Wetting of (0001) α-alumina single crystals by molten Mg–Al alloys in the presence of evaporation

Abstract

The wetting of (0001) α-alumina single crystals by Mg–Al alloys over a wide composition range at 1073 K was investigated using an improved sessile drop method in a flowing argon atmosphere. The initial contact angles are between 103° and 84°, 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 x Mg > 9 mol%, while MgAl2O4 when x Mg < 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.