Effects Of Ca And Mn On Microstructure And Mechanical Properties Of Mg-Zn Magnesium Alloy

Wrought magnesium alloys have enormous potential in the applicatin of structural material lightening due to the high specific strength and stiffness.Compared with other competing structural materials such as aluminum alloys and steels,the wide-scale commercialization of magnesium alloys are hindered by their low strength and poor ductility due to their hexagonal close-packed crystal structure,which results in an insufficient number of operative slip at room temperature.In order to expand the application of magnesium alloys,development of high ductility and high strength wrought magnesium alloys are needed.Addition of rare earth into Mg alloys can improve the ductility and strength simultaneously,that has been comfirmed by many research.Due to the high cost,none rare earth elements attracted much attention in rencent years.The present work aims to develop strength-ductile magnesium alloys as well as low cast.Mg-Zn-Ca alloys with varied Ca content were extruded into 3 mm thick sheets.Mg-Zn-Ca-Mn alloys with varied Ca and Mn content were extruded into bars.optical microscopy(OM),X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM),tensile tests were used to characterize the microstructures and mechanical properties of as-cast and as-extruded alloys.Dynamic recrystallization behaviors,strengthing and toughening mechanism in the as-extruded alloys have also been investigated.The main conclusions are list as following:(1)The main second phase of Mg-Zn-Ca alloys are Ca2Mg6Zn3 and MgZn.Perfect DRX occured during hot extrusion,leading to the formation of ununiform micstructure.The DRXed microstructure were refined by adding Ca into Mg-Zn alloy.The main DRX mechanism here was the particle stimulated nucleation(PSN),which was benefit for weakening basal texture.Ca additon reduced the c/a ration,improving the lattice symmetry,resulting in spliting of basal pole along ED,that is similar to the typical rare-earth texture.Finally,the basal texture of as-extruded alloy sheets got weakend.(2)The yield strength(YS)and ultra tensile strength(UTS)increased with the increasion of Ca.strengthing mechanism in as-extruded Mg-Zn-Ca alloys including grain strengthening,solution strengthening,second-phase precipitation strengthening and texture strengthening.Ca additon significantly enhanced ductility of Mg-Zn alloy due to refining grains and weakening texture.The alloy with 0.3wt.% Ca shown the highest fracture elongation(FE)with 27%.However,the ductility decreased when Ca content increased to 0.6wt.%.The anisortropy of as-extruded Mg-Zn-Ca alloy sheets were modified by Ca additon.It shown the best ductility when tensile tests were acted along extrusion direction(ED),secondly the 45°,while the transverse direction(TD)shown the worst ductility.Refined grains,weak basal texture and modified anisortropy make Mg-4Zn-0.3Ca the best candidate to obtain the balanced mechanical properties whose YS~121MPa,UTS~252MPa,FE~27%.(3)The grain size of as-cast Mg-Zn-Ca-Mn alloys decreased with Ca addition increasing to 1wt.%.The main DRX mechanism during hot extrusion was the particle stimulated nucleation(PSN).Coarse intermetallic compounds increased with increase of Ca and Mn content.High density of paritcles in nanoscale and submicron grade have precipitated at the same time,retarding the nucleation and growth of DRXed grains.Although additon of Ca can effectively weaken the basal texture of Mg alloy,the incease of unDRXed fraction resulted in the total basal texture intensity enhancing with the increase of Ca content.It followed the same law with the variety of Mn content.(4)Compared with Mg-Zn-Ca alloys,the YS and UTS of Mg-Zn-Ca-Mn alloys were significantly enhanced.The YS increased from 165 MPa to327MPa with the increase of Ca and Mn content.While the yield ratio increased as well.Both refined grains and strong texture in unDRXed area contrabuted to the improvement of YS.Meanwhile,the ductility of alloys decreased with the increase of unDRXed fraction.The alloys contain 0.3wt.% Ca shown superior ductility(uniform elongation varied from 25.6%~29.8%),resulting from refined grains and weakened texture.From the above,Mn additon into Mg-4Zn-0.3Ca-xMn and Mg-4Zn-0.6Ca-xMn alloys significantly improved the YS,slightly decreased the ductility.The YS,UTS and uniform elongation varied from 165~286MPa,270~320MPa and 29.8%~16.4% respectively.