| Mg-Zn series magnesium alloys have excellent comprehensive mechanical properties.However,it is difficult to mass produce the casting parts or deformed parts of Mg-Zn series magnesium alloys due to the formation of tiny voids in the preparation process,which greatly limits its application in practical production.The addition of rare earth elements（Y,Nd,Ce,La）in Mg-Zn series magnesium alloys can effectively enhance their mechanical performances,thereby expanding the applicability of such alloys.It has been demonstrated that Mg-Zn-Y alloys containing long period stacking ordered（LPSO）phase exhibit great potential in virtue of their excellent mechanical performance.Therefore,an as-extruded Mg-2.5Zn-4Y alloy containing LPSO phase was prepared and the deformation behavior and deformation mechanism of this alloy were studied in the present thesis.High-speed impact compression test was applied on the as-extruded Mg-2.5Zn-4Y alloy by using SHPB at different temperatures and strain rates,and the dynamic mechanical behavior of the alloy was studied.High temperature thermal compression experiments were carried out on the as-extruded Mg-2.5Zn-4Y magnesium alloy using a Gleeble thermal simulation experimental machine at different temperatures and strain rates,and the hot compression behavior of the alloy was studied.At the same time,the microstructure evolution of the alloy after static and dynamic deformation was observed by X-ray diffraction（XRD）,optical microscope（OM）,scanning electron microscope（SEM）,transmission electron microscope（TEM）,and electron backscatter diffraction（EBSD）,and the deformation mechanism of the alloy at different deformation conditions was explored.The research results indicated that the microstructure of the as-extruded Mg-2.5Zn-4Y magnesium alloy was mainly composed ofα-Mg and LPSO phase,and LPSO phase is distributed in strips along extrusion direction.Under dynamic deformation conditions,the experimental alloy exhibited significant positive strain rate reinforcement at a constant temperature.As the temperature increased,the flow stress decreased,and the strain rate sensitivity decreased.Also under the dynamic deformation condition,the plastic deformation mechanism of the as-extruded Mg-2.5Zn-4Y alloy was dominated by pyramidal<c+a>slip,{10（?）2}extension twinning,and the kinking of LPSO phase.At a temperature of 623 K and a strain rate of 1600 s-1,LPSO phase in the microstructure appeared as a"sandwich"structure of LPSO/α-Mg/LPSO,and this structure showed a clear"misalignment"under this deformation condition.At the same temperature,the higher the strain rate,the less twinning occured in the microstructure of the experimental alloy.Once the temperature increased to 623 K,the deformation mechanism changed from the twinning-dominated one to the slip-dominated one.At room temperature and high temperatures,the fracturing mechanism of the as-extruded Mg-2.5Zn-4Y alloy is mainly quasi-cleavage.The highest compression fracture strength of the alloy at strain rate of 2100 s-1 at room temperature reached 700 MPa.Under hot compression conditions,at relatively high strain rates(1 s-1),the dynamic recrystallization degree of the experimental alloy was low,the deformation was dominated by dynamic recovering.When the strain rate was low(0.001 s-1,0.01 s-1,and 0.1 s-1),the deformations were dominated by dynamic recrystallization.Under the same strain rate,flow stress and highest compression stressσp were inversely proportional to the temperature.At the same temperature,flow stress andσp were proportional to the strain rate.A critical strain prediction model was also established for dynamic recrystallization of as-extruded Mg-2.5Zn-4Y alloy in the present study.Investigation of the microstructure of the sample revealed that its recrystallization mechanism was dominated by continuous dynamic recrystallization and supplemented by discontinuous dynamic recrystallization.According to the processing map of the sample,the optimized processing conditions were the temperature range of 640-673 K and a strain rate of 0.001 s-1.Under such conditions,the sample exhibited good workability and the processed sample exhibited uniform microstructure and good mechanical.Additionally,a hyperbolic sinusoidal Arrhenius constitutive equation with the introduction of Z-parameter under thermal compression deformation conditions was established for the as-extruded Mg-2.5Zn-4Y alloy in the present study.In this study,the original and modified Johnson-Cook（J-C）constitutive equations for the as-extruded Mg-2.5Zn-4Y magnesium alloy under high-speed impact compression conditions were established.Through precision analysis of the modified J-C constitutive equation,it can be found that the predicted values of the modified J-C constitutive equation were in better agreement with experimental values,allowing better prediction of experimental values. |
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