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

Abstract

Aluminium foils (99.99% purity) and single crystals (99.999% purity) were charged with hydrogen using a gas plasma method and electrochemical methods, resulting in the introduction of a large amount of hydrogen. X‐ray diffraction measurements indicated that within experimental error there was a zero change in lattice parameter after plasma charging. This result is contradictory to almost all other face‐centred cubic (f.c.c.) materials, which exhibit a lattice expansion when the hydrogen enters the lattice interstitially. It is hypothesized that the hydrogen does not enter the lattice as an interstitial solute, but instead forms an H–vacancy complex at the surface that diffuses into the volume and then clusters to form H2 bubbles. Small‐ and ultra‐small‐angle neutron scattering (SANS, USANS) and small‐angle X‐ray scattering (SAXS) were primarily employed to study the nature and agglomeration of the H–vacancy complexes in the Al–H system. The SAXS results were ambiguous owing to double Bragg scattering, but the SANS and USANS investigation, coupled with results from inelastic neutron scattering, and transmission and scanning electron microscopy, revealed the existence of a large size 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 experimentally determined value.

Injection of Hydrogen and Vacancy-Type Defects during Dissolution of Aluminum

“…Because of the apparently large penetration of the D profiles and their independence on treatment time, we assign the D peak to mobile Vac-H defects. The results of Birnbaum and coworkers suggest that this form predominates over interstitial hydrogen after room-temperature charging (6)(7)(8) A large number of dissolution experiments were carried to follow the evolution of the AlD profiles in 99.99 % Al foils. Fig.…”

“…The subscript H is used because it is likely that all the mobile hydrogen in the metal is in the form of Vac-H defects (7). The origin of the x coordinate axis is on the interface, and the axis direction is toward the bulk Al.…”

“…Birnbaum other room-temperature charging procedures, accompanied by either no change or a small contraction of the metal lattice parameter (6)(7)(8). They concluded that H was introduced as H-vacancy complex defects (Vac-H defects).…”

Electron Microscopic Observations of Interfacial Voids in Aluminum Created by Alkaline Dissolution

“…Indirect microscopic evidence for interfacial voids was obtained using anodic oxide replicas of the void-containing surface layer (5). Buckley and Birnbaum co-workers found substantial hydrogen absorption accompanying the formation of voids in the interior of Al foils, due to extended periods of alkaline dissolution (6)(7)(8). They concluded that the hydrogen was injected during open circuit caustic dissolution as H-vacancy defects, which agglomerated to form hydrogen bubbles or voids.…”

Imaging and quantification of hydrogen isotope trapping.

“…As we demonstrated in our electropolishing work, alternative charging techniques could incorporate larger amounts of hydrogen isotopes in shorter times. Buckley and Birnbaum showed that protium plasmas could be used to charge aluminum samples, achieving atomic concentrations of up to 2880 appm, approximately three times typical levels obtained by electrolytic or chemical charging methods, and without the surface chemistry changes associated with those methods [16]. To test the efficacy of plasma charging, we used a Harrick Plasma Cleaner with an attached Plasmaflo.…”