Preparation And Microstructure Mechanical Properties Of Mg-Re Alloy Reinforced By Spherical Mg₃Zn₆Y Particles By

Mg alloys exhibit great application potential in the prospects of aerospace and transportation fields,because they possess super advantages including low density,high specific strength and stiffness,etc.However,the low absolute strength and poor high temperature performance severely restrict their wide application.Introduction of hard reinforcements can significantly elevate the mechanical properties of Mg alloys.Mg₃Zn₆Y quasi-crystal phase is one of the most important reinforcements in Mg matrix composites due to its high hardness,high thermodynamic stability,high modulus and good wettability with theα-Mg matrix.However,in the reported references,the Mg₃Zn₆Y quasi-crystal usually takes the large block,dendrite,eutectic lamellar and petal-like morphology with a particles size larger than 50μm and a low volume fraction.Consequently,the reinforcement performance was not well exploited.In the present work,the in-situ Mg₃Zn₆Y quasi-crystal master alloy was prepared by melt spin quenching method in the first step,and the effect of solidification rate on the morphology,volume fraction（V_f）and precipitation mechanism of Mg₃Zn₆Y phase were studied.Then,the atomized WE43 alloy powder was selected as the matrix,and the Mg₃Zn₆Y quasi-crystals as strengthening particles introduced by the master alloy prepared in the first step.The Mg₃Zn₆Y particles reinforced WE43 magnesium matrix composites were prepared by mechanical ball milling plus hot press sintering（HPS）.The effects of sintering temperature and the addition amount of Mg₃Zn₆Y quasi-crystal master alloy on the microstructure and mechanical properties of sintered WE43 alloy and composites were studied,respectively.The main results are as follows:（1）The effect of copper rolling speed on the morphology and volume fraction of Mg₃Zn₆Y quasi-crystal in Mg₅₈Zn₃₆Y₆master alloy prepared by melt spin method was studied.Under high cooling rates,the master alloys only contain high volume fraction Mg₃Zn₆Y phase and a littleα-Mg phase.The Mg₃Zn₆Y phase is formed from the supercooled melt through pseudo-eutectic reaction.When the rolling speed is up to 2000 r/min,the Mg₃Zn₆Y quasi-crystal phase takes spherical or nearly spherical morphology with a very fine average particles size（210 nm）and high V_f（90%）.（2）The effects of HPS parameters on the microstructure and properties of WE43 alloy were studied.The WE43 alloys sintered at 400oC～560oC are composed of Mg₄₁Nd₅,β-phase(Mg₁₄Nd₂Y),Mg₂₄Y₅andα-Mg phase（30MPa,60 min）.With the increase of sintering temperature,the volume fraction and size of pores decreased significantly in the WE43 alloy.For the WE43 alloy sintered at 400oC,a large number of pores were formed at the triangular grain boundaries,and the powders were only locally contacted.There are no obvious pores and the powders interface is tightly bonded,when the sintering temperature is higher than 440oC.The volume fraction and size of the precipitated phases increase with the sintering temperature increase.The grains coarsening is obvious when the sintering temperature is 560oC.In conclusion,the WE43 alloys could be sintered at 440oC～520oC possess good powder interface bonding,fine and dispersed distribution of precipitates,without obvious pores and abnormal growth of grains,the high relative density（>98%）.（3）The effects of sintering temperature（440oC,480oC,520oC）on the microstructure and mechanical properties of WE43-15wt.%Mg₃Zn₆Y composites were studied.When the sintering temperature is 440oC,the Mg₃Zn₆Y phase still exist and has not undergone phase transformation.However,no Mg₃Zn₆Y phase was detected in the composites sintered at 480oC and 520oC,because the Mg₃Zn₆Y transformed into the W-Mg₃Zn₃Y₂phase.With the increase of sintering temperature,the particle size of Mg₄₁Nd₅,Mg₂₄Y₅and Mg₁₄Nd₂Y phases in the composites increased and the volume fraction decreased significantly.The increase of sintering temperature resulted in a significant decrease in the microhardness and compressive strength of the composites,but all composites demonstrated a good plasticity with a high value compression deformation rate（15%～17%）.The composites sintered at 440oC have the most excellent comprehensive mechanical properties,namely the highest microhardness（96 HV）,ultimate compressive strength（370 MPa）and compressive yield strength（184 MPa）.It can be concluded that the strengthening effect of Mg₃Zn₆Y phase is better than that of W-Mg₃Zn₃Y₂phase.（4）The effect of the addition amount of Mg₃Zn₆Y quasi-crystal master alloy on the microstructure and mechanical properties of the WE43-x wt.%Mg₃Zn₆Y composites sintered at440oC was studied.With the addition of Mg₃Zn₆Y quasi-crystal master alloy,the microhardness of the composites increased significantly,the compressive yield strength and ultimate compressive strength increased firstly and then decreased.Interestingly,the overall compressive deformation rate decreased slightly with the increasing adding amount of Mg₃Zn₆Y quasi-crystal particles,which suggests that Mg₃Zn₆Y quasi-crystal almost would not deteriorate the plasticity of the HCP structured Mg-matrix.When the addition amount is 10%,the composite exhibits excellent comprehensive mechanical properties:the microhardness value is92 HV,the compressive fracture strength and compressive yield strength is as high as 471 MPa and 238 MPa.The compression deformation rate can also be as high as 15.8%.The addition of10wt.%Mg₃Zn₆Y particles can significantly improve the comprehensive mechanical properties of composites.The main reason is that the hard Mg₃Zn₆Y phase maintains a strong coherent interface withα-Mg matrix,which can effectively transfer the stress from matrix to Mg₃Zn₆Y phase particles during the plastic deformation process.Furthermore,the reinforcing particles can hinder the slip of dislocations.The strong interfacial bonding can also prevent the initiation of crack source during the plastic deformation process,thus improve the plastic deformation ability of composites.In conclusion,the introduction of Mg₃Zn₆Y particles significantly improves the strength and toughness of WE43 alloy.