Dynamic fracture and spall in aluminum with helium bubbles

Abstract

Investigation of the dynamic properties of aluminum targets with helium bubbles is presented. The targets were obtained by melting pure aluminum with 0.15% wt.¹⁰B powder. The solid targets were neutron irradiated to get homogeneous helium atoms inside the aluminum boron 10 matrix according to the reaction ¹⁰B + n → ⁷Li+⁴He. Helium atoms further accumulated into bubbles by diffusion in the bulk aluminum. Shock wave experiments were performed by accelerating the aluminum impactor into different targets: (1) pure aluminum, (2) Al-¹⁰B, and (3)Al-¹⁰B with different radii and concentrations of helium bubbles. The spall strength was calculated and analyzed from the free surface velocity measurements. It was found that the addition of ¹⁰B in pure aluminum reduces the spall strength of the material by 25–32%. However, irradiated sample with helium bubbles was found to have higher spall strength compared to samples without bubbles. This finding was reconstructed by numerical simulations. The impacted targets were collected after the impact experiments and examined by TEM. These targets were compared to TEM pictures before the impact. The number of helium atoms in the bubbles was calculated from the electron energy loss spectrum (EELS). TEM comparison between the pre-impacted and the impacted targets shows bubbles coalescence and EELS measurements demonstrate a reduction of the helium atoms concentration in the bubbles from ~10²⁸ m⁻³ before the impact to ~10²⁷ m⁻³ after the impact.