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Articles:

Appolaire, Benoit, Perevoshchikova, Nataliya, Teixeira, Julien and Denis, Sabine
Solid State Phenomena, 172-174(Part 1):1214-1219
2011
ISSN: 1012-0394

Resume: We have adapted the Quickhull algorithm with the general dimension Beneath-Beyond algorithm [6] for computing the convex hull of the Gibbs energy hypersurface of multicomponent two-phase alloys. We illustrate the salient features of our method with calculations of isothermal ferrite-austenite equilibria in Fe-C-Cr. Finally, successive equilibrium calculations in a Fe-C-Cr-Mo steel over a large temperature range show the benefit of computing the convex hull before performing the conventional Newton-Raphson search.

Equipe: Département SI2M : Microstructures et Contraintes

Pushkareva, Irina, Redjaimia, Abdelkrim, Moulin, Antoine and Valle, Nathalie
Solid State Phenomena, 172-174(Part 1):839-844
2011
ISSN: 1012-0394

Resume: A detailed analysis of the evolution of industrial Dual Phase (DP) steel microstructures is carried out as a function of various annealing and tempering conditions. Advanced characterization techniques such as Parallel Electron Energy Loss Spectroscopy (PEELS) in the TEM and high spatial resolution Secondary Ion Mass Spectrometry (NanoSIMS) are employed in order to provide qualitative and quantitative measurements of local carbon concentration in the martensite. For certain annealing and tempering conditions, it is observed that local variations in carbon levels have occurred inside the individual martensite islands. These carbon variations strongly influence the damage behaviour of the steel. During tensile tests, a clear dependence of the damage mode on the local martensite carbon content is observed. Better knowledge of the relationship between the microstructure evolution at the sub-grain level and the damage behaviour can facilitate the design of DP steels with improved damage resistance.

Equipe: Département SI2M : Microstructures et Contraintes

Dehmas, M., Kovac, J., Aeby-Gautier, E., Appolaire, B., Denand, B. and Da-Costa-Teixeira, J.
Solid State Phenomena, 172-174:396-401
2011

Equipe: Département SI2M : Microstructures et Contraintes

Koutsoukis, T., Redjaïmia, A. and Fourlaris, G.
Solid State Phenomena, 172-174(Part 1):493-498
2011
ISSN: 1012-0394

Resume: Superaustenitic stainless steels exhibit excellent corrosion resistance, at a wide variety of exposure temperatures, especially in chloride containing environments, coupled with desirable mechanical properties. Previous studies have shown that these steels are prone to precipitation of secondary phases, such as sigma phase (sigma), chi phase (chi). Laves-phase, carbides, nitrides or secondary austenite, when exposed at elevated temperatures, directly affecting their mechanical properties and corrosion behaviour. A detailed study of the effect of isothermal ageing on the microstructure of S32654 (Fe-24Cr-21Ni-7Mo-0.5N-0.013C) and S31254 (Fe-20Cr-18Ni-6Mo-0.2N-0.012C) superaustenitic stainless steels was carried out. Samples were aged within the temperature range of 650 degrees C to 950 degrees C for times up to 3000 h. Following ageing, precipitation of secondary phases was clearly observed with precipitates varying in volume fraction, size, shape and spatial distribution. Several secondary phases were identified via transmission electron microscopy (TEM) and electron diffraction (ED). The orientation relationships between the austenitic matrix and the secondary phases were identified. Interaction and also phase transformation among different types of precipitates, such as between precipitates and the austenitic matrix were observed and an attempt of understanding these phase transformations was carried out.

