First-principles Study Of Phase Interface Characteristics In Mg-Gd-Y Alloy

Rare earth elements have great influence on the high temperature mechanical properties,formability and corrosion resistance of magnesium alloy.The addition of trace gadolinium(Gd)and yttrium(Y)can not only refine the microstructure and im-prove the strength of magnesium alloy,but also improve the creep resistance,corro-sion resistance and casting formability of magnesium alloy.In Mg-Gd-Y alloy,Mg₂₄Y₅ phase and Mg₃Gd phase are common stable phases.Their interface with Mg alloy directly affects the interface properties and mechanical properties.The interface structure,energy and charge properties of Mg(001)/Mg₂₄Y₅(011)and Mg(001)/Mg₃Gd(111)have been studied by first principles density functional method.The ef-fect of Mg₂₄Y₅ and Mg₃Gd on the strength of Mg-Gd-Y alloy was analyzed.The rela-tionship between the interfacial bonding strength and the degree of mismatch,the in-terfacial spacing,the bulk crystal system and the terminal atoms was proposed as W_int=F(?,?,d,?).In this paper,the phase interface properties of magnesium alloy and its influencing factors were studied,hoping to shorten the research and development cycle and reduce the research and development cost.The main contents of this study are as follows(1)For the Mg(001)/Mg₂₄Y₅(011)interface,there are four different terminal in-terfaces:Mg₂+Y₁+Y₂,Mg₂,Mg₁ and Mg₂+Y₂-Mg(001)/Mg₂₄Y₅(011)interfaces.The interface separation work of Mg₂+Y₁+Y₂ terminal is the largest,which is 1.04 J/m²,and the interface spacing is 1.26(?),which is the most stable terminal interface.The results show that the most obvious phenomena are the aggregation degree of charge density at the interface of Mg₂+Y₁+Y₂,and the interaction between atoms at the in-terface is stronger than that of Mg matrix,but weaker than that of Mg₂₄Y₅ bulk phase.The interface strength is enhanced by the combination of Mg-Y and Mg Mg metal bonds,the hybridization of s,p orbitals of Mg atom and d orbitals of Y atoms.Mg-Mg and Mg-Y atomic bonding have different contributions to the interfacial bonding strength.The Mg-Mg force is similar to the weak Mg matrix,so it is mainly by introducing new Mg-Y bond to enhance the interfacial bonding strength.Although Mg(001)surface is most likely to form the optimal interface with the Mg₂₄Y₅(011)surface of the Mg₂+Y₁+Y₂ terminal,the weakest point of the interface is on the Mg(001)side.This indicates that the microcracks initiated at the grain boundary are more likely to propagate to the Mg end and cause the damage of the alloy.(2)There are seven kinds of terminals in Mg(001)/Mg₃Gd(111)interface:Gd-HB;Gd-T;Gd-B;Mg₁-T;Mg₁-HB;Mg₂-T;Mg₂-HB-Mg(001)/Mg₃Gd(111)in-terface.The separation work values of the seven terminal interfaces from larg-e to small are:Gd-B>Gd-T>Mg₁-HB>Mg₂-HB>Gd-HB>Mg₁-T>Mg₂-T.The interface separation work of Mg₁-T terminal is the minimum,which is 0.65 J/m²,while that of Gd-B terminal is the maximum,which is 1.32 J/m²,so it is the most stable terminal choice.The analysis of the density of charge states shows that the d orbitals of Gd atom and the s and p orbitals of Mg atom are obviously hybridized in the range of2.5?5 e V,and the s orbitals of Mg atom itself are greatly hybridized,which promotes the bonding between atoms and strengthens the bonding force of atoms at the inter-face.(3)According to the analysis results of the interfacial separation work of the most stable Mg(001)/Mg₂₄Y₅(011)and Mg(001)/Mg₃Gd(111),the interfacial separa-tion work of Mg matrix is about 0.8 J/m²,the interfacial separation work of Mg/Mg₂₄Y₅ is increased by 30%and Mg/Mg₃Gd is increased by 65%.For the former,the interface stability is better.The mechanical properties of Mg-12Gd alloy are im-proved by the interface strengthening effect of Mg₃Gd,while the stability of Mg₂₄Y₅alloy is weakened at the later stage of aging,and the improvement of interface strength is weaker than that of Mg₃Gd.Therefore,more Mg₃Gd phase are expected to improve the interfacial strength.According to the relationship between phase/magnesium interfacial separation work and mismatch degree,interfacial spac-ing,bulk crystal system and terminated atoms,the criterion of interfacial bonding strength is summarized as W_int=F(?,?,d,?)and the correctness of the theory was con-firmed.