Equilibrium And Thermodynamic Study On The Long-period Stacking Ordered Structure Phases In The Mg-RE(Y/Gd)-Zn Systems

Long-Period Stacking Ordered(LPSO) structure phases exist in the Mg-RE-TM(RE=Gd,Nd,Sm,Y;TM=Ag,Cu,Ni,Zn)systems,which can greatly improve the high temperature strength and creep resistance behavior of Mg-based alloys.The Mg-RE(Y/Gd)-Zn systems are especially focused on in the present thesis.The phase equilibrium relations are experimentally measured and the Gibbs energy expressions are thermodynamically assessed,so that the useful references can be provided for the composition design of the Mg-RE alloys and for the optimal selection of their heat treatment processes.Firstly,for the Mg-Y-Zn and the Mg-Gd-Zn systems,the phase equilibrium relations involving the LPSO phases in the Mg-rich sides of the Mg-Y-Zn and Mg-Gd-Zn systems were experimentally investigated.The solidification microstructures and the crystal structures,and compositions of the constituent phases of the as-cast ternary alloys were measured by means of X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM)and high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM).The solidification processes of the as-cast Mg-RE(Y/Gd)-Zn alloys were analyzed and the liquidus surface projections of the Mg-Y-Zn and Mg-Gd-Zn systems were constructed.On this basis,the as-cast alloys were isothermally treated for long enough time at 400 and 500?,so that the phase equilibrium relations and the isothermal sections of the Mg-rich sides were obtained.For the Mg-Y-Zn system,the isothermal equilibrium coexistences of(14H+18R)and(18R+10H)phases as well as the existence of(14H+18R+?-Mg)three-phase region were confirmed from atomic scales.The three-phase region of(14H+18R+Mg24Y5)reported by literature but not proved by experiments was negated.For the Mg-Gd-Zn system,there only exists one kind of stable LPSO phase 14H.The equilibrium phase relations involving the LPSO phase 14H with the other solid solutions and compounds in the Mg-rich sides at 500? were emphatically measured in the present thesis.According to both the available literatures and the present work,the solid solubilities of the LPSO phases were conspicuous and non-negligible.Secondly,according to the crystal structure characteristics of the LPSO phases,the 18R,14H and 10H phases in the Mg-RE-Zn(RE=Y,Gd)systems were remodeled in thermodynamics,and the corresponding thermodynamic parameters were optimized.Based on the atomic radius and the substitutions occurring in the(RE8Zn6Mg)clusters,the atomic replacement in the LPSO phases 18R,14H and 10H were described with four-sublattice model[(Mg2,YZn)x(Mg,Y)y(Mg,Va)z](Mg)w.In addition,the other solid solubility and compound phases were also modeled because of the latest experimental equilibrium information in the Mg-Y-Zn and Mg-Gd-Zn systems.The self-consistent and reasonable thermodynamic parameters were obtained through optimization on the basis of the phase equilibrium relations reported in the Refs.and the present work.The calculated isothermal sections,the liquidus surface projections and the vertical sections were well confirmed to the experimental data.Thirdly,the composition and phase constituent of the alloys and the heat treatment temperature were tailored under the assistance of the thermodynamic calculations of the Mg-Y-Zn system.The purpose of the tailoring was to obtain the alloy microstructure with different content of the LPSO phases and furher explore the effect of the LPSO phases on the mechanical performance of the desired alloys.The liquidus surface projection,the isothermal section and the vertical section of the Mg-Y-Zn system were calculated by CALPHAD method.Based on the calculations,double presentative alloys with the compositions of Mg95Y2.55Zn2.16 and Mg86.2Y7.7Zn6.1,as well as the heat treatment temperature at 500? were determined.The microstructures and compositions of the constituent phases of the alloys were tested and analyzed by using SEM-EDS,TEM and HAADF-STEM.The research results showed that the as-cast microstructure of the Mg95.3Y2.6Zn2.2 alloy including?-Mg,W_Mg3Y2Zn3 and 18R was transformed to the equilibrated microstructure including ?-Mg,14H and 18R after being heat treated at 500? for 240 h,while the as-cast microstructure of the Mg86.2Y7.7Zn6.1 alloy including ?-Mg,W_Mg3 Y2Zn3,18R and 14H was transformed to sole LPSO phases including 14H and 18R after the same heat treatment.The extrusion process with the extrusion ratio of 10:1 was exerted on the alloys after short-time homogenization and long-term isothermal treatment,in order to obtain small and dense microstructure.The results of the subsequent mechanical performance test indicated that the relative content of the LPSO phases should not be increased only for the purpose of high strength,but a certain content of ?-Mg should be retained in the microstructure of the alloys,so as to achieve good ductility as well as high strength in the same time.In addition,the detailed study on the microstructure evolution of the Mg86.2Y7.7Zn6.1 alloy indicated that the as-cast microstructure was transformaed to 14H through the solid-solid phase transition“18R+?-Mg+W_Mg3Y2Zn3?14H”.The 18R phase precipitated as primary phase was partly consumed,so the(14H+18R)two-phase alloy was obtained.The effective alloy tailoring in virtue of thermodynamic calculations showed that CALPHAD method could be applied to design alloy composition and heat treatment temperature purposefully to increase the predictability of alloy phase constituent.In addition,with consideration of the phase equilibrium dependency upon temperature(T)and pressure(P),the P-T type phase diagrams of Mg and Zr unary systems were constructed.The real solution model was applied to describe the molar Gibbs free energy of gaseous species by taking the fugacity coefficient into account.The temperature and pressure dependent Murnaghan equation was employed to describe the molar Gibbs free energy of condensed phases.Based on the literature data available,the thermodynamic parameters of constituent phases were optimized by use of Pandat Software Package.The calculated results of thermodynamic,thermophysical and physical properties including phase equilibrium,heat capacity,coefficient of thermal expansion,compression factor and molar volume were in good consistency with the reported experimental information,indicating the reasonability and reliability of the present work.The calculated results showed that when the pressure exceeded the magnitude of 109 Pa,the consideration of the contribution of pressure to the Gibbs free energy of condensed phases and the whole system was extremely necessary.The optimized and evaluated method to construct the P-T phase diagrams of Mg and Zr unary systems can be extended to other unary systems.The results of the present study can be applied to enrich the thermodynamic database of the Mg-RE alloy systems of multi-component,and especially referred by Mg-RE(Y/Gd)-Zn alloys with LPSO phases to optimize compositions and control processing to achieve the alloy materials with favorable performance.