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  • study of the ternary phase diagram Fe-Al-C (Networking) [achieved]
    Description: It has been largely reported that aluminium-alloyed steels can be hardened by a dispersion of kappa-Fe3AlC carbides. The micro-structure of such steels has been recently investigated in details. However, to control the precipitation micro-structures of steels and also to master the carburising process of Fe-Al alloys, it is necessary to rely on the thermodynamical properties of the iron-rich phases in the Fe-Al-C system, and to know the fundamental properties of these phases. However few and contradictory experimental and theoretical informations are present in the literature about the kappa carbide. As a matter of fact, if the phase equilibrium has been studied in high Mn containing steels, no detailed description of the iron-rich part of the Fe-Al-C phase diagram is available in the literature. First at all, we want to complete the CALPHAD databases (”Calculation of Phase Diagrams”) used to describe the ternary phase diagram Fe-Al-C. CALPHAD method consists in an optimization of the physical parameters on experimental data. Among these parameters, the evaluation of the formation energies of different phases in equilibrium in the phase diagram is needed. Ab initio calculations have been then used to determine them. In a second stage, we want to understand why experimenters don’t identify stoichiometric kappa-Fe3AlC structure but an off stoichiometric system Fe3AlC(1/2). In this way, we have studied the evolution of the formation energy of Fe3AlCx when x varies between 0 to 1.
    Results: We have already finished to generate the CALPHAD database for the Fe-Al-C phase diagram. It was necessary to calculate the formation energies of the different ordered systems in equilibrium in the phase diagram (kappa-Fe3AlC, Fe3Al-L12, Fe3Al-D03, L10-AlC etc.), but we have too simulated desorded binary FexAl(1−x) structures. To simulate such desorded FexAl(1−x) binaries (fcc and bcc structures), we have employed the SQS (‘Special Quasirandom Structures’) framework as proposed by A. Zunger. A Calphad description of the system has been then assessed which is compatible with general databases. We have too finished to study the evolution of the formation energy of the kappa phase with respect to the carbon concentration, and we shown that in accordance with experimental facts, Fe3AlC(1/2).
  • study of the quaternary Ni/Nb/Fe/Cr phase diagram (Networking) [achieved]
    Description: We propose to simulate the quaternary phase diagram of Ni-Nb-Fe-Cr by means of CALPHAD-type assessment. Emphasis was put on the phases that are stable in different ternary phase diagrams (Fe-Ni-Nb, Fe-Ni-Cr, Fe-Nb-Cr, Ni-Nb-Cr), namely fcc-A1, bcc-A2, C14-laves, D85, L12-fcc. We took also into account metastable C15-laves and D022-ortho. Besides ab initio calculations have been performed using VASP package in order to help for estimating enthalpies of formation of some CEF compounds.
  • First principle study of Raman in insolators (Networking) [planned]
    Description: Infrared and Raman spectroscopy is widely used to determine and investigate the structure of the finely divided mineral phases, which typically form in soils and weathering environments. It provides information ranging from the detection and identification of specific or minor mineral constituents, hardly accessible from X-ray diffraction techniques, to the determination of the stacking order and ordering pattern of substituting cations in clay minerals. Other applications include the mineralogical quantification in complex samples, the detection of mineral transformations in geological environments and the investigation of the interfacial properties of minerals. Infrared spectroscopy can be performed using a large variety of experimental geometry. We propose in this study to study by means of the density functional perturbative theory the Raman and Infrared spectra in insulators, and in particular oxides (Al-Y-Cr-O systems). By means of the DFPT, one can calculate and predict the vibrational spectra and then Raman active modes.The results will be then compared to experimental measurements done by collaborators (MEMO-CIRIMAT).



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    last update: 2008-11-18 16:23:10

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