Thermodynamic Study On The Phase Equilibria In The Mg-Tm-Gd (Tm=Cu,Ni) Systems

Magnesium alloys are the lightest metal structural materials. However, the low yield strength, ductility and poor elevated temperature creep limit the applications of the magnesium alloys. Therefore, the strengthening of the magnesium alloy has been the theme of magnesium alloy research, among which the precipitation strengthening is much more popular.The long period stacking ordered (LPO) phase has been reported to be a potential strengthening phase in the magnesium alloys. It has been proved that there exists the LPO phase in both Mg-Cu-Gd and Mg-Ni-Gd systems. In order to design LPO magnesium alloys better, knowledge of phase diagrams and phase equilibria of the involved systems are crucially necessary.In this paper, based on the earlier experimental investigations, thermodynamic calculations are carried out by the CALPHAD approach to the phase equilibria of the Mg-rich region in the Mg-Cu-Gd and Mg-Ni-Gd systems. The results should provide helpful information for the LPO magnesium alloy design. The main research results are as follows:(1) Based on the available experimental information, the LPO phase has been modeled by a three-sublattice model (Mg)o.86(Cu)o.o7(Gd)o.o7to describe the phase relationship and the thermodynamic stability of the LPO phase in the Mg-Cu-Gd and a three-sublattice model (Mg)o.89(Ni)o.os(Gd)o.o6to describe the phase relationship and the thermodynamic stability of the LPO phase in the Mg-Ni-Gd ternary systems.(2) The phase relationship in the Mg-Cu-Gd ternary system has been critically assessed. The isothermal sections of the Mg-Cu-Gd system at425and440℃have been thermodynamically calculated, and the calculated results agree with the experimental data. Especially, the thermodynamic calculation reproduces the four-phase invariant reaction L+(aMg)â†'X+Mg2Cu in the Mg-rich corner at436℃and the four-phase peritectic reaction L+(aMg)+MgsGdâ†'X in the Mg-rich corner at451℃.(3) The phase relationship in the Mg-Ni-Gd ternary system has been critically assessed. The isothermal sections of the Mg-Ni-Gd system at450and500℃have been thermodynamically calculated, and the calculated results are identical with the experimental data. Especially, the thermodynamic calculation reproduces the four-phase eutectic reaction Lâ†'X+(aMg)+Mg2Ni in the Mg-rich corner at483℃and the four-phase eutectic reaction Lâ†'X+Mg5Gd+(aMg) in the Mg-rich corner at529℃.