Effects Of Nd On Microstructure And Mechanical Properties Of ZM61Alloy

Mg-6wt.%Zn-1wt.%Mn（ZM61）is a new type wrought magnesium alloys withhigh content of zinc.The strength of ZM61alloy is equivalent to that of commercialZK60magnesium alloy after solution and two-stage ageing treatment,however,two-stage ageing process is complex and time-consuming and this shortcoming greatlyreduce the production efficiency.As an important alloying elements,rare earth canrefine the microstructure and changing the morphology of the second phase.Thanks tothese advantages,the research of wrought magnesium alloy containing rare earthelements become one of hot topics of magnesium alloys.The effects of Nd on microstructure of the ZM61alloys at different state（as-cast,as-annealed,as-extruded,T5,T4,T4+single-stage aging and T4+two-stageaging）were ivestigated by Optical microscop（eOM），X-ray diffraction analysi（sXRD），Differential thermal analysis（DSC），Scanning electron microscopy（SEM） and Energyspectrum analysis（EDS）and the mechanical properties of alloys at different state werealso tested.At last, the forms of existence of Nd in the alloys and its effect tomechanical properties were studied under different conditions. The results are:â‘ The microstructure of as-cast ZM61can be refined by adding Nd element andwith the content of Nd increasing,the refinement are becoming more obvious.As-castZM61alloy is consisted of α-Mg、Mg₇Zn₃and MgZn₂.With Nd addition,a ternary REphase T₂was found in the alloys and its chemical formula is （Mg,Zn）11.5Nd.Thedendrite segregation of as-cast ZM61can be eliminated by two-stage homogenizingannealing（330℃/16h+420℃/2h）.With Nd content increasing,the effect ofhomogenization is weakened.When Nd content is0.8%,the volume fraction of sccondphase in the alloy reaches7.35%.This is because the T₂phase has good thermal stabilityand we found the melting point of T₂is around467℃according to the differentialthermal analysis.â‘¡As-extruded ZM61magnesium alloy is consisted of α-Mg、MgZn₂andMn.when Nd content is low（0.2%,0.4%）,the RE-phase is T1in the as-extruded alloysand this phenomenon indicates that RE-phase is transformed from T₂into T1duringhomogenization treatment and hot extrusion process,however, this phase transformationis not obvious when Nd content increasing.When Nd content reaches0.8%, theRE-phase in as-extruded alloy remain T₂phase and T1phase has not been found.Dynamic recrystallization occurred obviously during hot extrusion process.Butthere are still many large-size unrecrystallized grains in ZM61alloy and the grain size isuneven.The grain size of alloys gradually decreased when adding Nd.The average grainsize of as-extruded ZM61is25μm and the size drop to10μm when Nd content is0.8%.At the same time,the microstructure become more uniform and the strength ofalloys increase. ZM61-0.4Nd alloy has optimum combination mechanical properties,with σ0.2=247Mpa,σb=312Mpa and δ=7.0%,respectively.Compared to the mechanicalproperties of as-extruded alloys,the tensile strength of direct aging（T5） alloys seems tohave no change,nevertheless,the yield strength and elongation is definitely improved.â‘¢ZM61alloy becomes a single phase solid solution after solution treatment at420℃for2h.When the Nd content increased from0.2%to0.8%,the volume fraction ofsccond phase in the alloy increased from0.43%to5%and this means the effect ofsolution treatment decrease with Nd content increasing. As for ZM61-0.8Ndalloy,RE-containing particles can not dissolve effectively into the matrix throughsolution treatment at420℃for2h,6h,10h and14h, respectively.Moreover,the grain sizeof ZM61-0.8Nd is increased from16μm to26μm with the prolongation of solutiontime.For ZM61-0.6Nd and ZM61-0.8Nd alloys, RE-containing particles can notdissolve even after second phase solution treatment in420℃/4h+460℃/2h.Whenincreasing second phase solution temperature to420℃/4h+480℃/0.5h，all the secondparticles dissolve into the matrix，however，high temperature cause grain coarsening andoverheating.â‘£The change rule of vickers hardness as the time prolongs of ZM61-xNd alloysis very similar by one-step ageing at180℃.The hardness is increasing quickly at thefirst10h and the hardness of all the alloys reach maximum value at16h and then thehardness of ZM61-0.2Nd is84HV,while the hardness of ZM61-0.8Nd is76HV lowerthan the rest. The results of tensile test show that the strength of as-one step ageingalloys has been significantly enhanced by adding Nd element. ZM61-0.4Nd alloy hasoptimum combination mechanical properties,with σ0.2=338Mpa,σb=359Mpa andδ=7.6%,respectively.With the Nd content increasing, precipitates decrease because ofthe consumption of Zn element by the ternary Mg-Zn-Nd phase and the strength of thealloys decrease.The strength of ZM61、ZM61-0.6Nd and ZM61-0.8Nd alloys can beimproved by two-step aging treatment（90℃/24h+180℃/16h）,however,the mechanicalproperties of ZM61-0.2Nd and ZM61-0.4Nd alloys do not change after two-step aging treatment. Considering all aspects,the as-one step aging ZM61-0.4Nd alloy have thebest mechanical properties and economic benefits.