Investigation Of The Phase Relationships And Solidification Microstructure Of High Strength And Heat Resistant Mg-Zn-RE Alloys

ABSTRACT:As the lightest metal structure material, magnesium alloys have been widely applied in automotive, aerospace and electronic industries because of a number of desirable features, such as light weight, high specific strength and stiffness, excellent electro-magnetic shielding, good damping characteristic, good machinability, etc. Magnesium alloys have great application prospect, known as the "21st Century Green Engineering Materials". However, the limited mechanical strength of magnesium alloys, especially at elevated temperature, has retarded their application. It has been demonstrated that rare earth metals (RE) are the most effective elements to improve the strength of magnesium especially at high temperatures. The RE addition in Mg-Zn based alloys would enhance the high-temperature strength and creep resistance. The precision thermodynamic description of the multi-component magnesium system is crucially necessary for the design and optimization of alloy compositions and manufacturing processes. Associated with the project "Study on the phase diagrams and alloying of Mg-based alloy systems" financed by National Science Foundation of China (No.50731002), experimental investigation and thermodynamic calculation (CALPHAD) were carried out on partial phase equilibria of Mg-Ce-Gd-La-Nd-Pr-Sm-Zn system. Based on the database, the equilibrium solidification and the solidification according to the Scheil model were calculated for some typical Mg-Zn-RE alloys. The major work of this study is as follows:(1) The phase relations of the Mg-Zn-Gd ternary system in the Mg-rich corner at673K were determined by means of alloying method, X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and electron-probe microanalysis (EPMA). Three ternary phases, X-(Mg]2ZnGd,14H long period stacking ordered structure), W-(Mg3Zn3Gd2, fcc) and I-(Mg3Zn6Gd1, icosahedral quasicrystal structure), were identified. The experimental results showed that there are three two-phase regions,(Mg)+X,(Mg)+W,(Mg)+I, and four three-phase regions,(Mg)+X+Mg5Gd,(Mg)+X+W,(Mg)+W+I, L+(Mg)+I. Mg5Gd, W and I have extended homogeneities along Gd to Zn.Based on the experimental results and literature information, the Gd-Zn binary system and Mg-Zn-Gd ternary system were optimized by means of CALculation of PHAse Diagram (CALPHAD) method. In particular, the order/disorder transition between BCC_B2and BCC_A2in the system has been taken into account and their Gibbs energies were expressed with a single expression. Reasonable agreement between the calculated and experimental data was achieved.(2) The phase relations of the Mg-Gd-Nd system in the Mg-rich corner at673K were determined using the same method as above. Ternary phase (3was identified. The experimental results showed that there are two three-phase regions,(Mg)+Mg5Gd+Mg41Nd5, Mg5Gd+Mg41Nd5+?, and three two-phase regions,(Mg)+Mg5Gd, Mg41Nd5+Mg5Gd,(Mg)+Mg41Nd5. Mg5Gd and Mg41Nd5have largely extended homogeneities along Gd to Nd.Based on the experimental results and literature information, the Gd-Nd binary system and Mg-Gd-Nd ternary system were optimized by means of CALculation of PHAse Diagram (CALPHAD) method. The calculated phase diagrams were in good agreement with available experimental data.(3) Based on available experimental data and literature information, the ternary systems, Mg-Zn-La, Mg-Zn-Ce, Mg-Zn-Nd, Mg-Zn-Pr and Mg-Zn-Sm ternary systems have been optimized using CALPHAD method. In order to treat the order/disorder transition between BCC B2and BCC_A2, the Gibbs energy of the ordered BCC_B2phase in all these systems was deseribed with a2-sublattice model and the same to the one of the disordered BCC_A2phase. A set of self-consistent thermodynamic parameters of individual ternary system was obtained.(4) The thermodynamic database of Mg-Ce-Gd-La-Nd-Pr-Sm-Zn has been established, using to design the composition and heat treatment process of magnesium alloys.(5) Using scanning electron microscopy (SEM) and X-ray diffraction analysis (XRD), the cast samples of some typical Mg-Zn-RE(RE=Ce, Nd, Gd and Sm), Mg-Gd-Nd alloys were observed. Based on the thermodynamic database, the equilibrium solidification and the solidification according to the Scheil model were calculated for these alloys to understand the experimental microstructures during solidification.