| dc.description.abstract | Magnesium and magnesium alloys are used in a wide variety of structural and nonstructural applications. It is commonly recognized that magnesium possesses poor formability because of its hexagonal close-packed structure. To make up for this shortcoming and further reduce weight, alloying magnesium with lithium of extremely low density, 0.534 g/cm3, can achieve both goals.
In recent years, the equal channel angular extrusion (ECAE) process is an innovative method to refine grain structure on many magnesium alloys. However, Mg-Li-Zn alloys were not much involved such that the relevant literatures are just two. Five Mg-Li-Zn alloys namely Mg-11%Li-1%Zn, Mg-9%Li-1%Zn, Mg-9%Li-1%Zn-0.2%Mn, Mg-9%Li-1%Zn-1%Al-0.2%Mn, and Mg-9%Li-3%Al-1%Zn-0.2%Mn were prepared. These alloys had been processed by equal channel angular extrusion (ECAE), and the subsequent mechanical properties and microstructures were studied. After ECAE process, the room temperature strength was significantly enhanced at a modest cost of elongation reduction.
The α phase mainly provides the strength and hardness on Mg-Li-Zn alloys and the β phase provides the ductility. In each of the Mg-Li-Zn alloy, the increase in Mn content raises the saltwater corrosion resistance and elongation but reduces the strength. The increase in Al content raises the air corrosion resistance and strength but reduces the elongation. From the results of these five Mg-Li-Zn alloys processed by ECAE, Mg-9Li-1Zn alloy processed under condition, 900-100℃-Bc4, shows the greatest increase in tensile strength about 41.8 MPa and the least decrease in elongation about 25%. With the same ECAE process, Mg-11%Li-1%Zn alloy shows the greatest increase in Micro-Vickers hardness of α phase about 22.4% and Mg-9%Li-1%Zn-0.2%Mn alloy shows the greatest increase in Micro-Vickers hardness of β phase about 28%. The microstructures of these five Mg-Li-Zn alloys were found out equiaxed subgrain structure and sub-micrometer grain size. The fine particles in Mg-11Li-1Zn alloy specimens processed under condition, 900-100℃-Bc4, had been identified by the TEM and EDS as the MgLiZn, which had been found out the annealing twins and (111) is twin plane. The fine particles in Mg-9Li-1Zn alloy specimens processed under condition, 900-100℃-Bc4, had been identified by the TEM and EDS as the MgZn2. The fine particles in LAZM9310 alloy specimens processed under condition, 900-175℃-C4, had been identified by the TEM and EDS as the ZnO. The grains in both α and β phases are recrystallized due to high strain that accumulated during ECAE processing and the fine recrystallized grains increases the grain boundary area, thereby enhancing grain boundary sliding and superplasticity. The specimens of LZ91 alloy processed under condition, 900-100℃-Bc8, were expected to show superplastic elongation and indeed the highest elongation was 350%, which was tested at 250℃ and initial strain rate of 1×10-4 s-1. All aged specimens of LZ91 alloy were checked by XRD and onlyα and β phases were obviously detectable showing peaks. The specimens treated at 50 ℃/100 hours and 100 ℃/10 hours had shown extra bump adjacent to the main peak ofα(0002). This is speculated to be the metastable (MgLi2Zn) phase. The micro hardness tests were done in all aged specimens of LAZM9310 alloy. The specimens treated at 100 ℃/10 hours had shown higher hardness of β phase. This is speculated to be the AlLi phase. | en_US |