SURFACE-ACTIVITIES OF MG IN AL-ALLOYS

The loss of Mg from Mg-containing Al alloys during heat treatments is a well-known metallurgical problem. Recent studies revealed that regardless of whether the loss of Mg is via formation of MgO in air or via evaporation of Mg in vacuum, the kinetics is primarily controlled by a surface reaction and is described by the relation dC/dt equals minus M (C minus C// infinity ) where C is the Mg concentration in the specimen, and M and C// infinity are temperature-dependent constants. However, the specimens used in these studies contained air-formed oxide, and their surface conditions were not characterized. In order to understand the detailed mechanisms, it is necessary to study the surface activity of Mg from well-characterized specimens, using surface sensitive tools. We have studied the loss of Mg from high-purity Mg-containing Al alloys using AES and XPS. The results show that the kinetics of Mg loss from an oxide-covered surface is actually controlled by the diffusion of Mg in the oxide, and that the loss of Mg in vacuum from an oxide-free surface is primarily limited by bulk diffusion. Furthermore, the results also show that the oxidation of a clean surface of these alloys is self-limiting, as in the case of pur Al metal; however, the reaction of Mg with oxygen persists beyond that of Al, resulting in an enrichment of MgO in the oxide layer. Results of other related issues, such as the reduction of Al//2O//3 by Mg, are also presented.