Phase equilibria of long-period stacking ordered phase in the ternary Mg-Y-Al alloys

Abstract

• The 18R and 10H have been confirmed as the thermodynamically stable phases in Mg-Y-Al system. • The phase transformation of 18R→10H have been well determined. • The coexistence of 18R and 10H are observed by the atomic-scale resolution identification. • Three isothermal sections focusing on the LPSO phases was constructed by the experimental data. Phase equilibria focusing on the long-period stacking ordered (LPSO) phases have been experimentally investigated in the Mg-Y-Al alloys. The microstructures of the eleven representative alloys annealed at 450, 500 and 550 °C were systematically investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Two thermodynamically stable LPSO polytypes were identified, including 18R and 10H structures. Meanwhile, the transformation of 18R → 10H was detected in Mg-9.0Y-1.7Al (at.%) alloy, while the coexistence of 18R and 10H was observed in Mg-14.2Y-12.8Al (at.%) alloy. It was found that the LPSO phases were non-stoichiometric compounds with ternary solubilities. The 18R can dissolve up to 7.21 at.% - 8.37 at.% Al and 10.75 at.% -12.01 at.% Y, while the 10H can dissolve up to 9.35 at.%-10.37 at.% Al and 14.50 at.%-14.94 at.% Y. All of the experimental results were used to construct the isothermal sections, which is conducive to the design of Mg-Y-Al alloys.