Helium bubbles formation in aluminum: Bulk diffusion and near-surface diffusion using TEM observations

Experimental and analytical investigation of helium bubble formation and growth in aluminum is presented. A pure aluminum with 0.15 wt% of 10B was neutron-irradiated in the Soreq nuclear reactor to get homogeneous helium atoms in the metal according to the reaction 10B+n→7Li+4He. Formation and growth of helium bubbles was observed in situ by heating the post-irradiated metal to 470 °C in TEM with a hot stage holder. It was found that above 400 °C the change in the bubble shape takes less than a second. In other experiments the Al–10B was first heated in its bulk shape and then observed in TEM at room temperature. In this case the helium bubble formation takes hours. Analytical evaluation of the diffusion processes in both cases was done to explain the experimental results. The number of helium atoms in a bubble was calculated from the electron energy loss spectrum (EELS) measurements. These measurements confirmed the hard sphere equation of state (EOS) for inert gases that was used in the analytical diffusion calculations.