Investigation On Phase Diagram Thermodynamics Of 7XXX Series Aluminum Alloy Containing Zr, Er And Y And Its Application In Homogenization

Al-Zn-Mg-Cu 7XXX series aluminum alloys exhibit high strength and toughness,which are widely used in rail transportation,aerospace and other fields.Zr,Er and Y are important addition elements for microalloying,which can refine grain size and inhibit recrystallization.However,a large number of alloying elements will result in the formation of non-equilibrium eutectic structure of aluminum alloy,which is harmful to the subsequent processing and improvement of alloy properties.Therefore,homogenization treatment should be used to eliminate or reduce the chemical composition and structure heterogeneity.Related phase diagrams and thermodynamic information as well as a complete thermodynamic database could provide important theoretical guidance for the design and preparation of high-performance aluminum alloys.In this work,CALculation of PHAse Diagrams（CALPHAD）and key experiments were combined to systematically study the phase diagram thermodynamics of 7XXX series alloy system containing Zr,Er and Y.The Al-Zn-Mg-Cu-Zr-Er-Y seven component thermodynamic database was constructed and its application in homogenization was investigated.The main innovative research results were as follows:（1）The phase equilibria of Al-Y-Zr,Al-Er-Zr and Al-Er-Y systems at different temperatures were systematically investigated.The solid solubilities of the third element in binary compounds were determined.The primary phase regions and solidification paths in three ternary systems were determined.The CALPHAD method was used to optimize and calculate the thermodynamics of these systems for the first time.Based on the consideration of the effect of the third element on the solid solubilities of binary compounds,the thermodynamic parameters for reasonably describing the phase equilibria of these systems were obtained.（2）The space groups and compositions of two ternary compounds（τ₁ andτ₂）in the Al-Er-Y ternary system were determined for the first time by X-ray diffraction,electron probe microanalysis and focused ion beam-transmission electron microscopy.The space group ofτ₁ was R 3 m and the composition was Al_75-76Er_15-22Y_2-10.The space group ofτ₂ was P6₃/mmc and the composition was Al_75-76Er_11-17Y_7-14.（3）Based on the experimental and calculation results of the sub-ternary Al-Y-Zr,Al-Er-Zr and Al-Er-Y systems,combining with the thermodynamic description of the Zr-Er-Y system from direct extrapolation,the Al-Zr-Er-Y quaternary system thermodynamic database was established.Combined with the thermodynamic parameters of binary and ternary systems reported in literature,the thermodynamic database of Al-Zn-Mg-Cu-Zr-Er-Y seven component system was constructed.（4）Based on the seven component system thermodynamic database established in this work,the thermodynamic calculations and experiments showed that the optimal homogenization treatment for Al-7Zn-1.5Mg-1.5Cu-0.1Zr-0.05Er（wt%）alloy was 743 K/24 h.After homogenization,the non-equilibrium phases（Al₂Cu Mg and Al Zn Mg Cu）almost disappeared,whereas the equilibrium phase Al₈Cu₄Er remained.After homogenization at 743 K/24 h,the Vickers hardness of the alloy decreased,and the electrical conductivity increased.The addition of Zr and Er improved the strength and plasticity of the alloys after homogenization.The results obtained in this work are important parts of the thermodynamic database of rare earth aluminum alloy,which will offer a theoretical basis for the selections of the addition of elements and heat treatment process,and also provide a feasible research method for improving the existing performance of aluminum alloy and designing new aluminum alloy in the future.