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

The 18R and 14H long-period stacking ordered (LPSO) structures formed in the as-cast and solution-treated Mg93.96Zn2Y4Sr0.04 (at.%) alloys prepared by conventional permanent mold casting were investigated systematically by XRD, DSC, SEM and TEM. The examination of microstructure evolution revealed that part of the X-phase dissolved gradually into the α-Mg matrix and a fine plate-like phase with 14H LPSO structure was presented in the matrix during solution treatment at 773 K for 35 h. Furthermore, the refinement of X-phase and increment in the volume fraction of the lamellar regions in this phase could be discerned upon the addition of trace Sr (0.13 wt.%) to the Mg94Zn2Y4 alloy. The improvement in ductility of the alloy could be ascribed to both the grain refinement and the appearance of stacking faults (SF) in the α-Mg matrix.