron Backscattering Diffraction (EBSD) provides important information to discriminate phase transformation products in steels. This task is conventionally performed by an expert, who carries a high degree of subjectivity and requires time and effort. In this paper, we question if Convolutional Neural Networks (CNNs) are able to extract meaningful features from EBSD-based data in order to automatically classify the present phases within a steel microstructure. The selected case of study is ferrite-martensite discrimination and U-Net has been selected as the network architecture to work with. Pixel-wise accuracies around ~95% have been obtained when inputting raw orientation data, while ~98% has been reached with orientation-derived parameters such as Kernel Average Misorientation (KAM) or pattern quality. Compared to other available approaches in the literature for phase discrimination, the models presented here provided higher accuracies in shorter times. These promising results open a possibility to work on more complex steel microstructures.

International audience

International audience

Published in IFAC-PapersOnLine, 54(11):55-60, 2021; International audience; The problem of reducing vibrations during continuous hot-dip galvanizing process is addressed. Using the finite difference method the concerned part of the steel production line is modeled by a state space version of the axially moving strip equation which takes into account disturbances that may affect its efficient functioning. The synthesis of an appropriate control law for this process aims to reduce the impact of these disturbances and its implementation requires a definition of the position and the number of the sensor/actuator allowing an optimal reduction. Some numerical simulation of the steel strip behavior are presented and discussed for different sources of vibrations with and without a control system.

Recovery of severely deformed ferrite was followed in situ by High Energy X Ray Diffraction during heating and isothermal holding experiments. Dislocation densities during annealing were determined by a modified Williamson Hall method. The deduced recovery kinetics was compared to post-mortem hardness measurements. A temperature dependent saturation of recovery was exhibited during holding. Dislocation density drop and saturation behavior cannot be reproduced simultaneously by the classical physically based models.

A strategy based on machine learning is discussed to close the gap between the detailed description of combustion chemistry and the numerical simulation of combustion systems. Indeed, the partial differential equations describing chemical kinetics are stiff and involve many degrees of freedom, making their solving in three-dimensional unsteady simulations very challenging. It is discussed in this work how a reduction of the computing cost by an order of magnitude can be achieved using a set of neural networks trained for solving chemistry. The thermochemical database used for training is composed of time evolutions of stochastic particles carrying chemical species mass fractions and temperature according to a turbulent micro-mixing problem coupled with complex chemistry. The novelty of the work lies in the decomposition of the thermochemical hyperspace into clusters to facilitate the training of neural networks. This decomposition is performed with the Kmeans algorithm, a local principal component analysis is then applied to every cluster. This new methodology for combustion chemistry reduction is tested under conditions representative of a non-premixed syngas oxy-flame.

International audience; The microstructural characterization of steels is key to optimize their processing route and ultimately better control their properties. It is of particular interest to classify, quantify and localize the different transformation products such as ferrite, pearlite, martensite and the different bainites inside the microstructure. This task requires a considerable amount of time and effort, and it is often performed by different experts, therefore carries a great degree of subjectivity. For years, the metallurgical industry has been looking for alternatives to accomplish this work, trying to minimize time, effort and subjectivity.Artificial intelligence (AI) based techniques are an upcoming and attractive option for materials characterization. Several studies have applied AI to this end on optical and scanning electron micrographs with good results in simple cases but moderate results in more complex cases 1–5. Microstructural images alone may not carry enough information to resolve ambiguous microstructures.Electron Backscattered Diffraction (EBSD) maps allow to access both the microstructural and crystallographic information of the microstructure. This allows one to exploit new features such as orientation relationship between the transformation products, habit planes or spatial distribution of the micro-constituents (packet, blocs, sub-block distribution) to improve the characterization 6–9. Few studies have started to implement machine learning on EBSD-based data for steels’ phase classification 10,11.The question we want to address is: would a neural network succeed in classifying the transformation products with EBSD maps and understand the intricate details that comes along with the orientation data? To answer this question, we propose the discrimination of ferrite and martensite in Dual Phase microstructures as a model study case. The present approach is based on a semantic segmentation strategy through the use of UNET architecture, a deep learning (DL) algorithm in the group of the convolutional neural networks (CNNs). Multiple models have been trained based on two main EBSD-based quantities: Pattern Quality Index and/or raw orientation data in the form of quaternions. Systematic assessment of the architectures hyper-parameters has been performed on a training set in order to optimize the performance. The trained models reached an average accuracy of 94% and above for the considered dataset. Good prediction/classification has been achieved by the models on microstructures with increasing complexity, providing promising results in several cases. Compared to other available approaches in the literature for phase discrimination, the model presented here provides higher accuracies.The proposed technique emerges as a powerful alternative to efficiently classify Dual Phase steel microstructures. Future studies will be dedicated to the extension of this approach to more complex microstructures where multiple transformation products appear.

