Effect of Annealing and Re-irradiation on the Copper-enriched Precipitates in a Neutron-irradiated Pressure Vessel Steel
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Effect of Annealing and Re-irradiation on the Copper-enriched Precipitates in a Neutron-irradiated Pressure Vessel Steel Weld M. K. Miller1 , K. F. Russell1 and P. Pareige 2 1 Microscopy and Microanalytical Sciences Group, Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6136, USA 2 Groupe de Physique des Matériaux, UMR CNRS 6634, Faculté des Sciences et INSA de Rouen, 76821 Mont Saint Aignan Cedex, France ABSTRACT An atom probe tomography study has been performed to estimate the matrix composition, and the size, composition and number density of the ultrafine copper-enriched precipitates that formed under neutron irradiation in a submerged arc weld (Weld 73W). This high copper (0.27 at. % Cu) weld was examined after a typical stress relief treatment of 40 h at 607°C, after neutron irradiation to a fluence of 1.8 x 1023 n m-2 (E > 1 MeV) at a temperature of 288°C, after irradiation and thermal annealing for 168 h at 454°C, and after irradiation, thermal annealing and re-irradiation to an additional fluence of 0.8 x 1023 n m-2 (E > 1 MeV). A high number density of ultrafine copper-enriched precipitates were found to form on irradiation. These precipitates were found to coarsen and decrease in number density after the annealing treatment at 454°C. These precipitates were present after re-irradiation and some additional subnanometer diameter copperenriched precipitates were also observed. Segregation of nickel, manganese, silicon, copper and phosphorus atoms to dislocations was also observed. INTRODUCTION The mechanical properties of pressure vessel steels used in nuclear reactors change during service. This exposure results in the embrittlement of the pressure vessel due to the interaction of the solutes in the vessel with the vacancies and other products created by the incident neutrons. Annealing the pressure vessel at elevated temperatures can restore some or all of the mechanical properties. Re-embrittlement of the vessel occurs on re-irradiation although at a different rate. A systematic atom probe tomography study has been performed on the matrix and the intragranular precipitates of pressure vessel steels in order to characterize the changes that occur in the microstructure after different sequences of neutron irradiation and annealing treatments. EXPERIMENTAL This atom probe tomography study was performed on a submerged arc weld from the fifth heavy-section steel irradiation (HSSI) series (Weld 73W). The bulk composition of this weld is Fe- 0.27 at. % Cu, 1.58% Mn, 0.57% Ni, 0.34% Mo, 0.27% Cr, 0.98% Si, 0.003% V, 0.45% C, 0.009% P, and 0.009% S [1]. Atom probe characterization of the weld material was performed on the following four treatments; after a typical stress relief treatment of 40 h at 607°C, after neutron irradiation to a fluence of 1.8 x 1023 n m-2 (E > 1 MeV) (I), after irradiation and an isothermal anneal of 168 h at 454°C (IA), and after reirradiation of the IA material to an additional fluence of 0.8 x 1023 n m-2 (E > 1 MeV) (IAR). The material was characterized
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