Effect Of Mn And Gd On The Properties Of Magnesium Alloy

Magnesium alloy has a series of advantages such as light weight,high specific stiffness,high casting productivity,and easy recycling,making it especially suitable for transportation,national defense and military industry,3C products,and many other fields.Wrought magnesium alloys are subject to the metallurgical quality and lattice types.Their low plasticity index is the bottleneck of its extensive application.The addition of alloying elements can improve their purity,coordinate their deformation mechanisms,etc.to enhance alloy plasticity.Mn and Gd are worth considering as additional elements.In this paper,the effect of Mn content on the iron reduction and corrosion sensitivity of Mg-Mn alloy and its mechanism was studied;the effect of Gd content on the basic characteristics of Mg-Gd alloy,such as site preference of solute atoms and lattice constant,was studied,and the variation of alloy properties and influence mechanism were analyzed;the control mechanism of the strength and toughness of Mg-4Gd alloy by pre-stretching and aging process was studied;the effect of Mn addition on the microstructure and properties of Mg-4Gd alloy was studied,and the evolution law of the second phase and influence mechanism of strengthening and plasticizing were analyzed;the effect of Gd content on the microstructure and properties of Mg-x Gd-0.8Mn alloy was studied,the strengthening and plasticizing mechanism,and corrosion mechanism of Mg-x Gd-0.8Mn alloy were analyzed.The ductility change rule of Mg-x Gd and Mg-x Gd-0.8Mn alloy were compared.The main conclusions are summarized as follows:(1)The Iron Changes(?C_Fe)of the melt can be significantly increased by using the low temperature treatment process,and the iron removal efficiency can be improved.The melt with the same Fe content has a higher degree of supersaturation at a lower temperature,which can promote the growth of Fe-rich particles and promote sedimentation.In this process,even without Mn addition,a significant iron removal effect can be obtained.After adding Mn,the degree of supersaturation of Fe in the Mg melt increases,which is conducive to the precipitation of Fe atoms.However,Mn can inhibit the growth of Fe-rich particles and reduce the coarsening rate(k_d)of Fe-containing particles.The corrosion sensitivity of Mg-Mn alloy mainly depends on the solid solubility of Fe in the Mg matrix.The free Fe-Mn intergrowth particles are formed in the iron-containing Mg melt after adding Mn,which can reduce the solid solubility of Fe,effectively reduce the harmful effect of Fe impurities on the magnesium alloy,and significantly reduce the corrosion sensitivity.When the mass ratio of Fe _{Solid solubility}/Mn _{Actual addition} is lower than 0.0083,the corrosion resistance of the alloy is better,in which the weight loss rate is 0.38±0.09?0.54±0.15mg/(cm²·d),the hydrogen evolution rate is 0.30±0.06?0.94±0.04 ml/(cm²·d),and the corrosion current density is3?9×10^-6 A/cm².(2)The effect of Gd content on the basic characteristics and properties of Mg-Gd alloy shows that with the addition of Gd(Gd content belows solid solubility,4 wt.%),the site preference of Gd solute atoms is prismatic plane(11-20).With the increase of Gd content,the axial ratio c/a of the extruded alloy first decreases and then increases,which is consistent with the change law of the alloy ductility.The solid solution strengthening and ductilizing effect of Mg-Gd alloy mainly occurs in the range of 1?4wt.%Gd.With the increase of Gd content in the low addition range,the starting resistance of basal slip increases sharply,while that of prismatic slip increases slowly.The difference between these two kinds of resistance gradually approaches to a certain extent with the increase of Gd content,which is beneficial to the coordination of basal slip and prismatic slip,and plays a plasticizing role.However,when the content of Gd exceeds a certain amount,the volume fraction of the second phase in the alloy is too high,which is unfavorable to the ductility.Under the high addition of Gd,when the starting resistance of prismatic slip and basal slip tend to be close and the difference is relatively constant,the hardening exponent also decreases to a relatively low stable value.(3)The addition of pre-stretching process to Mg-4Gd alloy after extrusion and before aging can promote the nucleation of the transition phase and its transformation to the equilibrium phase during the aging process.The average size of the equilibrium phase increases with the increase in the aging temperature.The aging treatment can improve the mechanical properties of pre-stretched specimens.Under the low temperature peak aging process(210?×24 h),the comprehensive mechanical properties of the alloy are the best,in which the yield strength,tensile strength and elongation are 137.4 MPa,245.4 MPa and 22.1%,respectively.Under the high temperature peak aging process(250?×8 h),the comprehensive mechanical properties are kept at a high level,in which the yield strength,tensile strength and elongation are130.9 MPa,241.5 MPa and 22.6%,respectively.The work hardening rate of the alloy is mainly affected by the pre-dislocations and the pre-twins resulted from the pre-stretching process.The subsequent aging treatment can adjust the work hardening rate of the alloy through the precipitation of the second phase and the elimination of part of the pre-dislocations.(4)The effect of Mn addition on the microstructure and properties of Mg-4Gd-x Mn alloy shows that the combination of Mn and different processes could adjust the solid solubility of Gd in the Mg matrix,thus optimizing the second phase morphology and properties of the alloy.When the Mn content increases from 0 to 0.8wt.%,the atomic utilization of Gd in Mg-4Gd alloy decreases from 61.5%to 51.8%,the second phase becomes coarser and more concentrated,and the volume fraction of the second phase increases from 0.35%to 1.18%after water-cooling casting;the atomic utilization of Gd in Mg-4Gd alloy increases from 73.8%to 81.0%,the number of needle-like second phase decreases obviously,the average size of particle phase decreases from 2.14 to 0.70?m,and the volume fraction of the second phase decreases from 1.84%to 1.22%after homogenization treatment;the atomic utilization of Gd in Mg-4Gd alloy increases from 72.3%to 84.1%,the average size of the particle phase and the volume fraction of the second phase are almost unchanged after hot extrusion.The addition of Mn has little effect on the Hall-Petch constant K_H and K_Y of Mg-4Gd alloy,while the increase of atomic utilization of Gd can significantly increase the value of?₀from 14.54 to 34.25 MPa.The corrosion resistance and mechanical properties of the extruded Mg-4Gd alloy are improved considerably by adding Mn.The increase of yield strength and ductility of the extruded Mg-4Gd-0.8Mn alloy is mainly due to grain refinement and the increase of solid solubility of Gd in the Mg matrix.(5)Gd element has a strong grain refinement effect on the extruded Mg-x Gd-0.8Mn alloy,and can effectively weaken the basal texture.When the Gd content increases from 0 to 6 wt.%,the asymmetry of the tensile and compressive yield of the alloy decreases continuously,in which CYS/TYS increases from 0.851 to 0.996,and SDE increases from-0.161 to-0.003.The Hall-Petch constant k under tension and compression mode is 308.05 MPa and 318.48 MPa?m^-1/2,and?₀ is 37.5 MPa and 29.7MPa,respectively.Compared with the extruded Mg-x Gd alloy,the limit composition point corresponding to solid solution strengthening and ductilizing of Mg-x Gd-0.8Mn alloy reduces from 4 to 2 wt.%.The corrosion resistance of the extruded Mg-x Gd-0.8Mn alloy mainly depends on the grain size and size of the second phases.The extruded Mg-6Gd-0.8Mn alloy has better corrosion resistance,in which the weight loss rate and hydrogen evolution rate are 1.79±0.26 mg/(cm²·d)and 1.39±0.18ml/(cm²·d),respectively.