Experimental Study Of The Mg-Zn-Gd Phase Equilibria At 300? Using Diffusion Couples And Key Alloys

Magnesium alloys have excellent properties,such as low density,specific strength and specific stiffness.However,magnesium alloys have poor corrosion resistance,low high-temperature strength and poor plastic deformation ability,which greatly limits their application range.The Mg-Gd alloy has excellent high-temperature mechanical properties.By adding Zn element to the Mg-Gd alloy,14H structure X-phase,icosahedral quasicrystal I phase,W phase,F phase and Z phase can be formed,which provides more possibilities to improve the performance of the magnesium-based alloy..The solid phase equilibrium of Mg-Zn-Gd system at 300? is very important for the alloy composition and heat treatment process design.However,the phase relationship is complicated at this temperature,and the research is still not perfect.There is a big difference in whether the quasicrystal I phase is a thermodynamically stable phase at 300?and whether the ternary compounds can be balanced with the magnesium-based solid solution.Therefore,this experiment combines the diffusion couple method with the equilibrium alloy method,and uses SEM,TEM,EPMA,XRD and DSC to study the phase equilibrium relationship and equilibrium phase composition of Mg-Zn-Gd system at 300?.The following conclusions were obtained:(1)In the Mg-Zn-Gd ternary system,the icosahedral quasicrystal I phase,the X phase of the 14H structure,the W phase and the F phase of face centered cubic structure,and the Z phase of the hexagonal structure are all thermodynamically stable phases.Among them,the quasicrystal phase I,W phase and X phase can coexist in equilibrium with the magnesium-based solid solution ?-Mg at 300?;while the ternary compound F phase and Z phase are not balanced with the magnesium-based solid solution.(2)The Mg-Zn-Gd system has three three-phase regions in the Mg-rich corner of the 300? isothermal cross-section phase diagram.With the increase of Gd content,the order is:?-Mg phase+Mg2Zn3 phase+I phase,?-Mg phase+I phase+W phase,?-Mg phase+W phase+X phase.At 300?,there are also phase equilibrium relations between the following compounds:F phase and W phase,F phase and I phase,I phase and MgZn2 phase,I phase and Mg2Zn11 phase,W phase and GdZn2 phase,W phase and GdZn3 phase,F phase and GdZn3 phase.(3)At 300?,the element Gd is solid-solved in the Mg2Zn3 phase of the binary compound,so that the Mg2Zn3 phase region extends from the Mg-Zn side to the ternary system,and forms a in the ternary system.The phase equilibrium between ?-Mg phase and Mg2Zn3 phase cannot exist in the Mg-Zn binary system.The solid solubility of Mg element in the binary compound GdZn3 can reach 10.8 at.%.(4)In the Mg91Zn7Gd2 alloy,a phase transition from the I phase to the F phase occurred at 324.4?.In the Mg91Zn7Gd2 alloy,during the process from 300? to 400?,the I phase disappears and the Z phase forms.