Abstract:The Y₂O₃ addition to Mg-5Si alloys has a good modification effect on the primary Mg₂Si. With 0.05% or 0.1% (mass fraction) Y₂O₃ additions, the primary Mg₂Si begins to change from coarse dendritic shape (about 100 mm) into small polyhedral shape and therefore the alloys exhibits sub-modified microstructure. With 0.2% Y₂O₃ addition, most of the primary Mg₂Si becomes polyhedral shape and its average size is only 25 mm or less. The Mg-5Si alloy exhibits modified microstructure. In addition, the experiments show that the reaction between Mg and Y₂O₃ cannot occur in the sintered Mg-6Y₂O₃ compact; however, the reaction among Mg, Si and Y₂O₃ can occur in the sintered Mg-5Si-6Y₂O₃ compact. Apart from the adsorption and poisoning manners, other mechanisms may exist in the modification of Y₂O₃ addition on the primary Mg₂Si.

Key words: microstructure; solidification; Mg₂Si; Y₂O₃; Mg-5Si alloy