Investigation On Thermodynamics And Corrosion Behavior Of The Mg-Al-Zn-Mn Alloys

As the lightest structural material,magnesium has the advantages of low density,good shock absorption performance and excellent electromagnetic shielding performance,which makes it have broad application prospects in the fields of automobiles,aerospace and national defense.However,the poor corrosion resistance greatly restricts its widespread application in various fields.This work focuses on the thermodynamic calculation and corrosion behavior of the Mg-Al-Zn-Mn alloys.The CALPHAD method was used to construct the thermodynamic database and design alloy composition.Microstructure characterization,weight loss and electrochemical tests were utilized to investigate the effects of Al,Zn and Mn addition on the corrosion behavior of Mg alloys.The electromotive force of each phase and the potential difference between the intermediate phases and the matrix phase were calculated using thermodynamic calculation methods,and the corrosion mechanism of the alloys was analyzed from a thermodynamic perspective.The main research of the present work is as follows:（1）The Mg-Al-Zn-Mn quaternary thermodynamic database was obtained by combining the CALPHAD method with experimental determination.The phase relationship of the Zn-rich side in the Zn-Mn system at 400°C was determined by the equilibrium alloying method and the structural information of theδ₁ phase was obtained by the Rietveld refinement method.The thermodynamic parameters of the Mg-Zn,Zn-Mn,Mg-Zn-Mn and Mg-Al-Zn systems were assessed based on the above results combined with experimental data in literature.Subsequently,the thermodynamic database for the Mg-Al-Zn-Mn quaternary system was constructed by integrating the aforementioned thermodynamic parameters and reported in the literature for Mg-Al,Mg-Mn,Al-Zn,Al-Mn,and Mg-Al-Mn systems.（2）Combining thermodynamic calculations with corrosion morphology observation,weight loss and electrochemical tests,the effects of Zn and Mn addition alone on the corrosion behavior of Mg alloys were investigated.（Mg）solid solution with different contents of Zn and Mn and Mg-Zn alloys containing Mg₁₂Zn₁₃ or Mg₅₁Zn₂₀ were designed based on the thermodynamic calculation.It was indicated that the corrosion resistance of（Mg）solid solution deteriorated with the increase of Zn content.When the content of Mn in（Mg）solid solution was 0.3 wt.%,the corrosion resistance of the Mg-Mn single-phase alloy is the most excellent.A small amount of the Mg₅₁Zn₂₀ phase is beneficial for reducing the corrosion rate of the Mg alloys;When the content is high,both Mg₁₂Zn₁₃and Mg₅₁Zn₂₀ phases accelerate the corrosion rate of the Mg alloys.The variation trend of alloy electromotive force calculated through thermodynamics is basically consistent with the variation trend of corrosion rate.The potential difference between the intermediate phase and（Mg）matrix calculated using the CALPHAD method is consistent with the SKPFM measurement results.It was indicated that the smaller the potential difference between intermediate phases and the（Mg）matrix,the smaller the trend of alloy corrosion reaction.（3）By integrating thermodynamic calculations with microstructure analysis and corrosion behavior characterization,the effects of the simultaneous addition of two elements,Al,Zn,or Mn on the corrosion behavior of Mg alloys were studied.Based on the phase diagram calculation,ternary（Mg）single-phase solid solutions with different element contents and with different intermetallics were designed.Corrosion behavior tests have shown that the Mg-Al-X（X=Zn,Mn）alloys containing intermediate phases have better corrosion resistance than single-phase alloys.The Mg₁₇Al₁₂ phase with network structure and a small amount of LTAl₈Mn₅ phase are beneficial for improving the corrosion resistance of Mg alloys and theφphase would accelerate the corrosion rate of Mg alloys.The higher the content of the Mg₁₇Al₁₂ phase in the alloy,the better the corrosion resistance.When the amount of Zn added in alloys is≤2 wt.%,the corrosion rate of the single-phase Mg-Zn-Mn alloys or alloys with Mg₅₁Zn₂₀ is close.The calculated trend of the electromotive force of the single-phase alloy is consistent with the trend of the corrosion rate of the alloys.The calculated potential difference between the intermediate phases(including Mg₁₇Al₁₂,LTAl₈Mn₅,φand Mg₅₁Zn₂₀)and the matrix is consistent with the trend of potential difference measured through SKPFM experiments.（4）Based on the influence of element solid solution and intermediate phases on the corrosion behavior of Mg alloys obtained from the previous two chapters,the solidification paths of various components of Mg-Al-Zn-Mn alloys were calculated using the CALPHAD method.The Mg-10Al-1.0Zn-0.05Mn alloy with the lowest tendency for galvanic corrosion was designed by analyzing the solid solubility of different elements in the（Mg）matrix,the type and content of intermediate phases,phase composition of the intermediate phases,and the potential difference between intermediate phases and（Mg）matrix.And two other alloy components were selected for comparison based on the calculation results.On this basis,the accuracy of the established database in predicting phase composition was demonstrated through microstructure analysis and phase characterization.The corrosion behavior of each alloy was analyzed through corrosion morphology observation,weight loss and electrochemical tests,verifying the feasibility of thermodynamic calculations in the study of Mg alloy corrosion behavior and alloy composition design.This thesis contains 121 figures,40 tables and 212 references.