| Mg-Li alloys are one of the lightest structural materials that also exhibit outstanding formability, high specific strength, and good damping ability. Mg-Li alloy provide a promising application for components and parts of automobile, aerospace and electronic products. Hence the use of this alloy for industrial applications is attracting more attention in both academic and technical fields. But, Mg-Li alloys exhibit low mechanical strength which is not beneficial for structural applications. Hence, it is important to improve the mechanical property of Mg-Li alloys while still maintaining its ductility.The Mg-Li-Al-Sr-Y and Mg-Li-Zn-Y alloys were prepared by adding Y and Sr in this research. The influence of Y and Sr on the microstructures and mechanical properties of Mg-Li alloys were researched; the effect of hot extrusion parameters on the microstructure and mechanical properties was investigated; the evolution and hot deformation behavior of material flow were studied. The constitutive model of Mg-Li alloy at elevated temperature was established and validated. The Mg-Li/Al composite was prepared by co-extrusion, the processing was simulated by FEM.The main research contents and conclusions are as follows:① The true stress-strain curves of dual-phase Mg-9Li-3Al-2Sr alloy were obtained by the compression tests in the range of temperatures(423–573K) and strain rates(0.001–1 s-1). At a certain strain, the flow stress increases with the decrease of deformation temperature and an increase of strain rate. Strain softening was seen at a critical strain and attributed to dynamic recovery and dynamic recrystallization. Serrated flow was sensitive for this alloy in particular at low strain rates and has been attributed to a dynamic strain aging(DSA) effect, the periodic shearing of dislocations by solute atoms Mg and Li in the duplex phase alloy. The flow stress curves of Mg-9Li-3Al-2Sr alloy predicted by the developed Zener–Hollomon model are in good agreement with experimental results. The correlation coefficient(R) and average absolute relative error(AARE) are 0.9970 and 4.41% respectively, which confirms the validity of the proposed model.② Constitutive analysis of Mg-9Li-3Al-2Sr-2Y alloy was developed by performing hot compression tests in a range of temperatures(423–573K) and strain rates(0.001–1.0 s-1). Three constitutive models based on the Ludwik, Zener-Hollomon and Hensel-Spittel equations were used to describe the plastic flow behavior of this alloy. All three constitutive equations successfully represented the constitutive behavior of the material for elevated temperature compression deformation. The values of R and AARE were determined as 0.9937 and 4.48% for the Ludwik equation, 0.9938 and 8.19% for the Zener-Hollomon equation and 0.9920 and 8.45% for the Hensel-Spittel equation, respectively. The Ludwik and Zener-Hollomon equations properly captured the work softening behavior of the material during deformation. On the other hand, Hensel-Spittel equation successfully described the hot deformation of this alloy under whole compression process, especially at small strains.③ The Mg-9Li-3Al-2Sr-2Y alloy was extruded into round bars by DCCAP. The β-Li and Al4 Sr phases are all refined during extrusion. The Al4 Sr phases mainly distribute along the α/β phase interface and β–Li matrix uniformly, part of Al2 Y solute into the matrix during hot extrusion. It was shown that grain size of matrix was reduced from 132 to ~3-8μm. The mid-length location along the extrusion part of Mg-9Li-3Al-2Sr-2Y alloy has the optimum strength and ductility compared to extrudate tip and as-cast material, due to DRX and homogeneous distributed α-Mg and Al4 Sr phases where the tensile strength and elongation are 246.6 MPa and 19.9%, respectively. DCCAP was confirmed to be a promising tool for processing of continuous Mg-Li alloy billets leading to a significant improvement of their mechanical properties.④ The DCCAP process was simulated by the commercial FEM package DEFORM-3D. The hardness distribution, damage and grain size evolution were relative to effective strain. Model predictions also confirmed considerable variation of deformation along the length of extrudate. Comparisons indicated that model predictions of the load and material flow pattern agree well with measured values.⑤ Microstructures and mechanical properties of LZ83-xY alloys containing I-phase and W-phase were investigated. The experimental results show that the content of I-phase and W-phase changes by varying Zn/Y mass ratio in the LZ83-xY alloys. The cohesion of I-phase/α-Mg eutectic pockets can enhance the strength in the as-cast LZ83-0.5Y and LZ83-1.0Y alloys, while the W-phase has no obvious strengthening effect on the LZ83-1.5Y alloy. In the extruded alloys, the I-phase and W-phase were extruded into the particles with nanoscale size in the β-Li matrix phase. The dispersion strengthening of W-phase was more obvious because of the higher volume fraction. The ultimate tensile strength of extruded LZ83-1.5Y alloy is up to 238 MPa while the elongation is up to 20%.⑥ A novel method for preparing Mg-Li/Al composite is introduced. The advantages of using this method can be expressed as: simple operation, energy conservation, good dimensional control, performing the coating process in one pass and larger bonding strength. The average thickness of coating on Mg-Li bar is about 20μm and the hardness is 220 HV. The compression strength of extruded bar is compared with the same product produced by casting and regular extrusion in room temperature and elevated temperatures. In room temperature, the compression strength of coated bar is about 335 MPa which is 32% more than as-cast one and 11% more than regular extruded bar. The eutectic coating have high thermal stability, while the compression strength of coated bar is double of as-cast ones in same compression conditions in elevated temperatures. |
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