Copper enrichment on aluminium surfaces after electropolishing and its effect on electron imaging and diffraction

Abstract

Electropolishing is the most common method of preparing samples of aluminium alloys for scanning and transmission electron microscopy, as it yields a good surface quality and large flat, electron-transparent areas. Like many chemical surface treatment procedures, the electropolishing process can create a Cu-rich layer between the aluminium matrix and the outermost aluminium oxide layer. The Cu layer is shown to be crystalline, which causes several distinct effects on electron images and diffraction patterns, that can easily be misinterpreted as originating from features inside the aluminium matrix. The layer is a modified θ’–Al2Cu phase, with a semi-coherent interface with aluminium. A θ’ superstructure suggested in this paper can explain lattice modulations with a 1 nm spacing that have been observed in the [011]Al zone axis of electropolished specimens. Evidence of Cu diffusion into the Al matrix was also found. The surface enrichment can also occur in Cu-free aluminium alloys, as small amounts of Cu can be introduced from outside sources through the electrolyte solution.