Investigation On Microstructure And Mechanical Properties Of As-cast, Extruded And Semisolid Mg-Li-Al Magnesium Alloys

With the developing of urgent needs for lightweight materials and components in modern industrial field, Mg-Li alloys have been displaying the first legs in many fields, such as auto industry, medical device industry and IT industry. However, there are many problems need to be solved before widely used in various fields. Although Mg-Li-Al and Mg-Li-Al-Zn alloys are the mainly used systems in commercial world, the overaging problems that cause by the Al and Zn are still far from being solved. For instance, the MgLi2Al metastable phase can be broken down into AlLi phase for the reason of overaging at room temperature. Thus, it is necessary to find stable strengthening phase by the method of powerful composition design. Accordingly, it also hopes to achieve new breakthrough in the key problems of technology. Moreover, the amount of Li contents directly influences the a-Mg phase lattice parameters and theβphase volume fraction, which effects mechanical behavior indirectly. Here it becomes very important to understand the micro-mechanism of co-deformation behavior of a phase andβphase. Until now, these kinds of researches are seldom reported.The microstructure and mechanical properties of the as-castα+βdual phase Mg-7Li-3Al-1Ca alloys, Mg-9Li-3Al-1Ca alloys and theβphase Mg-12Li-3Al-1Ca alloys were firstly studied. The Mg-11Li-3Al-1Ca alloy (Li content is designed at the point contact of lithium single-phase region in the Mg-Li equilibrium phase diagram) has also been concerned. The effect of extrusion process on the microstructure and mechanical properties of all the alloys were investigated. The strain rate sensitive and deformation mechanism of all the alloys were further studied. On the other hand, the Ce element (1 mass%) were added to the dual phase Mg-Li-Al-Ca systems to obtain Mg-xLi-3Al-1Ca-1Ce (x=6.5,8.5,10.5) alloys. The microstructure and mechanical properties of the as-cast and extruded Mg-xLi-3Al-1Ca-1Ce (x=6.5,8.5,10.5) alloys were studied. A new kind of Al2(Ca, Ce) compound, which were regarded as solution of intermetallic compound were confirmed. In the end, the extruded Mg-xLi-3Al-1Ca (x=7,12) alloys were hold at different isothermal holding temperatures with different isothermal holding times. To compared with Mg-12Li-3Al-1Ca alloys, through observation of microstructural evolution of semisolid Mg-7Li-3Al-1Ca alloy to find the evolution model of semisolid microstructure in this paper. The micro-mechanism of microstructural evolution of semisolid magnesium alloy had been studied. Based on these results, we have arrived at the following concusion:1. As-cast Mg-7Li-3Al-1Ca and Mg-9Li-3Al-1Ca alloys are composed of a-Mg phase,β-Li phase, MgLi2Al compound particles and needle like Al2Ca compounds. Mg-11Li-3Al-1Ca alloy matrix is composed ofβ-Li phases. Small pieces of a-Mg locate at grain boundaries. For the large grains, the distribution of compounds are not uniform, the there are more particles near the grain boundaries than those in the center area. Mg-12Li-3Al-1Ca has nearly the same microstructure except for the a-Mg phase. After extruding process, all the alloys exhibit fibrous extruding texture along the extrusion direction and the grains get fined obviously. A mount of equiaxed crystals had been found. The stress distribution may be not uniform in the extrusion process. So that for the soft P-Li matrix in the Mg-11Li-3Al-1Ca and Mg-12Li-3Al-1Ca alloys, both of the the grain sizes are quite different from each others in different parts. Some line like phases which were composed of MgAl2Li compounds had been confirmed along the extrusion direction.2. The yield stress increased with the Li addition rising. The dual matrix Mg-7Li-3Al-1Ca and Mg-9Li-3Al-1Ca alloys show strain hardening in the tension test. Serrated flow becomes more and more obvious as the Li content increasing. For the Mg-7Li-3Al-1Ca and Mg-9Li-3Al-1Ca alloys, all of the yield stress, ultimate tensile stress and elongation increased after extruding. For the Mg-11Li-3Al-1Ca alloys, although the yield stress and ultimate tensile stress had significantly improved and even show better values than those of other three alloys after extrusion process, the elongation of this alloy was little changed. On the other hand, for the Mg-12Li-3Al-1Ca alloy, the yield stress and ultimate tensile stress varies slightly after extruding, but the elongation increased obviously. The fracture mode of theβ-Li matrix alloys belong to intercrystalline fracture. A lot of micro-cracks had been found near the regions of fracture surface of Mg-7Li-3Al-1Ca and Mg-9Li-3Al-1Ca alloys. There is co-deformation relationship between the a-Mg phase andβ-Li phase. Mg-7Li-3Al-1Ca and Mg-9Li-3Al-1Ca alloys exhibit dual strain rate sensitivities. This value becomes higher when the