| Magnesium-lithium(Mg-Li)alloy is the metal structure material with the smallest density so far.Its low density,high specific strength and excellent electromagnetic shielding properties make it become one of the most ideal structural materials with great development potential in the fields of aviation,aerospace,weapon industry,nuclear industry,automobile,3C industry,medical equipment,etc.However,the existing Mg-Li alloy still has the problem of low strength.Especially,the strength of ?-phase Mg-Li alloy needs to be greatly improved,due to the coarse grain size.Accumulative roll bonding(ARB)is a typical severe plastic deformation(SPD)technique to obtain ultrafine-grained(UFG)and high-strength metallic materials.Combining with the excellent plastic deformation capability of ?-phase Mg-Li alloy,the deformation processing of ?-phase Mg-Li alloy by ARB is expected to obtain fine-grained microstructure and significantly improve the strength of the alloy.Firstly,the conventional two-layer accumulative roll bonding(TARB)was extended to four-layer accumulative roll bonding(FARB)and multi-layer accumulative roll bonding(MARB)by optimizing the conventional accumulative roll bonding process.The relevant mathematical relations were obtained by the derivation from the change rule of geometric size and strain rate in the process of FARB and MARB.Based on the theoretic analysis,the effects of different ARB processes on the microstructure and mechanical properties of the Mg-14Li-3Al-2Gd alloy(? phase Mg-Li alloy)were compared and analyzed,and the interface evolution and bonding mechanism,grain refinement mechanism and strengthening mechanism of Mg-14Li-3Al-2Gd alloy in the process of ARB were studied.Through the research,the following results were obtained.Compared with the TARB,the FARB has a higher accumulative efficiency in terms of accumulative layers,total number of interfaces,interface spacing,total deformation and equivalent strain.Therefore,the FARB processed sheets in lower cycles have the similar microstructure and mechanical properties of the TARB processed sheets in higher cycles.In addition,TARB process with a single pass in a single cycle possesses the deformation limitation that,if the deformation reduction is too low,the interface can not be well bonded,but if it is too high,the sheets are easy to crack.FARB process can avoid this drawback through adopting two-step rolling in one cycle with 50% deformation in one pass and 75%accumulative deformation in one cycle,which can effectively solve the problem of poor interface bonding of the latest interface brought by the last cycle,and thus significantly improve the phenomenon of unstable performance of the ARB-processed sheets.ARB process can effectively refine the grain of Mg-14Li-3Al-2Gd alloy.The refining effect is the most significant in the initial cycles,in which the grain refining rates of TARB1 and FARB1 are about 70.3% and 86.7%,respectively.The refining effect is greatly weakened in the middle cycles,and is recovered due to the rapid increase of the accumulative efficiency of total deformation in the higher cycles.The grain refining rates of TARB6 and FARB4 are about 36.3% and 53.8%,and the average grain size are about 14.5 ?m and 5.3 ?m,respectively.The refinement mechanism is the continuous dynamic recrystallization refinement mechanism based on dislocation evolution.It mainly includes two modes:(1)when the interface spacing is large,the dislocation substructure directly divides the deformed grains randomly,then forms the equiaxed subgrains and grains;(2)when the interface spacing is small,the dislocation first forms the lath-shaped dislocation substructure with parallel distribution and transverse the deformed grain,then forms lath-shaped subgrains with straight subgrain boundary.With the further deformation,necking and fracture occur in the lath-shaped subgrain or lath-shaped dislocation substructure,forming the equiaxed substructure with a small size.In the ARB process,the last cycle sheet always takes itself as the relatively independent evolution space in the next cycle,in which the interfaces accumulated by the previous cycles carry on the evolution behavior.Under this interface behavior,the microstructure evolution of Mg-14Li-3Al-2Gd alloy in the ARB process presents the cyclic evolution mode of single-layer evolution,mixed-layer evolution,multi-layer evolution and single-layer evolution.It should be noted that the single-layer evolution,mixed-layer evolution and multi-layer evolution do not exist independently in a certain form in a certain cycle,but with the continuous deformation in each cycle,but coexist in different regions at the same time or in the same region at different time with different proportions in different cycles.The interface bonding of the ARB-processed Mg-14Li-3Al-2Gd alloy is carried out in four stages.I stage: Physical contact,that is,surface layer fractures and nascent metal surface forms;II stage: Chemical action,that is,the deformation energy overcomes the energy barrier of metallurgical bonding,and activates the nascent metal surface and realizes the bonding of metallic bond;III stage: Dislocation behavior in the interface region,that is,the nascent metal surface evolves into the dislocation wall,then the grain boundary-interface structure is formed through the dynamic recrystallization behavior of the interface.VI stage: Dynamic recrystallization homogenization,that is,dynamic recrystallization occurs in the whole area of the combined sheet,the straight grain boundary-interface structure is replaced by the secondary dynamic recrystallization structure,and finally the homogeneous structure with the grain across the original interface is formed.Pressure bonding and metallurgical bonding provide physical basis and energy conditions for the final dynamic recrystallization bonding.The mechanical properties of Mg-14Li-3Al-2Gd alloy can be effectively improved by ARB process,which is mainly manifested in the continuous improvement of strength and the stability of elongation at a higher level.Compared with the as-cast alloy,the yield strength values of TARB6 and FARB4 increase by 194.1% and 236.4%,respectively,reaching 213.5MPa and 244.2 MPa,while the elongation values remain at 23.8% and 27.4%.The strengthening mechanisms of Mg-14Li-3Al-2Gd alloy in ARB process mainly include grain refinement strengthening,substructure strengthening,interface strengthening,dislocation strengthening,strain rate effect and second phase strengthening.Among these mechanisms,grain refinement strengthening and strain hardening are the most important strengthening mechanisms.The grain refinement strengthening can be realized by grain boundary retardation effect and orientation difference effect,and the severe plastic deformation at high strain rate provides the Mg-14Li-3Al-2Gd alloy with strong strain hardening effect. |
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