Calculation Of The Mg-Gd-Y-Zn Alloy Equilibrium Phase Diagram And Its Verification

As the lightest commercial metal structure material, magnesium alloys have been widely used in the aerospace automobile industry and become more and more popular in the material filed. Recently it has been discovered that the addition of some rare earth metals in the Mg-based alloys can improve its comprehensive performance and a lot of reports on RE magnesium alloys, especially the development and application of new type alloy containing rare earth are published. As everyone knows, phase diagram have important guiding significance for metal materials research and development, like making the exact theory of alloy’s phase composition, designing the alloy composition and formulating heat treatment process. At present, it is urgent to develop the uncompleted phase diagram of rare earth magnesium alloy to guide the development of this alloy system.In this study, horizontal cross-sectional and the vertical cross-sectional of Mg-Gd-Zn system alloy and Mg-Y-Zn system alloy phase diagram in Mg-rich corner was calculated with extrapolation method in detail by phase diagram calculation software Pandat and thermodynamic database for Mg alloys, this extrapolation phase diagram are verified and corrected in the diffusion couple method and alloy method.At first, eight alloys, Mg-x Gd-y Zn(x=6,9; y=6,8); Mg-x Y-y Zn(x=3,6; y=6,8) are chose in alloy method and the the equilibrium phase diagram are equilibrium heated for more than480hours at different temperatures and then DSC analysis was used for analyzing phase transition temperature, XRD analysis was used for analyzing phase composition, OM analysis was used for observing the formation of alloy and SEM/EDS was used to analyzing phase constituent.Comparative analysis shows, in addition to individual point measured by DSC and calculated equilibrium point is in the range of0.7%～9%. The calculated results that the room temperature equilibrium composition isa-Mg、Mg5Gd、MgZn、Mg24Y5phase and the test result of XRD shows experimental alloy consists ofα-Mg、Mg5Gd、MgZn、Mg24Y5. the Mg-Gd-Zn, Mg-Y-Zn phase diagram by Pandat software and thermodynamic database for Mg alloys can be used. after further observation, a-Mg is the main phase of the alloy, grain boundary is mainly a dendritic crystals and the second phase distribution of intergranular granular distribution. these equilibrium phase component replacement by SEM/EDS analysis was identified as a-Mg,Mg5(Zn,Gd),Mg(Zn,Gd),Mg(Zn,Y) Mg24(Zn,Y)5phase, based on that, equilibrium phase compositions are corrected and the modified Mg-Gd-Zn、Mg-Y-Zn equilibrium phase diagram obtained.In the experimental alloy also has determined few non-equilibrium phase, including MgZn2,Mg3Gd, including Mg3(Zn,Gd),Mg(Zn,Gd)2,Mg(Zn,Y)2phase by SEM/EDS analysis.The Mg-Zn-Gd and Mg-Zn-Y system alloy equilibrium phase diagram which calculated by the Pandat software is verified in diffusion couple method and do the energy spectrum analisis by SEM/EDS and EPMA. The result shows that at200℃, phase area distribution of Mg-2Zn/Gd and Mg-4Zn/Gd diffusion couples is α-Mg+Mg5Gd+MgZn�'MgZn+Mg5Gd+Mg2Zn3�'Mg2Zn3+Mg5Gd+Mg3Gd. The phase area distribution of Mg-2Zn/Y diffusion couple is a-Mg+MgZn�'α-Mg+MgZn+Mg24Y5�'MgZn+Mg24Y5�'MgZn+Mg2Zn3+Mg24Y5�'Mg2Zn3+Mg24Y5, the phase area distribution of Mg-4Zn/Y isα-Mg+MgZn�'α-Mg+MgZn+Mg24Y5�'MgZn+Mg24Y5�'MgZn+Mg2Zn3+Mg24Y5�'Mg2Zn3+Mg24Y5�'Mg2Y+Mg24Y5+Mg2Zn3and hey all are in accordance with the calculated phase diagram. analyze the energy spectrum of Mg-4Zn/Gd and Mg4Zn/Y diffusion couple by EPMA and the diffusion structure of Mg-4Zn/Gd is a-Mg+Mg5Gd+MgZn�'MgZn+Mg5Gd+Mg2Zn3�'Mg2Zn3+Mg5Gd+Mg3Gd and the phase area of Mg-4Zn/Y phase couple is α-Mg+MgZn�'α-Mg+MgZn+Mg24Y5�'MgZn+Mg24Y5�'MgZn+Mg2Zn3+Mg24Y5�'Mg2Zn3+Mg24Y5�'Mg2Y+Mg24Y5+Mg2Zn3. the accordance of phase boundary and phase diagram composition shows that the calculated phase diagram is precise.