Effect of Long-Period Stacking Order Phase and α-Mg Phase on Strength and Ductility of Mg-Zn-Y Alloy

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Mg alloys are lightweight structural alloys that normally have a good castability and machinability as well as an excellent specific strength and rigidity. However, the mechanical properties of Mg alloys are inferior to those of Al alloys, and their range of industrial applications is limited. Recently, Mg–Zn–Y alloy has been found to show a high tensile yield strength with a good elongation. The alloy has a long-period stacking order (LPSO) phase as the secondary phase in an α-Mg phase. In general, the tensile yield strengths of LPSO-type Mg alloy are known to be markedly enhanced by the formation of kink bands in the LPSO phase and by microstructural refinement of the α-Mg phase during plastic deformation. The separate roles of the LPSO phase and the α-Mg phase in relation to the mechanical properties of high-strength LPSO-type Mg alloy were investigated at ambient and high temperatures. For high strengths at ambient and high temperatures, it was important that the α-Mg phase consisted of a fine-grain region and a nonrecrystallized region, and that the LPSO phase remained as a block-type phase. On the other hands, it was necessary to change the LPSO phase from a block-type phase into a plate-type phase by heat treatment before tensile testing to improve the ductility of the alloy while maintaining its tensile yield strength. Microstructural control of the LPSO phase and the α-Mg phase is necessary to obtained Mg–Zn–Y alloy with superior mechanical properties at ambient-to-high temperatures.