International audience; The Cr/α-Al2O3(0001) interface has been explored by X-ray photoemission spectroscopy, X-ray absorption spectroscopy (XAS) and ab initio first-principles calculations of core level shifts including final state effects. After an initial oxidation via a reaction with residual surface OH but no reduction of the alumina substrate, Cr grows in a metallic form without any chemical effect on the initially oxidized Cr. However, Cr metal lacks crystallinity. Long-range (reflection high energy electron diffraction) and short-range (XAS) order are hardly observed. Thus photoemission combined with atomistic simulations becomes a unique tool to explore the chemistry and environment at the Cr/alumina interface. Cr 2p, O 1s and Al 2s shifted components are all explained by the formation of moieties involving Cr3+ and/or Cr4+ and of metallic Cr0, which supports the previously found Cr buffer mechanism for poorly adhesive metals. Beyond the situation under study, the present data demonstrate the ability of a combined experimental and theoretical approach of core-level shifts to exhaustively describe the general case of disordered metal/oxide interfaces.

International audience; Knowledge of phase transformation kinetics is a key point in designing steel grades, in particular modern high-performance grades, highly sought-after in energy and transportation applications. The design space for these grades is highly multi-dimensional given the numerous potential alloying elements. The characterization techniques that are usually relied on to assess transformation kinetics, such as metallography or dilatometry, are highly time consuming, due to their limitation to either a single transformation time or a single composition per experiment. The high-throughput approach showcased here overcomes those limitations by combining compositionally graded samples with time-and space-resolved in situ Xray diffraction, yielding full kinetic records over a range of compositions in a single run. Its application to low-alloy steel required addressing specific challenges related to the reactivity and thermodynamics of the material. The transformation of austenite into ferrite was chosen to illustrate its benefits. Using the rich resulting database, the transformation mechanism was examined quasi-continuously across sections of the composition space. Neither the paraequilibrium, nor the local equilibrium with negligible partitioning model, nor a transition from the former to the latter is applicable over the whole range of investigated conditions. Instead, the observed kinetics were explained by accounting for the solute drag exerted on the mobile interface. This work is a major contribution in accelerating the design of future low-alloy steel grades, involving the transformation of austenite into ferrite or any other transformation to which the present high-throughput methodology can be adapted.

International audience; Thanks to a dedicated modelling of intensities, the depth sensitivity of X-ray photoemission is used to probe the segregation profile of aluminium at the (110), (100) and (111) low index surfaces of the body-centred Fe 0.85 Al 0.15 random alloy. Sputtered surface composition is close to the nominal bulk one, thus excluding preferential sputtering. Surface enrichment in aluminium upon annealing starts at around 700 K before reaching a stationary state above 1000 K. The average surface composition is close to Fe 0.5 Al 0.5 , corresponding to the B 2 CsCl structure on the phase diagram. The impacted depth, that is in the range of 2.5-3.5 nm, is quite significant. Although not evidenced previously in surface science conditions at FeAl single crystal surfaces, it is qualitatively in agreement with the segregation at grain boundaries and shear planes of Al-alloyed steels. This segregation tendency is rationalized through ab initio calculations.

International audience

International audience; Cold rolled 0.1C-4.7Mn (wt-%) steel was submitted to double annealing. The holding time of second intercritical annealing at 650°C was varied between 3 min and 30 h. Tensile behavior after each treatment was measured and analysed. Microstructure characterisation was performed using field emission gun scanning electron microscope, transmission electron microscope and saturation magnetisation method. Moreover, interrupted tensile tests were done to obtain the kinetics of austenite destabilisation during straining. An important effect of soaking time on the microstructure and associate mechanical properties was revealed and analysed. Considering thermal and mechanical stability of retained austenite, the optimum combination of phases, providing the best strength-ductility balance, was found after 2 h holding.

