Helium solubility and bubble formation in a nanostructured ferritic alloy

The response of a nanostructured ferritic alloy to He implantation and post-irradiation annealing (PIA) at 750 C was characterized by atom probe tomography and transmission electron microscopy. The supersaturated He concentration in the ferrite at a dose of ∼2.1 displacements per atom was similar for the as-implanted, 75 ± 7 appm, and a 10 h PIA treatment, 71 ± 7 appm, but decreased to 38 ± 2 appm after a 100 h PIA treatment. Approximately 91-97% of the He bubbles were present as isolated bubbles in the ferrite and ∼1-5% on the surface of the nanoclusters in the ferrite. The remainder were associated with the grain boundaries with a small fraction on the surface of Ti(N,C,O) precipitates. Their average size and number density were similar for the as-implanted and 10 h PIA treatment with a small increase in the size and a significant increase in the number density after the 100 h PIA treatment. Swelling in the high dose region increased from ∼1% in the as-implanted and 10 h PIA conditions to ∼5% after the 100 h PIA treatment but the estimated number of He atoms per unit volume in the He bubbles decreased by an order of magnitude. Number densities increased from ∼8 × 10²³ m^-3 in the as-implanted to ∼15 × 10²³ m^-3 in the 10 h PIA condition, with little change (to ∼12 × 10²³ m^-3) in the 100 h PIA condition. This trend may indicate nucleation of new bubbles up to 10 h, with growth and possible consumption of the smaller bubbles between 10 and 100 h.