In the present study, the nonstoichiometric precipitation of the t-phase in as-cast Mg-xAl-lZn alloy was investigated from the viewpoint of solidification behavior. ~l-phase (Mg~TAI~2) has a nonstoichiometric composition of 28-41 wt.% AI and 6-12 wt.% Zn. Moreover, the chemical composition of the precipitate in MgxAl-lZn alloy locally varies depending on the A1 content in the alloy and the solidification rate. However, it still maintains the same a-Mn structure as the ~phase (Mg~TAI~2) in Mg-xA1 alloy. Also, the slope of the calibration curve between the volume fraction of precipitate and intensity ratio (I~r,/IM~) is 0.35343 and 0.31995 in Mg-xA1 and Mg-xAl-lZn alloy, respectively.

In the present study, the corrosion behavior of AZ91D as-cast alloy was investigated form the viewpoint of the distribution aspect of precipitate (Mg 17 Al 12 ) and the variation of Al concentration in the Mg-rich matrix. The dendrite arm spacing (DAS) of an as-cast specimen was measured as a function of degree which describes the distribution aspect of the precipitate, and the salt spray test was conducted for various grain-sized specimens for 20 days. The dendrite arm spacing increased as the grain size increased to about 150 μm, but a constant value is indicated when the grain size exceeds that range. Although the relationship between the corrosion rate and grain size is of a nonlinear type, the linear trend between the corrosion rate and the dendrite arm spacing is maintained for the overall range of dendrite arm spacing. Since the precipitate in the as-cast alloy is discontinuously distributed, this linear relationship means that the variation of Al-solute concentration in the Mg-rich matrix has a more potent effect than the protective action of the precipitate on the corrosion behavior of an as-cast alloy.