Research On The Hot Deformation Behavior And Microstructure Evolution Of Mg-Zn-Zr-Yb Alloy

Magnesium alloys have drawn a large amount of research attention in recent years as lightweight structural materials in the aerospace and automotive industries.However,the low strength and poor ductility have severely restricted the widespread application.In this work,a trace amount of rare earth element Ytterbium(Yb)has added to the commercial ZK60 magnesium alloy to improve the mechanical properties.In order to prepare a billet with reduced microstructure and composition segregation,the homogenization treatment has been performed and the microstructure evolution under different annealing temperatures and holding times has systematically investigated.Moreover,the hot deformation behavior and the dynamic recrystallization evolution of the studied alloy have investigated by using thermal simulation test.Based on the stress-strain data,the hot deformation constitutive equation,the thermal processing map,and the dynamic recrystallization mechanism have achieved subsequently.Finally,the isothermal extrusion test and the EBSD investigation have been carried out to verify the optimum hot deformation characteristics according to the prediction of the hot processing map.Main research results are as follows:The Mg-5.8Zn-0.5Zr-1.0Yb as-cast alloys are homogenized with different annealing temperatures and different holding times.Typical dendritic segregation is observed in the as-cast alloy.The undissolved Zr aggregates inside the grain boundary.The MgxZn5Yb ternary intermetallic compounds are mainly distributed at the grain boundaries.After the homogenization treatment,the dendrites in the alloy gradually disappear.Continuous reticular deposits evolve into fine and granular deposits.After homogenization treatment at 380 ℃ for 12 h,the percentage of residual phase area in the alloy decreased from 5.54%to 0.59%,and the microhardness reached a maximum of 67.81 HV.The ternary phase containing Yb remained in the form of a complete small particle.In the organizational structure,the structure is a very uniform and equiaxed grain with an average grain size of 31.46 μm.On the basis of a comprehensive analysis to the microstructure and mechanical properties,the parameters for the best homogenization heat treatment of the material is 380℃ and 12 h.Compression tests were carried out on a Gleeble-1500 thermal simulation machine at 250,300,350,and 400 0C with the strain rate of 0.001,0.01,0.1,and 1.0 s-1,respectively.The true stress-strain curves under the different deformation conditions were plotted.As the deformation temperature decreases or the strain rate increases,the peak flow stress and the corresponding strain value increase.It indicates that reducing the deformation temperature or increasing the strain rate is not conducive to the occurrence of dynamic recrystallization.Based on the relationship between the alloy flow stress and the deformation temperature,the constitutive relation of the material is determined:(?)= 3.328×1011[sinh(0.018σ)]3.670 exp(-158495/RT)According to the processing map,the instability regions of the Mg-5.8Zn-0.5Zr-1.0Yb alloy mainly occur in the localized regions with high strain rates.By comprehensive consideration of the DRX condition and the stable hot working regions,the optimum hot processing parameter ranges are determined as:(1)T=290-340 ℃,ε=0.1-1 s-1,(2)T=330-400 ℃,ε=0.001-0.1 s-1.According to the EBSD investigation,the mechanism of DRX is twinning-induced dynamic recrystallization in the hot working range of 290-340 ℃ and 0.1-1 s-1,while the mechanism of DRX in the range of 330-400 ℃ and 0.001-0.1 s-1 is continuous dynamic recrystallization.Based on the thermal simulation experiments,the critical strain of dynamic recrystallization of the studied alloy is obtained.The isothermal extrusion experiments were carried out under the optimum hot working conditions to verify the prediction of the hot processing map.A uniform and fine grain size is eventually obtained.The Yb-containing rare earth precipitates(Mg-Yb and Mg-Zn-Yb phases)were observed at the grain boundary,which acts as a significant barrier to hinder the grain growth.The average grain sizes of the as-extruded samples under two preferable hot working regions are 7.34 μm and 13.82 μm,respectively.Through tensile tests,the mechanical properties of the as-cast,homogenized and extruded specimens are compared.The average yield strengths are 204,264,301 and 303 MPa,respectively,and the elongations are 6.4,10.3,20.2 and 19.6%,respectively.The results give a good agreement between the prediction and experiment.