Calculation Of Phase Diagrams And Development Of Thermodynamic Database In The Mg-Al-Zn-Y-Th Magnesium Base Alloys

Magnesium alloys are the lightest metal structural material in engineering application. It will be widely used in aerospace, automobile and electronics industry for their outstanding characteristics, such as high strength-to-weight ratio, excellent cast ability, good heat conductivity, electromagnetic shielding property, machinability, damping property and recycle characteristics.To improve our understanding about the precipitating process and design alloy compositions, knowledge of phase diagrams and thermodynamic data of the involved systems are crucially necessary. In this paper thermodynamic calculation (CALPHAD) is carried out on the phase relations of Mg-Al-Zn-Y-Th system as described in the following.(1) Thermodynamic description of the Mg-Th and Al-Th binary systems are firstly optimized by using the CALPHAD method based on critically evaluated experimental data. Calculated results from the obtained thermodynamic parameters are in good agreement with the available experimental data.Combined with the optimized parameters in the literature for the Mg-Al binary system, the Mg-Al-Th ternary system was extrapolated. The isothermal sections of the Mg-Al-Th ternary system at 327℃, 427℃and 527℃were calculated.(2) The phase diagram in the Zn-Y binary system are evaluated by using the CALPHAD method with the experimental data of the phase equilibria and thermodynamic properties. The thermodynamic parameters in the Al-Zn-Y system were also evaluated by combining the experimental data with the parameters in three binary systems. The calculated phase equilibria and thermodynamic properties are in good agreement with the experimental data.(3) The Zn-Th and Y-Th binary systems are optimized by using the available experimental information on the phase diagram and thermodynamics based on the CALPHAD approach. The calculated phase diagram and thermodynamic quantities agree well with the experimental values. Combined with the optimized parameters in this thesis for the Zn-Y binary system, the Zn-Y-Th ternary system was extrapolated.(4) Using the present database, the Mg-Zn-Th, Mg-Y-Th,Mg-xAl-1Zn-1Y and Mg-xZn-1.8Th-1Y systems were also extrapolated; The equilibrium solidifications were calculated for the alloys Mg-9Al-1Zn-1Y, Mg-9Al-1Zn-1Y-1Th and Mg-2.2Zn-0.7Y-3Th.