Thermodynamic Study On The Phase Equilibria In The Mg-Cu-Ni-Zn-Y-Gd System

As the lightest structural metal materials in the engineering application,magnesium alloys have been widely used for their numerous prominent characteristics.However,the low yield strength and poor high temperature resistance make it difficult to practical applications.The long period stacking ordered(LPSO)phase has been reported to be a potential strengthening phase in the magnesium alloys.There exists the LPSO phase in Mg-TM-RE(TM=Cu,Ni,Zn;RE=Y,Gd)system.In order to design LPSO magnesium alloy better,knowledge of phase diagrams and phase equilibria of the involved systems are crucially necessary.In this work,thermodynamic calculations are carried out by CALPHAD on the phase equilibria of the Mg-rich region of Mg-Cu-Ni-Zn-Y-Gd systems.The main results are described as follows:(1)Based on the available experimental information,the LSPO phase has been modeled by a three-sublattice model(Cu)0.075(Gd)0.07(Mg)0.855 to describe the phase relationship and the thermodynamic stability of the LSPO phase in the Mg-Cu-Gd ternary system;The thermodynamic calculation reproduces a four-phase invariant reaction L+(?Mg)?X+Mg2Cu at 434? and a four-phase peritectic reaction L+(?Mg)+Mg5Gd? X at 453?.The calculated results are identical with the experimental data.(2)Based on the experimental phase diagram and thermodynamic parameters reported in the literatures,the Mg3Gd,Mg5Gd and Mg2(Gd,Ni)have been modeled by(Mg)0.75(Gd,Ni)0.25,(Mg)0.8333(Gd,Ni)0.1667 and(Gd,Mg,Ni)0.667(Gd,Mg,Ni)0.3333 model respectively;the YZn phase has been described by a two-sublattice model(Y,Zn)0.5(Y,Zn)0.5(Va)3.Mg-Ni-Gd and Mg-Zn-Y ternary systems have been thermodynamically optimized,the calculated results are identical with the experimental data.(3)The phase diagrams of Mg-Ni-Y,Mg-Cu-Y,Mg-Zn-Gd,Mg-Cu-Ni,Mg-Cu-Zn and Mg-Y-Gd ternary systems have been thermodynamically calculated,and a thermodynamic database of Mg-Cu-Ni-Zn-Y-Gd system has been established.Based on this,the Mg-rich region of the Mg-Zn-Y-Gd and Mg-Ni-Y-Gd quaternary systems have been calculated.