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Publications: Articles

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Morniroli, J. P., Redjaïmia, A. and Nicolopoulos, S.
Ultramicroscopy, 107(6-7):514-522
ISSN: 0304-3991

Resume: In a previous study, it was reported that the possible space groups of a crystal can be identified at a microscopic or nanoscopic scale, thanks to microdiffraction patterns obtained with a nearly parallel electron incident beam focused on a very small area of the specimen. A systematic method was proposed, which consists of the observation of a few microdiffraction patterns displaying at least two Laue zones. These microdiffraction patterns can also be obtained by using an electron precession equipment. In this case, the patterns display a very large number of reflections in the Laue zones whose intensity is the integrated intensity. These original features greatly facilitate the space group identification method and are particularly useful when the high-order Laue zones (HOLZ) are not visible on microdiffraction patterns or when very thin specimens are not available. (c) 2006 Elsevier B.V. All rights reserved.

Equipe: Département SI2M : Microstructures et Contraintes

Morniroli, J.-P. and Redjaïmia, A.
Journal of microscopy, 227(2):157-171
ISSN: 0022-2720

Resume: The possible space groups of a crystal can be identified from a few zone axis microdiffraction patterns provided the position (and not the intensity) of the reflections on the patterns is taken into account. The method is based on the observation of the shifts and the periodicity differences between the reflections located in the first-order Lane zone (FOLZ) with respect to the ones located in the zero-order Laue zone (ZOLZ). Electron precession microdiffraction patterns display more reflections in the ZOLZ and in the FOLZ than in the conventional microdiffraction patterns and this number increases with the precession angle. It is shown, from the TiAl example given in the present study. that this interesting feature brings a strong beneficial effect for the identification of the possible space groups since it becomes very easy to identify unambiguously the FOLZ/ZOLZ shifts and periodicity differences. In addition. the diffracted intensity on the precession patterns is the integrated intensitv and this intensity can also be used to identify the Laue class.

Equipe: Département SI2M : Microstructures et Contraintes

Aeby-Gautier, E., Bruneseaux, F., Teixeira, J. Da Costa, Appolaire, B., Geandier, G. and Denis, S.
Journal of Materials, 59(1):54-58
ISSN: 1047-4838

Resume: The prediction of microstructure during processing needs to characterize the phase transformation occurring during the thermal treatments and their kinetics. In-situ high-energy synchrotron x-ray diffraction experiments performed during temperature variations allow the characterization of the phase evolution. For some transformation conditions, the continuous recording of diffraction diagrams evidences clearly intermediate phases. The quantitative analysis of the diffraction diagrams gives the transformation kinetics of each phase as well as their cell parameters. Transformation kinetics obtained by this method are compared to results obtained by electrical resistivity.

Equipe: Département SI2M : Département SI2M : Microstructures et Contraintes

Hutchinson, C.R., Goune, M. and Redjaïmia, A.
Acta Materialia, 55(1):213-223
ISSN: 1359-6454

Resume: A physically based process model for the room temperature yield strength of an age-hardenable alloy subject to selected monotonic non-isothermal heat treatment schedules is developed and used to examine the possible efficiencies in processing and enhancements in strength that may be realistically achieved. `Processing Maps' are generated which illustrate both the yield strengths achievable and the processing routes necessary to achieve them, as well as estimates of the energy consumed in the thermal treatment. A Fe-2Cu (wt.%) alloy is used as an example and the results of physical experiments are compared with the model predictions. For the conditions examined, enhancements in yield strength of similar to 8% above those achievable in comparable isothermal treatments are illustrated with 65% of the energy input. Alternatively, similar yield strengths to those from comparable isothermal treatment are shown to be achievable with similar to 35% of the energy input. The physical origins of the observed effects and the potential implications of coupled process-property optimization are considered. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Equipe: Département SI2M : Microstructures et Contraintes

Teixeira, Julien Da Costa, Appolaire, Benoit, Aeby-Gautier, Elisabeth, Denis, Sabine, Cailletaud, Georges and Spath, Nadine
ISSN: 0921-5093

Resume: We have investigated the microstructure evolutions in the Ti17 near beta titanium alloy during heat treatments. The phase transformation has first been studied experimentally by combining X-ray diffraction analysis, electrical resistivity and microscopy observations. From a series of isothermal treatments. a IT diauram has been determined, which takes into account the different morphologies. Then, a Johnson-Mehl-Avrami-Kolmogorov (JMAK) model has been successfully used to describe the phase transformation kinetics during either isothermal or cooling treatments. Finally, the coupling of the JMAK model to the finite element software ZeBuLoN allowed us to investigate the evolution of the spatial distribution of the different morphologies during the cooling of an aircraft engine shaft disk after forging. (c) 2006 Elsevier B.V. All rights reserved.

Equipe: Département SI2M : Microstructures et Contraintes

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