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The effect of increasing levels of silicon on the microstructure and creep properties of high-pressure die-cast Mg-Al-Si (AS) alloys has been investigated. The morphology of the Mg 2 Si phase in die-cast AS alloys was found to be a function of the silicon content. The Mg 2 Si particles in castings with up to 1.14 wt pct Si have a Chinese script morphology. For AS21 alloys with silicon contents greater than 1.4 wt pct Si (greater than the ␣-Mg 2 Si binary eutectic point), some Mg 2 Si particles have a coarse "blocky" shape. Increasing the silicon content above the eutectic level results in an increase in the number of coarse faceted Mg 2 Si particles in the microstructure. Creep rates at 100 hours were found to decrease with increasing silicon content in AS-type alloys. The decrease in creep rate was most dramatic for silicon contents up to 1.1 wt pct. Further additions of silicon of up to 2.64 wt pct also resulted in significant decreases in creep rate.

This paper investigates the relationship between mechanical properties and microstructure in high pressure die cast binary Mg-Al alloys. As-cast test bars produced using high pressure die casting have been tested in tension in order to determine the properties for castings produced using this technique. It has been shown that increasing aluminium levels results in increases in yield strength and a decrease in ductility for these alloys. Higher aluminium levels also result in a decrease in creep rate at 150 C. It has also been shown that an increase in aluminium levels results in an increase in the volume fraction of eutectic Mg 17 Al 12 in the microstructure.