Papers by Author: Jean Luc Béchade

Abstract: To improve the efficiency of components operating at high temperatures, many efforts are deployed to develop new materials. Oxide Dispersion Strengthened (ODS) materials could be used for heat exchangers or cladding tubes for the new GENIV nuclear reactors. This type of materials are composed with a metallic matrix (usually iron base alloy for nuclear applications or nickel base alloy for heat exchangers) reinforced by a distribution of nano-oxides. They are obtained by powder metallurgy and mechanical alloying. The creep resistance of these materials is excellent, and they usually exhibit a high tensile strength at room temperature. Depending on the cold working and/or the heat treatments, several types of microstructure can be obtained: recrystallised, stress relieved…. One of the key challenges is to transform ODS materials into thin tubes (up to 500 microns thick) within a robust fabrication route while keeping the excellent mechanical properties. To prevent cracking during the process or to obtain a final product with low residual stresses, it is important to quantify the effect of the heat treatments on the release of internal stresses. The aim of this study is to show how residual stresses can be determined on different thin tubes using two complementary approaches: (i) macroscopic stresses determination in the tube using beam theory (small cuts along the longitudinal and circumferential directions and measurements of the deflection), (ii) stress determination from x-ray diffraction analyses (surface analyses, using “sin²Ψ" method with different hypothesis). Depending on the material and the heat treatment, residual stresses vary dramatically and can reach 800 MPa which is not far from the yield stress; comparisons between both methods are performed and suggestions are given in order to optimize the thermo-mechanical treatment of thin ODS tubes.

Abstract: X-ray diffraction is used in combination with tensile testing for measuring elastic properties of metallic thin films. Size effect, elastic anisotropy and grain morphologies are considered in all these experiments and supported by different kind of numerical simulations operating at different length scales. Such instrumental studies are time consuming even if synchrotron sources are used. New experiments are under progress for reducing acquisition data and improving precision on strain measurements. After introducing briefly the main principles and results of our techniques, first promising measurements on nanometric W/Cu multilayers using 2D CCD detectors and high monochromatic flux at the Advanced Light Source Berkeley (USA) on beam line 11.3.1 are presented. In addition, simulation experiments for analyzing elasticity in textured gold film are discussed.

Abstract: The overall plastic behavior of polycrystalline materials strongly depends on the microstructure and on the local rheology of individual grains. The characterization of the strain and stress heterogeneities within the specimen, which result from the intergranular mechanical interactions, is of particular interest since they largely control the microstructure evolutions such as texture development, work-hardening, damage, recrystallization, etc. The influence of microstructure on the effective behavior can be addressed by physical-based predictive models (homogenization schemes) based either on full-field or on mean-field approaches. But these models require the knowledge of the grain behavior, which in turn must be determined on the real specimen under investigation. The microextensometry technique allows the determination of the surface total (i.e. plastic + elastic) strain field with a micrometric spatial resolution. On the other hand, the white beam X-ray microdiffraction technique developed recently at the Advanced Light Source enables the determination of the elastic strain with the same spatial resolution. For polycrystalline materials with grain size of about 10 micrometers, a complete intragranular mechanical characterization can thus be performed by coupling these two techniques. The very first results obtained on plastically deformed copper and zirconium specimens are presented.

Abstract: The recrystallization of a Zr-2Hf alloy sheet deformed by plane strain compression at room temperature and then heat treated in the temperature range 500-650°C is studied. The microstructure, local and global crystallographic textures are investigated by EBSD and X-ray techniques. The as-deformed condition exhibits a heterogeneous microstructure composed of highly and less deformed zones, the EBSD indexing of the latter ones being more reliable. The asdeformed condition displays a (0001) < 0 1 10 > crystallographic texture. The evolution of the microstructure during recrystallization very much depends on the amount of local deformation. Recrystallization begins in highly deformed zones, new grains having two variants of texture components, {0001} < 0 1 10 > and {0001} < 0 2 11 >. Some change of preferred orientations concomitant with grain growth at 600 and 650°C has been observed with a decrease in the {0001} < 0 1 10 > component and an increase in the {0001} < 0 2 11 > component.