Effects of ultrasonic on morphologies of primary Mg2Si crystals in in-situ Mg2Si/A1 composite were investigated by metallographic microscopy and field emission scanning electron microscopy. The results show that the mean grain size of primary MgESi crystals is refined from 150 to 20 μm by high intensity ultrasonic, and the morphologies of primary MgESi crystals are changed as well. Optical microscopy reveals that primary MgESi crystals without ultrasonic vibration exhibit coarse particles with cavities, in which eutectic structures grow. However, primary Mg2Si crystals with ultrasonic vibration appear fine grains without any cavity. Three-dimensional morphologies of primary Mg2Si without ultrasonic vibration display octahedron and tetrakaidecahedron with hopper-like hole in the crystals. After ultrasonic vibration, primary Mg2Si particles become solid crystals with rounded comers and edges.