CO2 sequestration by reaction with abundant, reactive minerals such as olivine has often been considered. The most straightforward, direct process consists in performing the reaction at high temperature and CO2 pressure, in view to producing silica, magnesium and iron carbonates and recovering the traces of nickel and chromite contained in the feedstock mineral. Most of direct processes were found to have an overall cost far larger than the CO2 removal tax, because of incomplete carbonation and insufficient properties of the reaction products. Similar conclusions could be drawn in a previous investigation with a tubular autoclave. An indirect process has been designed for high conversion of olivine and the production of separate, profitable products e.g. silica, carbonates, nickel salts, so that the overall process could be economically viable: the various steps of the process are described in the paper. Olivine particles (120 μm) can be converted at 81% with a low excess of acid within 3 h at 95°C. The silica quantitatively recovered exhibits a BET area over 400 m2 g-1, allowing valuable applications to be considered. Besides, the low contents of nickel cations could be separated from the magnesium-rich solution by ion exchange with a very high selectivity.

International audience; Automotive industry asks for higher resistant steels to lighten parts and improve crash resistance. Keeping a good ductility while increasing tensile strength requires the development of new grades in which hardening mechanisms counteract the drop in elongation when enhancing mechanical resistance. This is mainly achieved with multiphase steels and completing dislocation hardening by twinning and martensite transformation during straining. This has led to high-strength steel families, some of them being already used in body in white (Dual Phase (DP) and TRIP steels). Others, still in development, will soon emerge on the market (Quenched and Partitioned (Q&P), medium-Mn steels or TWIP).; L'industrie automobile demande des aciers plus résistants pour alléger les pièces et améliorer la résistance aux chocs. Le maintien d'une bonne ductilité tout en augmentant la résistance à la traction nécessite le développement de nouvelles nuances dans lesquelles les mécanismes de durcissement compensent la baisse de l'allongement en augmentant la résistance mécanique. Ceci est principalement réalisé avec des aciers multiphasés et en complétant le durcissement par dislocation par du maclage et une transformation martensitique pendant la déformation. Ceci a donné naissance à des familles d'aciers à haute résistance dont certaines sont déjà utilisées pour la caisse en blanc (aciers Dual Phase (DP) et TRIP). D'autres, encore en cours de développement, apparaîtront bientôt sur le marché (aciers trempés et partitionnés (Q&P), aciers à moyenne teneur en manganèse ou TWIP).

International audience

International audience; A new generation of iron-based matrix composite reinforced by TiB2 particles was deformed in tension to investigate at a mesoscopic scale the localization of plastic deformation in relation with characteristic microstructural features of the composite (in particular, ferrite grain boundaries and particle/matrix interfaces). Large Electron Back-Scattered Diffraction (EBSD) maps with improved angular resolution were acquired to evaluate statistically the evolution of the geometrically-necessary dislocation (GND) density from the early stage of the deformation. GNDs were found to accumulate preferentially at matrix/particle interfaces with hot-spots located at the tips of elongated particles. Additional length-scale parameters derived from EBSD data evidenced the key influence of two main microstructural features: the particle morphology and the particle clustering. Finally, we present results of an advanced full-field micromechanical model that is best suited to capture these effects, based on an enhanced crystal plasticity elastoviscoplastic Fast Fourier Transform (EVP-FFT) formulation coupled with a phenomenological continuum Mesoscale Field Dislocation Mechanics (MFDM) theory. By taking the experimental TiB2 particle distribution into account, the model describes qualitatively the observed effect of particle morphological features on the heterogenous distribution of GNDs.

Acta materialia 212, 116920 (2021). doi:10.1016/j.actamat.2021.116920, The manufacturing of Dual-Phase steels includes as a crucial step the annealing of a cold-rolled ferrite-pearlite (F/P) microstructure, which involves numerous and interacting metallurgical mechanisms, namely recovery/recrystallization of ferrite, globularization, manganese enrichment, coarsening of cementite and finally austenite transformation. Present study focuses on the austenite transformation considering its interaction with the ferrite recrystallization and the influence of the chemical composition of the cementite. The behavior of a cold-rolled F/P microstructure is studied at three heating rates to induce weak and strong interactions between the mechanisms, in particular using post mortem microstructure observations but also in situ High Energy X-Ray Diffraction experiments on a synchrotron beamline. Slow heating leads to a necklace austenite distribution whereas fast heating conducts to a banded topology. This particular microstructure morphogenesis is explained by the presence of numerous intergranular (or isolated) carbides inside the ferrite matrix, inherited from the hot-rolling. Thermokinetic analysis accounting for the cementite composition shows that the pearlite islands transformation necessarily involves the partition of substitutional elements. Conversely, the dissolving isolated carbides undergo a partition/partitionless transition on heating. After the dissolution of the cementite, a final ferrite/austenite transformation takes place. The phase transformation kinetics increases with increasing heating rates, despite the thermal-activated nature of the austenite growth process. This is interpreted thanks to kinetic simulations with DICTRA software, which allow to analyze the austenite growth regimes involving or not the partition of the alloying elements., Published by Elsevier Science, Amsterdam [u.a.]

