Microstructures And Properties Of Multi-Element Microalloyed Al-Mg-Li Alloy

Al-Li alloys have two main advantages,including low density and high elastic modulus,and are considered to be the most ideal lightweight and high-strength structural materials for the aerospace field in the 21st century.Microalloying is one of the most effective ways to improve the comprehensive properties of Al-Li alloys.In this paper,the effects of composite addition of Sc,Zr,Er and Ti elements on the microstructure and mechanical properties of as-cast,homogenized and T6-aged Al-Mg-Li alloy were studied.The main conclusions are as follows:（1）The composite addition of Sc、Zr、Er、Ti elements can effectively reduce the segregation of the Al₂Mg Li phase at the grain boundary,and form micron-sized Al₃（Sc,Zr,Ti）primary phase in the alloy,the as-cast grains of the alloy were significantly refined by heterogeneous nucleation,and the grain size is about 19.7μm.The strength and ductility of the as-cast are simultaneously improved:compared with the Al-Mg-Li base alloy,the tensile strength and yield strength reach 312.1 MPa and 254.3 MPa,which are increased by 31.7%and16.16%respectively,and the elongation of the alloy is increased from 3.7%to5.5%.The homogenization treatment at 420℃/8 h+500℃/16 h can effectively improve the element segregation,and at the same time,a large number of L1₂type dispersed Al₃（Sc,Zr,Er,Ti）coherent phases will be precipitated in the matrix,with an average size is about 16.8 nm;however,the primary micron-sized Al₃（Sc,Zr,Ti）composite particles cannot be redissolved during the homogenization process.（2）The dynamic recovery process mainly occurs in Al-Mg-Li（1#）and Al-Mg-Li-Sc-Zr-Er-Ti（2#）alloys during hot rolling.The composite addition of Sc、Zr、Er、Ti inhibits the recrystallization process of the alloy,after as-quenched,the dynamic recrystallization of the 1#alloy without the addition of trace elements is basically completed,while the 2#alloy still retains non-recrystallized structure that is elongated in the direction of the alloy.The pinning effect of Al₃（Sc,Zr,Er,Ti）phase inhibits the recrystallization process of the alloy.（3）The optimal solution process of the alloy is 500℃/40 min.During the aging process,the main strengthening phase Al₃Li in the 1#alloy gradually coarsens,and the size of the Al₃Li phase is 15.2 nm at peak-aged;There are two main strengthening phases in 2#alloy:a part of Al₃Li phase,which size is 13.3nm;and a part of Al₃Li is coated on the surface of Al₃（Sc,Zr,Er,Ti）phase to form core-shell Al₃Li/Al₃（Sc,Zr,Er,Ti）structure phase,which has strong anti-roughening ability.Compound addition of Sc,Zr,Er,Ti elements inhibits the coarsening of Al3Li and the formation of PFZ.The yield strength and tensile strength of the based alloy at the peak-aged are 362.9 MPa and 426.4 MPa,respectively,and the elongation is 20.5%.The yield strength and tensile strength of the composite added Sc、Zr、Er、Ti alloy reached 427.3 MPa and 488.5 MPa,and the elongation was 9.7%.The fracture mode of the 1#alloy at peak-aged is mainly microporous aggregated fracture,and the 2#alloy is mainly fractured by intergranular stratification.The strengthening mechanism of core-shell Al₃Li/Al₃（Sc,Zr,Er,Ti）structure phase is mainly Orowan bypass strengthening.The combined effect of grain refinement strengthening,substructure strengthening,and Orowan strengthening mechanism can effectively improve the properties of the alloy.