Abstract: The phase constitutions of Mg-Zn-Zr-RE alloys with low Zn and high RE addi-tions have been studied.In the alloys containing high purity-Y, the X phase at the grain bounda-ries(GBs) and the small square shaped Mg_(24)Y_5 phase in the grain interior are presented.The structure of the X phase is determined as the 1 8 R modulated structure with periodicity of 0.26nm. In the alloys containing Y-or Ce-rich mischmetals,the net-shaped Mg_(12)RE phase at the GBs and the small square-shaped Mg_(24)Y_5 phase in the grain interior are presented. As compared with the MB25 alloy, the grain refining and the strengthening by the rare earth com-pounds at the GBs and in the grain interior may cause the high strength of this kind of alloy.

Key words: rare earth alloys, magnesium alloys, phases-constitution