Study On Solid Solution And Solid Solution Strengthening Of Elements In Mg-Al-Zn-Re(Ce,Y,Gd)Alloy

AZ magnesium alloy is an important series of magnesium alloys,and has been practically applied in many fields.In order to improve the mechanical properties of AZ magnesium alloys,many elements including rare earth elements are used for alloying treatment.These alloy elements exist in many forms in AZ magnesium alloy,and affect the mechanical properties of the alloy through various mechanisms.Solid solution is an important form of alloying elements in AZ magnesium alloy.The solid solution of the alloy element has a solid solution strengthening effect on the matrix,and the solid solution strengthening function is an important strengthening mechanism of the alloy,which is related to both the degree of solid solution of the element and the strengthening efficiency of the solid solution element.Therefore,it is the premise and foundation to understand the solid solution degree and strengthening efficiency of elements in AZ magnesium alloy.However,no matter AZ alloy or alloy after alloying with rare earth and other elements,there is lack of corresponding solid solution data at present.The reason of the change of the solid solution ability of the elements in the multi-element alloy is rarely studied,and the solid solution strengthening efficiency and mechanism of different elements are rarely reported.To this end,this work first established a method for determining the amount of solid solution of elements in AZ magnesium alloy,and actually measured the amount of solid solution of each element of the experimental magnesium alloy at various solidification rates?Die casting non-equilibrium solidification and temperature-controlled near-equilibrium solidification?.Secondly,the rules and causes of the influence of alloy composition and solidification speed on the solid solution of elements are discussed.Finally,the solution strengthening efficiency and strengthening mechanism of different elements are preliminarily explored.It is hoped that through the above work,it will provide solid solution data support for the research of element solid solution of AZ and AZ-RE magnesium alloys,alloy composition design and forming processing.Based on the property that the electrode potential of solid solution in magnesium alloy is lower than that of compound phase,a direct determination method of solid solution content of elements based on electrochemical phase separation is established.Organic electrolyte is used for electrochemical phase separation,and its composition is Ethanol?100ml?+Acetic acid?100ml?+Rosin acid?60mg?+ammonium benzoate?200mg?.The electrolyte has suitable conductivity and acidity,and is low sensitive to temperature and electrolysis process.Selecting-900m V as electrolysis potential,the?-Mg phase undergoes anodic reaction and is electrolyzed while the Mg₁₇Al₁₂ phase and Al-RE phase are retained without undergoing electrochemical reaction,thus realizing effective separation of?-Mg phase and compound phase.During the phase separation process,the electrolyte temperature is maintained at-15°C to suppress the chemical reaction rate of the alloy.After the electrolysis is completed,the solid-liquid phase in the reaction system is separated,and the liquid phase is converted into an aqueous solution that can be used for ICP measurement.The solid solution amount of the alloy element is calculated according to the content of the alloy element in the solution and the mass of the decomposed solid solution.The actual solid solution amounts of RE,Al and Zn elements in the experimental alloy were determined by the above method.The results show that no matter whether the alloy is non-equilibrium solidified or near-equilibrium solidified,the solid solution amount of RE is particularly low.The solid solution amount of Ce is tens of ppm,and the solid solution amount of Y and Gd is hundreds of ppm,and the solid solution amount of Gd is slightly larger than Y.The solid solution of Al is relatively large.Under die casting conditions,the solid solution of Al in AZ31-RE alloy can reach about 2/3 of its content in the alloy,about 1/2 of its content in AZ61-RE,and more than 1/3 of its content in AZ91-RE alloy.The solid solubility of RE and Al both increase with the increase of their content in the alloy,but decrease each other's solid solubility.The amount of Zn solid solution increased slightly with the increase of RE content,but slightly decreased with the increase of Al content.When the solidification rate is increased,the solid solution amount of each element generally shows an increasing trend,but the solid solution amount of Zn under near-equilibrium solidification conditions is higher than that of non-equilibrium solidification.Based on the Hume-Rothery solid solution rule,thermodynamic calculation and first-principles analysis,the equilibrium solid solution ability and its difference of alloy elements in Mg-X?RE,Al,Zn?system in Mg base were investigated.The results show that the above alloy elements can be dissolved in Mg,and there is indeed a difference in the solid solution capacity of the elements.Among them,the solid solution capacity of Y and Gd is much greater than Ce,and the solid solution capacity of Al is greater than Zn.The experimental alloy is regarded as Mg-Al-Zn-RE quaternary alloy,and the solid solution amount of each element in the non-equilibrium solidification solid state is calculated by using the thermodynamics software.The results show that the change rule is similar to the measured solid solution amount.Based on the calculation of the formation energy of Mg-Al-X?RE,Zn?solid solution and the activity interaction coefficient of elements in Mg-Al-RE solid solution,the interaction of elements in solid solution is explored,and the conclusion that Al and RE promote solid solution mutually,while Al and Zn inhibit solid solution mutually is drawn.Finally,the microstructure formation process and phase composition of the alloy were analyzed.The main reason for the low solid solubility of RE and the decrease of Al is that Al and RE preferentially form Al-RE compounds during the solidification of the alloy.It is also found that Zn is easier to dissolve in eutectic?-Mg₁₇Al₁₂ phase.The influence of alloy elements on the mechanical properties of Mg-based alloy is calculated and analyzed in this paper.The solid solution strengthening efficiency of elements and its causes were studied with the method of combining experiment and calculation.The results show that the order of solid solution strengthening efficiency of each element is Y>Gd>Zn>Al.Energy levels of elements and the electron orbital hybrid calculation shows that near the Fermi level,the 4d orbital electrons of Y and Gd have obvious hybridization with the 2s and 2p orbital electrons of Mg,showing strong covalent bond characteristics,while the hybridization between the 2s and 2p orbital electrons of Zn and Al and the 2s and 2p orbital electrons of Mg is relatively weak,and the covalent bond characteristics are not obvious.The difference in the strength of covalent bonds formed between the different elements and Mg may be one of the important reasons why the solid solution strengthening efficiency of rare earth elements is higher than that of Al and Zn.