| Mg-Li alloys are the lightest structural materials with low density, high specific strength, high specific stiffness, good damping capacity and good formability. So Mg-Li alloys have been certainly used in the fields of spaceflight, electricity, communication and weapon industry, etc. However, the commercial application of Mg-Li alloys has been limited because of low strength,poor heat stability and poor corrision resistance ability.In this thesis,Sr was added into Mg-Li-Al alloys and the effect of Sr on microstructure and mechanical properties of Mg-9Li-3Al alloys was studied. Mg-Sr master alloy, Mg-9Li-3Al alloy and Mg-9Li-3Al-xSr alloy were prepared by"mixture method". The as-cast, heat-treated and hot-extruded test alloys were analyzed by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and mechanical property test. The effect of Sr on microstructure and mechanical properties of Mg-9Li-3Al alloys was studied , the influence of heat treatment and hot extruding on microstructure and mechanical properties of Mg-9Li-3Al-xSr alloys was investigated and the fracture mechanism of the test alloys was analyzed.The results show that:①Mg-4Sr master alloy is consist ofα-Mg and Mg17Sr2. Mg17Sr2 is distributed along the boundary of matrix grain. Mg-9Li-3Al–xSr alloy is composed ofα-Mg,β-Li, Al4Sr ,a little AlLi and Mg17Al12 phase.②Sr has refining effect on magnesium alloys grains. When Sr content is 2.5wt%, the grains of Mg-9Li-3Al-xSr alloy are the finest. XRD and EDS shows that with the Sr addition new high-temperature and hard phase Al4Sr is formed in the alloys, which distributes along grain boundary. The result of mechanical property test indicates that containing 2.5wt% Sr, the as-cast Mg-9Li-3Al-xSr alloy reaches to the best mechanical properties, the tensile strength (reaches to the peak value 186.3MPa) and the yielding strength is respectively enhauced by 48.8% and 62%. Elongation decreases with Sr addition. The amount and distribution of Al4Sr has great effect on the mechanical properties of the test alloys. A little Al4Sr that disributes along the grain boundary has the grain boundary strengthening effect on the test alloys and the mechanical properties can be improved. That adding too much Sr could decreases the amount of Al element in matrix and cause the agglomeration of Al4Sr, which reduces the mechanical properties of Mg-9Li-3Al-xSr alloys. The fracture mechanism of the as-cast test alloy is dimple fracture and cleavage fracture.③After homogenization, the chemistry composition of Mg-9Li-3Al-xSr alloy distributes more uniformly , the grain size of test alloy has not significant change.④The dynamic recrystallization occurs during the hot-extruding and the grains are refined, so the grains of hot-extruded Mg-9Li-3Al-xSr alloys are more smaller than as-cast alloys, Al4Sr phase distributes dispersively in the form of block structure or particle structure. The result of mechanical properties test indicates that mechanical properties of hot-extruded test alloys are much better than the as-cast alloys, when Sr content is 2.5wt%, the tensile strength of the hot-extruded alloy reaches to its peak value 235.2MPa. The tensile strength, yielding strength and elongation respectively increases by 26.5%, 48.1% and 131% compared with the as-cast alloy. The fracture mechanism of the extruded alloy is dimple fracture.⑤The comparision result of the mechanical properties between hot-extruded Mg-9Li-3Al-2.5Sr alloy and several typical materials indicates that: the hot-extruded Mg-9Li-3Al-2.5Sr alloy performances good mechanical properties with good elongation and high specific strength. This new series Mg-Li-Al-Sr alloy holds promise to be developed as a new kind super-light wrought magnesium alloy.
|
PDF Full Text Request