Surface morphology of a die-cast Mg alloy

We have studied the microstructure, phase composition and morphology of the intermetallic b-phase in the surface layer of the most widespread magnesium alloy AZ91D Mg–9 wt.% Al–1 wt.% Zn.used in pressure die casting. Auger electron spectroscopy AES., energy dispersive spectroscopy EDS., scanning electron microscopy SEM.and optical microscopy have been applied. The surface of the AZ91D Mg alloy consists of an oxide-metallic film with a thickness of about 0.15 micron containing Mg and Al oxides. Prolonged aging in air at 2008C promotes an increase in the MgOrAl2O3 ratio from 9"2 to 14"2. The aluminum concentration gradient in the surface layer is rather significant. The external layer of as-cast specimens contains up to 32 at.% Al, whereas a-Mg grains in the bulk contain, on the average, about 5 at.% Al. Aging leads to a substantial increase in the surface concentration of aluminum at the expense of the acceleration of its diffusion and the intensification of supersaturated solid solution decomposition. During creep tests at elevated temperatures the morphology of the b-phase is significantly affected by strain and, to a lesser extent, by the casting temperature. q1999 Published by Elsevier Science B.V. All rights reserved