The knowledge of wettability of alumina by molten magnesium–aluminum alloys is very important in the preparation of Mg– or Al–matrix composites. However, little work has so far been carried out on this issue for the alloys with high Mg concentrations, basically because of the difficulty in the precise measurement of the wettability owing to enhanced evaporation of Mg at elevated temperatures. In this work, we investigated the wetting of (0001) a–alumina single crystals by Mg–Al alloys over a wide composition range at 1073 K using an improved sessile drop method. The experiments were preformed in a flowing argon atmosphere at a rate of 0.5 l·min–1. The initial contact angles are between 103o and 84o, almost linearly decreasing with increasing nominal Mg concentration, suggesting that the addition of Mg to Al improves the initial wettability. According to the evolution of contact angle and contact diameter, representative stages were identified to characterize the complex wetting behavior in the presence of evaporation. The wetting kinetics was dependent on the nominal Mg concentration in the alloy. Two patterns of “stick–slip” behavior were observed in the wetting process and interpreted by combining the effects of interfacial reaction and evaporation of magnesium. In addition, the dependence of the interfacial reaction on the Mg–Al alloy concentration was thermodynamically analyzed. The dominant reaction product at 1073 K should be MgO when xMg>9 mol.%, while be MgAl2O4 when xMg<9 mol.%. However, because of the continuous consumption of Mg due to the evaporation and reaction, its concentration in the alloy progressively decreased with time. As a result, MgO formed usually earlier while MgAl2O4 later even for the alloys with higher than 9 mol.% Mg.
Keywords: Wetting; Reaction; Magnesium; Alumina; Stick–Slip.