18R and 14H long-period stacking ordered structures in the Mg93.96Zn2Y4Sr0.04 alloy and the modification effect of Sr on X-phase

► Mg-Y-Zn alloy with LPSO structures was processed via conventional casting. ► Thickness of the X-phase decreased by addition of Sr. ► Simultaneous increase in both strength and ductility was obtained. ► 14H LPSO structure was generated in the α-Mg matrix when treated at 773 K.