International audience; The aim of this paper is to investigate the effective properties of Fe–TiB2 composites obtained after hot or cold rolling. The elastic moduli of both hot and cold rolled composites are measured experimentally using several methods. Microstructure analyses based on SEM observations are performed to characterize the distribution of particles and cracks, and are then used to generate 3D representative microstructures using the RSA method. This allows the numerical determination of the overall elastic behavior of Fe–TiB2 composites using full-field FFT-based simulations. In addition, Young’s moduli of the hot rolled Fe–TiB2 composites are also determined analytically using the mean-field homogenization scheme of Mori–Tanaka. The elastic properties determined experimentally, analytically and numerically are in a good agreement. Overall, a significant improvement of the specific stiffness in comparison to standard steels is achieved irrespective of the processing conditions

The iron and steel industry is one of the most energy-intensive industrial sectors, which makes a significant worldwide contribution to resource and energy consumption, as well as to carbon dioxide emissions. Up to date, much effort has been put into increasing the global efficiency of the internal production processes. Still, the potential for improvements tends to be more and more limited at the level of a steel plant. That is why many methodologies and tools have been, and are still being, developed to study industrial symbiosis in such areas, but most of them propose a static view. Indeed, typically integrated iron and steel plants have a significant role to play in industrial symbiosis. However, the question remains: how to evaluate the economic and environmental benefits or impacts of industrial symbiosis on such industrial sites? Therefore, the objective of this study is to propose a new methodology based on the concepts of system dynamics while providing a systematic approach for assessing these industrial symbiosis performances. This approach relies on a dynamic model resulting from the combination of a physicochemical sub-model and a system dynamics sub-model. This model built and applied a whole steel plant is fed by all resource and energy flows (i.e., not limited to one process but considering all processes). This dynamic model is tested and validated over a two-year simulation period using different statistical tests that show its relevance to industrial symbiosis scenario simulations involving complex systems fed by variable resource and energy flows. This proposal can thus be used to simulate the substitution or exchange of any material and energy streams of steel plants while allowing the economic and environmental benefits or impacts to be quantified from any industrial symbiosis configuration.

This paper presents an application to metal matrix composites (MMCs) of an enhanced elasto-viscoplastic Fast Fourier Transform (EVP-FFT) formulation coupled with a phenomenological continuum Mesoscale Field Dislocation Mechanics (MFDM) theory. Contrary to conventional crystal plasticity, which only accounts for plastic flow and hardening induced by statistically stored dislocations (SSDs), MFDM-EVP-FFT also describes the evolution of polarized geometrically necessary dislocation (GND) density and its effect on both plastic flow and hardening. Numerical results for a Fe–TiB2MMC made of a ferrite matrix (α-Fe) and elastic ceramic particles (TiB2) are presented. Full-field simulations are performed using synthetic periodic unit cells representative of the MMC, with single-crystalline and polycrystalline matrix, for different particle interspacing distances. A strong dependence of the predicted equivalent stress, cumulated plastic strain and GND density fields with particle interspacing distance is observed, in contrast with conventional crystal plasticity. Correlations between these mechanical fields and microstructural features, and their influence on local and global mechanical behavior are examined for the different MMC microstructures.

International audience; Ground granulated blast furnace slags (GGBS) are glasses (>99%) of the CaO-Al 2 O 3-SiO 2 compositional system and are widely used as supplementary cementitious materials. Differences in reactivity of GGBS were screened by modifying the content of 11 minor elements (namely Ba, Ce, Cs, Cr, Mn, P, Sn, Sr, Ti, V, Zr). SEM observations showed that most elements entered the silicate glass matrix, only Sn was reduced to its metallic form and P accumulated in minor minerals. Mortar strength tests showed that 2d-compressive strength was reduced by > 50% for a TiO 2 content of 2.5 wt.% in the slag. At 28 days the loss in compressive strength was still > 40%. Calorimetric tests on other element additions showed that network modifiers (Ba, Cs and Sr) and GGBS reactivity are positively correlated, whereas Ce, Cr, V and Zr significantly decreased reactivity. It is shown that these effects can be well estimated by the weighted field strength of the added element.

