Analyse statistique par calculs EF de la transition d'échelle dans les polycristaux hexagonaux

Dans la démarche auto-cohérente classique où chaque phase est en interaction directe avec le milieu homogène équivalent, de nombreuses règles de transition d'échelle ont été proposées. L'absence de solutions exactes pour un comportement élastoviscoplastique pose la question de leur validité, surtout si l'anisotropie est marquée. Nous proposons une démarche statistique, fondée sur des calculs EF d'agrégats, permettant de construire une solution numérique « de référence» des champs moyens dans différentes phases. Ces résultats sont confrontés à différentes règles de transition

Localized Surface Plasmon Resonance of Metallic Nanoparticles--Optical Property Characterization for Rational Applications

在光的激发下金属纳米结构中的自由电子能够发生群体性的振荡，进而产生表面等离激元（SPP）。发生等离激元共振时，金属纳米结构会将光束缚在表面，并在表面产生极强的电场增强。表面等离激元有两种类型：一类具有传播的特点，其表面等离激元能够在表面传播，称之为propagatingSPP；另一类不具有传播性，共振局域在一个很小的金属结构中，称之为localizedSPP，即局域表面等离激元共振（LSPR）。金属纳米颗粒就具有很强的表面等离激元共振的（LSPR）性质，使其对光产生增强的吸收和增强的散射，并表现出相关的热、光电场增强和热电子等效应。近年，随着纳米科技的发展，金属纳米粒子的LSPR效应已经成为一...The collective oscillation of free electrons in metal nanostructures excited with light is called surface plasmon polaritons (SPP). The light will be confined to a small area on the surface under the resonance condition (SPR), thus a giant enhancement in the electric field will be produced. There are two kinds of surface plasmon polaritons (SPP): one is propagating plasmon polaritons (PSPP), which...学位：理学博士院系专业：化学化工学院_物理化学(含化学物理)学号：2052010015366

A Re-examination of the Portevin-Le Chatelier Effect in Alloy 718 in Connection with Oxidation-Assisted Intergranular Cracking

In Alloy 718, a sharp transition exists in the fracture path changing from an intergranular brittle mode to a transgranular ductile mode which is associated with a transition of flow behavior from smooth in the dynamic strain aging regime to a serrated one in the Portevin-Le Chatelier (PLC) regime. In order to better understand both deformation and rupture behavior, PLC phenomenon in a precipitation-hardened nickel-base superalloy was carefully investigated in a wide range of temperatures [573 K to 973 K (300°C to 700°C)] and strain rates (109^-5 to 3.2910^-2 s^-1 ). Distinction was made between two PLC domains characterized by different evolutions of the critical strain to the onset of the first serration namely normal and inverse behavior. The apparent activation energies associated with both domains were determined using different methods. Results showed that normal and inverse behavior domains are related to dynamic interaction of dislocations with, respectively, interstitial and substitutional solutes atoms. This analysis confirms that normal PLC regime may be associated to the diffusion of carbon atoms, whereas the substitutional species involves in the inverse regime is discussed with an emphasis on the role of Nb and Mo

Etude comparative de différents superalliages base Ni pour ressorts de systèmes de maintien

Les systèmes de maintien situés sur les structures assemblages-combustibles des réacteurs nucléaires à eau sous pression (REP) sont constitués d'un empilement de lames qui agissent à la fois, comme élément accommodant les incompatibilités thermiques résultant des différences de coefficients de dilatation Acier, alliages de zirconium et principalement, comme système permettant de limiter les effets hydrodynamiques induits par le passage du fluide caloporteur à travers les assemblages. Actuellement, l'alliage 718 est le matériau constitutif de ces ressorts à lames. Il présente les performances en service nécessaires et suffisantes pour répondre aux sollicitations de ces systèmes ainsi qu'aux exigences des autorités de sûreté (dans les conditions actuelles de fonctionnement des REP). Or, dans le cadre de l'augmentation des performances générales des assemblages combustibles, l'emploi d'autres matériaux, dont les propriétés de relaxation sous flux neutronique sont supérieures à celles du 718, est envisagé par AREVA. Les matériaux étudiés sont principalement des superalliages base Ni, tels que les nuances 625+ et 725 qui à l'instar de l'alliage 718 durcissent par précipitation de phases secondaires, ainsi que des nuances d'alliage 718 riche en Molybdène. Cependant, bien que ces nouveaux matériaux présentent une relaxation sous flux neutronique améliorée, ils doivent répondre également à un cahier des charges strict, propre à leur utilisation en centrale : des propriétés mécaniques équivalentes, une bonne résistance à la corrosion sous contrainte (CSC) et une bonne résistance à la fragilisation par l'hydrogène (FPH) en milieu primaire de REP. Chacune de ces propriétés a été étudiée avec attention dans le double but de comparer ces matériaux entre eux et afin de cerner les paramètres clés contrôlant leur différence de comportement aussi bien en CSC qu'en FPHHold-down systems used in the fuel assembly of Nuclear Pressurized Water Reactor (PWR) are constituted by stiff springs. The role of the hold-down springs is to ensure the bond between the fuel assembly and the lower plate of the intern structure of the core, thus holding down the assembly on the bottom plate of the reactor, during all the exploitation and maintenance periods. Nowadays, alloy 718 is the constitutive material of these hold-down springs. Its properties in terms of mechanical behaviour, corrosion resistance fill in the specifications required for such application in the present service conditions. However, in order to improve the common efficiency of fuel assemblies, the upgrading of their design as well as the use of new materials are advocated by the nuclear power plant company, AREVA. Though other Ni-base superalloys known for their good behaviour under neutronic radiation can be proposed as new materials, those superalloys must fill in all the application specifications in order to substitute alloy 718. So, sufficient mechanical properties, good resistance to Stress Corrosion Cracking (SCC) and good resistance to Hydrogen Embrittlement (HE) are also required to allow the replacement. All of these properties are carefully studied with the double aim to characterize and compare different superalloys, and to determine key parameters governing the SCC and HE behaviours of such alloys in primary water of PWRTOULOUSE-INP (315552154) / SudocSudocFranceF

