Atomic Level Characterization of Neutron Irradiated Pressure Vessel Steels
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Atomic Level Characterization of Neutron Irradiated Pressure Vessel Steels M. K. Miller1 and P. Pareige 2 1 Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6136 2 Groupe de Physique des Matériaux, Equipe de Recherche Technologique,UMR CNRS 6634, Faculté des Sciences et INSA de Rouen, 76821 Mont Saint Aignan Cedex, France ABSTRACT Atom probe tomography provides one of the most effective tools to characterize the solute distribution and precipitation that occurs in pressure vessel steels and associated model alloys during irradiation. The three-dimensional atom probe is able to experimentally determine the elemental identities of the atoms and their spatial coordinates with near atomic resolution so that their distribution within small volumes of the specimen can be reconstructed and analyzed. This technique together with conventional atom probe field ion microscopy has been applied to many different types of pressure vessel steels and model alloys and has revealed and characterized several different nanostructural transformations. These radiation induced or enhanced processes lead to the formation of copper-nickel-manganese-silicon-enriched precipitates, and solute segregation to dislocations, dislocation loops, nanovoids and boundaries. INTRODUCTION The mechanical properties of the pressure vessel steels that are used in nuclear reactors can change dramatically during service due to the interaction of the solutes in the vessel with the vacancies and other products created by the incident neutrons. It is therefore important to be able to accurately characterize the microstructure and correlate the changes that occur during the initial processing of the steel and service with the mechanical properties. Many diverse techniques have been used to characterize the microstructure of these steels and related model alloys. Two of the most powerful techniques are atom probe field ion microscopy (APFIM) [1,2] and atom probe tomography (APT) [3,4]. Both of these techniques provide information on the redistribution of solute including solute segregation to dislocations and boundaries, and precipitation on the atomic scale. Many applications of these techniques have been performed on a variety of different reactor pressure vessel (RPV) steels and the results have been reviewed [5,6]. These atom probe studies established that a high density of ultra-fine copper-, manganese-, nickel- and silicon-enriched precipitates form during neutron irradiation. These studies have also documented the size and compositions of some of the coarse precipitates including cementite, Mo2 C and vanadium carbonitrides precipitates that are present in various steels and model alloys. ATOM PROBE TOMOGRAPHY In both APFIM and APT techniques, atoms are field ionized and field evaporated from a needle-shaped specimen by the application of a high voltage pulse superimposed on a standing voltage. The flight times to a single atom sensitive detector at the end of a time-of-flight mass spectrometer are measured. The mass-to-charge
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