The Mg-Gd-Y System Alloy Phase Diagram Calculation And Validation By Diffusion Couple Technique

Magnesium alloys are the lightest metal structure materials in the world, and have broad application prospects, which have been attracted more and more attention in research and industrial field. Magnesium-rare earth alloys, especially Mg-Gd-Y system alloy, have all superiorities of ordinary magnesium alloys, and have the incomparable heat-resistant properties, and creep properties, and are becoming a hot research field of high-strength and heat-resistant magnesium alloy magnesium alloy. Moreover phase diagram has an irreplaceable role in new materials development, the research about the phase diagram of Mg-Gd-Y system alloys, however, are very few, which restricts the development of magnesium-rare earth alloys.In this paper, the method of diffusion couple was first introduced to the phase diagram research of Mg-Gd-Y system alloys. The preparation of diffusion couple, isothermal equilibrium heat treatment at vacuum condition, equilibrium phase were systematic analyzed by OM,SEM,EDS,XRD and TEM, and Mg-Gd-Y system alloys phase diagrams which were got by calculation of multiphase equilibrium technology were validated by the method of diffusion couple.Phase figure, phase and phase composition of the Mg-Gd alloy, Mg-Y alloy, and Mg-xGd-3Y alloy were calculated by commercial equilibrium calculation software (Pandat software) and thermal database of magnesium alloy. Comparing the Phase transition temperature of Mg-20Y,Mg-9Gd-3Y alloys of testing by DSC with calculating by Pandat software, the differences were very small, which confirmed the Pandat software and thermal database of magnesium alloy were reliable.The microstructures of Mg-20Gd, Mg-20Y, Mg-9Gd-3Y alloys at as-casted, solid solution, homogenization states were observed by the method of OM,XRD. The results indicate that the microstructure at room temperature of Mg-20Gd alloy was composed by a-Mg and Mg5Gd, the microstructure at room temperature of Mg-20Y alloy was composed by a-Mg and Mg24Y5, the microstructure at room temperature of Mg-9Gd-3Y alloy was composed by a-Mg, Mg5Gd and Mg24Y5, which confirmed the Pandat software and thermal database of magnesium alloy were reliable.The microstructures of three alloys which were homogenized at 520℃for 8h,and aged at 300℃for 144h were analyzed by TEM. The results indicate: Precipitation phases of Mg-20Gd alloy are refinement bamboo-shaped Metastable phase Mg3Gd, and uniformed distributed in the matrix, and coexisted with the phases of Mg5Gd, which confirmed the state of heat treatment at 300℃for 144h didn't reach to balance state. Precipitation phases of Mg-20Y alloy are rod-shaped stable phase Mg24Y5. Precipitation phases of Mg-9Gd-3Y alloy are parallelogram-shaped and uniformed distributed in the matrix, which were proved to be stable phase Mg5(Gd, Y)and Mg24(Gd, Y)5.The preparation process of diffusion couple by rivet method was simple, and applicable to rare earth magnesium alloys were found at the process of phase diagram calculation and validation. Phase equilibrium compositions of diffusion couple sample at 400℃and 450℃of Mg-Gd and Mg-Y alloy were analyzed. The results indicate: Diffusion layers of Mg-Gd are composed of five double-phase zones and two single-phase zones. Diffusion layers of Mg-Y are composed of four double-phase zones and two single-phase zones, which were conformed to the results of phase diagram calculation, and which confirmed the Pandat software and thermal database of magnesium alloy were reliable.