In Situ Ageing with the Platform Preheating of AlSi10Mg Alloy Manufactured by Laser Powder-Bed Fusion Process

AlSi10Mg alloy is mainly produced by laser fusion on a powder bed. It offers a good compromise between easy processing and good mechanical properties. These properties depend on the manufacturing parameters, including the preheating temperature, as this alloy hardens by precipitation. This study explored the effect of preheating to 170 °C on the mechanical properties and microstructure of this alloy as a function of the manufacturing time. The mechanical properties were characterized by tensile, hardness and impact strength tests as a function of the sample height. An anisotropic behavior was confirmed: the horizontal orientation showed higher deformation and fracture energies. In addition, a gradient of properties appeared as a function of the distance from the platform; the closer the sample was to the platform, the higher its fracture energy and the lower its hardness. The hardness values followed the same evolution as a function of the distance to the platform as that of the hardness curve after ageing post-treatment. It was therefore shown that the preheating of the platform generated in situ ageing with respect to the manufacturing height: a hardness peak was obtained at a certain distance from the plateau (40 mm—10 h of remaining manufacturing time) and over-aging near the plateau was induced by long exposure times at 170 °C

In Situ Ageing with the Platform Preheating of AlSi10Mg Alloy Manufactured by Laser Powder-Bed Fusion Process

AlSi10Mg alloy is mainly produced by laser fusion on a powder bed. It offers a good compromise between easy processing and good mechanical properties. These properties depend on the manufacturing parameters, including the preheating temperature, as this alloy hardens by precipitation. This study explored the effect of preheating to 170 °C on the mechanical properties and microstructure of this alloy as a function of the manufacturing time. The mechanical properties were characterized by tensile, hardness and impact strength tests as a function of the sample height. An anisotropic behavior was confirmed: the horizontal orientation showed higher deformation and fracture energies. In addition, a gradient of properties appeared as a function of the distance from the platform; the closer the sample was to the platform, the higher its fracture energy and the lower its hardness. The hardness values followed the same evolution as a function of the distance to the platform as that of the hardness curve after ageing post-treatment. It was therefore shown that the preheating of the platform generated in situ ageing with respect to the manufacturing height: a hardness peak was obtained at a certain distance from the plateau (40 mm—10 h of remaining manufacturing time) and over-aging near the plateau was induced by long exposure times at 170 °C

Evolution of the mechanical properties of two precipitation hardening stainless steels after ageing at 400°C comparison between 15-5PH and 13-8PH

International audienc

Local mechanical and microstructural characterization of electron beam welded 15-5PH stainless stell

International audienc

Relationship between the Yield Strength-Fracture Toughness Balance and the Multiscale Microstructure of a Maraging Stainless Steel for Aircraft Applications

International audienceTwo grades of Fe-Cr-Ni-Al-Ti-Mo maraging steels, with a different titanium content, were investigated. Particular attention was given to the correlation between the precipitated phases and the yield strength. Synchrotron X-ray diffraction, small-angle neutron scattering and atom probe experiments were performed to determine the crystal structure, shape, size distribution, chemical composition, particle number density and volume fraction of precipitates. Both alloys show a strong increase in strength after an aging treatment, which is attributed to the co-precipitation of two different intermetallic phases. Strengthening by a single precipitation of β-Ni(Al,Ti) particles induces a saturation of yield strength around 1600 MPa above a volume fraction of 6 %. The improvement of yield strength is then obtained by introducing a nanoscale co-precipitation of η-Ni 3 (Ti,Al) phase