Effects Of RE/Zn-controlled LPSO On Microstructure And Mechanical Properties Of ECAP-state Mg-Gd-Nd-Zn-Zr Alloys

Mg-Gd-Nd series alloys have excellent solution strengthening effect and good aging hardening response.The addition of quantitative Zn elements(RE/Zn>1(at))to Mg-RE alloys can form(Long period stacking order structure)LPSO structural phases with strong toughness,which is one of the effective ways to improve the strength and ductile of the alloy.But,the formation of LPSO phase will consume RE elements and excessive LPSO will reduce the mechanical properties of the alloy.At the same time,the addition of Zn element will reduce the solubility of RE elements in magnesium matrix,which is not conducive to the subsequent aging precipitation effect of the alloy.Therefore,exploring the optimum RE/Zn content ratio in Mg-Gd alloys has theoretical significance for obtaining Mg-RE alloys with excellent comprehensive properties.This project designs Mg-9Gd-2Nd-x Zn-1Zr(x=0,1.1,1.6,2.3,4.6wt.%),RE/Zn atoms are?(without Zn),1:1,2:1,3:1 and 4:1,respectively.Through OM,SEM,XRD,TEM,EBSD,Vickers hardness test and tensile performance test,the effects of different RE/Zn ratios on microstructure and mechanical properties of cast alloys were systematically studied,and the regulation and formation mechanism of lamellar LPSO phase during solid solution treatment were investigated.The as-soluted alloy with the best mechanical properties was subjected to different passes of ECAP extrusion.The microstructure evolution and mechanical properties of alloy III(Mg-9Gd-2Nd-1.6Zn-1Zr)after ECAP with different passes was studied.The effect of the eutectic phase on the grain refinement mechanism and the second phase precipitation mechanism and the synergistic of LPSO phase on alloy deformation during the ECAP process were revealed.The synergistic strengthening mechanism of eutectic phase and LPSO relative alloy was analyzed.The specific research content is as follows:Without Zn,the as-cast alloy microstructure is composed of?-Mg,Mg₅RE and RE-rich phase.With the addition of Zn elements,the Mg₅RE phase is transformed into(MgZn)₃RE phase,and the eutectic phase morphology is transformed from skeleton to continuous network,and the volume fraction of the second phase is significantly improved.The second phase was completely dissolved in the matrix after solid solution treatment of the as-cast alloy without Zn.While with the increase of Zn content,the residual eutectic phase at the grain boundary increased gradually,and lamellar LPSO structure phase was formed in the grain.When RE/Zn=3:1(alloy III),the yield strength,tensile strength and elongation of the alloy are 122MPa,228MPa and 12.5%,respectively,which have the best comprehensive mechanical properties.The as-soluted alloy III(RE/Zn=3:1)was subjected to different ECAP passes.The study found that with the increase of the extrusion pass,the dynamic recrystallization volume fraction of the alloy increases,the grain size is obviously refined,and the residual eutectic second phase is completely broken.The dynamic recrystallization via particle-induced nucleation mechanism mainly occurs at(MgZn)₃RE phase and grain boundary.However,few dynamic recrystallized grains were found near the lamellar LPSO phase.The dynamic recrystallization volume fraction and average grain size of the alloy change little with the increase of extrusion pass after 4 passes extrusion.The alloy has the best comprehensive mechanical properties after 4 ECAP passes.Its yield strength,tensile strength and elongation are 321 MPa,378 MPa and 14.4%,respectively.The alloy with 4 passes ECAP was subjected to aging treatment.The aging temperature is200?,225?and 250?respectively.With the increase of aging temperature,the time of reaching the peak aging of the alloy is obviously shortened,which is 8h,6h and 3 h.respectively.The precipitates volume fraction in the matrix increased significantly after 200?,225?and250?peak aging treatments,corresponding to ultimate tensile strength was 386 MPa,403MPa and 381 MPa,yield strength was 355MPa,348 MPa and 345 MPa,elongation was7.7%,10.8%,7.6%,respectively.Compared with ECAPed alloy,the mechanical properties of the ECAPed alloy after aging treatment are significantly improved.The main reason is that a large amount of?₁ phases were precipitated during the aging process,which can effectively pin the movement of dislocations.Compared with ECAPed alloy,the mechanical properties of the ECAPed alloy are obviously improved after aging treatment.The main reason is that a large number of₁ phases are precipitated in the aging process,which can effectively pin the dislocation movement.