Eutectic morphologies of as-cast Mg-Al alloys with various aluminum contents were observed by optical microscope（OM）

scanning electron microscope（SEM）

X-ray diffraction（XRD）.The formation mechanism of the eutectic structure and its influential factors were described.The results indicate that β phase is prior to precipitate from the eutectic structure in the Mg-Al alloy.With increasing in aluminum content

eutectic structure in the Mg-Al alloy is converted from divorced eutectoid into granular

and into fibrous

and then to lamellar structure.With increasing in cooling rate

eutectic structure becomes finer and finer

and developed primary α phase can be observed.The eutectic morphologies of Mg-Al alloys in solid solution range is significantly affected by the primary α dendrite morphology.