Study On Deformation Mechanism Of Mg-Li Binary Alloys At Room Temperature

Magnesium(Mg)alloys exhibit a poor plasticity,which hinders their industrial applications.The addition of Lithium(Li)to Mg alloys remarkably improves the room temperature ductility.Mg-Li alloys possess three different micro-structures,depending on the Li content.The alloy is only composed of hexagonal close-packed(HCP)structure with a content of Li below 5.5 wt.%.The alloy with 5.5-10.3 wt.% Li exhibits a twophase structure,consisting of HCP-structured α phase and body-centered cubic(BCC)structured β phase.A Li content over 10.3 wt.% generally yields a single-phase BCCstructured alloy.The deformation behavior of Mg-Li alloys is significantly affected by the micro-structure.In the present study,the deformation behavior of the Mg-Li binary alloys with different Li content was systematically studied,and the mechanisms of the ductility enhancement of Mg-Li alloy have been explained.Firstly,the effect of <c+a> slip on the ductility of a HCP-structured Mg-4.5Li alloy was studied.A quantitative analysis reveals that Li-addition can only slightly increase the activity of <c+a> slip during compression along the normal direction(ND)of a hot-rolled Mg-4.5Li plate,with a relative activity of approximately 18%.Although the limited activity of <c+a> slip cannot accommodate a large plastic strain,it effectively reduces the number of {101?1}-{101?2} double twins,which are believed to be favorable sites for crack initiation.The evidently reduced activity of {101?1}-{101?2} double twins leads to a lower cracking tendency,and therefore improves ductility.Secondly,the effect of Li addition on the strain distributions in Mg-4.5Li alloy during tension was studied.It is found that the enhanced ductility of Mg-4.5Li alloy is related to the homogenous strain distribution during deformation.The strain distribution in Mg alloys was affected by the deformation transfer behavior,and the difference in activation stress(?stress)for slip between two neighboring grains is an important parameter for characterizing the deformation transfer behavior.The homogenous distribution of ?stress in Mg-4.5Li plate accounts for the homogeneous deformation transfer,which yields the homogenous plastic strain distribution.ΔStress is severely affected by the critical resolved shear stress(CRSS)ratio.A much lower CRSS ratio for prismatic <a> slip to basal <a> slip(1.3:1)in Mg-4.5Li alloy is estimated based upon crystal plasticity finite element(CPFE)simulations.Thirdly,the deformation mechanisms of dual-phase Mg-x Li alloy(x=7;8;9 wt.%)during tension were studied.In the dual-phase Mg-Li alloys,the area fraction of β phase increases with the Li content,which effects the deformation behaviors.The yield elongation phenomenon appears in the Mg-7Li alloy during tension.With the increase of Li content,this phenomenon weakens in the Mg-8Li alloy,and finally disappears in the Mg-9Li alloy.The β phase yields first during tension,which can result in the stress concentration at the phase boundary.The stress concentration can promote the movement of dislocation in α phase without increasing the applied load,thus the yield elongation phenomenon appears.With the content of β phase increasing,the stress concentration is weakened,resulting in the weakening of the yield elongation phenomenon.In addition,the β phase is completely plastic after yielding,thus the strain hardening rate of dualphase Mg-Li alloy decreases with the increasing of β phase content.Finally,with the aim to investigating the mechanisms for texture evolution in dualphase Mg-Li alloy,an as-extruded dual-phase Mg-8Li plate was rolled along the extrusion direction(ED-rolling)and transverse direction(TD-rolling),respectively.The results show that the texture evolution of α phase in Mg-8Li alloy is affected by the initial texture and the phase boundary.The basal poles of grains close to the TD in the α phase of the initial plate slightly rotate from TD towards the normal direction(ND)during ED-rolling,while the basal poles rotate to ND after TD-rolling.The variation of rolling path hardly alters the texture of β phase.Annealing does not change the main feature of rolling texture of α and β phases,but reduces the maximum intensity of texture.The mechanisms for texture evolution during rolling were studied by visco-plastic self-consistent(VPSC)simulations.It is shown that the main features of the texture of the rolled plates is effected by the initial texture of each phase,while α/β phase boundaries can retard the texture evolution.In addition,the mechanical anisotropy in Mg-8Li alloy is significantly improved after TD-rolling and annealing.