Experimental Study Of Multi-phase Mg-Li-Zn-Y Alloys:Composition,Microstructure And Deformation Behavior

The depletion of energy resources and environmental pollution have emerged as two major bottlenecks in the current social development of our country.Lightweight design is considered an effective strategy to achieve energy conservation and emission reduction.Mg-Li alloys,which have low density,good plasticity,high specific strength,and abundant reserves,stand out as the most ideal lightweight materials.However,the relatively low absolute strength and modulus of traditional Mg-Li alloys significantly limit their industrial applications.Typically,the introduction of high-strength rare earth phases into Mg-Li alloys,such as the Ⅰ-phase（Mg₃Zn₆RE）and the W-phase（Mg₃Zn₃RE₂）,can effectively enhance the strength of the alloy.Nevertheless,stable rare earth phases tend to aggregate at grain boundaries in a reticulated structure and exhibit continuous distribution,resulting in material embrittlement and reduced alloy ductility.Therefore,how to adjust the microstructure of Mg-Li alloys to optimize the distribution of the second phase and improve the strength-ductility amalgamations of Mg-Li alloys is key to the research of high-performance Mg-Li alloys.Mg-Li-Zn-Y alloys are chosen as the research object.There is a structural transition betweenβ-Li（BCC structure,a solid solution phase in which Mg is dissolved in Li）andα-Mg（HCP structure,a solid solution phase in which Li is dissolved in Mg）in Mg-Li alloys.By changing the Li content,Mg-Li alloys with different phase structures can be formed.Combining DSC and phase diagram analysis,the formation mechanism of the Mg-Li alloy structure was explored.Through room temperature stretching,we explore the mechanical behavior of the alloy during stretching and reveal the strengthening mechanism of Mg-Li alloys.Combining in-situ stretching,EBSD analysis,TEM analysis,and DIC（digital image correlation）technology,the intergranular slip transfer is analyzed to reveal the deformation coordination mechanism of Mg-Li alloys.The main conclusions obtained are as follows:（1）Through the construction and analysis of phase diagrams,the study investigates the influence of the Zn/Y ratio and the Li,Zn,and Y contents on the alloy phase composition.The phase transformation sequence of the Mg-Li-Zn-Y alloy during solidification is predicted.By combining modulus-phase composition,phase composition-composition,and density-composition correlation models,a range of low-density,high-modulus（ρ≤1.6 g/cm³,E≥45 GPa）Mg-Li alloy compositions are obtained,specifically:Zn/Y（wt.%）:4.4～6.7,Li（wt.%）:6.7～10.3,Y（wt.%）:1～2,Zn（wt.%）:4～10.Within this preferred range,two compositions,Mg-8.5Li-6.6Zn-1.5Y（LZW861）and Mg-9.5Li-8.8Zn-2Y（LZW982）,are selected for experimental study.The cast LZW861 alloy comtains β-Li,α-Mg,reticular Ⅰ-phase,（Li,Mg）₃Zn nanoscale precipitates and a small amount of W-phase.In contrast,the LZW982 alloy lacksα-Mg and is composed of β-Li,Ⅰ-phase,（Li,Mg）₃Zn phase,and a small amount of W-phase.（2）The cast LZW861 and LZW982 alloys studied exhibit higher strength compared to conventional Mg-Li alloys.The alloy strengthening is mainly attributed to nanoscale（Li,Mg）₃Zn precipitates dispersed in theβ-Li matrix and micron-scale reticular Ⅰ-phases enriched at interfaces.The（Li,Mg）₃Zn phase exhibits low lattice misfit with the matrix,presenting orientations of（200）β-Li//（200）（Li,Mg）₃Zn and[110]β-Li//[110]（Li,Mg）₃Zn.TEM reveals that sliding dislocations within the matrix intersect the（Li,Mg）₃Zn phase,resulting in continuous slip planes between the two phases,significantly enhancing the alloy matrix without sacrificing ductility.The reticular Ⅰ-phase acts as a barrier to dislocation motion,leading to the accumulation of numerous dislocations along the I/βinterface,thereby generating a strengthening effect.However,the mismatch in geometric deformation between the hard I phase and the soft matrix results in the non-coordinated development of plastic deformation at the interface and an increase in interface stress concentration,thereby promoting Ⅰ-phase cracking,which simultaneously strengthens the alloy while affecting its ductility.（3）Compared to theα-Mg phase-free LZW982 alloy（EL=5.1%）,the（β+α）dual-phase matrix of the LZW861 alloy（EL=17.8%）demonstrates a superior strength-ductility balance.This is attributed to the significant increase in interface density upon the introduction of theα-Mg phase.The high-density interfaces disperse the distribution of the Ⅰ-phase along the interfaces,reducing the size of the Ⅰ-phase and alleviating the deformation mismatch between the Ⅰ-phase and the matrix.Simultaneously,during tensile deformation,dispersed Ⅰ-phases form diffuse microcracks,delaying the process of damage evolution.（4）Regarding intragranular deformation behavior,no twinning was observed within theα-Mg phase,and plastic deformation is primarily regulated by dislocation slip.Dislocation configuration analysis indicates that the main modes involved in coordinated deformation are basal<a>slip and prismatic<c+a>slip.The strain coordination near grain boundaries is influenced by the geometric coordination of slip systems on both sides of the grain boundary.Based on the orientation relationship{110}Li//{0001}Mg and<（?）11>Li//<11（?）0>Mg betweenα-Mg andβ-Li,possible slip systems for both phases are constructed.The slip planes{123}Li-{11（?）2}Mg and{112}Li-{11（?）2}Mg exhibit higher continuity at the interface than other plane pairs.The{11（?）2}planes of theα-Mg show slip continuity with the{123}and{112}planes of theβ-Li,facilitating the deformation coordination between the two phases.（5）During the heat treatment process,theα-Mg phase dissolves at high temperatures and reprecipitates and grows during subsequent slow cooling,while smaller secondary nanoscale（Li,Mg）₃Zn particles form within theβ-Li phase.The dissolution of theα-Mg and Ⅰ-phases at high temperatures results in the matrix being in a supersaturated state.During the subsequent cooling process,the solubility of Mg and Zn atoms in the Li matrix decreases with decreasing temperature,further enhancing the elemental supersaturation and forming a high phase transformation driving force,promoting the nucleation ofα-Mg and（Li,Mg）₃Zn phases in large quantities.（6）Before and after heat treatment,the alloy’s different deformation behaviors can be attributed to different phase morphologies.The smaller-sizedα-Mg phase is fully enveloped by theβ-Li matrix.Compared to the larger-sized blockyα-Mg phase,its phase boundary influence zone is proportionally greater.This can activate a large number of slips along the phase boundaries,resulting in the formation of fine and uniform slip lines within theα-Mg phase,thereby inducing more extensive deformation of theα-Mg phase and optimizing the deformation coordination between the two phases.