A 3-DIMENSIONAL STUDY OF THE MICROSTRUCTURE OF AN ALUMINUM-ALLOY AS REVEALED WITHIN ITS THICK ANODICALLY FORMED OXIDE

Most of the microscopically visible constituents in the aluminium alloy Hiduminium 48 have been observed to persist, with unchanged morphology, in the relatively transparent coatings formed by anodic oxidation in a sulphuric acid electrolyte. This has made it possible to study the features of the original microstructure, in three dimensions, using the optical microscope to view detail directly through the top surface of the coating. This internal examination of the distribution of the micro-constituents has been found to be particularly helpful in the assessment of residual "ingotism". It can also aid the study of machining-induced disturbance of "near surface" microstructure by avoiding the problems presented by section preparation, with the special requirement to preserve the specimen edge. The influence of the disturbed surface layer, as induced by machining or other forms of surface deformation, on both the fatigue and the stress corrosion behaviour of aluminium alloys, is well known. The approach described in this paper should provide a better understanding of the way this disturbance affects these properties.