The Microstructure And Properties Of Mg-Zn-Mn Magnesium Alloys With Sn And Ca Addition

Magnesium alloys have become one of the most promising green metal materials due to their low density,high specific strength,high specific stiffness,good damping,high electromagnetic shielding and strong noise reduction ability.Low absolute strength and poor plasticity are major bottlenecks hindering the application of magnesium alloys.Therefore,it is very important to develop new high performance and low cost magnesium alloys and expand their application.Alloying is one of the important methods to improve the strength and plasticity of magnesium alloys.In this work,the influence of addition of Ca and Sn element on the microstructure and mechanical properties of Mg-6Zn-1Mn（ZM61）wrought magnesium alloy are studied to develop new high performance wrought magnesium alloy.The microstructure evolution,morphology of the second phase,mechanical properties at room temperature and thermal deformation behavior of Mg-6Zn-1Mn-4Sn-xCa（ZMT614-xCa,x=0,0.2,0.5,0.8,wt.%）and Mg-6Zn-1Mn-0.5Ca-x Sn（x=0,1.0,2.0,3.0,wt.%）alloys are mainly researched.In the ZMT614-xCa（x=0,0.2,0.5,0.8,wt.%）alloy series,the second phase of the as-cast ZMT614 alloy distributes as coarse and continuous net-work,which consists ofα-Mn,Mg-Zn and Mg₂Sn phases.With addition of Ca,needle-like Ca Mg Sn appeared,and the as-cast microstructure is obviously refined.When the content of Ca exceeds0.5wt.%,the refining effect is weakened and microstructure is even coarsened.With the increase of Ca content,the volume fraction of Ca Mg Sn phase increases,and the morphology of parts of this phase changes from needle-like to rod-like,while the volume fraction of Mg₂Sn phase decreases.In the Mg-6Zn-1Mn-0.5Ca-x Sn（x=0,1.0,2.0,3.0,wt.%）alloy system,with the addition of Sn,the second phases composition changes fromα-Mn,Mg-Zn and Ca₂Mg₆Zn₃ toα-Mn,Mg-Zn,Ca₂Mg₆Zn₃ and Ca Mg Sn.The fractional volume of Ca₂Mg₆Zn₃ phase decreases with the increasing of Sn content.The formation enthalpy of Ca Mg Sn is obviously lower than that of other ternary and binary phases which demonstrated by thermol calculation.Therefore,Ca Mg Sn phase is preferred to form in the solidification process for Mg-Zn-Mn-Sn-Ca magnesium alloy.During the extrusion process,the grains are refined obviously for ZMT614-xCa（x=0,0.2,0.5,0.8,wt.%）alloys,and complete dynamic recrystallization occurs for all alloys.The recrystallized grain sizes are 7.65μm,4.72μm,4.85μm and 4.81μm,respectively.In the Mg-6Zn-1Mn-0.5Ca-x Sn（x=0,1.0,2.0,3.0,wt.%）alloy system,incomplete dynamic recrystallization occurs for Mg-6Zn-1Mn-0.5Ca alloy at 350℃,and elongated and deformed grains exist in extruded alloy.The addition of Sn can significantly promote the dynamic recrystallization,and the recrystallized grain sizes are 2.7μm,3.5μm,2.8μm and 3.9μm,respectively.After solution treatment,the grain grows up obviously,and a large amount of Ca Mg Sn in the two alloy systems cannot be dissolved into the matrix.This is because that the total solid solubility in the magnesium matrix is limited at this solution temperature,and the solid solubility and diffusion rate of Zn in magnesium are much higher than that of Ca and Sn.The most important strengthening phases of aged ZMT614-xCa（x=0,0.2,0.5,0.8,wt.%）alloy and Mg-6Zn-1Mn-0.5Ca-x Sn（x=0,1.0,2.0,3.0,wt.%）alloy are Mg-Zn phases,including rod-likeβ’phase being parallel to<0001>direction and disk-likeβ’phase being parallel to（0001）substrate.With the addition of Ca/Sn element,the number of Mg-Zn phases increases,the size decreases and the distribution becomes more dispersed.For ZMT614-xCa（x=0,0.2,0.5,0.8,wt.%）alloy,the yield strength and tensile strength are increased by 96MPa and 90MPa respectively,to 345 MPa and 378 MPa,and elongation of the aged ZMT614-0.5Ca alloy are 6.5%.The performance improvement of the aged alloy is attributed to the refinement and dispersion of the Mg-Zn phase.For Mg-6Zn-1Mn-0.5Ca-x Sn（x=0,1.0,2.0,3.0,wt.%）alloy,the yield strength,tensile strength and elongation of extruded Mg-6Zn-1Mn-0.5Ca-2Sn alloy are299 MPa,366 MPa and 9.2%,respectively,which are attributed to the second phase strengthening.The yield strength,tensile strength and elongation of aged Mg-6Zn-1Mn-0.5Ca-2Sn alloy are 379 MPa,407 MPa and 7.5%,respectively.For Mg-6Zn-1Mn-0.5Ca and Mg-6Zn-1Mn-0.5Ca-1Sn alloys,Zn first forms Ca₂Mg₆Zn₃phase with Ca in the aging process,and the number of Mg-Zn phase decreases,so the properties of aged alloys decreases greatly compared with the extruded alloys.The fracture mode of extruded ZMT614-xCa（x=0,0.2,0.5,0.8,wt.%）and Mg-6Zn-1Mn-0.5Ca-x Sn（x=0,1.0,2.0,3.0,wt.%）alloys is mainly a mixture of ductile fracture and brittle fracture.After aging treatment,cleavage steps appear and the fracture mechanism of two series alloys is mainly brittle fracture.The flow stress-strain curves of the homogenized Mg-6Zn-1Mn-0.5Ca and Mg-6Zn-1Mn-2Sn-0.5Ca alloys belong to the typical dynamic recrystallization type.The deformation temperature and deformation rate are important factors affecting the flow stress of the alloy.The peak stress of the alloy increase with the decrease of the deformation temperature or the increase of the strain rate.Under the same deformation temperature and deformation rate,the peak stress of Mg-6Zn-1Mn-0.5Ca-2Sn alloy is always higher than that of Mg-6Zn-1Mn-0.5Ca alloy.This is because needle-like Ca Mg Sn phase can effectively block the dislocation movement.The activation energy of Mg-6Zn-1Mn-0.5Ca alloy and Mg-6Zn-1Mn-0.5Ca alloy are 199.654 k J/mol和276.649 k J/mol,respectively.The optimal processing areas for the Mg-6Zn-1Mn-2Sn-0.5Ca alloy can be concluded in the following sections:T=300～350℃,?=0.05～0.001 s^-1.