Preparation And Properties Of LPSO Hybrid Reinforced Magnesium Alloys

Due to its low density,high specific strength and good shockabsorbing performance,magnesium alloy has a wide application prospect in lightweight weapons and equipment,rail transit,aerospace and other fields that are in urgent need of weight reduction,energy saving and emission reduction.Mg-RE alloy has excellent strength and creep resistance due to theβ′phase precipitated in the aging process,but due to the influence of the dense hexagonal crystal structure（HCP）and the hard and brittleβ′phase,the plasticity of the series of alloys is poor,and the elongation is often very low,which greatly restricts its application.It is reported that a new reinforcing phase Structure——Long Period Stacking Ordered Structure（LPSO structure）,can be generated by adding Zn,Cu,Ni and other elements in Mg-RE alloy under the conditions of composition process preparation.The phase has high hardness,high elastic modulus and high thermal stability,and forms a coherent interface with magnesium matrix.The interface energy is low,which is conducive to load transfer,inhibit crack initiation and propagation,and its existence can reduce the c/a ratio of Mg and activate the prismatic plane<c+a>Slip system.The plastic deformation ability of magnesium alloy can also be improved by torsion deformation.Five kinds of Mg-9Gd-3Y-x Zn-0.6Zr（x=0.5,1.1,1.87,2.2,3.75,wt%）alloys were prepared by iron casting.The effects of Zn content on the microstructure and mechanical properties of the alloys under non-equilibrium solidification conditions were systematically studied by different heat treatments and hot pressing sintering processes.The content and type ofβ′,Mg₅Gd and LPSO phases were regulated to optimize the strength and toughness of the material.The main research results are as follows:（1）The effect of Zn content on the microstructure and mechanical properties of as-cast Mg-9Gd-3Y-x Zn-0.6Zr alloy was studied.As-cast alloy is made up ofα-Mg substrate and distributed in the branches of the grain boundary Mg₂₄（Gd,Y,Zn）_5,Mg₅（Gd,Y,Zn）eutectic phase and layered organization 18R phase composition.When the content of Zn is 0.5%,a small number of 2H stacking faults and lamellar 18R phases are formed at the grain boundary.With the increase of Zn content,the eutectic phase content decreased gradually,and the 18R phase increased gradually.The morphology changed from lamellar phase to block phase distributed along grain boundaries.The volume fraction of 18R phase in 3.75 Zn alloy is up to 40%,the tensile strength is 222 MPa,the yield strength is 138 MPa,the elongation is 8.4%,and the hardness is 88 HV.（2）The effect of heat treatment on the microstructure of Mg-9Gd-3Y-x Zn-0.6Zr alloy was studied.At 480℃,the meshed eutectic structure with 0.5～1.87 Zn content was partially dissolved,and the lamellar and massive 18R phase was formed at the dendrite boundary.3.75Zn alloy of the eutectic phase is completely dissolved,and the morphology of the massive 18R phase at the grain boundary does not change significantly.When the solution temperature increased,the eutectic phase dissolved more obviously at 500℃,and the LPSO phase content was higher than that at 480℃.When the solution temperature is further increased to 520℃,the eutectic phase of 0.5 Zn alloy is basically dissolved into the matrix,while the microstructure of 1.1 Zn alloy changes from lamellar texture to linear texture.The LPSO phases at the grain boundary of 1.87 Zn and 2.2 Zn were decomposed in solid solution,showing the characteristics of lamellar phase.A large number of bright white rare earth phases are distributed in the grain boundary and intragrystalline of 3.75 Zn alloy,and the continuously distributed massive 18R phase is decomposed into discontinuous massive phase and spherodized particle phase W-Mg₃Zn₃（Gd,Y）₂.The solution treatment at 480℃～500℃ is conducive to the dissolution of eutectic phase and promote the formation of 18R phase.High solution temperature will lead to the decomposition of 18R phase.The ratio of two phases can be controlled by the content of1.1 Zn.After solution at 520℃ for 24 h and aging at 225℃ for 63 h,a large number of closely arrangedβ′phases are formed in the matrix,and 18R phases are distributed parallel to the base surface of the magnesium matrix,and the orientation relationship is perpendicular to each other,the base plane formed LPSO phase co-existed.3.75 Zn has no aging precipitatedβ’was found in alloys.（3）The effect of heat treatment on mechanical properties of Mg-9Gd-3Y-x Zn-0.6Zr alloy was studied.Compared with the as-cast alloy,the tensile strength and elongation of the alloy after solution treatment are increased to a certain extent,up to 233 MPa and 14.5%.The tensile strength and yield strength of 2.2 Zn alloy can reach 280 MPa and 210 MPa,respectively,and the elongation is 5.3%.At the solution temperature of 520℃,the microstructure of 0.5 Zn alloy becomes solid solution completely,the aging driving force is large,and a large amount ofβ′phase is precipitated in the alloy,so the strengthening effect is good,the tensile strength（268MPa）and yield strength（242 MPa）are high,but the elongation is as low as 2%.As for 1.87 Zn alloy,the coexistence ofβ′phase and LPSO phase was formed by aging at 500℃.The LPSO phase was strengthened at grain boundary,β′phase was strengthened in grain,the tensile strength（261 MPa）,yield strength（200 MPa）,elongation（5.3%）,and the comprehensive mechanical properties were excellent.（4）The microstructure and mechanical properties of Mg-9Gd-3Y-x Zn-0.6Zr alloy prepared by melt rotation quenching and hot pressing sintering were studied.GW93 alloy strip samples prepared by melt rotation quenching method are composed ofα-Mg solid solution with the size of about 1μm and Mg₃Gd particle phase with the size of 2.3μm.The microstructure of GW93series alloy after hot pressing sintering is composed ofα-Mg,Mg₅Gd and 14H phases,The hot pressing sintered samples have small grain size（13.45μm）and 14H volume fraction is higher.The grain size,precipitated phase size（200 nm）and 14H volume fraction of the alloy samples have little difference,so the mechanical properties of the alloy are not significantly different,but they can maintain high compressive strength.The alloys prepared by melt suspension quenching and hot pressing sintering have excellent mechanical properties.It has high density（96.15%）,microhardness（80-97 HV）,compressive fracture strength（413-461 MPa）and compressive yield strength（143-197 MPa）,and its elongation is also kept at a high level,about21.31-33.51%.