The effect of helium on swelling in stainless steel: Influence of cavity density and morphology

The interaction of displacement damage and transmutant helium is thought to be an important factor in the evolution of cavity microstructures. A plausible sequence of events involves the growth of bubbles to a critical size or critical number of helium atoms. Above this critical size, determined by the local point defect fluxes, the cavities grow rapidly as voids. Cavities growing at matrix-precipitate interfaces convert to voids at a lower helium content than matrix cavities due to surface energy and other effects. Such precipitate-associated voids are the major source of swelling in austenitic stainless steels in fast reactors. Because of several competing mechanisms, the increased helium to displacement (He/dpa) ratios characteristic of fusion and mixed-spectrum fission reactors can lead to either significant increases or decreases in net cavity swelling when compared with fast reactors. Cavity density is believed to be a critical factor and it is increased at higher He/dpa ratios. We use a calibrated rate theory swelling model to examine the combined effect of cavity density and helium generation rate; the influence of precipitates is also modeled. At very high cavity densities, which may be produced at high He/dpa ratios, a bifurcation in the swelling path is observed: precipitate-associated voids give way to matrix bubbles and reduced swelling. At lower cavity densities swelling is enhanced at higher helium levels, primarily due to reduced incubation exposures. Most significantly, the theory predicts non-monotonic swelling behavior as a function of the He/dpa ratio. These results have important implications with respect to use of fast spectrum (low He/dpa ratio) and mixed-spectrum (high He/dpa ratio) fission reactor data to predict swelling in fusion spectra at intermediate He/dpa ratios. Indeed, the model suggests the peak swelling is near the fusion He/dpa ratio for some austenitic stainless steels.