Formation process, microstructure, and mechanical properties of an ultrafine dual-phase alloy formed through phase transition of 18R-type long-period stacking ordered in Mg₈₅Zn₆Y₉ under high pressure

The 18R-type long-period stacking ordered (LPSO) structure in Mg₈₅Zn₆Y₉ exhibits phase transition into D0₃ and α-Mg (D0₃/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 D0₃/α-Mg decreases with pressure. A further increase in the temperature decreases the energy difference between LPSO and D0₃/α-Mg. Therefore, the free energy variations due to the effects of both pressure and temperature cause phase transition. The D0₃/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 D0₃/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 solidified above 5 GPa also comprises D0₃/α-Mg phases. The alloy solidified at 5 GPa exhibits a typical eutectic lamellar structure. However, spherulite-like polycrystalline microstructures containing ultrafine grains are obtained when the alloys are solidified at 10 and 15 GPa. The microstructure variations are caused by an increase in the degree of supercooling due to atomic diffusion suppression with the increasing pressure. The alloys containing D0₃/α-Mg exhibit both low modulus and ultrahigh strength. The Young's modulus is related to the presence of the D0₃ and α-Mg phases in the alloy structure. In contrast, the increase in σ_y can be attributed to the grain refinement strengthening.