【Citation Format】
CHEN Xiaoliang, YANG Bo, GU Yicheng, et al. Research Progress on Magnesium-Lithium Alloy Strengthening of Ultralight Body Centric Cube (BCC) Structure[J]. Special Casting & Nonferrous Alloys,2024,44(4):457-464.
CHEN X L，YANG B，GU Y C，et al. Research progress in strengthening of ultralight body-centered cubic （BCC） structured Mg-Li alloys［J］. Special Casting & Nonferrous Alloys，2024，44（4）：457-464.
Guide
Mg-Li alloy has excellent properties such as low density, high specific strength and specific stiffness, and good formability, and has broad application prospects in aerospace, electronic communication and other fields. When the amount of Li is greater than 10.3% (mass fraction), Mg-Li alloy has a single-phase body-centered cubic (BCC) structure, which is one of the lightest low-density metal structural materials at present, but due to its high Li content and low strength, it limits its application in the engineering field. This paper reviews the research progress of strengthening methods and strengthening mechanisms of Mg-Li alloys with BCC structure at home and abroad, focuses on the problem of low formability of alloys strengthened by nano-precipitated phase, and finally points out the existing problems and looks forward to the development direction.
【Background】
MG is a close-packed hexagonal (HCP) structure, with few slip systems and poor machinability, which limits its application range. The addition of Li to Mg to form the Mg-Li alloy reduces the critical shear stress and c/a axial ratio of the magnesium crystal, increases the slip system, improves the ductility, and has good machinability. The density of Mg-Li alloy is only 1/2 of that of aluminum alloy and 3/4 of that of traditional magnesium alloy, which is called ultra-light alloy. Due to its light weight, high specific strength and specific stiffness, good electromagnetic shielding performance and damping characteristics, as well as excellent cutting performance, it has broad application prospects in the fields of military, aerospace and 3C products.
According to the difference of Li content (mass fraction) and crystal structure, Mg-Li alloy can be divided into three types: (1) Li content is less than 5.7%, which is composed of α-Mg monophase (solid solution of Li in Mg) and has a hexagonal close-row structure. The addition of a small amount of Li reduces the critical shear stress and c/a axial ratio of magnesium alloy, increases the non-base slip (cylindrical or cone slip), and further improves the ductility at room temperature, but the alloy is still HCP structure, with relatively few slip systems, showing the characteristics of high strength and low shaping. (2) The Li content is between 5.7%~10.3%, which is composed of α-Mg and β-Li duplex structure, among which the β-Li phase is a solid solution of Li-Mg with BCC structure, and the coexistence of the two phases has both the medium strength of the α-Mg phase and the excellent ductility of the β-Li phase, so it has attracted much attention. (3) The Li content is greater than 10.3%, and the matrix phase is completely β-Li phase (a solid solution of Mg in Li). The alloy has only a single BCC structure with more slip systems, which improves formability and reduces texture anisotropy, but the strength is low.
【Research Highlights】
A large number of studies have shown that the BCC structure Mg-Li alloy not only reduces the density but also improves the plastic deformation ability, but sacrifices the strength to a certain extent and does not meet the demand for high strength. It is difficult for the strength of the alloy to exceed 250 MPa under casting, heat treatment and plastic deformation, so it is very important to improve its strength, and on the basis of high Li content, strengthen the strength of Mg-Li alloy and optimize its mechanical properties, which can achieve a good combination of ultra-low density and high strength. However, the BCC structure of Mg-Li alloy also has the disadvantages of poor high temperature stability and easy corrosion. At present, the main improvement methods are alloying, fine-grain strengthening, deformation strengthening, solution strengthening and composite strengthening.
The joint research team of Anhui University of Technology and Huafu Technology Co., Ltd. published an article entitled "Research Progress on Magnesium-Lithium Alloy Strengthening of Ultralight Body Center-Centered Cube (BCC) Structure" in the journal "Special Casting and Non-ferrous Alloys" Volume 44, Issue 4, 2024. The aim of this study is to provide a reference for the preparation of ultra-light and high-strength Mg-Li alloys.