The evolution of the dislocation densities in martensite and in austenite during the quench of a low-carbon (0.215 wt% C) steel is investigated in situ by the mean of a High Energy X-Ray Diffraction experiment on a synchrotron beamline. The line configuration offers an excellent time resolution well adapted to the studied martensitic transformation kinetics. The mean density of dislocations in martensite increases as the transformation proceeds confirming that dislocations are not homogeneously distributed between the laths in agreement with some recent post-mortem observations. The resulting spatial distribution of dislocations and the associated strain-hardening support the views assuming that lath martensite is a heterogeneous microstructure and behaves as a “multiphase” aggregate. In austenite, the increase in dislocation densities is even more significant meaning that austenite in martensite is also a hard phase, contradicting some recent theories attributing to films of retained austenite a major role in the plasticity of martensite.

International audience; We present a time-of-flight tomography method for exceeding the naïve axial (i.e., depth) resolution limit of terahertz (THz) deconvolution by autoregressive spectral extrapolation (AR) based on the modified covariance method (AR/MCM). In contrast to Wiener filtering combined with wavelet denoising, AR/MCM does not discard any frequency components in the low signal-to-noise (SNR) regions of the measured data, and unlike the AR approach based on the Burg method (AR/BM), no peak splitting (single peaks in the impulse response function appearing as double peaks) as well as frequency bias (spectral peaks shifted with respect to their correct positions) is observed after deconvolution. After verifying the advantages of AR/MCM over Wiener filtering in conjunction with wavelet denoising as well as over AR/BM, using synthetic data, AR/MCM is employed to reconstruct a single layer of mill scale on a steel coupon from experimental THz time-of-flight tomography data. The reconstruction shows good agreement with the film thickness obtained from destructive cross-sectional measurements. In addition, unlike AR/BM, optimizing the parameters to obtain stable reconstruction is straightforward relying of Akaike’s information criterion suggesting that AR/MCM can be an easier to implement for THz nondestructive characterization of stratigraphy under noisy conditions, particularly when estimates of the stratigraphy may not a priori be available.

International audience; We measure in a nondestructive and noncontact fashion the thicknesses of three scale films with thicknesses 28.5 ± 1.4 μm, 13.4± 0.9 μm, and 5.1 ± 0.3 μm on steel substrates employing terahertz time-of-flight tomography combined with advanced signal-processing techniques. Wüstite is the dominant phase in the scale films, though magnetite and hematite are also present. Because wüstite is electrically insulating, the incident terahertz electromagnetic pulses largely penetrate into the scale film; however, the pulses are entirely reflected by the underlying electrically conductive steel substrate. Because the film layers are thin, in some cases optically thin, the distinct pulses reflected at the air/scale and scale/steel interfaces overlap in time and thus are not visually evident in the reflected terahertz signal, necessitating the use of deconvolution techniques to recover the sample structure. We compare the merits of three deconvolution techniques, one unsuccessful (frequency-wavelet domain deconvolution) and two successful (sparse deconvolution and autoregressive extrapolation), to characterize the thicknesses of these scale films.

International audience; The formation of an ultra-thin aluminum oxide film at Fe0.85Al0.15(110) surface (A2 random alloy) has been studied by a variety of surface sensitive techniques (X-ray photoemission, low-energy electron diffraction, surface X-ray diffraction and scanning tunneling microscopy) supplemented by ab initio atomistic simulations. Since iron is not oxidized in the used conditions, the study focused on the coupling between aluminum oxidation and segregation processes. Compared to the bare surface, whose average composition (Fe0.6Al0.4) is closer to the B2-CsCl structure over a ∼ 3 nm depth, the oxidation hardly affects the subsurface segregation of aluminum. All the structural and chemical fingerprints point to an oxide film similar to that found on NiAl(110). It is a bilayer (∼ 7.5Å thick) with a composition close to Al10O13 and a large (18.8 × 10.7)Å 2 nearly rectangular unit cell; an almost perfect match between substrate periodicity and the (1 × 2) oxide supercell is found. Nevertheless, microscopy reveals the presence of anti-phase domain boundaries. Measured Al 2p and O 1s core level shifts match calculated ones; their origin and the relative contributions of initial/final state effects are discussed. The ubiquity of the present oxide on different supports asks for the origin of its stability.

International audience; Annealing of cold-rolled high-strength steels leads to various microstructural changes such as recrystallization, cementite precipitation, microalloying elements precipitation and austenite formation. These transformations are expected to interact with each other. Understanding how and where austenite forms in a microstructure is of prime importance to avoid formation of banded microstructures, which are detrimental to good in-use properties. In this work, a mean-field model is used to simulate concomitant recrystallization, cementite precipitation, microalloying elements precipitation and austenite formation kinetics, as well as their interactions during 1 and 10 °C/s heating. Excellent agreement with experimental data is obtained only if cementite pinning effect on recrystallized grain boundaries is considered. It is shown that cementite exhibits a much stronger delaying effect on recrystallization kinetics than microalloying elements, leading to the formation of banded microstructures. Carbon nominal content of a steel appears to be the most important parameter to acknowledge to understand recrystallization kinetics.

