Abstract
Specimens of a model austenitic alloy (Fe-17Cr-16.7Ni-2.5Mo) in the solution-annealed and 20% cold-worked conditions have been implanted with 10 to 50 appm helium at room temperature and subsequently annealed at temperatures between 600 and 900°C. These specimens and unimplanted specimens were irradiated in the Oak Ridge Research Reactor to a dose of about 4.7 dpa at 350, 550 and 600°C. The irradiation resulted in the production of about 65 appm helium by transmutation reactions.Transmission electron microscopy has revealed that the as-implanted microstructure consists of a high density of small point defect clusters that evolve into a mixed bubble and faulted loop microstructure during postimplantation annealing. Bubbles become visible after annealing for one hour at temperatures greater than about 700°C. Up to 750°C, Frank faulted interstitial loops grow and the loop size distribution coarsens. The upper cut-off temperature for observing loops is dependent on the level of helium implantation. The major influence of the helium pre-injection in the specimens irradiated to 4.7 dpa in the as-implanted condition is to suppress void formation relative to the unimplanted or the implanted and aged specimens. This suppression is apparently the result of helium being distributed and trapped on a fine scale due to the high initial sink density in the as-implanted specimens. However, when the microstructure coarsens during the postimplantation anneals, the helium coalesces into large bubbles that can convert to voids during the subsequent irradiation. This sequence is consistent with the theoretical explanation of void formation from bubbles which grow by gas and vacancy accumulation until they reach a critical size beyond which further growth is bias-driven rather than due to gas. An interesting new observation was that a high density of large stacking fault tetrahedra were observed in these specimens after irradiation at 550 and 600°C.
Published Version
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