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[Graphic Analysis]
Focusing on the addition of alloying elements, precipitation strengthening and the influence of precipitation phases at grain boundaries, the research progress of magnesium-lithium alloy strengthening of ultralight body centroid (BCC) structure was introduced. The specific contents include the mechanical properties of BCC structure Mg-Li alloy at home and abroad in recent years, the solid solubility of alloying elements in BCC structure Mg-Li alloy, the strengthening effect of some alloying elements in BCC structure Mg-Li alloy, the uniform distribution of nano-size precipitated phase in the matrix can be generated through heat treatment after adding alloying elements, which can significantly improve the alloy strength, and the precipitated phase of BCC structure Mg-Li alloying and post-alloying heat treatment process can improve the strength of the matrix. Plasticity decline was analyzed. In addition, the addition of Y, Gd, and Er elements to Mg-Al-Zn alloys (Zn and Y, Zn and Gd, Zn and Er in different proportions) can generate icosahedral quasicrystalline phases (I phases) with high hardness, good thermal stability, high corrosion resistance and small friction factors in situ.
Fig.1 TEM image of D03-Mg3Al particles precipitated in LA113 alloy (bright white arrows are superlattice diffraction points; The white arrows are rod-shaped white precipitates)
Fig.2. TEM diagram of S-LZ116 alloy
Fig.3. Tensile stress-strain curves of as-cast Mg-14Li-xAl-yY alloy
Fig.4. Fracture morphology of as-cast LAZx32-0.5Y alloy
Fig.5. Schematic diagram of the grain boundaries of BCC structure Mg-11Li-3Al alloy
Fig.6. Backscattered electrons (BSE) image of Mg-14Li-6Zn-1Y alloy
The role of deformation strengthening in the strengthening of ultralight body-centered cubic (BCC) magnesium-lithium alloys. The strengthening mechanism was explored by studying the changes in strength and plasticity of different alloys after plastic deformation, as well as the mechanism of precipitate phase and grain refinement. At the same time, the effects of different rolling processes on the microstructure and mechanical properties of magnesium-lithium alloys were also studied, and the strengthening mechanism was analyzed.
Fig.7. Mechanical properties curves of Mg-16Li-4Zn-1Er alloy rolled at room temperature and cold-rolled under different pressure levels
Composite strengthening adds hard ceramics or other materials (grains, fibers or whiskers) as reinforcements to the Mg-Li alloy, and these reinforcements are diffusely distributed in the matrix, which improves the comprehensive mechanical properties of the alloy and thus obtains composite materials with excellent performance. The main preparation methods include powder metallurgy, pressure impregnation, stirring casting, thin film metallurgy and in-situ synthesis. At the same time, the types of reinforcements used in the preparation of Mg-Li matrix composites and their advantages and disadvantages were summarized.
Fig.8. High-resolution diagram of the interface between Al2Y particles and matrix
【Conclusion and Prospects】
This paper reviews the research progress of improving the low strength of Mg-Li alloy with BCC structure in three aspects: alloy strengthening, deformation strengthening and composite strengthening, and puts forward the existing problems: (1) the density of rare earth elements is high, and the density of rare earth elements is significantly affected by the addition of alloys and the production cost is high; (2) Only through alloy strengthening and deformation strengthening will lead to poor formability, and the balance between strength and plasticity cannot be achieved; (3) The use of a strengthening method (alloy strengthening, deformation strengthening or composite strengthening) does not increase the strength of the material to the greatest extent; (4) The external reinforcement of Mg-Li alloy is easy to agglomerate in the matrix, float on the surface and do not wet the interface with the matrix, which affects the enhancement effect, and even reacts with the matrix, significantly reducing the strength.
In the future, the following aspects can be considered to improve the strength of the ultra-light BCC structure Mg-Li alloy: refine the grains and develop a process that can be quickly cooled; External or in-situ generation of graphene, due to the low density of graphene, meets the needs of ultra-light, and has excellent mechanical properties, which can effectively improve the strength, but it is necessary to solve the interface problem with the matrix; The development of a homogeneously distributed nano-sized precipitated phase requires the exploration of new material systems and new processes. The focus of future research and development is to improve the plasticity, corrosion resistance and simplify the production process (develop a process that is easy to produce on a large scale and have a low cost) on the basis of realizing the ultra-light and high-strength BCC structure Mg-Li alloy, so that its comprehensive mechanical properties will be continuously improved, and then it will be more widely used in the fields of engineering, biology and aerospace.