Surface morphology of a die-cast Mg alloy

We have studied the microstructure, phase composition and morphology of the intermetallic beta-phase in the surface layer of the most widespread magne sium alloy AZ91D (Mg-9 wt.% Al-1 wt.% Zn) used in pressure die casting. Aug er electron spectroscopy (AES), energy dispersive spectroscopy (EDS), scann ing electron microscopy (SEM) and optical microscopy have been applied. The surface of the AZ91D Mg alloy consists of an oxide-metallic film with a th ickness of about 0.15 micron containing Mg and Al oxides. Prolonged aging i n air at 200 degrees C promotes an increase in the MgO/Al2O3 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 alpha-Mg grains in the bulk contain, on the average, about 5 at.% Al. Aging leads to a substantial increase in the surface conc entration of aluminum at the expense of the acceleration of its diffusion a nd the intensification of supersaturated solid solution decomposition. Duri ng creep tests at elevated temperatures the morphology of the beta-phase is significantly affected by strain and, to a lesser extent, by the casting t emperature. (C) 1999 Published by Elsevier Science B.V. All rights reserved .