SANS study of the aluminum-hydrogen system

X-ray diffraction measurements indicate that the introduction of solute hydrogen into polycrystalline aluminum of 99.99% purity results within experimental error to a zero change in lattice parameter. Almost all FCC materials exhibit a lattice expansion when the hydrogen enters the lattice interstitially. It is hypothesised that the zero change in lattice parameter is due to the formation of a H-vacancy complex at the surface which diffuses into the volume. Polycrystalline Al was charged with hydrogen to high levels of supersaturation and then studied using the SANS technique. The SANS results indicated scattering from a range of particle sizes up to micron size particles. Subsequent SEM measurements revealed the existence of bubbles on the surface? as well as in the bulk and preliminary inelastic neutron-scattering experiments show that hydrogen resides in the bulk in the form of H-2 molecules. Precision density measurements were used to calculate the volume concentration of vacancy sites and from these results a small lattice expansion was calculated, which was within experimental error equal to the zero-change measured from X-ray diffraction. The pressure of the H, in the bubble is also calculated. (C) 1998 Elsevier Science B.V. All rights reserved.