Characterization of H defects in the aluminium-hydrogen system using small-angle scattering techniques

Aluminium foils (99.99% purity) and single crystals (99.999% purity) were c harged with hydrogen using a gas plasma method and electrochemical methods, resulting in the introduction of a large amount of hydrogen. X-ray diffrac tion measurements indicated that within experimental error there was a zero change in lattice parameter after plasma charging. This result is contradi ctory to almost all other face-centred cubic (f.c.c.) materials, which exhi bit a lattice expansion when the hydrogen enters the lattice interstitially . It is hypothesized that the hydrogen does not enter the lattice as an int erstitial solute, but instead forms an H-vacancy complex at the surface tha t diffuses into the volume and then clusters to form H-2 bubbles. Small- an d ultra-small-angle neutron scattering (SANS, USANS) and small-angle X-ray scattering (SAXS) were primarily employed to study the nature and agglomera tion of the H-vacancy complexes in the Al-H system. The SAXS results were a mbiguous owing to double Bragg scattering, but the SANS and USANS investiga tion, coupled with results from inelastic neutron scattering, and transmiss ion and scanning electron microscopy, revealed the existence of a large siz e distribution of hydrogen bubbles on the surface and in the bulk of the Al -H system. The relative change in lattice parameter is calculated from the pressure in a bubble of average volume and is compared with the experimenta lly determined value.