Growth of Al₂O₃/Al composites from Al-Zn alloys

Abstract

Observations are presented here of the initiation and growth of an Al₂O₃composite by the directed oxidation of a molten binary Al-Zn alloy with and without preforms. The oxidation behaviour into free space begins with the formation of ZnO on the melt surface followed by a second stage of relatively high growth rate associated with the constant presence of ZnO and a final region of slow growth rate during which the surface consists of both ZnO as well as Al₂O₃. Composite formation is explained on the basis of a cyclic formation and reduction by molten aluminium of ZnO. Oxidation was carried out with ternary Mg additions into Al₂O₃ preforms of different particle sizes. The infiltration of an Al₂O₃ preform is governed by reaction induced wetting between alloy and ZnO. Nucleation of the alumina is epitaxial with respect to particles of the preform and growth rates are higher than that for composite growth into free space.