Effects Of Ca On Microstructures And Mechanical Properties Of Mg-Li-Al Alloys

Magnesium-Lithium alloys, as the lightest structural materials, possess many advantages, such as good processing properties, good magnetic screening and shock resistance ability. Therefore, they are widely used in the fields of aerospace, electronic industry, military and so on.According to the Mg-Li binary phase diagram, when the Li content is less than about 5.5 wt.%, the alloy is composed of a singleα(Mg) phase with hcp structure. When the Li content is between 5.5 wt.% and 11 wt.%, the hcpα(Mg) phase coexists with bcc structuredβ(Li) phase, while the microstructure has only a single bcc structuredβ(Li) phase when the Li content is more than 11 wt.%. An addition of lithium below 5.5 wt.% leads to a reduction of c/a axial ratio, which causes the decrease of the critical shearing stress of the prism slipping, and the slipping systems are more easily activated.In this thesis, the as-cast Mg-5Li-3Al-xCa (x=0,0.5,1,1.5, wt.%) were prepared with vacuum induction melting furnace, then processed by hot extrusion. The as-extruded alloys were annealed at 400℃for 4 hours and then quenched in air. Chemical composition of alloys was measured with inductively coupled plasma spectrometer. The microstructures of as-cast, as-extruded and annealed alloys were characterized by optical microscope (OM), scanning electronic microscope (SEM). Phase analysis was conducted by X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS). The tensile properties were measured by tensile tester at room temperature.The creep behavior of as-extruded Mg-5Li-3Al-xCa (x=0.5,1, wt.%) was researched at different temperatures (373 K,398 K,423 K) and different stresses (35 MPa,55 MPa,75 MPa). The stress exponents, creep activation energies and material constants were obtained by creep power-law equation, and the creep mechanisms were discussed combining with the microstructures before and after creep tests.The results indicate that:The grains of as-cast alloys were refined gradually with the increase of Ca content from 0.5 wt.% to 1wt.%, while the Ca content increases to 1.5 wt.%, the grain size increases. The dynamic recrystallization happened during hot extrusion and the microstructures of investigated alloys were further refined. Both as-cast and as-extruded Mg-5Li-3Al-0.5Ca alloys have the highest mechanical properties, which are mainly attributed to the grain refinement caused by the addition of Ca, solution strengthening effect of Al element and the formation of strengthening phase, Al4Ca. When the addition of Ca is up to 1～1.5 wt.%, the mechanical properties of alloys are worsened due to the excessive (Mg, Al)2Ca eutectic phase forming at grain boundary.Under the test conditions, the steady creep rates of as-extruded Mg-5Li-3Al-0.5Ca and Mg-5Li-3Al-1Ca alloy increase with the increase of the temperature and stress. For as-extruded Mg-5Li-3Al-0.5Ca, the averages of exponent (n) and creep activate energies (Qa) are 1.92 and 59.36 KJ/mol respectively. Meanwhile, there are many twins can be found on OM graph after creep test, therefore, the creep mechanism of as-extruded Mg-5Li-3Al-0.5Ca is controlled by grain boundary sliding and twinning deformation. Further analyses show that the creep mechanism of as-extruded Mg-5Li-3Al-1Ca is controlled by boundary sliding.