Study On Phase Transitions And The Related Phase Equilibrium In The Mg-Zn-Gd System

As we all known,Magnesium is the green engineering material in 21st century.Magnesium alloy has the advantages of high specific strength,small density,high modulus of elasticity,good shock resistance,strong ability of impact load,and better corrosion resistant performance.However,the disadvantages of heat resistant and high temperature creep resistance of magnesium alloy limit its wide application,the magnesium alloy can not meet the requirements of industrial development on the material properties.Some studies have found that the rare earth elements possess the solid solution strengthening and precipitation strengthening,and can improve the corrosion resistance,the high temperature tensile,and creep properties of magnesium alloys.Some studies have found that Gd element can effectively improve the mechanical properties and corrosion resistance of alloy,but Mg-Zn-Gd ternary phase diagram is imperfect.Especially the phase diagram of Mg-rich part and Gd-poor part is incomplete,These still need a lot of experiments.The related quasicrystal phase transitions and equilibria in ternary Mg-Zn-Gd system alloy are studied by SEM,EPMA,TEM,XRD and DSC,determining Gd-poor part at 400? isothermal section diagram,with the conclusions as follows:Not only can the hexagonal structure Z phase,face-centered cubic H and W phase coexist with ?-Mg solid solution in Gd-poor part of Mg-Zn-Gd system,but quasicrystal I phase can also do this.And the stability of W phase is highest,followed by H phase,the worst is I and Z phase.Also,the temperature of W phase can reach at least to 518?.In Mg-Zn-Gd system,there exists four-phase reaction:the quasi-peritectic reaction occurs at 427.5?:?-Mg+I?H+L;another quasi-peritectic reaction occurs at 445.4?:?-Mg+H?W+L;the peritectic reaction occurs at 518.6?:?-Mg+W?L,and at this time,W phase is melting.With the increasing of Gd/Zn atomic ration,there orderly exists four three-phase regions consisting of ?-Mg+Z+L,?-Mg+Z+I,?-Mg+I+H,?-Mg+H+W in the isothermal section of Gd-poor part in Mg-Zn-Gd system at 400?.Also,there exists two two-phase regions consisting of ?-Mg+W,?-Mg+X.When Gd/Zn<0.1248,the system is in liquid-state phase equilibrium,locating in ?-Mg+Z+L region;When Gd/Zn>0.143 and<0.223,I phase appears and the region is ?-Mg+I+H;When Gd/Zn>0.224 and<0.491,I phase does not exist,W phase appears and the region is ?-Mg+W+H;When Gd/Zn>0.58 and<1.3,there exists ?-Mg+W+X region;When Gd/Zn>0.491 and<0.58,there exists ?-Mg+W region.