Effect of cryolite on formation of Al2O3/Al composites produced by oxidation of molten Al-Mg alloy

The oxidation of molten Al-5 wt-%Mg and Al-5 wt-%Mg-0.5 wt-% cryolite (Na3A lF6) has been investigated by carrying out crucible corrosion tests under t hermal gradient conditions. Two morphologies of Al2O3/Al composite were obs erved: 'large balls' in the absence of cryolite; and small mushrooms in the presence of this salt. Microstructural analysis and thermogravimetric meas urements were also carried out to study the effect of cryolite on the oxida tion mechanism. The results showed that both composite morphologies are the result of the same directed metal oxidation (DIMOX) process (DIMOX is a tr ademark of Lanxide Corporation, Newark, DE). Thermogravimetric analysis als o confirmed that two morphologies formed under different oxidation kinetic conditions.