Microstructure And Mechanical Properties Of As-extruded Mg-Li-Zn-Y Alloys

Magnesium alloys are dubbed "green engineering materials of the 21 st century" because of their low density,high specific strength,high specific stiffness,and good electromagnetic shielding.The lightest metal is lithium.Magnesium may be made lighter and lighter by adding lithium to it.The formed magnesium-lithium alloy,whose density is usually between 1.4-1.7 g/cm3,is the lightest metal structural material and will be used in transportation,aerospace and 3C industry and other fields have huge application prospects in the future.In magnesium-lithium alloys,when the Li element content exceeds 5.7 wt.%,the body-centered cubic(bcc)β-Li phase will form in the α-Mg phase of the original hexagonal close-packed structure(hcp).At 10.3 wt.%,the single-phase β-Li phase structure is completely formed.The β-Li phase of the bcc structure has good plasticity,but its modulus is low,resulting in low hardness and strength of the alloy material.Therefore,it is not enough for magnesium-lithium alloys to be widely used only by virtue of their low density and good plasticity,and their physical and mechanical properties need to be improved.In this paper,the Mg-Li-ZnY as-cast alloys were prepared by the alloying method,which were numbered LZW861 and LZW982 respectively.The microstructure is regulated by plastic deformation and heat treatment to further improve its mechanical properties.The microstructure evolution and strengthening mechanism of magnesium-lithium alloys under different processes were studied,and the correlation model between the microstructure and mechanical properties of magnesium-lithium alloys was initially established.The research results can provide a reliable theoretical basis for the design and development of magnesium-lithium alloys.The main contents are summarized as follows:(1)After the hot extrusion process,the α phase of LZW861 alloy changes from the as-cast irregular block structure to the elongated structure distributed along the extrusion direction after extrusion,but the α-Mg phase in some areas,still coarse and uneven organizational structure.Some α-Mg phases are precipitated in LZW982 alloy after extrusion.The β phase of the two alloys has obvious grain refinement,showing an equiaxed grain shape,and the average grain size is 12.2 μm and 10.9 μm,respectively.The quasicrystal phase of the network structure of the two alloys was broken and some W phase and impurity phase were distributed along the extrusion direction.The extruded LZW861 and LZW982 alloys are mainly composed of α-Mg,β-Li and I phases.The Vickers hardness of the two alloys as extruded is 65.3 HV and66.9 HV,respectively,the yield strength is 175 MPa and 189 MPa,the tensile strength is 212 MPa and 220 MPa,and the elongation is 29.4% and 27.7%,respectively.LZW982 alloy has higher Zn and Y element content,higher volume fraction of precipitation phase,and smaller grain size,which brings greater effect of solid solution strengthening,precipitation strengthening and fine grain strengthening.Therefore,compared with LZW861,the hardness and strength are higher,the plasticity is lower.(2)Most of the α-Mg phase and I phase and some impurity phases of the two alloys were dissolved into the β matrix by solution treatment,and the grains grew,and the average grain sizes were 121.8 μm and 121.7 μm,respectively.Due to the diffusion of the Zn element,the I phase and the W phase are transformed into the Mg Y phase.The hardness of the two alloys are 85.3 HV and 91.2 HV,respectively,which are 30.6%and 36.3% higher than those in the as-extruded state.The yield strengths of the two alloys are 202 MPa and 264 MPa,respectively,the tensile strengths are 249 MPa and355 MPa,and the elongations are 28.7% and 15.9%,respectively.Compared with the extruded state,the hardness of the α-phase and β-phase in the solid solution state increases,and the hardness of the I-phase and W-phase is slightly decreased after the transformation into the Mg Y phase,and the overall hardness of the alloy is improved.Solution treatment can effectively control the phase structure of the alloy,homogenize,eliminate impurities,strengthen the β matrix,greatly improve the hardness and strength of the alloy,but also reduce the plasticity.(3)The mechanical properties of LZW861 alloy and LZW982 alloy in different states change with the change of microstructure.In the extrusion state,grain refinement strengthening and strain hardening are the main strengthening mechanisms,and in the solid solution state,solid solution strengthening and second-phase strengthening are the main strengthening mechanisms.The correlation model between the microstructure and mechanical properties of the two alloys was initially established by theoretical calculation.However,the theoretical calculation failed to accurately calculate the strength contribution of various strengthening mechanisms.The results of this paper show that the Mg-Li-Zn-Y alloy can effectively control the microstructure and improve the mechanical properties through plastic deformation and heat treatment.