Effect of silicon on phase selection of ternary compounds during solidification of ZA84 magnesium alloy

Effect of silicon on the phase selection between τ phase (Mg32(Al, Zn)49) and ϕ phase (Al2Mg5Zn2) in ZA84 (Mg-8Zn-4Al-0.3Mn) magnesium alloy produced by steel mold cast was studied using X-ray diffractometer, scanning electron microscope and differential scanning calorimeter. The results show that with increasing Si addition in ZA84 alloy, the liquidus temperatures of the alloys and the solidification temperature ranges decrease. The ternary compound in ZA84 alloy is mainly τ phase and a little ϕ phase. When adding Si to ZA84 alloy, the preferential precipitation sequence of the ternary compounds changes, ϕ phase preferentially forms, whereas τ phase is suppressed. The solidification kinetics study of phase selection indicates that there is a critical degree of undercooling of the melt. If the undercooling exceeds the critical degree, τ phase preferentially forms while ϕ phase is suppressed; otherwise, ϕ phase preferentially forms while τ phase is suppressed.