Studies On Microstructures And Mechanical Properties Of Super-Light Mg-Li Alloy

Mg-Li alloy is a kind of super light alloy which is the most outstanding among the engineering materials, it has high specific strength and stiffness, good damping capacity and high energy particle resistance. The density of the alloy decreases due to the addition of the element Li. When the Li content is higher than 5.7%, the crystal structure transfers from hcp to bcc, which is unique and improves the plastic deformation ability. And some Mg-Li alloys can have a elongation of 30%. While Mg-Li alloy has been one of the most promising structural material in the area of aerospace, nuclear industry,3C industry and medical instruments, it also has some shortcomings, among which the low strength is the most urgent to be solved. This research chose the Mg-8Li-4Al-3Zn-La alloy as the material which has quite good comprehensive mechanical properties as it has hcp+bcc dual-phase structure. To improve the strength of the alloy and at the same time we can analyze the changes in the microstructures and the mechanical properties, the alloy was first treated by solid solution and annealing, then the original alloy was treated by rolling and the solid-soluted alloy was treated by HPT and ECAP. Optical microscope, SEM, XRD, EDS, EBSD, FIB, TEM were employed to observe the microstructures, phase distribution and the deformation structure. The micro-hardness test and uni-axial tensile test were performed on the alloys at room temperature to evaluate their mechanical properties. The strength of the alloy is improved and the relationship between the mechanical properties and the microstructures is analyzed. The results are showed as follows:The original alloy is composed of α-Mg, β-Li, MgAlLi2, AlLi and Al2La phase due to the addition of Al, Zn, La. These new phases not only improve the mechanical properties but also have great effects on the microstructures during the heat treatment and deformation treatment.After solid solution treatment and annealing treatment, the phases in the alloy have great changes. The unstable phase MgAlLi2 and AlLi are dissolved in the matrix after solid solution, the Al2La phase still exist in the interface of the two phases due to the high melting point. The a phase changes little. The strength is improved by 25%, and the elongation is decreased by 50%. After annealing treatment, the β phase transfer to a phase partly and the content of AlLi phase increases much. The strength decreases a little and the elongation decrease from 18% to 10%.After rolling, ECAP and HPT treatment, some deformation twins appear. The little particle phases appear again in the β phase of the solid solution treated sample when it is deformed at room temperature, and this makes the solid solution strengthen effect weaken. The hardness improved a lot after HPT and rolling, there is gradient hardness from the center to the edge for the HPTed sample and the hardness of the edge improved almost by double compared with original alloy.