Equipe: Département SI2M : Microstructures et Contraintes

Daoudi, M., Triki, A. and Redjaïmia, A.
Journal of Thermal Analysis and Calorimetry, 104 (2):627-633
2011

Equipe: Département SI2M : Microstructures et Contraintes

Berveiller, S., Malard, B., Wright, J., Patoor, E. and Geandier, G.
Acta Materialia, 59(9):3636-3645
2011
ISSN: 1359-6454

Resume: Two synchrotron diffraction techniques, three-dimensional X-ray diffraction and Laue microdiffraction, are applied to studying the deformation behaviour of individual grains embedded in a Cu(74)Al(23)Be(3) superelastic shape memory alloy. The average lattice rotation and the intragranular heterogeneity of orientations are measured during in situ tensile tests at room temperature for four grains of mean size similar to 1 mm. During mechanical loading, all four grains rotate and the mean rotation angle increases with austenite deformation. As the martensitic transformation occurs, the rotation becomes more pronounced, and the grain orientation splits into several sub-domains: the austenite orientation varies on both sides of the martensite variant. The mean disorientation is similar to 1 degrees. Upon unloading, the sub-domains collapse and reverse rotation is observed. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Equipe: Département SI2M : Microstructures et Contraintes

Hell, Jean-Christophe, Dehmas, Moukrane, Allain, Sebastien, Prado, Juscelino Mendes, Hazotte, Alain and Chateau, Jean-Philippe
ISIJ International, 51(10):1724-1732
2011
ISSN: 0915-1559

Resume: We elaborated two carbide-free bainitic steels with different microstructures through specific alloy design and austempering process. Microstructural characterizations were performed by means of EBSD analysis and in-situ high energy synchrotron diffraction in order to evaluate the phase fractions and the carbon content in the retained austenite, as well as the microtextures. These microstructural features were correlated to the tensile properties. Both steels exhibited an excellent compromise between high strength (above 1 250 MPa), good ductility (uniform elongation up to 14%) and high fracture strain (reduction of area up to 46%). The volume fraction of MA blocks (blocks of retained austenite partially transformed into fresh martensite during the final cooling at room temperature) was a key relevant parameter that strongly influenced the work-hardening at the expense of the damaging processes at high strain.

Equipe: Département SI2M : Microstructures et Contraintes

Borgese, L., Bontempi, E., Gelfi, M., Depero, L. E., Goudeau, P., Geandier, G. and Thiaudière, D.
Acta Materialia, 59(7):2891-2900
2011
ISSN: 1359-6454

Resume: Amorphous TiO2 thin films were deposited by means of atomic layer deposition on Kapton substrates and then crystallized ex situ by annealing at 300 degrees C to obtain the anatase phase. The morphology, structure and microstructure of films treated for 12, 24, 72 and 90 h were investigated. The local Ti coordination changes were studied by X-ray near-edge structure (XANES). On the basis of X-ray diffraction residual stress calculations, the elastic anisotropy of the films is experimentally determined for the first time (A*(comp) = 0.07, A*(shear) = 0.03). The film macro-strains increased with the time of treatment, while the micro-strains decreased. This effect may be correlated with the incipient anatase-to-rutile transformation as suggested by the changes observed in the XANES pattern of the film treated for 90 h. However, the contribution of the substrate cannot be excluded. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Equipe: Département SI2M : Microstructures et Contraintes

Di Napoli, P., Appolaire, B., Aeby-Gautier, E. and Bénéteau, A.
Solid State Phenomena, 172-174:1044-1049
2011

Equipe: Département SI2M : Microstructures et Contraintes

Grigorieva, Raisa, Drillett, Pascal, Mataigne, J. -M. and Redjaïmia, A.
Solid State Phenomena, 172-174(Part 1):784-790
2011
ISSN: 1012-0394

Resume: Nowadays more and more hot stamped steel sheets dedicated for the automotive body-in-white structure are pre-coated to prevent the steel surface against iron oxidation and decarburization during the austenitization step. For these applications, the coating is deposited by continuous hot-dipping the steel in an Al-Si bath. The Al-Si coating, at the delivery state, contains Al-grains, Al-Fe-Si ternary phases, Al-Fe binary phases. During the austenitization, the Al-Si coating transforms completely by inter-diffusion and solidification reactions. The mechanisms of Al-Fe-Si phase transformations at high temperature are almost unknown. The phase transformations occurring during austenitization define the final coating microstructure responsible for the in use properties of the product like spot welding, painting adherence or corrosion behavior. It is the aim of this paper to propose a new way of understanding the mechanisms of phase transformation in the Al-Si coating during the austenitization step (between 900 and 930 degrees C) before the transfer into the hot-stamping press.

Equipe: Département SI2M : Microstructures et Contraintes

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