Investigation On Phase Equilibria In Mg-rich Corner Of Mg-Zn-Al System At300-400℃

As one of the most potential creep resistant systems with lower cost, Mg-Zn-Al system has been increasingly attended. However, the thermodynamic data is very limited, and the phase equilibria of Mg-rich corner haven’t been understood well, which constrains the alloying process of this system. Therefore, accurate and reliable information of the phase diagram has great theoretical and practical importance for the design of ZA series magnesium alloys. Though the CALPHAD technology has greatly developed, the database obtained must be based on the data from experiments. In this paper, the isothermal section of Mg-Zn-Al ternary system at350℃, the phase boundary of Mg17Al12and α-Mg phases and the thermally stable temperature range of R phase have been determined by the equilibrated alloy method and diffusion couple technique with the analysises of XRD, SEM and energy dispersive spectropy of X-ray(EDS), in order to design the new type of heat-resistant Mg alloys and provide the based information for the establishment of the relationship between the microstructure and property.Six three-phase regions have been identified, i.e., Mg+L+Φ, Mg+Φ+γ, L+Φ+Ï„, L+Ï„+MgZn, y+Φ+Ï„,γ+R+β. There are three intermetallic compounds, i.e., L, Φ, γ, in equilibrium with the α-Mg solid solution at350℃. Both Zn and Al can be soluble together in the a-Mg matrix. The solubility of Al in the α-Mg solid solution in equilibrium with y phase decreases due to dissolution of Zn, while the solubility of Zn increases for the addition of Al when the Liquid/a-Mg phases are in equilibrium, compared to the Mg-Al and Mg-Zn binary systems. The whole composition range of Φ phase has also been obtained at350℃, i.e.,～52.7-57.2at.%Mg,～17.7-30.7at.%A1,～15.8-27.7at.%Zn.The content of Al in Mg17Al12phase has been confirmed to be36.3at.%by alloying method when Mg17Al12compound are in equilibrium with the α-Mg phase at35O℃and400℃. The present results is3at.%lower than that accepted in Mg-Al binary phase diagram. R phase has been verified to be thermally stable from300-400℃through the analysis of microstructure and composition of the Mg/Al and Mg/Al-10at.%Zn diffusion couples after annealing at300-400℃. This temperature range is much wider than that of the accepted Mg-Al binary phase diagram, i.e.,320-370℃. It means that the Zn element can stabilize the R phase. Additionally, the diffusion path from pure Mg to Al-10at.%Zn alloy has been founded to be α-Mgâ†'γâ†'Râ†'βâ†'Ï„â†'Al(Zn).