Phase stability of long-period stacking structures in Mg-Y-Zn: A first-principles study

The phase stability of the long periodic structures in Mg has been investigated at finite temperature by means of first-principles calculations. Free-energy calculation, including the lattice vibration effect, clearly reveals that 14H and 18R type long periodic structures become more stable than 2H-Mg. Furthermore, the stacking fault energies from a structure of ABA (hcp) to ABC (fcc) were calculated for the isotropic lattice variation. We found that the stacking fault energy decreased by lattice expansion and went to nearly zero upon 10% expansion of the lattice. These two calculated results provide important information about the formation of long periodic stacking "ordered" (LPSO) structures in a Mg-Y-Zn system. It has been suggested that the substituted large atoms and temperature effect cooperatively generate a metastable long periodic stacking faults structure that precede LPSO formation.