Effect Of Ca,Mn And Ce Microalloying On Microstructure And Properties Of Pure Mg And Mg-6Zn During Hot Deformation

Magnesium alloy has a broad research prospect,due to its low density,high specific strength and specific stiffness,easy cutting processing,recyclable utilization,etc.Compared with other alloys such as aluminum alloy,the K value of magnesium alloy in Hall-Petch is higher,which has better grain refinement and strengthening effect,and grain refinement can also improve the plasticity of the alloy.However,the two commonly used methods for preparing fine grained magnesium alloys,namely the severe plastic deformation method and medium-low temperature deformation method,which are less industrially applied due to complex process,high cost and difficultly in producing large-size parts.Studies that a certain size second phase can stimulate nucleation and pinning of grain boundaries during hot deformation.The solid solubility of Ca,Mn and Ce in pure Mg is extremely small.Mg₂Ca,?-Mn and Mg₁₂Ce are formed during the solidification process,respectively.These second phases can stimulate nucleation and pinning grain boundaries during hot deformation,which can refine recrystallized grains.In this paper,the effects of different contents of Ca,Mn and Ce on the microstructure and properties of pure Mg as-cast and extruded were studied;the microstructure evolution and recrystallization mechanism of Mg-xCa/Mn/Ce alloy under different deformation conditions were studied;the microstructure and properties of Mg-6Zn-xCa/Mn/Ce alloy as-cast,solid solution and as-extruded were studied.Firstly,the microstructure and mechanical properties of Mg-xCa/Mn/Ce alloy(x=0.25,0.5,1,2 wt.%)as-cast and as-extruded were studied.The results as show:Ca has the best grain refinement effect on as-cast pure Mg,followed by Ce and Mn.The addition of Ca,Mn and Ce plays an important role in the dynamic recrystallization of pure Mg,but the grain size of Mg-xCa/Mn/Ce alloy does not decrease with the increase of Ca,Mn and Ce contents.Except for Mg-0.25Mn and Mg-0.25Ce alloys,the grains of the other 10 extruded alloys are fine and the average grain size is less than 5?m.The addition of Ca,Mn and Ce has a significant effect on the texture of extruded Mg-xCa/Mn/Ce alloy.The addition of 0.25-2%Ca and 0.25%Ce can weaken the texture of the alloy,while the addition of Mn can enhance the texture of the alloy.Moreover,the weakening effect of“rare earth”elements on the texture of wrought magnesium alloy is not absolute,and the“rare earth texture”does not necessarily appear.With the increase of Ce content,the texture of Mg-x Ce(x=0.5,1,2 wt.%)alloy is obviously strengthened,showing strong{0002}-<011^—0>texture.The microstructure evolution of Mg-0.5Ca,Mg-2Mn and Mg-1Ce alloy under different deformation conditions was studied by hot compression.The results as show:the effect of Ca,Mn and Ce on the deformation resistance of Mg are obviously different.The deformation resistance of Mg-2Mn alloy is obviously lower than Mg-0.5Ca and Mg-1Ce alloy.beacause the larger size of Mg₂Ca and Mg₁₂Ce has more obvious hindrance to dislocation slip and twinning.At 275? low temperature deformation,a large number of twins will be produced in all three alloys.The grain size of Mg-2Mn alloy is larger,and the twins will be coarser.With the increase of strain rate and deformation temperature,the width of twin becomes narrower.By EBSD analysis shows that Mg-0.5Ca alloy with large deformed grains has strong basal texture.Effect of Zn on Mg-0.5Ca,Mg-2Mn and Mg-1Ce alloy were studied.The results as show:The as-cast microstructure of Mg-0.5Ca,Mg-2Mn and Mg-1Ce alloys is obviously refined by adding 6 wt.%Zn.The second phases in the three alloys are Mg₆Zn₃Ca₂ phase,Mg Zn phase,Mg Zn phase and Mg Zn₂Ce phase,which are distributed in the dendrites and grain boundaries in a network.After solution treatment,Mg₆Zn₃Ca₂ and Mg Zn₂Ce phase are difficult to dissolve into Mg matrix.After extrusion,Mg-6Zn-0.5Ca,Mg-6Zn-2Mn and Mg-6Zn-1Ce alloys show obvious recrystallization structure.The recrystallized grain size of extruded Mg-6Zn-0.5Ca and Mg-6Zn-2Mn is about 2?m,while Mg-6Zn-1Ce alloy is 8?m.The tensile strength and compressive strength of Mg-6Zn-0.5Ca and Mg-6Zn-2Mn alloys are 309.5 MPa,489.2 MPa and 328.7 MPa 429.9 MPa,respectively,due to solution strengthening,grain refinement and second phase strengthening.However,the mechanical properties of Mg-6Zn-1Ce alloy are decreased,because the grain size of Mg-6Zn-1Ce alloy is obviously coarse,and the Mg Zn₂Ce phase with large size.The tensile yield strength and compressive yield strength are 193.9 MPa and 124.7 MPa,respectively.