Microstructure And Mechanical Properties Of α/β Alternative Mg-Li Laminated Composites Based On Accumulative Roll Bonding

In the magnesium-lithium(Mg-Li)alloy material system,low-lithium content Mg-Li alloys(α-based single-phase alloys)and high-lithium content Mg-Li alloys(β-based singlephase alloys)both have individually significant advantages and disadvantages(α-based singlephase alloys have relatively high strength and poor ductility,while β-based single-phase alloys have higher ductility and poorer strength).In order to obtain ultra-light Mg-Li alloy materials with good comprehensive properties,α/β alternative Mg-Li alloy layered composite sheets were prepared by accumulative roll bonding(ARB)with Mg-5Li-1Al(LA51)alloy and Mg14Li-1Al(LA141)alloy in this dissertation.In this dissertation,firstly,the evolution of the microstructure,texture and mechanical properties of conventional ARB composite sheets were studied.Then,the rules of hard layer necking,microstructure evolution,and the coordinate deformation mechanism of two Mg-Li alloys in ARB composite sheets with different thickness ratios of α/β sheets during the ARB process were studied.Subsequently,the composite sheet was processed by the continuously lowered temperature ARB,and the evolution of the microstructure and the effect of grain size and texture on mechanical properties of the two alloys were studied.Finally,the α/β composite sheet with gradient structure was processed by introducing annealed alloys in each pass,and the microstructure,texture evolution,and heterogeneous deformation-induced(HDI)hardening mechanism of ARB composite sheets with gradient structure were investigated.With the increase of ARB passes,the grains of the two alloys are gradually refined.The ultrafine grains in LA51 alloy and the coarse grains of LA 141 alloy form a typical bimodal grain structure.Due to the poor plastic deformation compatibility between the two alloys,the necking of the LA51 alloy in the ARB composite sheet becomes more severe with the increase of the ARB pass.In ARB composite sheet,the typical basal plane {0002} texture is dominant in LA51 alloy,and {110} texture is dominant in LA141 alloy,and the two textures weaken with the increase of ARB pass.With the increase of ARB passes,the microhardness,yield strength(YS)and ultimate tensile strength(UTS)of LA51 alloy and LA141 alloy increased first and then decreased,and the comprehensive mechanical properties of ARB3 composite sheet were relatively higher,which are 72.1 HV,53.0 HV,204 MPa,and 216 MPa,respectively.The strengthening mechanisms of ARB composite sheets are mainly grain refinement strengthening and dislocation strengthening.The ARB composite sheet has good interfacial bonding,and its average interfacial shear strength remains between 16.5MPa and 18.1 MPa.The fracture elongation of the ARB composite sheet is always high and remains around 20%,because of the longer strain hardening stage III during the tensile deformation of the composite sheet.In ARB composite sheets with different initial thickness ratios of α/β sheets,with the increase of ARB passes and the decrease of initial thickness ratio H(0.5-3),the necking of LA51 alloy is gradually serious.In the LA51 alloy,there exist many shear bands and twins.In the LA 141 alloy,fine equiaxed grains exist near the surface and interface,and the internal structure is band-like elongated grains.With the increase of the initial thickness ratio,the grains of the two alloys show a gradual refinement trend,and the dislocation density gradually increases in the LA51 alloy,but first decreases and then increases in the LA141 alloy.The difference of microhardness between the two alloys gradually decreases with the increase of initial thickness ratio.With the increase of the initial thickness ratio,the strength of the composite plate first increases and then decreases,and the plasticity shows a gradual increase trend.During the ARB process,the coordinate deformation mechanism of the two alloys is that the LA141 alloy is work hardened first,then the work hardening of the LA51 alloy layer increases significantly,and then the recrystallized grains grow in the LA141 alloy,and the two alloys work harden simultaneously.ARB-A,ARB-B and ARB-C composite sheets were prepared by successively lowering temperature 0,15℃ and 30℃ for each pass.In the lowered temperature ARB composite sheet,a large number of twins are formed near the surface and the interface in the L A51 alloy,and twin dynamic recrystallization(TDRX)occurs.The twins are formed inside the alloy and gradually increase with the decrease of temperature,and the dynamic recovery(DRV)is transformed into TDRX.Fine DRX grains are formed on the surface and interface of LA141 alloy,and the recrystallization mechanism is discontinuous dynamic recrystallization(DDRX),while elongated DRX grains are formed inside,and the mechanism is continuous dynamic recrystallization(CDRX).In the process of lowered temperature ARB,with the decrease of the ARB temperature,the basal texture of the LA51 alloy is gradually enhanced,and the {110} and{200} textures of the LA141 alloy are enhanced first and then weakened.With the decrease of ARB temperature,the microhardness of LA51 alloy and LA141 alloy in ARB-C composite sheet gradually increases,and its uniformity along ND is better.The strength of the composite sheet increases with the increase of ARB pass and lowered temperature amplitude.The grain refinement and the strengthening of basal texture and {110} texture of the two alloys caused by continuous lowering temperature promote the prolongation of the Ⅲ stage of the tensile process.Accordingly,the lowered temperature ARB composite sheet maintains a higher strength and elongation.The gradient ARB composite sheet was prepared by introducing the annealed alloy in each pass,and then the lowered temperature gradient ARB composite sheet was prepared by combining successive lowered temperature in each pass.The stepped distribution of shear strain in ARB composite sheet leads to the gradient distribution of grain and texture,and the continuous lowering temperature makes the grain gradient in the composite sheet more obvious.The dislocations density increases near the interface of the ARB5 composite sheet.In the gradient ARB5 composite sheet,the basal texture and {100},{110} textures are gradually strengthened with the increase of rolling times.With the increase of the gradient ARB pass,the microhardness of the two alloy layers increases gradually,while the microhardness of the two alloy layers first decreases and then increases during the lowered temperature gradient ARB composite process.The strength of the gradient composite sheet increases with the number of ARB passes,and its elongation is slightly reduced.The strength of lowered temperature gradient ARB composite sheet increases further,and its elongation increases gradually.The gradient structure leads to the increase of hetero-deformation induced(HDI)stress in the gradient ARB composite sheet,indicating that HDI hardening promotes the strengthening and toughening.