Effect Of Gd On The Microstructure And Mechanical Properties Of Mg-Zn-Mn Wrought Magnesium Alloy

In this paper,the influence of Gd addition on the microstructure and mechanical properties of Mg-6Zn-1Mn（ZM61）,Mg-6Zn-1Mn-4Sn（ZMT614）and Mg-6Zn-1Mn-0.5Ca（ZMX6105）wrought magnesium alloys were investigated to develop the high-performance wrought magnesium alloys.The microstructure evolution,change of mechanical properties at room temperature,strengthening and toughening mechanisms of ZM61-x Gd（x=0,0.2,1.0,3.0,wt.%）,ZMT614-x Gd（x=0,0.2,0.6,1.0,wt.%）and ZMX6105-x Gd（x=0,0.2,1.0,wt.%）alloys were analyzed.The role of Gd element in different alloys was determined and the alloy composition was optimized.The dendrites were obviously refined with the addition of Gd.The phase of as-cast ZM61 alloy was mainly composed of Mg₇Zn₃ phase,α-Mn andα-Mg solid solution.Mg₃Zn₆Gd phase（I-phase）was detected in the ZMG610 and ZMG611 alloys,while Mg₃Zn₆Gd phase（I-phase）and Mg₃Zn₃Gd₂ phase（W-phase）coexisted in the ZMG613alloy.The formation of rare earth phases consumed the Zn element,leading to the reduction of Mg₇Zn₃ phase in as-cast alloys.Homogenization treatment（330℃/16 h+420℃/2 h）was beneficial to the dissolution of Mg₇Zn₃ phase,but it had little influence on the dissolution of I-phase and W-phase.The addition of Gd hindered the dynamic recrystallization（DRX）,and led to the finer DRXed grains and weak basal texture.During the extrusion process of Gd containing alloys,the nano-size I-phases were precipitated,which not only acted an obvious strengthening effect,but also helped to improve the plasticity.Therefore,both of the strength and ductility were enhanced with the increase of Gd addition.Gd addition had a significant effect on the aging precipitation of ZM61 alloy.A trace of Gd addition（0.2 wt.%）promoted the precipitation ofβ₁′phase,leading to the improvement of aging strengthening effect.With the further increase of Gd addition,the formation of rare earth phases consumed large amount of Zn elements,which significantly reduced the concentration of Zn element in Mg matrix after solid solution treatment,resulting in a decrease in the precipitation ofβ₁′phase in the subsequent aging process.Thus,the aging strengthening effect was not obvious for ZMG611 and ZMG613alloys.Aging treatment could effectively enhance the strength of alloys with minor Gd addition,but it was of little significance for the alloys with high Gd content.The peak-aged ZMG610 alloy exhibited optimal mechanical properties,the yield strength（YS）and ultimate tensile strength（UTS）were 360 MPa and 390 MPa.With Gd addition to as-cast ZMT614 alloy,phases of alloy had a transformation fromα-Mg+Mg₇Zn₃+Mg₂Sn+Mn toα-Mg+Mg₇Zn₃+Mg₂Sn+MgSnGd+Mn.The formation of MgSnGd phase during solidification consumed Sn element,which led to a decrease in the content of Mg₂Sn phase.The Mg₇Zn₃ phases was dissolved after homogenization treatment（330°C/16 h+420°C/2 h）,but some of Mg₂Sn phases and MgSnGd phases remained on the matrix.Gd addition refined the DRXed grains,hindered the DRX and enhanced the basal texture,but the strength of as-extruded alloys increased only a little due to the reduced strengthening effect of Mg₂Sn phase.Gd addition had a significant effect on the aging strengthening effect of ZMT614alloy.Owing to the pinning effect of MgSnGd phase,some dislocations were preserved during solution treatment,and the retained dislocations could act as the heterogeneous nucleation sites for the rod-likeβ₁′precipitates,thus led to promote the precipitation ofβ₁′phase.A trace of Gd addition（0.2 wt.%）had little effect on the precipitation of Mg₂Sn phase,and thus peak-aged ZMT614-0.2Gd alloy exhibited the optimum mechanical properties with YS of 373 MPa and UTS of 405 MPa.The fraction of Mg₂Sn precipitates during aging was significantly reduced when the content of Gd was high,resulting in a decrease in the aging strengthening effect.The phase composition had a transformation fromα-Mg+Mg₇Zn₃+Ca₂Mg₆Zn₃+Mn toα-Mg+Mg₇Zn₃+Ca₂Mg₆Zn₃+I+Mn with Gd addition to the as-cast ZMX6105alloy.The fraction of I-phase and Ca₂Mg₆Zn₃ phase increased with the increase of Gd,while that of Mg₇Zn₃phase decreased.Mg₇Zn₃ phase was fully dissolved after homogenization treatment（330°C/16 h+360°C/12 h+400°C/2 h）,but some of Ca₂Mg₆Zn₃ phases and I-phases still remained in the matrix.Gd addition refined DRXed grains,hindered DRX and increased the fraction of deformed grains,which led to the formation of strong basal texture.The fraction of Ca₂Mg₆Zn₃ phase increased with the addition of Gd,resulting in an increase in the number of nano precipitates during extrusion.Thus,the strength was enhanced with Gd addition,which was mainly ascribed to the texture strengthening,second phase strengthening,fine grain strengthening and dislocation strengthening.As-extruded ZMX6105-1.0Gd alloy exhibited the optimal mechanical properties with YS of 336 MPa,UTS of 395 MPa and EL of 9%,respectively.Gd addition had little influence on the mechanical properties of peak-aged ZMX6105alloy.After solid solution,some lamellar phases were distributed along the grain boundaries,and the formation of Ca₂Mg₆Zn₃ and I phase consumed Zn element,which reduced the density ofβ₁′phase during aging,thus the aging strengthening effect was poor.The ZMT614-0.2Gd alloy in the different states（as-cast,homogenized and aging）before extrusion had a significant effect on the microstructure and mechanical properties of the alloy after extrusion.The as-extruded samples for as-cast alloy and pre-aged alloy exhibited fine DRXed grains,sufficient recrystallization structure and weak basal texture,while samples for homogenized alloy had coarse grains,insufficient recrystallization and relatively strong basal texture.The number of nano particles in the matrix of as-extruded samples for pre-aged alloy was high and the strengthening effect was the best,the YS,UTS and EL were 251 MPa,380 MPa and 15.0%.