Research On Microstructure And Properties Of Fine Grained Mg-Mn-Al-Ca-Zn Alloys

As a kind of magnesium alloy,Mg-Mn based alloys have the merits of good weldability,corrosion resistance and etc,while the shortcomings of poor mechanical properties exist.This work has carried out the corresponding researches on property improvements of Mg-Mn based alloys through alloying,hot extrusion,heat treatments and etc,including designing and fabricating Mg-1Mn-0.5Al-0.5Ca-x Zn(x=0,0.5)and Mg-1Mn-x Al-0.5Ca-0.5Zn(x=0.5,1,2),conducting extrusion and heat treatment tests.The microstructure was analysed by optical microscope(OM),scanning electron microscope(SEM),transmission electron microscope(TEM)and etc.the room-temperature tension and compression performances were tested and the interior associations of composition-process-structure-properties were discussed.The main works and results in the work include:(1)With the flux free melting,Mg-1Mn-0.5Al-0.5Ca-x Zn(x=0,0.5)was fabricated for the samples annealed at 350?for 11h,researching the microstructure and property.The results indicate that,annealed microstructure is dendritic,and refined by an addition of 0.5%Zn in annealed Mg-1Mn-0.5Al-0.5Ca-x Zn.The ultimate tensile strength for Mg-1Mn-0.5Al-0.5Ca-x Zn(x=0,0.5)is 130MPa and 152MPa,and the elongation for them is 3.9%and 4.7%respectively;compressive strength for them is215MPa and 249MPa respectively.The addition of 0.5%Zn produces effects of grain refinement and precipitation strengthening,improving the machanical properties.(2)With the flux free melting,Mg-1Mn-x Al-0.5Ca-0.5Zn(x=0.5,1,2)was fabricated for the samples annealed at 350?for 11h,researching the microstructure and property.In as annealed Mg-1Mn-x Al-0.5Ca-0.5Zn(x=0.5,1,2),with the increase of x value the microstructure is in refinement,and the intermetallic compounds in grain boundaries are of increase,such as Al₈Mn₅,(Mg,Al)₂Ca.The ultimate tensile strength of annealed Mg-1Mn-x Al-0.5Ca-0.5Zn(x=0.5,1,2)is 152MPa,168MPa and 199MPa,respectively.An increase of Al₈Mn₅ due to the Al content increase leads to the obstacle of grain growth,which improves the mechanical properties.(3)With the hot extrusion used in this work,Mg-1Mn-0.5Al-0.5Ca-x Zn(x=0,0.5)was fabricated,and their microstructure and properties were studied.Compared to as cast samples,Grain size decreases obviously and basal plane texture with{0001}almost perpendicular to ED is weakened remarkably in as-extruded alloys.The average grain size is 2.6?m and 1.2?m for Mg-1Mn-0.5Al-0.5Ca-x Zn(x=0,0.5)rexpectively,the deformation of the alloys is dominated by the slips of nonbasal<a>and<c+a>instead of basal<a>.As x=0.5 deformed grains dramatically decrease to only 1.3%of the total while that occupy 67.5%as x=0.An addition of 0.5%Zn into Mg-1Mn-0.5Al-0.5Ca makes its as-extruded mechanical properties improved dramatically with?_b=332MPa,?_s=314MPa and?=13.9%.(4)With the hot extrusion used in this work,Mg-1Mn-x Al-0.5Ca-Zn(x=0.5,1,2)was fabricated,and their microstructure and properties were studied.After hot extrusion,grains are refined remarkably.For as-extruded Mg-1Mn-x Al-0.5Ca-0.5Zn(x=0.5,1,2)grain size is 1.2?m,2.2?m and 2.1?m,respectively;the proportion of deformed grain increases with the x value.Especially,when x=2,that is 27.2%of the total.Prismatic{10-10}texture parallel to ED appears fiber shape and has the intensity of 5.3 in as-extruded Mg-1Mn-2Al-0.5Ca-0.5Zn.the mechanical properties decrease with the increase of x value,as-extruded Mg-1Mn-0.5Al-0.5Ca-0.5Zn has the mechanical properties with?_b=332MPa,?_s=314MPa and?=13.9%.The compressive yield strength of as-extruded Mg-1Mn-0.5Al-0.5Ca-0.5Zn is 205MPa.Grain refinement and precipitation are two main strengthening mechanism in the alloy.(5)With the artificial aging treatment used in this work,the aged samples were fabricated,and their mechanical properties were investigated.After aging treatment at200?for 1h,the strength has remarkable improvements while the ductility has no changes.With the Al content increasing,the increment of strength is increasing in aged alloys.It is interesting that the yield strength is 358MPa for the Mg-1Mn-2Al-0.5Ca-0.5Zn after aging treatment at 200?for 1h.