Helium in metals-diffusion and equation of state

In the first part of this paper experimental and analytical investigation of helium diffusion and bubble formation and growth in aluminum is presented [1;2]. A theoretical model for equation of state of aluminum with helium bubbles and its validation in shock wave experiments are presented in the second part [3]. A pure aluminum with 0.15% wt of ¹⁰B was neutron-irradiated in a nuclear reactor to get homogeneous helium atoms in the metal according to the reaction¹⁰B+n→⁷Li+ ⁴He . Formation and growth of helium bubbles was observed in situ by heating the postirradiated metal to 470°C in TEM with a hot stage holder. It was found that above 400°C the time scale for bubble's shape change is seconds. 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. It was also found that the helium-rich area expands due to helium migration. Electron beam diffraction revealed that the preferred orientation of the helium atoms migration is normal to plane₍₀₂₂₎. The results are consistent with models for helium atoms migration between interstitial sites for an fcc metal. At the second part of this paper a theoretical model for equation of state of aluminum with helium bubbles is presented. Based on this equation of state, the influence of helium bubbles on shock loading is examined. The Hugoniot curve (temperature vs. pressure as well as shock velocity vs. particle velocity) for aluminum containing bubbles is calculated for various bubbles mass, bubbles percentage and helium equation of state models. The bubble mass and concentration seem to affect measurably the Hugoniot curve. The equation of state model implied for the helium in the bubbles has minor significance, which means the model is not sensitive to the details of the helium EOS. These findings were confirmed in shock wave experiments.