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Articles:

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

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

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

Faurie, D., Renault, P. -O., Geandier, G. and Le Bourhis, E.
Thin Solid Films, 520(5):1603-1607
2011
ISSN: 0040-6090

Resume: Synchrotron x-ray radiation was used for in situ strain measurements in gold films on polyimide substrate during uniaxial deformation tests. We have used an area detector that allows inspecting multiple directions in the polycrystalline thin film without serial sectioning during straining. We show in this paper the configuration used and the attainable orientations on a pole figure for which the x-ray strains are measured. Moreover, we show the effect on the x-ray strains of the onset on plasticity, which was not detected by optical (macroscopic) strain measurement. (C) 2011 Elsevier B.V. All rights reserved.

Equipe: Département SI2M : Microstructures et Contraintes

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