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-deposit ed Al-(1.9-22) at% W-(0.6-0.3) at%Cu alloys has been examined, using t ransmission electron microscopy and Rutherford backscattering spectros copy, in order to understand the enrichment and oxidation of alloying elements at the alloy/film interface. Anodic oxidation of the Al-1.9 a t% W-0.8 at%Cu alloy results in initial prior oxidation of aluminium a toms and the accumulation of both tungsten and copper atoms in an allo y layer, about 2 nm thick, immediately beneath the amorphous alumina f ilm, similar to enrichment behaviour in dilute binary Al-Cu and AI-W a lloys. Tungsten atoms are then oxidized and incorporated into the film at about 12 V, and copper atoms are oxidized and incorporated subsequ ently, at about 150 V. After initial incorporation of tungsten into th e anodic film, the concentration of tungsten atoms in the enriched lay er decreases, in association with the progressive accumulation of copp er atoms in the layer, until the steady-state composition for incorpor ation of both tungsten and copper atoms is achieved. In contrast to th is relatively dilute alloy, tungsten enrichment, but no significant en richment of copper, occurs following anodic oxidation of ternary alloy s containing 15 at%, or more, tungsten. For each of the ternary alloys of the study, the compositions of the enriched alloy layers can be in terpreted using the compositions of the enriched alloy layers of corre sponding binary alloys. The anodic film compositions and morphologies are dependent upon the anodizing voltage, and are related readily to t he delayed oxidation of alloying elements through enrichment, and the subsequent mobility of incorporated copper and tungsten ions. (C) 1997 Elsevier Science Ltd.