Effect Of Cold-rolled And Annealing Treatment On Microstructure And Mechanical Properties Of Mg-Li Binary Alloys

Based on the energy crisis and the progress of industrial technology in modern times,lightweight and environmentally-friendly materials are urgent needed with the development of society and industry.Magnesium-lithium alloys are the most lightweight metal structural material at present,with low density,high specific strength and specific stiffness,good electromagnetic shielding and excellent ability of room temperature deformation.It has shown great potential in aerospace,weaponry and 3C industries.However,the disadvantages such as low absolute strength,poor corrosion resistance and creep resistance of Mg-Li alloys limit its application.Deformation strengthening and alloying are effective ways to improve the mechanical properties of Mg-Li alloys.In this paper,the cast Mg-5Li,Mg-9Li and Mg-9Li-1Zn alloys were rolled at room temperature and then annealed.The microstructure and mechanical properties of Mg-Li alloy plates with different alloy components under different cold rolling deformations and annealing processes were tested by means of the optical electron microscope,scanning electron microscope,transmission electron microscope,X-ray diffractometer and room temperature extension and hardness tests equipment.The following conclusions were drawn by comparing and analyzing:?1?The as-cast Mg-5Li alloy consists of 96% ?-Mg phase and 4% ?-Li phase.After cold rolling,the two-phase structures are elongated along the rolling direction.The as-cast Mg-5Li alloy's tensile strength,hardness and elongation are 74 MPa,40.6 HV and 6.5%,respectively.The work hardening occurs during cold-rolling.When the cold reduction is 50%,the tensile strength is 190MPa which is 2.6 times that of the as-cast alloy.Annealing treatment reduces the strength and improves the plasticity of the alloy.At 60%reduction,the elongation of the annealed alloy is 2.6 times that of the cold-rolled alloy and the hardness decreased by 9.8HV.The alloy is severely cracked due to its poor plasticity when cold-rolling reduction is 60%.The fracture morphology is quasi cleavage fracture.?2?The Mg-9Li alloy is composed of 28% of?-Mg phase and 72% of?-Li phase,in which the?phase is plate-like at the cast state.The structures are refined and elongated along the rolling direction.The greater rolling reduction,the more obvious the phenomenon is.The tensile strength,hardness and elongation of the as-cast Mg-9Li alloy are 100MPa,41.4HV and 22.6%,respectively.When the amount of cold deformation up to 75%,the tensile strength of the alloy is 179 MPa?which is 1.8 times that of the as-cast alloy?,the hardness is 84.4 HV?which is 2.0 times that of the as-cast alloy?,but the elongation is only 10.5%.The elongation of the annealing alloy is 20.6%which is 2.0 times that of the 75%cold-rolling alloy.The plasticity of Mg-9Li alloy is great,there is no serious cracking at 75%deformation,and there are more dimples in tensile fracture morphology,which shows ductile fracture.?3?The as-cast Mg-9Li-1Zn alloy is composed of?-Mg,?-Li and MgLi₂Zn metastable phase in which the?phase is plate-like and?-Mg:?-Li?1:3.After cold-rolling,the structures are refined along the rolling direction,and the MgLiZn phase appears in the alloy.The alloy begins to recrystallize at the boundary of the?phase during annealing.At annealing state,the recrystallized grains in the alloy are little under25%reduction,then increased and significantly coarsened under the 50%deformation.When cold-rolling reduction is 75%,a large number of fine recrystallized grains are forming in the alloy during annealing.In 75%-deformation cold-rolled alloy,the?-phase shows a?0002?basal texture and?phase exhibit?110?and?200?planes stronger texture.The tensile strength,hardness and elongation of the as-cast Mg-9Li-1Zn alloy are 113 MPa,45.7 HV,and 19.2%,respectively.At 75%deformation,the tensile strength of the alloy was 190 MPa,and the hardness was 71.8 HV.At 75%cold-rolling reduction and then annealed treatment,the Mg-9Li-1Zn alloy has excellent comprehensive mechanical properties?hardness is 57HV,tensile strength is 177MPa and elongation is 23.2%?.The plasticity of Mg-9Li-1Zn alloy is superior,and the edge cracking of rolled plate is not obvious under 75%deformation.The tensile fracture morphology is ductile fracture.?4?After 95% severe cold-rolling and annealing at different temperatures,Mg-9Li-1Zn alloy is composed of matrix phases??-Mg and?-Li?and two compound phases?MgLiZn and MgLi₂Zn?,in which the structures are significantly refined and elongated.With the increasing of annealing temperature,the recrystallization completed gradually and the grains grow up.When annealing at 125?,the recrystallization is completed and a large number of fine equiaxed grains??3?m?are evenly distributed on the matrix.There are MgLi₂Zn and MgLiZn phases at the grain boundaries and the dislocation is less.So the alloy has the best mechanical properties at the state?56.1%elongation,157MPa tensile strength,and 51.2HV hardness?.As the annealing temperature continues to increase,the grains within the?phase begin to coarsening and the strength and plasticity of the alloy are decrease.The?-phase shows basal texture in the cold-rolled state.After annealing at 125°C,the pole figures of the alloy have no obvious change.The fracture morphology of Mg-9Li-1Zn alloy under various conditions is rock-like torn edges and dimples with uneven sizes,which are ductile fractures.