2013 Volume 54 Issue 5 Pages 661-667
It has been proposed that a long-period stacking ordered (LPSO) structure is responsible for the excellent mechanical properties of lightweight alloys of Mg–Zn–RE (RE: rare earth elements) system. The phase separation of the metastable hexagonal close-packed (hcp) phase in the Mg–Y–Zn alloy was simulated by means of the phase-field method to discuss the mechanism of formation of the LPSO structure. Near the Mg-corner of the Mg–Y–Zn ternary system, metastable spinodal decomposition occurs before conventional spinodal decomposition, i.e., the supersaturated solid solution of Mg–7 at% Y–7 at% Zn alloy separates into two phases: Mg–12 at% Y and Mg–17 at% Zn. The resulting microstructure has a lamellar morphology, elongated along the  direction of the hcp phase, with a wavelength of ∼7 nm. The calculated orientation of the lamellar is completely different from that of the LPSO structure. Therefore, it is difficult to explain the formation of the LPSO structure directly in terms of a spinodal decomposition of the hcp phase in the Mg–Y–Zn ternary system.