Abstract:Effects of ultrasonic on morphologies of primary Mg₂Si crystals in in-situ Mg₂Si/Al composite were investigated by metallographic microscopy and field emission scanning electron microscopy. The results show that the mean grain size of primary Mg₂Si crystals is refined from 150 to 20 μm by high intensity ultrasonic, and the morphologies of primary Mg₂Si crystals are changed as well. Optical microscopy reveals that primary Mg₂Si 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 corners and edges.

Key words: ultrasonic; Mg₂Si/Al composite; Mg₂Si; morphology; grain refinement