Based on the evolution of the positions and intensities of the diffraction peaks, high energy X-ray diffraction (HEXRD) is recognized as the ultimate method to follow quantitatively in situ phase transformations in steels. However, the possible asymmetricity of diffraction peaks is seldom considered, and is known to bear information. A procedure for quantifying their skewness is proposed. In the case of a third generation high strength steel obtained by quench and partitioning (Q&P), the skewness is shown to be due to carbon heterogeneities at austenite/martensite interfaces developed at nanoscale, in agreement with prior post mortem atom probe tomography (APT) investigations.

International audience; Alsingle bondSi coated boron steels have been of interest for many years because of their superior mechanical properties and their excellent oxidation resistance. These high strength steels are particularly used in hot stamping processes during which the Alsingle bondSi coating undergoes multiple microstructural transformations. In this study, morphology and structure transformations are investigated using an original approach. Most of the time, the coating evolution is studied in cross section, by ex situ characterization of its different layers after heat treatment. In this work, the evolution of the surface is explored for the first time using in situ high-temperature Environmental Scanning Electron Microscopy (HT-ESEM). The austenitization step reproduced in the ESEM chamber allows a precise description of several surface changes occurring, depending on the temperature range. Among them, the main change in morphology is occurring between 650 and 800 °C and is linked to different reactions between aluminium and bi- and ternary Fe-Al-Si phases. The heating rate is also pointed out as a key parameter that affects the surface morphology. In fact, with low heating rates the surface is mostly composed of hexagonal and rectangular elements coming from τ5 (Fe2Al8Si) structuration, while needle-shaped FeAl3 structures are found for higher heating rates. In addition, it is observed that the heating rate also affects the surface roughness depending on the surface morphology. Finally, the origin of the presence of micrometrics pores appearing between 800 and 900 °C at the coating surface is discussed. Our hypothesis supported by ex situ characterization is that they result from formation of a crystallized oxide layer, probably close to α-Al2O3 structure.

International audience; Formation of mixed compounds at interfaces between materials often has significant impact on the properties of the junction. With the goal of assessing if interfacial oxide mixing improves the adhesion of anti-corrosive zinc coatings on silica, we report an ab initio study of the structural, electronic, and adhesion characteristics of interfaces between zinc and zinc silicate. We show that, regardless the precise thermodynamic conditions, ZnO-rich zinc sili-cate interfaces are more stable than SiO 2-rich ones because their formation does not require breaking strong Si-O bonds. Moreover, we find that zinc adhesion at such ZnO-rich interfaces is by at least a factor two larger than that of the zinc/silica contact. Due to a dense network of interfacial Zn-O bonds, this improvement does not produce any unfavorable decohesion effects in the zinc deposit. The formation of an interfacial silicate layer enables the suppression of the weak zinc/silica interface, which is responsible for the poor zinc adhesion. The cohesive cleavage within zinc predicted for the complex zinc/silicate/silica junction offers a promising improvement of performances of anti-corrosive zinc coatings of Si-rich steel grades, of crucial importance in steel-making and automotive industries.

International audience; Ground granulated blast furnace slag (GGBS) is a glassy by-product of pig iron production and is commonly used in concrete industry to replace cement and thereby lower the carbon footprint of the material. Large variations in reactivity exist depending on the GGBS physical and chemical features. Here we investigate the ability of three rapid calorimetric methods to evaluate the reactivity of GGBS. On a set of 16 industrial GGBS, we show that 24 h heat release, using the R3-protocol, correlates well with 2d compressive strength of standard mortars using 75 wt% GGBS. The correlation of R3-test results (R2 = 0.87) is better than for traditional reactivity indices calculated from chemical composition. Furthermore, we present data on the repeatability of the test protocol and show that the R3-protocol is very sensitive to sample fineness. Finally, XRD patterns show that slight differences in phase assemblage exist between the most and least reactive GGBS.

