Study On The Solid Solution Characteristics And Influence Mechanism Of Alloying Elements In Mg-Zn-Y Series Magnesium Alloys

ZK series magnesium alloy is an important series of magnesium alloys,it is currently one of the high-strength magnesium alloy material systems with excellent comprehensive performance,it has the advantages of high specific strength,excellent resistance to stress corrosion,and heat treatment strengthening.In order to improve the performance of ZK series magnesium alloy,many elements including rare earths are used for alloying treatment,alloying is an important method to improve the properties of magnesium alloys,alloying elements affect the performance of the alloy by affecting the structure.Solid solution is an important state of alloying elements,amount of solid solution of the element directly affects the degree of solid solution strengthening of the alloy,also indirectly affects the structure type,quantity and grain size of the compound and other organizational factors,thereby affecting the alloy structure and bringing about changes in performance.Y is a rare earth alloying element commonly used in ZK series alloys,but from the existing related research work,the research on the solid solution of Y in ZK series magnesium alloys and the influence on the microstructure and properties is mostly based on qualitative analysis,at the same time,there is rarely an in-depth analysis of the reasons for the changes in tissue properties from the perspective of Y solid solution properties.For this reason,this subject uses ZK31-Y and ZK61-Y as the research alloy system,using a method that combines theoretical calculations and experiments,under different solidification and cooling modes,the solid solution characteristics of Y and Zn and the corresponding changes in the structure are analyzed,from the perspective of element solid solution,the influence of alloy structure and solid solution mechanical properties is analyzed and discussed.In equilibrium solidification and Scheil-Gulliver non-equilibrium solidification mode(S-G mode),the solid solubility and element distribution coefficient of Y and Zn in Mg are calculated and analyzed,to understand the solid solution characteristics of Y and Zn elements in Mg.The results show that both alloying elements can be dissolved in Mg,but under the two different solidification modes,the solid solubility and solute distribution coefficient of Y are both greater than that of Zn;In S-G mode,alloying elements are prone to supersaturated solid solution,and the amount of solid solution at room temperature is greater.Using first-principles to calculate the formation energy and charge transfer of solid solution,it turns out that,the formation energy of Mg-Y solid solution is less than that of Mg-Zn solid solution,that is,Y is easier to dissolve into the Mg matrix,Y and Mg share more charges than Zn and Mg,the bond is stronger and the solid solution is more stable.In equilibrium solidification and S-G non-equilibrium solidification modes,calculate and analyze the solid solubility of Y and Zn elements in Mg-Zn-Y-Zr alloy,under near-equilibrium solidification conditions,the amount of solid solution of Y and Zn elements in Mg-Zn-Y-Zr alloy was determined experimentally.The results show that whether in equilibrium solidification or S-G non-equilibrium solidification mode,solid solubility of Y and Zn are significantly lower than their corresponding values in Mg;In equilibrium solidification,S-G non-equilibrium solidification and near equilibrium solidification alloys,Y and Zn all have a mutual inhibition of solid solution relationship.The activity interaction coefficients of Y and Zn in the alloy system are calculated,found that the activity interaction coefficient is positive,that is,in thermodynamics,the Y and Zn elements coexisting in the solid solution repel each other to increase the activity,which leads to the decrease of the respective solid solution capabilities.In equilibrium solidification and S-G non-equilibrium solidification modes,the phase composition of Mg-Zn-Y-Zr is calculated and analyzed,actually observe the structure and phase composition of the alloy.Calculations show that the solute partition coefficient of Zn and Y is less than 1,will be enriched at the front of the solid-liquid interface,when Zn and Y are enriched to a certain extent,the ternary phase begins to precipitate.The type of ternary phase is related to the Zn/Y ratio in the remaining melt,the lower the Zn/Y,the higher the content of the ternary phase with the lower Y measurement.The Zn/Y of the remaining melt is not only related to the alloy composition,also affected by the solid solution ability of Y and Zn.The reasons for the changes in the alloy structure are analyzed from the perspective of element solid solution.Calculation and analysis of the effect of element solid solution on the elasticity and plasticity of solid solution,solid solution strengthening efficiency of the elements is studied by a combination of experiment and calculation.The results show that,both Y and Zn dissolved in Mg can increase the elastic modulus of Mg,indicating that the rigidity of the solid solution becomes larger,but both Y and Zn dissolved in Mg will reduce the plasticity of the solid solution,that is,the ductility will be worse;Regardless of elasticity and plasticity,Y has a stronger influence,solid solution strengthening efficiency of Y is significantly higher than that of Zn.The 4d orbital of Y has a wider distribution and higher energy at the Fermi level,stronger hybridization with 2s and 2p orbitals of Mg,showing stronger covalent bond characteristics.When Y and Zn exist in the Mg matrix at the same time,electron orbital between Zn-Y is also hybridized,which makes the covalent bond characteristics in solid solution stronger than binary alloys.