Formation Behavior of 14H Long Period Stacking Ordered Structure in Mg-Y-Zn Cast Alloys with Different α-Mg Fractions

Phase compositions and microstructure evolutions of three Mg-Y-Zn cast alloys during isothermal annealing at 773 K have been systematically investigated to clarify the formation behavior of 14H long period stacking ordered (LPSO) structure from α-Mg grains. The annealed microstructure characteristics indicate that the 18R phase is thermal stable in Mg86Y8Zn6 alloy where 18R serves as matrix, and 14H lamellar phase only forms within tiny α-Mg slices (less than 1% for volume fraction). The α-Mg grains in Mg88Y8Zn4 and Mg89Y8Zn3 alloys exhibit cellular shape, and 14H phase forms and develops into lamellar shape in these cellular grains after annealing. The results suggest that the presence of α-Mg grains is a requirement for the generation of 14H phase. The nucleation and growth rates of 14H lamellas are accelerated in α-Mg grains with higher concentrations of stacking faults and solute atoms. Moreover, the 14H lamellas are parallel to adjacent 18R plates in Mg86Y8Zn6 alloy, but the 14H phase precipitated in cellular α-Mg grains of Mg88Y8Zn4 and Mg89Y8Zn3 alloys exhibits random orientation relationship with surrounding 18R phase, indicating that the orientation relationship between 14H and 18R phases depends on the relationship between α-Mg grains and 18R phase.