International audience

International audience; An original mean field model for the nucleation and the growth of new recrystallized grains during annealing treatments of deformed, low-carbon ferritic steels is proposed in this paper. The model was calibrated on two steels extensively studied in the literature under both isothermal annealing and continuous heating schedules. It permits one to predict not only recrystallization kinetics but also advanced microstructural features (such as dislocation density, dislocation cell size and grain size) during complex heat treatments. Once calibrated, the model was applied to the case of a third ferrite/pearlite steel and was shown to accurately capture the effect of cold-rolling ratio on the recrystallization kinetics.

International audience; Based on previous photoemission findings, the influence of newly observed aluminum surface segregation at the low (110), (100) and (111) index surfaces of the A 2 body-centered Fe 0.85 Al 0.15 random alloy is explored. Scanning Tun-neling Microscopy, Low-Energy Electron Diffraction and Grazing-Incidence X-Ray Diffraction are combined to explore surface structures and topogra-phies. The formation of a surface composition close to the B 2 Fe 0.5 Al 0.5 occurs with the appearance of (i) a long-range pseudo-hexagonal incommen-surate superstructure with a periodicity of ∼ 18Å on the (110) surface, (ii) a (1 × 1) termination on the (100) surface and (iii) a intense vicinal faceting of the (111) surface in the form of triangular pits.

International audience; The understanding of interfacial effects and adhesion at oxide-metal contacts is of key importance in modern technology. Metal-silica interfaces specifically are relevant in electronics, catalysis and nanotechnology. However, adhesion at these interfaces is hindered by a formation of siloxane rings on the silica surface which saturate the dangling bonds at stoichiometric terminations. In this context, we report a thorough density functional theory study of the interaction between β-cristobalite and selected 3d transition metals under different oxygen conditions. For any given interface stoichiometry, we find a progressive decrease of the metal/silica interaction along the series, following the increase of metal electronegativity. Crucially, in presence of early transition metals (Ti or Cr) the surface siloxane rings are spontaneously broken, allowing for strong adhesion. Late transition metals interact only weakly with reconstructed surfaces, similarly to what was found for zinc. In absence of reconstruction, stoichiometric silica/metal contacts behave similarly to alumina/metal contacts, but display larger interactions across the interface. Based on these results, we show that early transition metal or stainless steel buffers can significantly improve the weak adhesion between silica and zinc, responsible for a poor performance of anti-corrosive galvanic zinc coatings on modern advanced high strength steels.

International audience; High resolution imaging by secondary ion mass spectrometry and atom probe tomography have been employed to investigate boron segregation at austenite grain boundaries (γ GBs) after soaking in a high-strength low-carbon steel. The combined use of these two analytical techniques is shown to be powerful for quantifying solute and segregated boron levels. Quenching was performed after soaking aiming to clarify the temperature effect on boron distribution under thermal equilibrium. Boron depletion in the γ GBs vicinity was observed in the as-quenched states from high temperatures, suggesting that the cooling rate was not fast enough to limit boron diffusion during cooling. We found that boron segregation at γ GBs increases with temperature. This is due to the increase of solute boron concentration in the grains, resulting from boride precipitate dissolution. It appears that the segregation magnitude still follows the equilibrium laws as a function of temperature. From our investigations, it was possible to determine the boron equilibrium segregation enthalpy. These results have important practical consequences for controlling the levels of segregated boron in steels.

The mechanisms behind the carbon enrichment of austenite during quenching and partitioning are still a matter of debate. This work investigates the microstructural evolution during the quenching and partitioning of a model Fe&ndash, C&ndash, Mn&ndash, Si alloy by means of in situ high energy X-ray diffraction (HEXRD) atom probe tomography, and image analysis. The ultra-fast time-resolved quantitative information about phase transformations coupled with image analysis highlights the formation of carbide-free BCT bainite, which is formed within a very short range during the reheating and partitioning step. Its transformation rate, which is a better indicator than the intrinsic volume fraction, depends on the quenching temperature (QT). It is shown to decrease with decreasing QT, from 45% at QT = 260 &deg, C to 20% at QT = 200 &deg, C. As a consequence, a significant part of the carbon enrichment observed in austenite can be attributed to bainite transformation. Furthermore, a large part of carbon was shown to be trapped into martensite. Both the formation of Fe2.6C iron carbides and the segregation of carbon on lath boundaries in martensite were highlighted by atom probe tomography. The energy for carbon segregation was determined to be 0.20 eV, and the carbon concentration on the lath boundaries was obtained to be around 25 at %. Therefore, the carbon enrichment of austenite is the result of competitive reactions such as carbon partitioning from martensite, bainite transformation, and carbon trapping in martensite.

