Influence of stacking fault energy on formation of long period stacking ordered structures in Mg-Zn-Y-Zr alloys

The influence of alloying elements on the stacking fault energy (SFE) of Mg-Y-Zn-Zr alloys was calculated by using first-principles, and the microstructure of as-cast Mg-1.05Y-0.79Zn-0.07Zr (mole fraction, %) alloy prepared by conventional casting was investigated by SEM, TEM and HRTEM. The block-like long period stacking ordered (LPSO) phase, the lamellar LPSO phase and stacking faults were observed simultaneously and the lamellar LPSO structure and stacking faults were both formed on (0001)(alpha-Mg) habit plane and grown or extended along [01 (1) over bar0](alpha-M)g direction. The calculation results by the first-principles showed that the addition of Y can sharply decrease the stacking fault energy of the Mg-Zn-Y-Zr alloy, while Zn slightly increases the stacking fault energy of the alloy. The influence of stacking fault energy on the formation of LPSO was discussed. It shows that LPSO may nucleate directly through stacking faults and the lower stacking fault energy was in favor of formation of LPSO.