Abstract: The equal-width lath-shaped Zn_2Zr_3 phase and the irregular lump MgZn_2 phase were ob- served in the Zr-and Zn-enriched nucleus region and along the grain boundaries of the as-cast alloy.The very fine ZnZr_2 phase occurs in the grains and the superfine MgZn phase disperses in the α-Mg solid solution.In extruded shapes,the coarse grains of Zn_2Zr_3 phase and a great many dispersively distributed ZrH_2 phase and MgZn_2 phase particles appear in the Zr-and Zn-enriched lath region.They play an important role in impeding the proceeding of recrystallization.The precipitation of Mg_2Zn_3 phase may be led to the non-lath region by ageing at 170℃ for 10h.

Key words: magnesium alloy, phase constitution, microstructure