Ultra-fine grain sized mechanically alloyed surface layers on aluminium alloys

There is significant interest in high shear processing of aluminium alloys to develop ultra-fine grain sized microstructures that have interesting combinations of high strength and ductility. It is less well known that this transformation of microstructure also occurs during normal fabrication procedures used in the manufacture of aluminium products. Specifically, ultra-fine grain sized surface layers are produced during hot and cold roiling and by mechanically grinding or machining of the aluminium surface due to the high levels of surface shear experienced. Such surfaces have been examined in detail using a combination of ultramicrotomy and transmission electron microscopy (TEM). The ultra-fine grains in the transformed surface layers are stabilised by fine particles of magnesium oxide in the same way as aluminium alloys made from mechanically alloyed powders. A critical feature of the transformed surface layers is preferential precipitation compared to the bulk alloy. This can result in the surface layers having much higher corrosion susceptibility than the bulk microstructure and this can dramatically increase susceptibility to filiform corrosion under paint films.