LPSO Phase Reinforced Lightweight Mg-Lialloy Design,preparation,and Characterization

Magnesium alloy has the characteristics of low density,high specific strength,and high specific stiffness,thus having broad application potential in the fields of aerospace,3C digital,biomedical,and military equipment.As the lightest metal material,Mg-Li alloy not only greatly improves the specific strength of the material but also significantly improves the poor plasticity of magnesium alloy.However,the low absolute strength of Mg-Li alloy severely limits its application in the field of engineering structural materials.At present,the common composite material reinforcement in Mg-Li alloy has some problems,such as easy aging transformation,poor interface bonding and poor particle dispersion,which leads to unsatisfactory strengthening and toughening effect.In recent years,a long period stacked ordered structure（LPSO-phase）formed in situ by matrix attached stacking faults（SFs）has been discovered in Mg alloy,which has a completely coherent interface with the matrix.Not only does it have high hardness and elastic modulus,but it can also coordinate the plastic deformation of the alloy through twisting and pinning dislocations.In the study,it was found that the precipitation and transformation of LPSO phase in Mg alloy are mainly influenced by alloy composition,heat treatment process,and atomic ratio.In this paper,Pandat phase diagram was used to calculate and design the composition and content of the alloy conducive to the precipitation of LPSO phase.The alloy is prepared using conventional casting iron mold pouring method and heat treated in an argon protected environment.Firstly,by comparing Mg-5Y-2.7Zn with Mg-6.5Li-5Y-2.7Zn as cast and heat treated alloys,the effect of Li element on LPSO phase precipitation and phase transformation during heat treatment process of the as cast alloy was studied,as well as the improvement of LPSO relative alloy properties.Secondly,different heat treatment temperatures and holding times were used to study the effect of heat treatment processes on the precipitation amount of LPSO phase in Mg-6.5Li-5Y-2.7Zn alloy,and to analyze the precipitation process of LPSO phase during the heat treatment process.Finally,the effect of Y/Zn ratio on the precipitation content and mechanical properties of LPSO phase in magnesium lithium alloys in the two-phase region was studied by preparing Mg Li Y-Zn alloys with different Y/Zn ratios.The research results are as follows:（1）According to Pandat phase diagram calculation,it was found that the formation of LPSO phase is beneficial when Y/Zn≥1（at%）in Mg-Y-Zn.In the Mg-Y-Zn and Mg-6.5Li-Y-Zn alloy system,the highest content of LPSO phase precipitated by equilibrium solidification is Mg-5Y-2.7Zn and Mg-6.5Li-5Y-2.7Zn.at the same time,the effects of Y/Zn（at%）ratios of 1:1,2:1 and 3:2 on the precipitated content of LPSO phase in Mg-6.5Li-5Y-x Zn alloy were compared from the perspective of phase diagram calculation.It was found that the calculated equilibrium solidification of Y/Zn=3:2（at%）precipitated the highest LPSO phase content.（2）The microstructure and mechanical properties of Mg-5Y-2.7Zn and Mg-6.5Li-5Y-2.7Zn alloys as cast and 500℃-8h heat treated alloys were studied.It was found that 18R-LPSO phase can be formed in Mg-5Y-2.7Zn alloy,but the addition of Li inhibits the growth of Mg dendrites and the precipitation of 18R-LPSO phase,and Mg₂₄（Y,Zn）₅eutectic phase is formed.In Mg-5Y-2.7Zn alloy,18R is dissolved and converted to 14H by solution treatment,while in Mg-6.5Li-5Y-2.7Zn alloy,Mg₂₄（Y,Zn）₅phases are dissolved and transformed into slat or block 18R and 24R phases.The presence of LPSO phase can not only improve the tensile strength of the alloy,but also significantly improve the plasticity of the alloy,and the fracture elongation is up to 17.3%.（3）The effect of heat treatment temperatures（450℃,500℃,550℃）on the microstructure of Mg-6.5Li-5Y-2.7Zn alloy was studied.The LPSO phase was formed at both 450℃and500℃,but the content of LPSO phase was higher at 500℃,and Mg₂₄（Y,Zn）₅eutectic dissolution was more adequate.Theα-Mg grains and Mg₂₄（Y,Zn）₅eutectic grow rapidly during overburning at 550℃.Therefore,the effect of heat treatment time（4h,6h,8h,10h,15h）on the microstructure of the alloy was studied at 500℃.It was found that the granular Mg₂₄（Y,Zn）₅phase grew up and formed strip structure along theα-Mg boundary,and dissolved into strip18R-LSPO phase at the same time.The content of LPSO phase increases and coarsening occurs over time.Due to the consumption of Y and Zn atoms inα-Mg andβ-Li,the Vickers hardness of the alloy decreases with the increase of LPSO content.（4）The microstructure and mechanical properties of as-cast and heat-treated Mg-6.5Li-5Y-3.68Zn and Mg-6.5Li-5Y-1.8Zn alloys with Y/Zn（at%）ratio of 1:1 and 2:1 were studied.It was found that Mg₂₄（Y,Zn）₅eutectic phase was formed in the as-cast Mg-6.5Li-5Y-x Zn alloy when Y/Zn（at%）<1,and a small amount of strip 18R-LPSO phase was formed by heat treatment.When Y/Zn（at%）=1,the as-cast alloy formed skeleton-like W phase（Mg₃Zn₃Y₂）and granulated and crushed after heat treatment.The broken W phase did not strengthen and toughen the alloy,but the LPSO phase increased the tensile strength and improved the plasticity of the alloy.