Irradiation-produced defects in austenitic stainless steel
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J. L. STRAALSUND,
J. J. HOLMES,
AND
J. F. BATES
The microstructure of annealed AISIType 304 and Type 316 stainless steels has been characterized by transmission electron microscopy as a function of fast reactor irradiation at fluence levels from 4 • 102' to 7 • 1022 n per sq cm (E > 0.i mev) and at irradiation temperatures from 370~ to 700~ Several irradiation produced defect types where found: voids, Frank faulted loops, perfect loops, dislocation networks, and precipitates. Void number density obeys a power law relationship to fluence, wherein the exponent increases with increasing temperature from 0.8 to 1.4 over the irradiation temperatures investigated. The void size is nearly independent of fluence and increases with increasing temperature. The upper limit irradiation temperature for void formation is about 650~ to 700~ The density and size of Frank faulted loops followed trends similar to those found for voids to temperatures of ~550~ where unfaulted loops, perfect loops, and dislocation networks coexist. These experimental results do not confirm predictions of recently advanced models of void formation. The major deficiency of these models appears to be the nucleation rate. Accordingly, empirical nucleation rates were used to formulate a diffusion-controlled void growth model. This model was found to closely describe experimentally determined void growth kinetics.
THE
objective of this study was to characterize the defects which develop in austenitic stainless steels during irradiation to high neutron fluence levels at temperatures between 0.38 and 0.58 Tin. Recent studies have shown that the defect state in this temperature range consists primarily of voids and interstitial Frank dislocation loops.''2 The volume increase and change in mechanical properties induced by void and loop formation greatly affect fast reactor design and may pose a limitation on the development of fast breeder reactors. 3'4 In order to fully understand the effects of neutron fluence and irradiation temperature upon volume and mechanical property changes, a mapping of the underlying defect concentrations and sizes is needed. Bloom and Stiegler s have studied the relationship between void number density and fluence at irradiation temperatures of 370~ and 450~ but as yet no complete mapping has been given. EXPERIMENTAL
PROCEDURE
Transmission electron microscopy was used to reveal the defect structure in AISI Types 304 and 316 stainless steel in the solution annealed condition. The specimens examined in this study were obtained from a number of sources. Most specimens were obtained from various components irradiated in the Experimental Breeder Reactor II (EBR-II) and included a safety rod thimble, a control rod thimble, and an experimental subassembly thimble. Additional specimens were also taken from several EBR-II irradiation experiments. The chemical composition of all specimens was within AISI specifications for Types 304 or 316 stainless steel, Table I. The irradiation temperature for each specimen was calculated from ther
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