Helium nanobubbles are a common form of radiation damage that degrades mechanical and electrical properties of exposed metals. They impede desired performance in ion-implanted semiconductors, nuclear reactors, tritium storage materials, and nuclear waste containers. Knowledge of the mechanisms of initial bubble formation and subsequent evolution is critical to predicting the lifetime of these materials, but much remains unknown. The microscale deformation and creation of defects in the metal in response to bubble expansion are poorly understood. Additionally, the number, pressure, and distribution of bubbles cannot be predicted for a metal subjected to a specified radiation dose. Current theory holds that bubble distribution and size are based on rapid nucleation of all bubbles in a narrow time window followed by diffusion of interstitial helium atoms to the nearest bubble. To investigate these questions, we harnessed transmission electron microscopy to characterize the three-dimensional configuration of helium bubbles in a palladium-nickel alloy aged as a tritide for 3.8 years. We generated a three-dimensional image of the sample by tomographic reconstruction of projections obtained at a series of tilt angles. We determined the number, volume, and spatial distribution of helium nanobubbles in the metal. By measuring the electron energy loss spectra of an ensemble of individual bubbles, we established a pressure range of a few GPa. We then calculated the total helium content trapped in the metal lattice as nanobubbles. We found the prevailing explanation of bubble formation, which holds that the capture volume of a bubble determines its size, to be incomplete. Density functional theory and molecular dynamics simulations we performed suggest that interstitial helium transport is rapid in the absence of defects, and that trapping sites, such as vacancies or impurities, must be considered to yield a more complete and predictive model.

Three-dimensional maps of helium nanobubbles in a palladium alloy

256th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.