Defect Structures during Incubation Period of Void Swelling in Austenitic and Ferritic Alloys Studied by Positron Annihi
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Defect Structures during Incubation Period of Void Swelling in Austenitic and Ferritic Alloys Studied by Positron Annihilation Spectroscopy S.Huang1, K.Miyawaki1, K.Tsujikawa1, M.Horiki1, T.Yoshiie1, K.Sato1, Q.Xu1 and T.D.Troev2 1 Research Reactor Institute, Kyoto University, Osaka, Japan 2 Institute for Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences, Tzarigradsko, Chaussee 72, Sofia 1784, Bulgaria
ABSTRACT In order to understand the vacancy behavior during incubation period before steady state void swelling, positron annihilation lifetime measurements was performed after isochronal annealing of austenitic stainless steel (Ti added modified SUS316SS) and ferritic stainless steel (F82H) irradiated by neutrons and electrons to a dose of 0.2 dpa. By electron and neutron irradiations below 363 K, vacancies and nano-voids containing of few vacancies were formed in both alloys. By increasing annealing temperatures, the lifetime decreased without forming nanovoids. The change of lifetime during the annealing indicated the formation and growth of staking fault tetrahedra (Ti added modified SUS316SS) and the annihilation of vacancies at precipitates (F82H). INTRODUCTION Austenitic and ferritic stainless steels are both important nuclear materials. The former has been used as nuclear reactor core materials primarily because they are highly corrosion resistant. The latter is expected for structural materials in fusion and fast breeder reactors [1-4]. In the irradiation damage process of steels, there exists an incubation period, a transient stage before steady-state void swelling. Understanding point defect behaviors in the incubation period is important because it determines the service lifetime of steel components in nuclear systems. Though there have been a lot of theoretical analyses on the period (recent examples [5,6]), only a few experimental studies have been performed for the detection of vacancies and their clusters [7,8]. It is due to the fact that transmission electron microscopy (TEM) has been employed for most of these studies. Point defects and their small clusters in the incubation period are below the microscope’s resolution limits and therefore are impossible to detect. For the observation of irradiation damage by TEM, a conventional diffraction constant image that is formed by a single beam passing through an objective aperture is taken. A wide variety of crystal lattice defects can be observed by this method. The contrast mainly comes from crystal lattice distortions, it is impossible to observe cavities less than 2 nm [9]. The positron is the only probe that can sensitively detect vacancy-type defects in most materials [10,11]. High-energy positrons generated from the β+ decay of a radioactive source (e.g., 22Na) are injected into materials. These positrons are rapidly thermalized and annihilated with electrons. If there are open spaces in the lattice, such as vacancies, where positively charged nuclei are absent, the positrons are briefly trapped there, resulting in a longer lifetime
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