Research On Microstructure And Mechanical Behaviors Of Mg-Li Alloys

The phenomenon of plastic instability or Portevin-Le Chatelier(PLC)effect shows a serrated wave on the stress-strain curve and the wrought magnesium alloys often present crystallographic texture and mechanical anisotropy.Microstructures of the extruded Mg-4Li?Mg-6Li?Mg-4Li-6Zn-Y?Mg-6Li-6Zn-Y are investigated by optical microscope(OM),scanning electron microscope(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD)and electronic backscattered diffraction(EBSD)in this work.Effect of microstructure on the serrated phenomena,texture evolution and mechanical anisotropy of Mg-Li alloys is also explored.For the extruded and solid solution Mg-4Li-6Zn-Y alloys reinforced by the quasicrystalline phase,the PLC effect occurrs after a certain critical strain.However,the aging treatment can restrain the PLC effect and the strength is improved.Yield point elongation appears after a certain amount of accumulated strain for the extruded and solid solution alloys.Moreover,the more cumulative strain,the less the critical strain.The aged alloy should have not the PLC effect but can still emerge after a certain amount of accumulated strain.Meanwhile,the type of serration is different from the extruded and solid solution alloys.On the one hand,the Mg-4Li-6Zn-Y alloys tensiled with different orientations present the PLC effect but no twins On the other hand,the Mg-6Li alloys tensiled with different orientations do not present the PLC effect but twins appear.Thus,there is no direct relationship between the twins and the PLC effect.The strength decreases and the enlongation increases with the increase of lithium content in Mg-Li alloys.However,both the strength and the enlongation increase when quasicrystalline phase is formed in Mg-Li alloys.Basal texture is intense for the extruded Mg-4Li alloy but the Mg-4Li-6Zn-Y shows weak basal texture.The basal texture of ?-Mg for the duplex structured Mg-Li alloys is similar with the single-phase structured Mg-Li alloys,only the Mg-6Li-6Zn-Y alloy shows the weaker texture including ?-Mg and ?-Li.Thus,both the quasicrystalline phase and(3-Li phase can weaken the texture of Mg-Li alloys.Moreover,the single-phase structured Mg-Li alloys containing texture present the serrated yielding on the stress-strain curves for the different orientations.However,the duplex structured Mg-Li alloys show smooth stress-strain curves for the different orientations.Thus,there is no relationship between the texture and PLC effect,and(3-Li phases can also eliminate the PLC effect.Mechanical anisotropy,especially the elongation,is obvious for the extruded Mg-4Li and Mg-6Li alloys,which is attributed to the(0002)basal texture.I-phase acts as nucleation positions of dynamic recrystallization and improves recrystallized structure during the thermal deformation.As a result,the texture is weak.Beisides,?-Li phase can also weaken the texture of Mg-Li alloys.Therefore,the mechanical anisotropy almost does not exist for the Mg-6Li-6Zn-Y alloy.