Atom Probe Tomography characterization of the microstructural evolution of a low copper reactor pressure vessel steel un
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Atom Probe Tomography characterization of the microstructural evolution of a low copper reactor pressure vessel steel under neutron irradiation H.F. Huang1,, B. Radiguet1*, P. Todeschini2, G. Chas3, P. Pareige1 1 Groupe de Physique des Matériaux, UMR CNRS 6634, Université et INSA de Rouen, Avenue de l’Université, 76801 Saint Etienne du Rouvray, France 2 EDF-R&D, Materials and Mechanics of Components Department, Site des Renardières-Ecuelles 77818, Moret-sur-Loing cedex, France 3 EDF-Production and Engineering Branch, CEIDRE/DLAB, CNPE de Chinon, BP 23, 37420 Avoine, France ABSTRACT A low copper reactor pressure vessel steel was characterised by atom probe tomography after neutron irradiation at different fluences. The specimens were irradiated within the frame of the Surveillance Program of a production reactor. Roughly spherical clusters enriched in nickel, manganese, silicon and, in a lesser extent, phosphorus and copper were observed at all fluences. The chemical composition of these clusters shows no evolution with fluence, as well as their diameter, close to 3 nm. Their number density increases linearly with the neutron fluence. A continuous segregation of the elements found in the clusters is also observed along dislocation lines, with similar enrichments. INTRODUCTION Neutron flux causes an evolution of the microstructure of reactor pressure vessel (RPV) steels, and results in their hardening and embrittlement [1-7]. The evaluation and prediction of radiation embrittlement of vessel steels is of particular importance for ensuring the safe operation of nuclear power plants during their planned lifetime. In order to predict the embrittlement of vessel steels, it is necessary to understand the submicro-processes that occur under irradiation. The origin and mechanisms of the microstructure changes were largely apprehended in the last thirty years on model alloys and vessel steels irradiated under laboratory conditions (electrons or ions) or in nuclear reactors [8-16]. However, there is still a lack of experimental data at the atomic scale on low copper (1MeV) in the frame of the regulatory surveillance program of the investigated RPV material. Four different neutron fluences were available for this study: 1.7, 3.7, 5.6 and 7.6 1023 m-2 (E>1MeV). APT samples (thin needles with an end radius smaller than 50 nm) were prepared by standard electro-polishing method [17] from rods of approximately 15 mm in length and 0.3×0.3 mm2 square section. Rods were cut from Charpy test samples at EDF-CEIDRE (Chinon, France). APT characterisations were performed at the Groupe de Physique des Matériaux (Rouen, France) with two CAMECA atom probes: an Energy Compensated Tomographic Atom Probe (ECoTAP) and a Laser-Assisted Wide-Angle Tomographic Atom Probe (LAWATAP). The high mass resolution (FWTM†~800 on main peak of Fe) of ECoTAP allows very accurate measurements of matrix composition and detection limit down to 10 at.ppm. However, the analyzed volume (10×10×100 nm3) is limited. The mass resolution of the LAWATAP (FWTM~
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