Investigation On Phase Diagram Thermodynamics And Diffusion Dynamics Of Mg-Al-Zn-O System

As an important light alloy material,magnesium alloys have a very wide range of applications and development prospects in automotive,electronic communications,aerospace and other fields.AZ alloy is one of the most widely used alloys due to its excellent casting properties.However,the use of magnesium alloys is severely limited by their easy reaction with oxygen in air during casting and cutting,poor resistance to high temperature oxidation,and other shortcomings.The oxidation behavior of magnesium alloys is influenced by a number of factors,such as the P-B ratio,ion diffusion,and compounds.In this thesis,by studying the thermodynamics and diffusion kinetics of Mg-Al-Zn and Mg-Zn-O system phase diagrams,a thermodynamic and kinetic database of Mg-Al-Zn-O system was established,and the diffusion and movement of elements in intermetallic compounds of Mg-Al-Zn system were analyzed,as well as the oxidation behavior of AZ and Mg-Zn alloys.It provides fundamental data to support the improvement of the oxidation resistance of magnesium alloys.For the phase diagram of Mg-Al-Zn system,the traditional phase diagram test method,scanning electron microscopy（SEM）,X-ray diffraction（XRD）,differential scanning calorimetry（DSC）,electron probe analysis（EPMA）and other experimental instruments were used to obtain the experimental data of the phase composition and phase relationship ofεphase.The local phase diagram near theεphase at 350℃was obtained by thermodynamic calculation,and the accurate phase composition and phase relationship were determined and verified by comparing them.It is found that the maximum solid solubility of Zn inεphase increases to 7.7 at.%at 350℃,and Zn can improve the stability ofεphase at high temperature in the range of solid solubility.A more refined phase diagram of the Mg-Al-Zn system has been obtained by combining experimental data and computational results.For the element diffusion behavior of intermetallic compounds（IMCs）in Mg-Al-Zn system,the temperature-dependent kinetic coefficients of IMCs were studied by combining diffusion couple method,phase stability diagram,in situ observation technique and numerical inversion method（NIM）.The parabolic growth constant（PGC）and mutual diffusion coefficient of the Mg solid phase withγ-Mg₁₇Al₁₂,β-Mg₂Al₃,ε-Mg₂₃Al₃₀,Mg Zn₂,Mg₂Zn₃,τ-Mg₃₂（Zn,Al）₄₉,andφ-Mg₅Zn₂Al₂ in the Mg-Al-Zn alloy system are determined.By comparing the current experimental data and the calculated results,the temperature dependent diffusion rate controlling factors ofφ,τandεternary IMCs in Mg-Al-Zn system are discussed in detail.It is shown that the diffusion of Zn and Al elements is the key factor controlling the growth of IMCs.For the phase diagram of Mg-Zn-O system,CALPHAD thermodynamic calculation method and Thermo-Calc software were used to optimize the phase diagram,and a set of reasonable thermodynamic parameters were obtained.The thermodynamic database of Mg-Al-Zn-O system was established based on the literature data,and the oxidation behavior of AZ alloy and Mg-Zn alloy was analyzed.The surface of Mg-Zn alloy is Mg O-Zn O oxide thin film,in which the Mg matrix is oxidized first,followed by Zn.The surface of AZ alloy is Mg O-Al₂O₃-Zn O oxide thin film mainly composed of Mg O.Due to the addition of Al,Mg₁₇Al₁₂ phase is formed,which accelerates the oxidation of AZ alloy and reduces the oxidation resistance of AZ alloy.