La cuve des réacteurs à eau sous pression forme la seconde barrière de confinement de l'assemblage combustible nucléaire. Dans les centrales françaises, elle est constituée d'acier 16MND5 faiblement allié (équivalent de la nuance ASTM A508 Cl.3). Diverses techniques expérimentales (microscopie électronique, diffraction des rayons X etc.) sont mises en oeuvre lors d'essais de tractions in-situ afin de mettre en évidence les hétérogénéités mécaniques apparaissant au sein du matériau. Ces mesures se font en cours de sollicitation pour diverses basses températures [-150°C;-60°C]. Les hétérogénéités mécaniques sont principalement dues aux deux aspects "polycristallins" et "composite"(effet des amas de cémentite) de la microstructure. Des écarts de contraintes résiduelles interphases (jusqu''a 150 MPa en moyenne entre bainite et ferrite), et intraphases (jusqu''a 100 MPa en moyenne par orientation pour la ferrite) sont mis en évidence. Une modélisation complexe est mise en oeuvre afin de représenter le comportement. Elle inclut une loi micromécanique, un modèle de transition d'échelle, et une représentation par éléments finis d'agrégats 3D, le tout associé dans une démarche multi-échelles. L'identification se fait sur le comportement à différentes températures, et permet de reproduire les hétérogénéités de contraintes mises en évidence expérimentalement. Cette modélisation sert de base à l'application déterministe d'un critère local de rupture fragile micromécanique et cristallographique. L'utilisation de divers tirages de répartitions de carbures réalistes permet d'obtenir une probabilité de rupture du volume élémentaire en accord avec les hypothèses formulées par l'approche locale de la rupture. A ceci près que contrairement aux approches habituelles, on ne suppose pas de dépendance de cette probabilité par rapport au chargement ou à la microstructure, celle-ci est naturellement introduite.

There is a strong industrial driving force to find alternative production technologies in order to make the production of aero engine components of superalloys even more efficient than it is today. Introducing new and nonconventional machining technologies allows taking a giant leap to increase the material removal rate and thereby drastically increase the productivity. However, the end result is to meet the requirements set for today's machined surfaces.The present work has been dedicated to improving the knowledge of how the non-conventional machining methods Abrasive Water Jet Machining, AWJM, Laser Beam Machining, LBM, and Electrical Discharge Machining, EDM, affect the surface integrity. The aim has been to understand how the surface integrity could be altered to an acceptable level. The results of this work have shown that both EDM and AWJM are two possible candidates but EDM is the better alternative; mainly due to the method's ability to machine complex geometries. It has further been shown that both methods require post processing in order to clean the surface and to improve the topography and for the case of EDM ageneration of compressive residual stresses are also needed.Three cold working post processes have been evaluated in order to attain this: shot peening, grit blasting and high pressure water jet cleaning, HPWJC. There sults showed that a combination of two post processes is required in order to reach the specified level of surface integrity in terms of cleaning and generating compressive residual stresses and low surface roughness. The method of high pressure water jet cleaning was the most effective method for removing the EDM wire residuals, and shot peening generated the highest compressive residual stresses as well as improved the surface topography.To summarise: the most promising production flow alternative using nonconventional machining would be EDM followed by post processing using HPWJC and shot peening.

Within the collaborative research center SFB 920 “Multifunctional Filters for Metal Melt Filtration”, the thermally induced formation of metastable aluminum oxides and related microstructural changes were investigated. It was confirmed that the γ-Al₂O₃ phase possesses a defective spinel structure containing Al vacancies that preserve the stoichiometry of this phase. The presence of vacancies fragments apparently the γ-Al₂O₃ crystallites into nanocrystalline domains, which are separated by non-conservative antiphase boundaries (APBs) of the type {100} ¼<110>. These APBs form a 3D network that is randomly distributed over all crystallographically equivalent lattice planes. This phenomenon causes a starlike (and hkl-dependent) broadening of the reciprocal lattice points that correspond to the aluminum sublattice. It was shown that the extent of the broadening of the reciprocal lattice points can be predicted by employing the phase shift factors. With increasing degree of the APBs ordering, the initial streaks representing the broadened reflections start to split, forming superstructure reflections. This superstructure of γ-Al₂O₃ is commonly known as δ-Al₂O₃. Between the ordered APBs, the crystal structure of δ-Al₂O₃ is closely related to the crystal structure of monoclinic θ-Al₂O₃. The phase transition of γ-Al₂O₃/δ-Al₂O₃ to θ-Al₂O₃ proceeds via migration of just three Al³⁺ cations to the neighboring tetrahedral and octahedral sites in the cubic close packed (ccp) oxygen sublattice. The general migration vector is ⅛<111> (γ-Al₂O₃). Diffraction effects associated with different intermediate states can be explained by an improper long-range ordering of equivalent APBs or certain Al³⁺ cations and the local formation of θ-Al₂O₃ within the δ-Al₂O₃ superstructure. The formation of θ-Al₂O₃ is accompanied by an increase of the occupancy of the tetrahedral sites in the oxygen sublattice by the Al³⁺ cations. In surrounding local γ-Al₂O₃ domains, however, some cations migrate from the tetrahedral to the octahedral sites. Thus, the local formation of θ-Al₂O₃ is nearly invisible for the ²⁷Al 1D magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. Still, it was recognized by the 2D multiple quantum (MQ) MAS NMR spectroscopy. A continuous formation of the θ-Al₂O₃ domains was confirmed by the Raman spectroscopy, X-ray diffraction (XRD) and selected area electron diffraction (SAED). The proposed microstructure and transformation models helped to explain the thermal stabilization of the metastable alumina phases by Si-doping. For investigation of the thermally induced phase transitions in metastable alumina phases, boehmite (γ-AlO(OH)) was chosen as the starting compound. However, the metastable alumina phases were also observed in endogenous inclusions present in solidified steel melts. For identification of these phases, a procedure for reconstruction of spherical Kikuchi maps from recorded EBSD patterns was developed.