Research On Microstructure And Properties Of Mg-Li-Zn-Mn Alloys

As the lightest metallic structal materials at present,Mg-Li series alloys enjoy the most"green and environmental protection"revolutionary material in the future,Mg-Li alloys with high specific strength,good forming performance,have shown great potential in aerospace,weaponry and 3C industries.However,the disadvantages such as low absolute strength and poor corrosion resistance of binary Mg-Li alloys limit its application.A large number of studies have shown that alloying and deformation strengthening are effective ways to improve the properties of Mg-Li alloys.In this paper,based on the dual phase Mg-Li-Zn alloy,a new Mg-Li-Zn-Mn alloys were designed and constructed.The as-cast and extruded Mg-6Li-4Zn-xMn alloys were prepared.The microstructure and mechanical properties of Mg-6Li-4Zn-xMn alloys were investigated by optical microscopy,scanning electron microscopy equipped with energy dispersive spectrometer and X ray diffractometer.The corrosion resietance of the extruded Mg-6Li-4Zn-xMn test alloys was studied by electrochemical test and immersion test.The following conclusions were drawn:?1?The microstructure and mechanical properties of as-cast Mg-6Li-4Zn-xMn alloysw were systematically studied.The microstructure of as-cast Mg-6Li-4Zn-xMn alloys is mainly composed of?-Mg phase and?-Li phase,and MgLi₂Zn phase distributed in the?-Li phase and?-Mg/?-Li phase boundary.As the Mn content increases,the distribution and size of the compounds in the alloy change significantly.The tensile results show that the strength of the as-cast Mg-6Li-4Zn-xMn alloys first increases and then decreases with the increase of Mn,while the elongation decreases,which is attributed to the content of Mn and the volume fraction of?-Li phase of the as-cast Mg-6Li-4Zn-xMn alloys.Adding a small amount of Mn contributes to the refinement of the structure.When the content of Mn is 1.2%,the excessive addition of Mn results in agglomeration,which reduces the mechanical properties of the alloy.In addition,the gradual decrease of the volume fraction of?-Li phase in the alloys results in the gradual decrease of the elongation.The fracture morphology shows that the as-cast Mg-6Li-4Zn-xMn test alloys present brittle fracture characteristics.?2?The microstructure and mechanical properties of extruded Mg-6Li-4Zn-xMn alloy were systematically studied.After hot extrusion,the?-Mg and?-Li phases are elongated parallel to the extrusion direction.The intermetallic compounds in the alloys are distributed at the grain boundary and the?-Mg/?-Li phase boundary.During the hot extrusion process,the alloys undergoes dynamic recrystallization.Compared with as-cast alloys,the grain size of as-extruded alloys is obviously refined,and the grain size of as-extruded alloys decreases with the increase of Mn content.The tensile results show that the strength of the as-extruded alloys improves with the increase of Mn element,while the elongation of the alloys decreases gradually.The yield strength and tensile strength of the extruded Mg-6Li-4Zn-1.2Mn reached the maximum values of250 MPa and 321 MPa,respectively,and the tensile strength is 30%higher than that of the extruded Mg-6Li-4Zn.The tensile fracture morphology shows that the extruded Mg-6Li-4Zn and Mg-6Li-4Zn-0.4Mn alloys are ductile fracture.With the increase of Mn content,the extruded alloys gradually change from ductile fracture to brittle fracture.?3?The corrosion resistance of extruded Mg-6Li-4Zn-xMn alloys were systematically tested.The results of dynamic polarization curves show that the extruded Mg-6Li-4Zn-1.2Mn alloy has the highest self-corrosion potential and the smallest corrosion current density.The electrochemical impedance?EIS?data show that the extruded Mg-6Li-4Zn-1.2Mn alloy has the highest impedance and the smallest corrosion area.The results of the immersion test show that the weight loss rate and hydrogen evolution rate of extruded Mg-6Li-4Zn-1.2Mn alloy are the smallest among the four alloys.Therefore,the extruded Mg-6Li-4Zn-1.2Mn alloy has better corrosion resistance.?4?The surface corrosion morphology of as-extruded alloys immersed in 3.5wt%NaCl solution for 72h was systematically observed.The results show that the surface corrosion of extruded Mg-6Li-4Zn-1.2Mn alloy is the weakest,which was attributed to the fact that the corrosion product film generated on the surface of the alloy prevented the corrosion from developing to the depth.The corrosion mechanism of extruded dual-phase Mg-6Li-4Zn-xMn alloy is analyzed.Firstly,the calxanic couple is formed between the second phase of the alloy and the matrix,which results in the corrosion of the alloy starting from pitting corrosion,and then corrosion pits appear.Secondly,because the phase boundary between the?-Mg phase and?-Li phase is a sensitive spot for local corrosion,local corrosion occurs at the phase boundary of the alloys.