Intermetallic Compounds In ZK-RE(Ce,Y,Gd)Magnesium Alloy And Their Evolution

As the lightest metal structural material that has been commercialized,magnesium alloy has wide development potential,and ZK magnesium alloy is one of the most widely used commercial magnesium alloys.For the mechanical properties of ZK magnesium alloy that still have low strength,alloying modification is undoubtedly an important method.One of the important purposes of magnesium alloy alloying is to form intermetallic compounds,and the second phase strengthening mechanism is used to achieve alloy strengthening.At present,rare earth elements are widely used in the alloying treatment of ZK alloys,but in ZK-RE alloys composed of added rare earths,the compounds will become more diverse and their functions more complicated.It affects not only the mechanical properties of the alloy,but also the processing properties of the alloy.These effects depend not only on the structure of the compound,but also on the changes of the compound during the processing.Therefore,accurately determining the structure of the compound and grasping its evolution in such as hot working is an important prerequisite for the role of rare earth in ZK magnesium alloy.This subject is aimed at the practical application of ZK series magnesium alloys treated with rare earth alloys.ZK31 and ZK61 with the highest degree of commercialization are selected as master alloys.Typical rare earth elements Ce,Y,Gd are added and the amount of addition is changed It constitutes the ZK-RE（Ce,Y,Gd）experimental alloy system,which is processed into ingots by semi-continuous casting and heat-treated according to the conventional homogenization process of the ZK magnesium alloy.On this basis,the phase structure of intermetallic compounds in as-cast and homogenized alloys was identified by a combination of transmission electron microscopy electron diffraction and extraction phase X-diffraction;For the content determination,the number of compounds in the as-cast and homogenized alloys was analyzed.At the same time,the changes in the structure and number of the compounds after homogenization were studied.The intermetallic compound in the as-cast ZK61 and ZK31 magnesium alloys is MgZn₂,while the intermetallic compound in the as-cast ZK61-Ce and ZK31-Ce magnesium alloys is the ternary compound（MgZn）₁₂Ce.The content of Zn remains unchanged,the content of rare earth increases,and the number of intermetallic compounds increases;the content of rare earth remains unchanged,the number of intermetallic compounds with a Zn content of 6%is higher than that of 3%.After homogenization treatment,the intermetallic compound phase of ZK31 and ZK61 alloy is still MgZn₂,and the intermetallic compound phase of ZK-Ce alloy is still（MgZn）₁₂Ce;MgZn₂ decomposes during homogenization treatment,and MgZn₂ after homogenization treatment The mass fraction decreases,（MgZn）₁₂Ce only changes the element relative ratio in the homogenization,and the mole fraction remains basically unchanged.In the as-cast ZK31-Y alloy,the intermetallic compounds are mainly Mg₃Zn₃Y₂and MgZnY,while in the as-cast ZK61-Y alloy,the intermetallic compounds are mainly W-Mg₃Zn₃Y₂ and I-Mg₃Zn₆Y,of which I-Mg₃Zn₆Y is the quasicrystalline phase.The increase in the content of Zn and Y not only increased the number of compounds in the as-cast state,but also changed the type of compound.In the study,only I-Mg₃Zn₆Y was found in the as-cast ZK61-Y as a quasi-crystalline phase.After homogenization,MgZnY disappeared in ZK31-Y alloy,and only W-Mg₃Zn₃Y₂ phase existed;I-Mg₃Zn₆Y quasicrystalline phase disappeared in ZK61-Y alloy,and only W-Mg₃Zn₃Y₂phase existed.Some of the elements after the decomposition of MgZnY and I-Mg₃Zn₆Y will form a stable W-Mg₃Zn₃Y₂ phase,but due to the reduction of Y/Gd provided by their decomposition,the number of compounds after homogenization of the ZK-Y/Gd alloy is reduced.In the as-cast ZK31-Gd alloy,the intermetallic compounds are mainly Mg₃Zn₃Gd₂and MgZn Gd,while in the as-cast ZK61-Gd alloy,the intermetallic compounds are mainly W-Mg₃Zn₃Gd₂ and I-Mg₃Zn₆Y,of which I-Mg₃Zn₆Gd is the quasicrystalline phase.The amount of compound in the as-cast alloy varies with composition and the type and amount of compound after homogenization is basically the same as that of ZK-Y alloy.