Microstructure And Mechanical Properties Of ZK60 Based Wrought Mg Alloys With Ce-Cu-Gd Additions

Wrought Mg alloys have a bright application prospect in transportation,aerospace and electronics industries for their prominent advantages such as low density and high specific strength as typical lightweight materials.However,the strength of the commercial wrought Mg alloys are still low,which restricting their further application.Therefore,it is of practical significance to develop new wrought Mg alloys with high strength and ductility.Based on the multi-microalloying designing thought,by investigating the effects of trace Cu,Ce and Gd elements on the microstructural evolution and mechanical property of the industrial ZK60 basic alloy,two new Mg-6Zn-0.5Zr-0.5Cu-0.5Ce and Mg-6Zn-0.5Zr-1.0Gd-0.5Ce alloys with high strength and ductility have been developed in the present paper.Firstly,the effects of Cu,Ce and Gd addition on the solidification microstructure of ZK60 alloy were studied.Results show that it formed MgZnCu phases with a high-melting point and a C15 Laves structure when Cu addition.By adding Ce,it formed Mg-Zn-Ce ternary phases(T phases).With the addition of Gd,Mg2Zn6Gd(I phase)and Mg3Zn3Gd2(W phase)appeared.Gd was more effective than Cu in refining solidified grains,which was ascribed to the different restriction effects on the growth of a-Mg dendrites.Based on the results of single microalloying,Cu/Ce or Gd/Ce co-addition was chosen to investigate the synthetic effects on ZK60 alloy.Results show that the solidified grains were refined,while the phase constituents were not changed and no new intermetallics were observed after the co-addition of Cu/Ce or Gd/Ce.The co-addition of Gd and Ce may promote the formation of W phase.Secondly,the effects of Cu,Ce and Gd single addition or Cu/Ce and Gd/Ce co－addition on the microstructure and texture evolutions of as-extruded ZK60 alloy under the condition of extrusion temperature of 350 ℃,extrusion ratio of 39 and extrusion rate of 0.5 mm/s were studied.Results show that Cu and Ce single addition or Cu/Ce and Gd/Ce co-addition could decrease the average dynamic grain size of ZK60 alloy.The dynamic recrystallization fraction decreased with the increase of Cu content,while Ce could promote the dynamic recrystallization.Minor Gd addition(<1%)inhibited dynamic recrystallization,while 2%addition of Gd promoted it.Thus the degree of dynamic recrystallization was influenced by the solute atoms,fine precipitates and secondary phases remained after the homogenization treatment.Cu,Ce and Gd elements could influence the texture of the as-extruded ZK60 alloy.All the studied alloys exhibited basal texture while the texture strength slightly weakened only when minor Ce or Gd additions(<1%).When Cu/Ce or Gd/Ce co-addition,the texture strength increased significantly.In addition,no rare earth texture(<1-211>//ED)was found in any studied alloys in the dissertation.The single addition of Cu or Gd or co-addition of Cu/Ce or Gd/Ce could refine the nano-scale precipitates in the as-extruded ZK60 alloy.The length and diameter of the[0001]α rods or the diameter of the sphere-like precipitates would be refined to about 1/2 to 1/3 compared with those in the ZK60 alloy.Besides,it formed many basal precipitates in Gd-containing alloys.Moreover,new[0001]α needle-like precipitates were also observed in the Mg-6Zn-0.5Zr-1.0Gd-0.5Ce alloy.Finally,the tensile properties of as-extruded ZK60 alloy after multi-microalloying were investigated.It was found the co-addition of Cu/Ce or Gd/Ce would more effectively improve the tensile property of ZK60 alloy compared with those of single additions.The tensile yield strength,ultimate tensile strength and elongation of the as-extruded Mg-6Zn-0.5Zr-0.5Cu-0.5Ce and Mg-6Zn-0.5Zr-1.0Gd-0.5Ce alloys were 303 MPa,347 MPa and 19.5%and 329 MPa,361 MPa and 20.2%,respectively.The yield strength increased 86 MPa,51 MPa and 69 MPa compared with that in ZK60,Mg-6Zn-0.5Zr-1.0Gd and Mg-6Zn-0.5Zr-0.5Ce alloys,respectively.The yield strength,ultimate tensile strength and elongation of the two alloys reached 319 MPa,354 MPa and 15.8%and 345 MPa,363 MPa and 20.5%after T5 heat treatment,respectively.The significant improvement of tensile properties resulted from the grain refinement,precipitation strengthening by secondary phases,together with texture strengthening.