Research On The Corelation Between Mechanical,Corrosion Behaviors And Microstructure Of High-performance Mg-Li Alloys With I-phase

Magnesium-lithium(Mg-Li)alloys present a significant perspective as ultra-lightweight metal materials in the field of automotive,aerospace and electronics due to their low density,high specific strength and excellent formability.However,the low absolute strength,mechanical abnormity,poor thermostability and corrosion resistance of Mg-Li alloys strictly limit its application and development.Regarding of the addition of Li,the crystal structure of Mg-Li alloys can be altered with Li content:hexagonal close-packed(HCP,less than 5.5 wt.%Li)→HCP+BCC(more than 5.5 wt.%Li but less than 10.3 wt.%Li)→ body-centered cubic(BCC,more than 5.5 wt.%Li).Therefore,mechanical and corrosion behaviors of three different crystal-structured Mg-Li alloys were investigated in detail to reveal the underlying correlation between the mechanical strength/corrosion resistance and the microstructure in this work.Meanwhile,the surface film on the high-corrosion-resistant Mg-Li alloy was also analyzed in detail.Through investigating and comparing the microstructure and mechanical properties of as-extruded Mg-4Li and Mg-4Li-6Zn-1Y alloys,it demonstrated that although the formation of I-phase(Mg3Zn6Y,icosahedral structure)could weaken the texture and improve the mechanical strength,the mechanical anisotropy in terms of yielding strength remained for the Mg-4Li-6Zn-1Y alloy,which was ascribed to the zonal I-phase particles distributed along the extrusion direction.Moreover,Portevin-Le Chatelier(PLC)phenomenon of as-extruded and solid solution treated(T4,300℃/2h+400℃/1h)Mg-4Li-6Zn-1Y alloys could be obviously observed on the tensile stress-strain curves.The salient PLC phenomenon in T4 samples suggested that the occurrence of PLC phenomenon was closely related to the super-saturation degree of solute atoms in the matrix.As most of solute atoms were consumed for the formation of MgZn2 precipitates(ξ1’ and β2’ during the subsequent ageing treatment(T6,T4+200℃/4h),the PLC phenomenon of T6 sample was eliminated.Meanwhile,due to the pinning effect of the formed precipitates on the mobile dislocations,the tensile strength of T6 sample was higher than those of the other two conditions.In addition,both the two alloys displayed filiform corrosion with wide filiment extending along the extrusion direction inMg-4Li alloy,whereas the thin corrosion filament propagated randomly in Mg-4Li-6Zn-1Y alloy,which was correlated with its weaken texture effect.Furthermore,the galvanic effect between the broken I-phase particles and Mg matrix contributed to the higher corrosion rate of Mg-4Li-6Zn-1Y alloy in NaCl solution.Through investigating and comparing the microstructure and mechanical properties of Mg-6Li and Mg-6Li-6Zn-1 Y alloys,it revealed that the volume fraction of β-Li phases formed in Mg-6Li-6Zn-1Y alloy was two times higher than that in Mg-6Li alloy.Texture analysis demonstrated that I-phase particles could stimulate nucleation of reerystallization(PSN)and the formation of higher volume fraction of β-Li phases could remarkably weaken the basal texture of α-Mg phases,resulting in a weak mechanical anisotropy.Natural ageing response of Mg-6Li and Mg-6Li-6Zn-1Y alloys revealed that the precipitation and coarsening of α-Mg particles could occur in β-Li phases of both the two alloys during ageing process.Since a certain amount of Mg atoms in β-Li phases were consumed for the precipitation of abundant tiny LiMgZn particles,the size of α-Mg precipitates in Mg-6Li-6Zn-1Y alloy was much smaller than that in Mg-6Li alloy.With the ageing time increasing,the β-Li phases of Mg-6Li alloy exhibited a slight age-softening response,whereas the age-softening response ofβ-Li phases in the Mg-6Li-6Zn-1Y alloy was much more profounded.Failure analysis demonstrated that cracks were preferentially initiated at α-Mg/β-Li interfaces in Mg-6Li alloy,but it occurred at both α-Mg/β-Li interfaces and slip bands in α-Mg and β-Li phases of Mg-6Li-6Zn-1Y alloy.Through investigating and comparing the corrosion behavior of as-cast Mg-7.5Li and Mg-7.5Li-6Zn-1Y alloys,it demonstrated that corrosion attack initiated at α-Mg/β-Li interface and then propagated preferentially toward α-Mg phase.However,the I-phase pockets were helpful to improve the corrosion resistance of duplex Mg-7.5Li alloy,which was attributed to their physical obstacle to the corrosion process.However,the I-phase pockets were broken into particles after extrusion and acted as more cathodic phases to accelerate the dissolution of anodic Mg matrix,resulting in a lower corrosion resistance of as-extruded Mg-7.5Li-6Zn-1Y alloy than that of as-extruded Mg-7.5Li alloy.Therefore,the distribution of I-phase in matrix was closely related to the corrosion resistance of Mg-Li alloys.Through investigating and comparing the microstructure and mechanical properties of Mg-14Li and Mg-14Li-6Zn-1Y alloys,it showed that the as-extruded Mg-14Li alloy possessed a stronger intensity of texture,resulting in an obvious mechanical anisotropy.However,the mechanical anisotropy of Mg-14Li-6Zn-1Y alloy was ascribed to the distributed morphologies of I-phase particles alonged with the extrusion direction.Regarding of corrosion behaviors,the broken I-phase particles acting as cathode accelerated the dissolution of anodic matrix in the Mg-14Li-6Zn-1Y alloy,resulting in a lower corrosion resistance than that of Mg-14Li alloy.In the case of the protective surface film on the binary Mg-14Li alloy,double-layer structure could be formed and maintain intact completely after 24 h immersion in 0.1 M NaCl solution.The outer layer was a mixture of LiOH(·H2O)and Mg(OH)2,and the inner layer was enriched with Mg element(α-Mg,Li0.184Mg0.816).The layer of outer LiOH(·H2O)could easily be transformed into Li2CO3 in air,which reduced the corrosion performance of Mg-14Li alloy.As a result,the LiOH(·H2O)embedded with Mg(OH)2 contributed to the excellent protective function of surface film on the Mg-14Li alloy.Through investigating and comparing the mechanical and corrosion properties of different crystal-structured Mg-Li alloys,it demonstrated that the mechanical properties of Mg-Li alloys were determined by the volume fraction of β-Li phase and texture components of matrix,and then the addition of Zn and Y elements could improve the mechanical performance effectively.Moreover,the corrosion resistance of binary Mg-xLi(x=4,7.5,14,wt.%)alloys increased in the order of Mg-7.5Li<Mg-4Li<Mg-14Li,and the corrosion performance of BCC Mg-14Li alloy was improved with the immersion time increasing,which was ascribed to the protective film gradually deposited on the surface of Mg-Li alloy,whereas it was reversed for the HCP Mg-4Li and(HCP+BCC)Mg-7.5Li alloys.