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

Magnesium alloys have become more and more popular in the material science. Recently it has been discovered that the addition of some rare earth metals in the Mg-based alloys is effective for the improvement of mechanical behavior at high temperature. The rare earth magnesium alloy has not only low mass but also creep resistance at high temperature. And the most important characteristic is the high heat-resisting strength. So it has been widely used in astronavigation and aviation field, and attract extensive attention in the material science. Because my country is rich in magnesium and rare earth resources. Because the shortage of the mineral resources, the research on the new heat resistant high strength magnesium alloy has become our urgent task, but the phase diagram is very important in the new materials research. There is an uncompleted phase diagram data on the Mg-Gd-Y-Zn alloy which limits the development of the rare earth magnesium alloy.In this study, the equilibrium phase diagram of Mg-Gd-Y-Zn system was calculated using Pandat software. The diffusion couple method and the alloy method as well as thermal analysis were used to verify the calculated phase diagram.At first, the thermodynamic model was described and the Mg-Gd(Y) alloy equilibrium phase diagram as well as the multigroup isothermal section and vertical cross-section phase diagram of Mg-Gd-Y、Mg-Gd(Y)-Zn alloys have been characterized using the multiple phase equilibrium calculation software (Pandat) and the magnesium alloy thermodynamic database.The binary and ternary diffusion couples were made by the rivet method. Then the diffusion couple method was used to verify the Mg-Gd-Y-Zn alloy equilibrium phase diagram which calculated by the software (Pandat). The results show that Mg-Gd binary diffusion couples have the same distribution of phase area in their diffusion layer after equalization treatment at300℃and400℃, from Mg matrix to Gd matrix can be divided into five double-phase zones and two single-phase zones. In addition, Mg-Y binary diffusion couples also have the same distribution of phase area in their diffusion layer after equalization treatment at300℃and400℃, from Mg matrix to Y matrix can be divided into four double-phase zones and two single-phase zones, the test results are completely consistent with the results of phase area distribution in the calculated phase diagram which confirmed the Pandat software were reliable for the Mg-Gd and Mg-Y alloy phase diagram calculation. The two diffusion couples of Mg-xZn/Gd (x=2,4)(wt%) have similar distribution of phase area in the diffusion layer after equalization treatment at200℃, it can be detected several three-phase areas successively from magnesium-rich side in the diffusion layer, which are α-Mg+Mg5Gd+MgZn�'Mg5Gd+Mg3Gd+Mg2Zn3�'Mg3Gd+Mg2Gd+Mg2Zn3�'Mg2Gd+MgGd+Mg2Zn3�'MgGd+Mg2Zn3+hcp(Gd). In addition, Mg-xZn/Y(x=2,4)(wt%) two diffusion couples have similar distribution of phase area in the diffusion layer after equalization treatment at200℃, it can be detected several three-phase areas successively from magnesium-rich side in the diffusion layer, which are a-Mg+Mg24Y5+MgZn�'Mg24Y5+Mg2Y+Mg2Zn3�'Mg2Y+MgY+Mg2Zn3�'MgY+Mg2Zn3+hcp(Y), the results are basically consistent with the results of the phase area distribution in the calculated phase diagram, which confirmed the Pandat software were reliable for the Mg-Gd(Y)-Zn alloy phase diagram calculation in the low-Zn side at200℃.Using the alloy method to verify the calculated phase diagram, there is a (α-Mg+Mg5Gd+MgZn) three-phase region in the magnesium-rich corner of Mg-Gd-Zn ternary alloy phase diagram, and a (a-Mg+Mg24Y5+MgZn) three-phase region in the magnesium-rich corner of Mg-Y-Zn ternary alloy phase diagram at200℃. The equilibrium phase is no longer a single constant in the quaternary alloy, and it will be formed by many atoms aggregation and replacement. So the MgZn phase in Mg-9Gd-3Y-3Zn quaternary alloy should be Mg(Zn0.55, Yo.o5,Gdo.4), while Mg5Gd should be Mg5(Gd0.7, Zno.3) phase at200℃. There are the long strip Mg5Gd phase and the lenticular MgZn phase in the magnesium-rich corner of Mg-Gd-Zn ternary alloy phase diagram, whose zone axis are [088] and [2421] at300℃. And there is a square shape Mg24Y5phase in the magnesium-rich corner of Mg-Y-Zn ternary alloy phase diagram, whose zone axis is [112] at300℃. The experimental results are accordance with the calculated phase diagram.DSC analysis was used to verify the phase-transition temperature in the magnesium-rich corner for the calculated phase diagram of Mg-Gd-Zn and Mg-Y-Zn alloys. The results show that the calculated value of the phase-transition temperature are basically consistent with the experimental value, which confirmed the Pandat software were reliable for the calculated value of the phase-transition temperature of the Mg-Gd(Y)-Zn alloy phase diagram calculation.