Direct evidence of the formation of Al₂Au nanocrystals at the alloy-film interface of anodized Al-Au alloys

Abstract

Anodic oxidation of many dilute aluminium alloys results in alloy layers about 1-2 nm thick, just beneath the anodic film, suggested to contain clusters rich in alloying elements, although direct evidence of their existence is lacking. The present high-resolution transmission electron microscopy study discloses discrete Al₂Au nanocrystals in the gold-enriched layer on anodized Al-Au alloys containing 0·4 and 3·8at.% Au. For the single-phase 0·4at.% Au alloy, the nanocrystals form as a consequence of enrichment of gold in the alloy through initial oxidation of aluminium atoms. For the 3·8at.% Au alloy, composed of fcc aluminium and Al₂Au phases, the nanocrystals are derived from the bulk alloy by the retreat inwards of the alloy-film interface and are also produced following preferential oxidation of aluminium in the fcc matrix phase.