Influence of grain orientation on zinc enrichment and surface morphology of an AlZn alloy

Zinc is an important alloying element in the 7000 series aluminium alloys. It is also an element that may enrich near the alloy surface during treatments of aluminium alloys by processes such as electropolishing, alkaline anodic etching and alkaline etching. The enrichment may occur since the change in Gibbs free energy per equivalent for formation of ZnO is less negative than that for formation of Al2O3. The enriched alloying element is present in an alloy layer up to ∼5 nm thick located immediately beneath the alloy/film interface. In the present study, the dependence of the enrichment of zinc on the grain orientation of the alloy is investigated for a solid solution Al-1.1at.%Zn alloy. The enrichment of the zinc is developed by alkaline etching of the alloy. The grain orientation is determined by electron backscattering diffraction, with enrichments quantified on selected grains by Rutherford backscattering spectroscopy and medium energy ion scattering. The morphologies of the surfaces of the etched grains are characterised by scanning electron microscopy and atomic force microscopy. The findings reveal that the zinc enrichment ranges from 1.7 × 1015 atoms/cm2 to 3.9 × 1015 atoms/cm2, with the greatest enrichment occurring on a grain of (100) orientation, while differing surface topographical textures are developed on the various grains.