The interactions of polymers (corn starch, dextrin from maize starch, humic acid sodium salt and sodium carboxymethyl cellulose) with iron oxides (hematite and magnetite) have been investigated by measuring adsorption isotherms and by electrophoresis. According to the electrophoresis measurements at pH 7 both iron oxides present negative surface charge and positive at pH 5. The equilibrium adsorption isotherms were then determined at pH 7 for all the adsorbates except for humic acid which was studied at pH 5, due to its anionic characteristics. The equilibrium data of both iron oxides were studied using Freundlich and Langmuir models and it was found to best fit to the Freundlich one. The values for Freundlich constants indicate that the mechanism that contributes most to the adsorption process in all experiments was the hydrogen bonding. However, the coexistence of more than one adsorption mechanism is what best explains the process itself, in addition to explaining the differences found amongst theories over the years. Keywords: Hematite, Magnetite, Depressant, Adsorption, Isotherm

International audience; Natural rubber (NR) is the most commonly used elastomer in the automotive industry thanks to its outstanding fatigue resistance. Strain-induced crystallization (SIC) is found to play a role of paramount importance in the great crack growth resistance of NR (Lindley, Int J Fracture 9:449–462, 1973). Typically, NR exhibits a lifetime reinforcement for non-relaxing loadings (Cadwell et al., 1940; Ruellan et al., 2019). At the microscopic scale, fatigue striations were observed on the fracture surface of Diabolo samples tested in fatigue. They are the signature of SIC (Cadwell et al., 1940; Le Cam et al., 2013; Le Cam et al., 2004). In order to provide additional information on the role of SIC in the fatigue crack growth resistance of NR, striations are investigated through post-mortem analysis after fatigue experiments using loading ranging from −0.25 to 0.25. No striation was observed in the case of tests performed at 90 °C. This confirms that the formation of striation requires a certain crystallinity level in the material. At 23 °C, two striation regimes were identified: small striation patches with different orientations (Regime 1) and zones with large and well-formed striations (Regime 2). Since fatigue striations are observed for all the loading ratios applied, they are therefore not the signature of the reinforcement. Nevertheless, increasing the minimum value of the strain amplified the striation phenomenon and the occurrence of Regime 2.

International audience; A novel methodology is developed for predicting the thermal impact of fouling in Steam Generators (SG). The originality of this methodology is to resort to fractal and statistical theories to depict the porous structure of the deposits. The proposed Statistical Fractal methodology (SF) accounts for the heat transfer driven by the liquid-vapor phase change inside the deposits. It simulates the complex intricate networks of sinuous open pores of different scales, with liquid inflows (capillaries) and vapor outflows (steam-chimneys). The multi-layered representation of fouling deposits allows to mimic aging mechanisms such as densification which occur during SG operation.The SF predictions are consistent with experimental data. The deposit thickness and the profile of porosity are found to be the most influential fouling properties on the heat exchange. The methodology is capable to simulate the experimentally observed heat transfer enhancement for thin and porous deposit as well as the heat exchange decline for thick and dense deposit.

International audience; Spectral emissivity measurement (λ = 8–12 μm) of TRIP alloy steel during annealing in HNx atmosphere is carried out. Emissivity variation is contributed by the selective oxidation that takes place along with the surface topography from pre-annealed state. Comparing results from physicochemical characterization (Fourier transform infrared spectroscopy, scanning electron microscopy and optical surface profiler) on post-annealed samples and examining the global and relative spectral emissivity separately established correlation of roughness to the former and oxide signatures to the peak features in the latter. Intensity of emissivity peaks at 10 μm and 10.5 μm vary with ratio of Mn to Al. Meanwhile, the influence of local texture on emissivity is demonstrated.

International audience; An experiment was developed to simulate the interface between liquid high manganese carbon steel and solid ultra-low alloyed steel in a thermal gradient, corresponding to the nugget / sheet interface of resistance spot welds. Despite the large difference in the interaction time between the experiment and the welding process, identical microstruc-tures and phases were observed in both. The time and temperature control of the experimental setup indicates that the solute diffusion in the solid and liquid phase together with the interface temperature when cooling determines the microstructure formation and morphology. The presence of-martensite requires a γ / liquid interface with a manganese-driven dissolution. A criterion based on manganese supersaturation is proposed and shows that with the studied steels the nucleation and growth of-martensite is almost unavoidable when using resistance spot welding. The cellular patterns observed during solutal melting at low interface temperature increase the interface surface and the amount of martensite. The origin of the cells is the carbon enrichment in the manganese diffusion zone of the γ phase that provokes a solidus temperature gradient in the thermal gradient.