| Based on the purpose of environment protection and energy saving, the development oflight weight materials has received extensive attention in recent years. Among numerouskinds of light weight materials, magnesium-lithium (Mg-Li) alloys can be used as idealstructural materials for the applications in the fields of military defense, aerospace,transportation and electronic industry because of their many advantages, such as low density,high specific stiffness, high specific strength, good damping ability, high electromagneticshielding ability, good machinability and easily recycle. However, magnesium-lithium alloyshave some outstanding drawbacks in the application, for example, the microstructure andperformance of the basic Mg-Li-Al and Mg-Li-Zn alloy systems are unstable, it is necessaryfor these alloy systems to further study for the unstable properties of Mg-Li alloys. To obtainhigh-quality superlight magnesium lithium alloys, the means, such as the addition of alloyingelements and the improvement of ageing process, are used to enhance the material properties.In this thesis, Mg-5Li-3Al-2Zn-xCu alloy with α single phase was prepared by vacuuminduction melting method. The effects of Cu addition on the microstructure and mechanicalproperties of as-cast and extruded alloys were systemic studied. Based on the results, theprocess of solid solution and ageing in Mg-5Li-3Al-2Zn-xCu alloy was intensively studied,and the mechanism of the ageing precipitation behavior was revealed. The results show that:1. With increasing Cu content, the AlCuMg and Al2Cu phases formed inMg-5Li-3Al-2Zn-xCu alloy gradually increase, whereas the lamellar eutectic structures (α-Mgphase+AlLi phase) relatively decrease. The ultimate tensile strength, yield strength,elongation and Brinell hardness of as-cast Mg-5Li-3Al-2Zn-xCu alloy increase withincreasing Cu content. When the Cu content increased to2%, the alloy exhibits goodcomprehensive mechanical properties.2. After the hot extrusion, the eutectic structure (α-Mg phase+AlLi phase) of as-castMg-5Li-3Al-2Zn-xCu alloy is squeezed into smaller eutectic structure, the AlCuMg phaseslocated at the grain boundaries are also broken, the grain is refined, and a typical (0002) basaltexture is formed. The Mg-5Li-3Al-2Zn-xCu alloy has good mechanical properties after thehot extrusion, and the mechanical properties of the alloy increase with increasing Cu content.Three kinds of strengthening mechanism are used to explain the improvement of the mechanical properties. Firstly, due to the grain refinement, the impeditive effect on themovement of dislocation increases. Secondly, the (0001)<1120> basal slip is prohibitedbecause of the formation of (0002) basal texture, which causes the increase of yield strength，and the elongation increase because the non-basal slip systems are activated. Thirdly, themore the second phase is broken, the greater the impeditive effect on the deformation is, thehigher the strength of alloy is.3. With increasing solution temperature, the AlLi phases distributed in theMg-5Li-3Al-2Zn-xCu alloy gradually dissolve into the α-Mg matrix, whereas the AlCuMgand Al2Cu phases do not dissolve. After the solution treatment (390oC×5h), the AlLi phasescompletely dissolve into the α-Mg matrix, and the a and c values of the lattice constants inα-Mg phase decrease. The hardness of Mg-5Li-3Al-2Zn-xCu alloy gradually increases withincreasing solution temperature. The hardness changes of alloy at different solutiontemperatures are the results that the effects of strengthening and softening are superposed.4. The natural ageing treatment slightly affects the precipitation of the AlLi phases inMg-5Li-3Al-2Zn-xCu alloy, while the artificial ageing treatment makes the AlLi phases occurwith discontinuous and continuous precipitation from the supersaturated solid solution.Discontinuous AlLi precipitate with lamellar shape distributes in the vicinity of grainboundaries, and continuous AlLi phase is precipitated in the grain interior. At different ageingtemperatures, the precipitation time of AlLi phase from the supersaturated solid solution inMg-5Li-3Al-2Zn-xCu alloy is not the same. With the prolong of ageing time, thediscontinuous AlLi phase controlled by grain boundary diffusion stops precipitation, while thecontinuous AlLi phase controlled by the volume diffusion begins precipitation. During theprocess of ageing treatment, the Cu addition restrains the ageing precipitation ofdiscontinuous AlLi phase of Mg-5Li-3Al-2Zn alloy.5. In the ageing process, the continuous precipitates of Mg-5Li-3Al-2Zn-xCu alloyobserved from the [0001] direction of α-Mg phase parallel to electron beam have threemorphologies: plates, laths and asymmetric-lozenges, while the morphologies of continuousprecipitates have a small amount of plates and a large number of needles from the [2110]direction of α-Mg phase. The orientation relationship between the matrix and continuousprecipitate is (0001)α//(110)p，[1210]α//[111]pat the ageing of150℃for438h. HRTEManalysis indicates that, at the ageing of150℃for438h, the layers of atomic disorder occur between the precipitates with plates, laths and asymmetric-lozenges morphologies and thematrix interface, and the lattice fringes of the matrix and precipitates from the [220] and [111]direction are different, so the relationship between the precipitates and the matrix isincoherent.6. As the ageing temperatures are different, the change trends of the hardness-time curvein Mg-5Li-3Al-2Zn-xCu are different. At the natural ageing, the alloy hardness has an upwardtrend with increasing ageing time. At the ageing of100℃, the hardness-time curves of thealloys only occur a hardness peak; whereas the hardness-time curves both appear doublehardness peaks at the ageing of150℃and200℃. With increasing ageing temperature, theincubation period of hardness curve in Mg-5Li-3Al-2Zn-xCu alloy becomes shorter duringthe initial ageing, the value of peak hardness decreases, and the time reaching the peakhardness advances. Compared with the alloy without Cu addition, the addition of Cu delaysthe time to reach the peak hardness of ageing, which restrains the over-ageing phenomenon. |
PDF Full Text Request