Study On The Strengthening Mechanism And Control For Microstructure,Mechanical Properties Of Novel High Strength Mg-Li Alloys

Considering the global energy shortage and environmental pollution,the lightweight material and environmental protection are required urgently by many industrial fields.As the lightest metallic structural material,Mg-Li alloys have attracted more and more attentions.For its low density,high specific stiffness,specific strength,excellent seismic resistance and high-energy particle penetration resistance,Mg-Li alloys are the very ideal and potential structural material in the fields of aerospace,automobile,3C electronic industry and weapon industry and so on.However,at present the poor mechanical properties and some technical problems,such as melting,casting and plastic deformation,long limited its application.Therefore,the preparation for the novel pure and high strength ultra-light Mg-Li alloy and the design for a good preparation process suitable for industrial production are very significant.In the work,a high strength Mg-Li alloy(?-Mg matrix,?-Mg+?-Li matrix,?-Li matrix)was prepared through the composition design,alloy melting,casting,homogenization,plastic deformation and heat treatment.Meanwhile,the microstructural evolution and mechanical properties were investigated including the solidification mechanism,plastic deformation mechanism,recrystallization mechanism and orientation transformation mechanism.Simultaneously the alloy strengthening nature and mechanism of the secondary phase during annealing were also investigated.In the end,the effects of AlTiB on the microstructural refinement and the corrosion resistance of the three alloys were further studied.The main results are as follows:(1)For preparing the high strength Mg-Li alloy,based on the Mg,Li,Al,Zn and Y elements,the microstructural evolution,textures and mechanical properties of Mg-(5,8,11)Li-3Al-2Zn-0.2Y alloy in the preparation process were investigated.The results showed that,with increasing Li content,the ?-Mg matrix(hcp)was transformed to the ?-Li matrix(bcc).The brittle fracture was transformed gradually to the ductile fracture.Simultaneously the density and hardness both decreased.Additionally,the static recrystallization emerged in the deformed ?-Mg and ?-Mg+?-Li alloys during annealing.However,the secondary recrystallization emerged in the deformed ?-Li alloy during annealing.In addition,with increasing temperature,the {11-20} recrystallization texture was found in the ?-Mg phase,and the basal texture decreased gradually.After annealing,the recrystallization softening could be observed in the ?-Mg alloy.However,the strengthening effect could be found in the?-Mg+?-Li and ?-Li alloys.The strengthening reason was the solid solution strengthening of Al atom in the ?-Li phase after AlLi decomposition.Comparing to the ?-Mg+?-Li alloy,the strengthening degree in ?-Li alloy was relatively high.(2)Aiming to enhance the mechanical properties,the Mg-(5,8,12)Li-5Al-2Zn-0.5Y alloy was prepared and studied based on adding the proper content of Li,A1 and Y to the former alloying composition.The results showed that the solidified microstructure was refined and the mechanical properties of the extruded alloys were improved significantly.Compared to the ?-Mg+?-Li and ?-Li alloys,the mechanical properties of the ?-Mg alloy were affected slightly by extruding and annealing temperatures.Additionally,the mechanical properties of the ?-Mg+?-Li and ?-Li alloys were significantly improved during annealing.After annealing,the optimal mechanical properties and density of the ?-Mg+?-Li alloy were as follows:ultimate tensile strength 235.1 MPa,yield strength 217.6MPa,elongation 23.1%,density I.56g/cm3;the optimal mechanical properties and density of the ?-Li alloy were as follows:ultimate tensile strength 329.2MPa,yield strength 329.1 MPa,elongation 5.4%,density l.41g/cm3.(3)Based on adding different contents of AlTiB(0.1%,0.5%,1.0%)to the Mg-(5,8,12)Li-5Al-2Zn-0.5Y alloy,the effects of AlTiB on the microstructure,mechanical properties and corrosion resistance of the alloy were studied.The results showed that the solidified microstructure of ?-Mg phase was refined obviously while the ?-Li phase was not refined.The densities of the three extruded alloys increased linearly with increasing AlTiB content.In the solidified,extruded and annealed microstructures,increasing AlTiB content enhanced the AlLi number in ?-Li phase,but the mechanical properties have not been significantly improved.However,the mechanical properties of the ?-Mg and ?-Mg+?-Li alloys increased with increasing AlTiB content.After annealing,the optimal mechanical properties and density of the ?-Mg+?-Li alloy were as follows:ultimate tensile strength 269.6MPa,yield strength 244.7MPa,elongation 11.8%,density 1.61g/cm3;the optimal mechanical properties and density of the ?-Li alloy were as follows:ultimate tensile strength 336.8MPa,yield strength 334.3MPa,elongation 4.3%,density 1.52g/cm3.Additionally,increasing AlTiB content did not influence the corrosion resistance of the three alloys.Simultaneously,under the same conditions,the corrosion resistance of the ?-Mg+?-Li alloy was the worst.Meanwhile the corrosion resistance of the ?-Mg alloy was slightly better than that of the ?-Li alloy.