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The 18R-type long-period stacking ordered (LPSO) structure in Mg 85 Zn 6 Y 9 exhibits phase transition into D0 3 and α-Mg (D0 3/α-Mg) before it transforms into a liquid phase with the increase in temperature at 7.8 GPa. First-principles calculations suggest that the energy barrier between 18R-type LPSO and D0 3/α-Mg decreases with pressure. A further increase in the temperature decreases the energy difference between LPSO and D0 3/α-Mg. Therefore, the free energy variations due to the effects of both pressure and temperature cause phase transition. The D0 3/α-Mg phases formed at high pressure can be recovered at 0.1 MPa and room temperature. The phase transition from 18R-type LPSO structure to D0 3/α-Mg causes inner-grain diffusion and generates a lamellar structure. The thickness of the lamellar structure decreases with the increasing pressure until it reaches 65 nm at 15 GPa. Moreover, the alloy …