Papers by Author: Françoise Bley

Abstract: Fully austenitic steels of the Fe-Mn-C system can show extensive deformation twinning (TWIP effect). The deformed microstructure of such steels has been analysed using X-ray diffraction at the European Synchrotron Radiation Facility (ESRF). The experimental diffractograms, recorded using a 2D CCD camera, are analysed in terms of Bragg peak profiles (broadening and asymmetry) and position (shift from the reference [undeformed state] position) leading to an estimation of dislocation and stacking faults densities.

Abstract: This paper presents a physically based precipitation model which aims at describing precipitation kinetics when it occurs exclusively on dislocations. We present specific nucleation, growth and coarsening equations, which are integrated in a set of differential equations. This model is successfully applied to the case of precipitation of NbC in a ferritic steel, whose kinetics has been determined by small-angle neutron scattering.

Abstract: Carbon aerogels are very promising substrates for electrocatalyst deposition involved in fuel cells. Their advantage over high surface area carbon blacks currently used, is the porous monolithic structure yielding large pore volumes with controlled pore sizes. By changing the synthesis parameters, it is possible to adjust their multi-scale structure which is strongly related to the electrochemical performances. The aim of the lecture is to give a survey of information about the multi-scale structure that can be obtained by small and wide angle X-ray scattering (SAXS and WAXS) techniques combined with contrast variation (CV). To this end, a series of SAXS experiments on carbon aerogels are described and the analysis of the experimental data is explained. Particular attention is paid to the determination of the specific surface area, SSAXS, and to the reasons why WAXS curves combined to SAXS ones make this determination more pertinent. The physical meaning of similarity or difference between SSAXS and surface area determined by gas adsorption, SADS, is discussed and information obtained by using contrast variation (CV) is described for two carbon aerogels prepared in different conditions.

Abstract: This paper presents two studies illustrating the possibilities of Small-Angle X-ray Scattering for characterising quantitatively the state of precipitation in aluminium alloys. In the first example, maps are presented, of precipitate size and volume fraction in the cross-section of friction stir welds of AA7449 alloy. It is shown that the influence of welding speed on the distribution of mechanical properties can be understood using this microstructural data. In the second example, the precipitation kinetics in an Al-Zr-Sc alloy is evaluated by in-situ small angle X-ray scattering. Evidence is given for the heterogeneous chemical structure of the Al3(Zr,Sc) precipitates, consisting of a Zr-rich shell surrounding a Sc-rich core. It is shown that this particular distribution results in a very good resistance to coarsening of the precipitate microstructure.

Abstract: The deformation mechanisms of an Fe-Mn-C TWIP steel have been investigated as a function of deformation and deformation temperature, using synchrotron X-ray diffraction at the European synchrotron radiation facility. Using the Warren theory, it is possible to reach a good qualitative understanding of the deformation mechanisms. We have confirmed that the deformation mechanisms shifted from the formation of martensite at very low temperature, to twinning around room temperature and dislocations at higher temperatures. Although some quantification of the density of crystalline defects can be reached using simple parameters such as peak shift and broadening, the complexity of defects present in this material require the development of more advanced data interpretation models. First results are shown, using shift and broadening of the peak and fit of intensity by a pseudo-voigt function, as well as the study of the asymptotic behavior of the intensity.