Influence Of Zn On The Mg-Gd-Y Alloy Phase Equilibrium

Magnesium alloy have a wide application in Defense-industry and Civil-industry for its advantage as high strength weight ratio, easy recycling, etc.Phase diagram is important for metal material design, which can explain the phase composition of alloy by theory, and allow researcher to make reasonable and correct judgments about design the composition of the alloy, heat treatment process, etc. To strengthen magnesium alloy by the precipitates strengthening phase, some metal elements was added in pure magnesium. It is particularly important that find out the influence of metal element on the alloy. So,the research on the precipitate process is urgent. In this paper, the CALPHAD phase diagram was used to studied the influence of Zn on Mg-Gd-Y alloy system.First of all, The Mg-Zn, Mg-Gd-Y, Mg-3Gd-3Y-0.5Zr-xZn, Mg-6Gd-3Y-0.5Zr-xZn, Mg-9Gd-3Y-0.5Zr-xZn, Mg-12Gd-3Y-0.5Zr-xZn alloys phase diagram and phase ratio varies with temperature was calculated with a phase diagram calculation software Pandat.The result shows that The addition of Zn element in Mg-Gd-Y alloy promoted the formation of Mg5Gd and result to the fall of liquid temperature.The increase of Gd content, promoted the formation of Mg5Gd phase and Mg24Y5 phase, but less affected to the MgZn phase.In the foundation of CALPHAD, the phase-chage temperature of Mg-xGd-3Y (x=3,6,9wt%),Mg-9Gd-xY(x=1,5,7wt%),Mg-3Gd-3Y-xZn-0.5Zr(x=0,1,3,6wt%)and Mg-9Gd-3Y-xZn-0.5Zr(x=1,3,6 wt%) alloys, was tested with a DSC instrument to verified the calculated phase diagram. The result shows that tested result close to that calculated in ternary alloys, because of the large phase region, the broad interval between phase transition point and larger phase transition energy fluctuations. But, in complicate alloys, with the increase of phase region amount, a number of solid-phase transitions energy are too small to test by DSC, some process could not be well verified by this method. Anyway, the Pandat soft is practical and reliable.At last, the microstrcture of some MG-Gd-Y-Zn system alloy was observed with the OM, SEM, EDX, and TEM.The results show that, MgZn phase mainly distributes in the grain boundaries in the as-cast Mg-Zn binary alloys, and manifold with the zinc content increases. After 520℃for 8h and 200℃for168h treatment, MgZn phase almost disappeared in less Zine content alloy, but in mass zinc content alloy, there is a certain residue in the grain boundary. In the Mg-Gd-Y ternary alloy, no matter gadolinium or yttrium to increase, the second phases were increased. Mg-Gd phase diffuse distribution in cast alloy, aggregated when the Gd content increased. Mg-Y phase mainly distributed along the grain boundary distributed. Gd content with more than 9% after treatment at 520℃for 8h for 400℃,168h, short rod-like Mg5Gd phase was found. Four groups of different Zn (x=0,1,3,6 wt%)content alloy, Mg-3Gd-3Y-xZn-0.5Zr, Mg-6Gd-3Y-xZn-0.5Zr, Mg-9Gd-3Y-xZn-0.5Zr, Mg-12Gd-3Y-xZn-0.5Zr were compared and analyzed. In the as-cast alloy, the network-like second phase manifolded with the zinc content increases, and distributed along the grain boundary eventually. After 520℃for 8h and 200℃for 168h treatment, the second phase general decline in the 6% zinc content alloy, plate shape,strip shape also a granular Mg (Zn, Gd, Y) phase was found. With the increase of Gd and Zn content, the second phase increased, and inhibited the grain growth. Gd content was less affect to formation of MgZn phase,the increase in Zn content would promote the formation of Mg24Y5 phase, Mg5Gd phase. Test result well confirm that with the increase in the amount of Zn element a number of rich Gd and Y second phase appear in the grain boundaries.