Early stages of composite formation by oxidation of liquid aluminium alloys

Abstract

Microstructure evolution during the early stages in the directed oxidation of molten Al-Mg and Al-Mg-Si alloys was investigated to provide needed insight into the origins of the incubation period and its practical elimination by SiO₂ additions. Oxidation experiments were performed primarily in thermogravimetric balances and microstructures were analyzed by optical and scanning electron microscopy. Continuous heating above the alloy liquidus produces first a thin MgO layer and then a brief rapid growth of a spinel + metal mixture within a temperature range which depends on the alloy Mg content and the heating rate. The initial rapid oxidation terminates abruptly with the formation of a dense spinel layer at the surface, leading to a long incubation period of negligible weight gain. The surface MgO regenerates in this regime, while the metal channels slowly advance upward by dissolution of the dense spinel, eventually reaching the MgO and inducing the formation of composite nodules. These consist initially of spinel + metal upon which the conventional Al₂O₃+ metal growth starts after the Mg in the near-surface alloy is depleted to a critical level. SiO₂ surface additions promote composite nucleation by locally hindering surface passivation, acting as an O source for continued spinel growth, and modifying the local chemistry to facilitate the formation of Al₂O₃.