First-principles Calculation Of The Strengthening Mechanism Of Rare Earth Elements Y And La In Mg-Zn Alloy

As a lighter metal material for industrial applications,magnesium?Mg?alloys have been widely used in automobile manufacturing,aerospace 3C industries rail transit and biomedicine based on their good performance advantages.However,due to the increasing variety and complexity of application forms,the performance requirements of Mg alloys are also increased.The alloying with rare earth?RE?elements is an effective way to improve the mechanical properties of Mg-based alloys.Mg-Zn-RE alloys have always gained much attention due to their excellent mechanical properties and unique crystal structures.Adding Y and La elements into Mg-zinc?Zn?alloy can not only enhance the mechanical properties by improving the casting properties and forming a stable quasicrystal strengthening phase,but also improve the high temperature strength and creep resistance.In addition,the rare earth elements in the Mg alloy can also highly improve the elongation and impact toughness of Mg alloy.In order to investigate the strengthening mechanism of RE elements Y and La on Mg-Zn alloy.The first-principles calculation method based on density functional theory with the generalized gradient approximation is used to investigate the thermodynamic stability,mechanical properties,and electronic structure of Mg-Zn-Y?La?intermetallic compounds in this paper.Meanwhile,the performance of the compounds under high pressure,the strengthening effect and mechanism in Mg-based alloy after Y and La dissolved were all studied by using the first-principles pressure and tensile test methods.Firstly,the physical and electronic properties such as thermodynamic stabilities,mechanical properties,melting points,hardness and electronic structures of the compounds MgZn₂,Mg₂Y,Mg₂La,Mg₃La and Mg₃Zn₃Y₂ were analyzed.The results show that MgZn₂,Mg₂Y and Mg₂La are ductile phases and Mg₃La,Mg₃Zn₃Y₂ are brittle phases.The alloying ability and thermodynamic stability are enhanced by adding Y and La into the Mg-Zn alloy.Deformation resistance,shear resistance and stiffness of ternary compound Mg₃Zn₃Y₂ are stronger than binary compounds?MgZn₂,Mg₂Y,Mg₂La and Mg₃La?,which indicates that the mechanical properties of Mg-Zn alloy can be improved by generating ternary alloy.In addition,the hardness and melting point of the compounds containing Zn and Y are higher than other compounds referenced in this paper.Secondly,the performances of Mg₃Zn₃Y₂,MgZn₂,Mg₂Y,Mg₂La and Mg₃La under high pressure were calculated based on the first-principles pressure method.The results show that the compression resistances of these compounds become stronger under high pressure and their alloying ability and stability are decreased with increasing pressure.When the pressure is increased to 10GPa or 20GPa,these compounds cannot be formed through the normal exothermic reactions.Induced transformation does not happen to Mg₃Zn₃Y₂ under high pressure,For Mg₃Zn₃Y₂,the metal properties are still excellent,however the conductivity is weakened under high pressure.The heat resistance and wear resistance of Mg₃Zn₃Y₂ are increased under high pressure.Deformation resistance,shear resistance and stiffness of these compounds?Mg₃Zn₃Y₂,MgZn₂,Mg₂Y,Mg₂La and Mg₃La?are improved under high pressure.Under high pressure environment,the mechanical properties of MgZn₂ are stronger than that of Mg₂Y,and the mechanical properties of Mg₂La are stronger than that of Mg₃La.The plasticity of Mg₂La,Mg₃La,and Mg₃Zn₃Y₂.can be improved under high pressure.The plasticity of MgZn₂ shows a weakening trend under high pressure.The ductility and toughness of Mg₂Y are stronger than that of MgZn₂,the ductility and toughness of Mg₂La are stronger than that of Mg₃La under high pressure.Finally,the strengthening effect of Zn,Y and La dissolved in the Mg-based were studied based on first-principles tensile test method.In the experiment,Mg atoms were replaced by Zn,Y and La atoms to form Mg₅₃X₁ and Mg₅₂X₂ solid solutions with 1.8 at.%and 3.7 at.%,respectively.The cohesive energy,differential charge density,electronic state density,and stress of these solid solutions under a strain of 0-20%were calculated and the mechanism of solid solution strengthening was also analyzed.The experimental results show that the solid solution containing Y and La atoms has a more stable structure under tensile strain,and the stability of Y and La in strengthening the Mg-based is stronger than that of Zn.When the X content is 1.8 at.%,the effects of the Y and La on improving the yield and tensile strength of the solid solution are stronger than that of the Zn.When the X content is 3.7 at.%,the strengthening effect of Zn and Y on the solid solution is stronger than that at 1.8 at.%,however the strengthening effect of La on the solid solution is disappeared,which will cause the decrease of tensile properties of the Mg-based alloy.