Preparation And Study On Microstructure And Properties Of Mg-Li-Al-Si Magnesium Alloys

With the increasing development of the lightweight alloys,magnesium alloys,alight engineering structure material,have been paid more attention.Mg-Li alloys,as thelightest structure materials,have many advantage such as the low density, high specificstrength, excellent electromagnetic shielding,compared with other alloys.those attractmore attention in the aerospace,automobile industries.However,Mg-Li alloys also arecomposed of some disadvantages such as unstable properties,poor corrosion resistanceand low heat resistance,which limit the development of Mg-Li alloys in many fields.In this study,Mg-8Li-3Al-xSi,a new hot resistance alloy,are introduced by theaddition of Si to Mg-8Li-3Al alloy.The effect of different content of Si on themicrostructure,phase constitution and the mechanical properties of the alloys areinvestigated by optical microscopy, scanning electronic microscopy (SEM), X-raydiffraction (XRD) and mechanical properties testing. The as-cast alloys are extruded toanalyze to the microstructure evolution of the as-extruded alloys.Effect of Si on themicrostructure and mechanical properties of as-extruded alloys is studied.Finally,effectof Y and Sr addition on the alloys is investigated,the results reveal:Mg-8Li-3Al-xSi alloys are composed of α-Mg、β-Li、Mg17Al12、Mg2Si phases.Thegrain size of α-Mg decreases after the addition of Si but increase with the addition of Siincreasing.With the addition of Si increasing,the Granular Mg2Si phase is replaced by coursedendritic phase.The Mg2Si phase is firstly distributed uniformly and then segregationoccurs.The mechanical properties tests of as-cast Mg-8Li-3Al-xSi alloys are carriedout.The results show that the addition of Si can improve the tensile strength of thealloys.When the content of Si is0.1wt%,the tensile strength is186.8MPa.With Siaddition increasing,the tensile strength decreases gradually and the elongation shows thesame trend as the tensile strength.Combining the tensile fracture and microstructure ofthe alloys,it can be deduced that a trace of Si addition is beneficial to the improvementof the tensile strength.However,when more Si are added,the tensile fracture exhibits thecharacteristic of brittle fracture because of the existence of course dendritic Mg2Si.Thisleads to the decrease of the tensile strength of the alloys.As-cast Mg-8Li-3Al-xSi alloys are extruded.It can be seen that the microstructure of the alloys are improved obviously compared with that of as-cast alloys.The dynamicrecrystallization occurs in the extrusion and the grain is refined.Mg2Si phase is brokenup and diffusely distributed the matrix alloys,which is good for the improvement oftensile strength.The tensile strength and elongation of Mg-8Li-3Al-0.1Si are enhancedby54.1MPa and5.9%,respectively.When the content of Si is0.3wt%,the tensilestrength reached the maximum value by252.3MPa.The microstructure of fracturepresents plastic fracture.The tensile strength tests are carried out at150℃，the tensilestrength increases with the addition of Si increasing.When the content of Si is1.0wt%,the tensile strength reaches to the max value by188.0MPa.This indicates thatthe addition of Si is beneficial to the improvement of the alloys at high temperature.A certain amount of Y and Sr are added to the alloy as modificator respectively,the result show that the modificator Y can effectively improve the microstructure of thealloys, reduce the size of Mg2Si phase, refine the crystalline structure and enhance itsmechanical properties.While the modification effect of Sr is not obvious.