The co-enrichment of alloying elements in the substrate by anodic oxidation of Al-Cu-W alloys

Anodic film formation, at high Faradaic efficiency, on sputter-deposited Al-(1.9-22) at% W-(0.6-0.3) at%Cu alloys has been examined, using transmission electron microscopy and Rutherford backscattering spectroscopy, in order to understand the enrichment and oxidation of alloying elements at the alloy/film interface. Anodic oxidation of the Al-1.9 at% W-0.8 at%Cu alloy results in initial prior oxidation of aluminium atoms and the accumulation of both tungsten and copper atoms in an alloy layer, about 2 nm thick, immediately beneath the amorphous alumina film, similar to enrichment behaviour in dilute binary Al-Cu and AI-W alloys. Tungsten atoms are then oxidized and incorporated into the film at about 12 V, and copper atoms are oxidized and incorporated subsequently, at about 150 V. After initial incorporation of tungsten into the anodic film, the concentration of tungsten atoms in the enriched layer decreases, in association with the progressive accumulation of copper atoms in the layer, until the steady-state composition for incorporation of both tungsten and copper atoms is achieved. In contrast to this relatively dilute alloy, tungsten enrichment, but no significant enrichment of copper, occurs following anodic oxidation of ternary alloys containing 15 at%, or more, tungsten. For each of the ternary alloys of the study, the compositions of the enriched alloy layers can be interpreted using the compositions of the enriched alloy layers of corresponding binary alloys. The anodic film compositions and morphologies are dependent upon the anodizing voltage, and are related readily to the delayed oxidation of alloying elements through enrichment, and the subsequent mobility of incorporated copper and tungsten ions. (C) 1997 Elsevier Science Ltd.