stretching rate increasing from 10-3S. The Mg-11Li-3Al-1Ca alloy has the highest strain rate sensitivity value, i.e.0.078. The strength of Mg-9Li-3Al-1Ca and Mg-11Li-3Al-1Ca alloys decreased deeply with the increasing temperature, at the same time the elongation did not changed much. As the co-deformation relationship between a phase andβphase became worsen, the fracture behavior occured with the increasing temperature. The ductility of Mg-9Li-3Al-1Ca alloys is always in the range of 50%-60% at different temperature. Both the elongation and strength of Mg-11Li-3Al-1Ca alloys decrease faster than that of Mg-9Li-3Al-1Ca at high temperature.3. As-cast Mg-6.5Li-3Al-1Ca-1Ce and Mg-8.5Li-3Al-1Ca-1Ce alloys are dual matrix (a-Mg phase andβ-Li phase) and as-Cast Mg-10.5Li-3Al-1Ca-1Ce alloys has only oneβ-Li matrix. MgLi2Al compound particles, needle like Al2Ca compounds and a new kind of Al2 (Ce, Ca) compound are found in all the alloys. In geometry terms, needle like and small particles have better enhancement impact than the larger massive compound. Non equilibrium exhalation Mg2Ca compounds along the grain boundaries were observed in the TEM result. The orientation relationship was observed between the a phase and Al2(Ce, Ca) phase, i.e., [112]//[7253]α, (111)//(0111)α,an addition 0-0.5°rotation of the cubic lattice around [112]//[7253]a axis. After extruding process, all the alloys exhibit fibrous extruding texture along the extrusion direction and the grains get fined obviously. The Mg) phase in eutectic structure that cannot be seen in the as-cast Mg-10.5Li-3Al-1Ca-1Ce alloy began to show up and assemble in the form of white long strip along the extruding direction.4. Mg-xLi-3Al-1Ca-1Ce (x=6.5,8.5,10.5) shows a little better strength value than the Mg-xLi-3Al-1Ca (x=7,9,11,12) alloys do. However the elongation appeared to decline slightly. For the Mg-6.5Li-3Al-1Ca-1Ce and Mg-8.5Li-3Al-1Ca-1Ce alloys, all of the yield stress, ultimate tensile stress and elongation increased after extruding. For the Mg-10.5Li-3Al-lCa-1Ce alloy, although the yield stress and ultimate tensile stress had significantly improved after extrusion process, the elongation of this alloy was little changed and the fracture mode of thisβ-Li matrix alloy belong to intercrystalline fracture. The results showed that addition of Ce have no obvious impact on the mechanical properties of Mg-xLi-3Al-1Ca (x=7, 9,11,12) alloys at high temperature. The ductility of dual phase alloys is always in the range of 50%-60% at different temperature. Phase boundary softening and P-Li softening mechanisms both act in temperature range of 323K-425K. As the co-deformation relationship between a phase and P phase get worsen, the fracture behavior occured with the increasing temperature. The crack origins located at phase boundaries. With the Li content increasing, the microhardness data drop slightly. The microhardness data increase after extruding process. In the same alloy, the microhardness data of a-Mg phase is nearly of the same with that ofβ-Li phase, perhaps due to the fact that the presence of precipitated compounds in both of the two phases. 5. The microstructural evolution of semisolid a+(3 daul phase Mg-7Li-3Al-1Ca alloy as follow:after extrusion form fibroid structure and dendrite rupture→fibroid structure has disappeared, take placed recovery and recrystallization→Low melting point regions with higher Li content melted firstly when the isothermal temperature is slightly higher than the solidus temperature→the liquid phase saturated into high-energy recrystallization grain boundaries, isolated solid particle appeared→the three kinds of (a-Mg phase,β-Li phase andα+βphase) solid particle rounded→solid particle coarsing by atoms diffusion. Except for recrystallization grains, some a-Mg fibroid structure had also been found in some solid particles. The liquid phase could not saturate into the phase boundaries between the a-Mg fibroid structure andβ-Li phase. So that these fibroid structures were kept in someβ-Li solid particles. Theβ-Li solid particles in the singleβ-Li matrix alloys and the liquid molten bath in solid particles also both grow up quickly. This phenomenon results in the high equivalent solid phase fraction and the solid particles will break away when the liquid molten bath became too much with increasing isothermal holding time or temperature. So that the singleβ-Li matrix Mg-Li alloys is not fit for SIMA method to obtain semi-solid materials. Atom diffusion effect exist both a-Mg parts andβ-Li parts in the same solid phase. Also for the reason of atom diffusion effect, the big solid particle will grow up larger and small particle will disappear. For the Mg-7Li-3Al-1Ca alloy with 72% deformation degree, the appropriate isothermal holding temperature is 538℃-562℃and corresponding isothermal holding time is 10min-30min to obtain 50% solid fraction in this experimental condition.