| Mg-Li alloy is the lowest density metal structure material at present,it has high specific strength,stiffness and perfect electromagnetic shielding property.It has great application potential in aerospace,weapon industry,3C and other fields.However,the absolute strength of Mg Li alloy is low,and the existing mechanical properties mainly focus on the static tensile behavior,while the research on the service behavior under cyclic load is very few,which limits the popularization and application of Mg-Li alloy.Therefore,it is of great significance to explore the method of improving the strength of Mg-Li alloy under cyclic loading.Based on this,the Mg-Li-Al-Zn(-Y)system alloy with better comprehensive properties developed by the research group is taken as the research object in this paper.The Li content is selected as 8wt.%with dual phase structure,and the total Al and Zn content is 5wt.%.The static mechanical properties and dynamic fatigue properties of Mg-Li-Al-Zn(-Y)alloy were further improved by means of multi-element microalloying,solution heat treatment and extrusion deformation without significantly increasing the alloy density(ρ≤1.55 g/cm~3),this research provided theoretical foundation for the further application of Mg-Li alloys.The main research results are as follows:In this paper,the effects of Al/Zn ratio and solution heat treatment process on the tensile properties and microstructure of Mg-8Li-xAl-yZn(x+y=5)alloy were investigated,and the best Al/Zn ratio and heat treatment process were optimized.Mg-8Li-xAl-yZn alloy with Al/Zn ratio of 1:4,2:3,3:2,4:1 was prepared.Then four different solid solution processes(300℃,325℃,350℃,375℃)were designed for4h.The results show that for as cast alloy,when the Al/Zn ratio is less than 2:3,the secondary phase is mainly composed of MgLiZn phase and AlLi phase,when the Al/Zn ratio is more than 3:2,the secondary phase is mainly composed of Al-Li phase,and the secondary phase in the alloy with different composition is systematically identified.In Mg-8Li-xAl-yZn(x+y=5)alloy,the optimal Al/Zn ratio is 3:2,and the yield strength(YS),tensile strength(UTS)and elongation(EL)of Mg-8Li-3Al-2Zn(LAZ832)alloy are 143.0 MPa,183.0 MPa and 26.6%,respectively.The best solution treatment process is 350℃×4h.After the solid solution treatment,the yield strength and tensile strength of LAZ832 alloy increased to 272.5 MPa and 315.0 MPa respectively,increased by90.6%and 72.1%,and the elongation decreased to 3%.The effect of Al and Zn elements on the type and formation mechanism of the secondary phase and the mechanism of solid solution strengthening of the alloy are discussed.The decomposition of Mg Li Zn phase at 300°C and solid solution into the matrix are the main factors to improve the mechanical properties of LAZ alloy.The decomposition of Al-Li phase and the spheroidization ofα-Mg matrix are the main factors to improve the mechanical properties of LAZ alloy when the solution temperature is over 350℃.Based on the above optimal Al/Zn ratio and solution treatment process,in order to further improve the service performance and thermal stability of this kind of Mg-Li alloy,0.5wt.%rare earth element Y was added to the LAZ alloy.The tensile mechanical properties and high cycle fatigue properties of as-cast,solid solution treated and as-extruded LAZ832-0.5Y alloy were studied and compared with that of LAZ814-0.5Y alloy.The results show that the tensile and fatigue properties of all the alloys are higher than those of the alloys.Compared with as cast LAZ832-0.5Y alloy(YS=198.1 MPa,UTS=222.5 MPa,EL=12.3%),YS and UTS of solid solution LAZ832-0.5Y alloy increased to 274.7 MPa and 321.7 MPa,respectively,38.7%and 44.6%,while EL decreased to 4.9%.YS,UTS and EL of as-extruded LAZ832-0.5Y alloy increased to282.4 MPa,319.3 MPa and 19.3%respectively,which were 42.6%,43.5%and 56.9%higher than as-cast.Under R=-1 and 10~7 high cycle fatigue cycles,the fatigue strength of as-cast,solid solution treated and as-extruded LAZ832-0.5Y alloy is 65 MPa,90MPa and 105 MPa respectively.The strengthening mechanism of the fatigue properties of Mg-Li alloys with different compositions under different states was explored.The fatigue cracks of the as-cast alloy mainly originated from the secondary phase of the stress concentration on the subsurface.Compared with the as-cast alloy,the fatigue cracks of the solid solution treated alloy mainly initiated on the subsurface of the sample,the improvement of fatigue strength is mainly related to solid solution strengthening,and the crack propagation is mainly hindered by lattice distortion,which is shown as intergranular and transgranular propagation.The fatigue cracks of the as-extruded alloy mainly originate on the surface of the specimen.The increase in fatigue strength is mainly due to the refinement of the structure,the dislocation stacking,and the secondary phase dispersion strengthening. |
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