| Lightweight high-performance magnesium alloys are urgently needed materials in the automotive and aerospace industries,due to their significant advantages in reducing energy consumption and improving mobility.Mg-RE alloys are a series of high-strength alloys that have been applied in engineering,but their plasticity is generally poor and could not meet the application requirements of key load-bearing structure parts.By adding proper Zn to Mg-RE alloys,some novel structure such as LPSO structure and stacking faults could be introduced into the alloy,and the plasticity of alloys are found significantly improved.However,the research on the microstructure evolution and toughening mechanism of Mg-RE-Zn alloy is still inadequate.Based on this,the alloys Mg-8Gd-4Y-0.6Zn-0.5Zr,Mg-8Gd-4Y-1.1Zn-0.5Zr and Mg-8Gd-4Y-1.6Zn-0.5Zr(referred to as 0.6Zn,1.1Zn and 1.6Zn,respectively)are designed in this paper.The microstructure,structure,strengthening and toughening mechanism of the alloys under different heat treatment and deformation conditions are systematically studied and the formation of LPSO structural phase and stacking fault as well as their influence on the microstructure evolution and properties are systematic analyzed.The microstructures of three as-cast alloys are all composed of equal-axial a-mg matrix,non-equilibrium eutectic phase at grain boundary and lamellar stacking faults near grain boundary.The second phases at grain boundaries of the 0.6Zn alloy are composed of Mg24(Gd,Y)5 and(Mg,Zn)3(Gd,Y),and they are divorced distribution.While the second phases in the alloys 1.1Zn and 1,6Zn are both composed of(Mg,Zn)3(Gd,Y)and 18R-LPSO structure phase,and they are coupled distribution.Mg24(Gd,Y)s is in body-center-cubic(BCC)structure,with the similar lattice constant with that of Mg24Ys.The(Mg,Zn)3(Gd,Y)phase is in face-center-cubic(FCC)structure,with the lattice constant 0.72nm,slightly less than that of the Mg3Gd phase.While the 18R-LPSO structure phase is in monocline structure,and its c axis is about 93.5 degrees from the bottom surface.The heat treatment is carried out at 480?,500? and 520?,and the microstructure evolution of the three alloys all includes processes:(1)rapid decrease of non-equilibrium phase at grain boundary,(2)increase of LPSO structure phases and(4)decomposition of LPSO structure phases.Good homogenization effect can be achieved for the alloys after heat treatment at 500? for 48 hours.After homogenization,the LPSO structure phases in the 0.6Zn alloy is completely recovered,while 14h-LPSO structure phases found in the grain boundary of the 1.1 Zn and 1.6Zn alloys.Desorption occurs during the cooling treatment after homogenization,and lamellar LPSO structure phases or stacking faults are precipitated.The plastic deformation mechanism of the alloy with these lamellar structures is mainly the basal slip in the matrix between the lamellar structures.The lamellar structures consume the solute atoms RE and Zn in the matrix,thus weaken the resistance of the matrix to dislocation slip and crack growth,leading to the reduction of strength.The influence of LPSO structure phase on the hot compression deformation behavior and dynamic recrystallization in homogenized Mg-8Gd-4Y-xZn-0.5Zr(x=0.6?1.1?1.6)alloy is studied.The peak stress is positively correlated with Zn content and strain rate,and negatively correlated with deformation temperature.While deformation activation energy decreases with the increase of Zn content.The interfaces of LPSO structure phases are favorable positions for dynamic recrystallization nucleation.However,because the dislocation concentration can be released by the self deformation of LPSO structure phases,the ability to promoting dynamic recrystallization for LPSO structure phase is weaker than that of grain boundary.When the deformation degree is low,the recrystallization near the interface of LPSO structure phase is not obvious,and increasing the deformation degree is beneficial to the dynamic recrystallization at the interface of LPSO structure phases.Obvious recrystallization occurrs after extrusion in Mg-8Gd-4Y-xZn-0.5Zr(x=0.6,1.1,1.6)alloys,and with the increase of Zn content,the average size of dynamic recrystallization grains decreased.Some lamellar structure precipitates in dynamic recrystallization grains,while some minor LPSO structure phases within 1 ?m appear in the dynamic recrystallization grain boundaries.There is obvious basal texture in 0.6Zn alloy,compared with 1.1Zn and 1.6Zn alloy.The 1.1 Zn and 1.6Zn alloys exhibit significantly higher strength than 0.6Zn alloy,but lower plasticity.Comprehensive analysis shows that the failure behavior of 1.1Zn and 1.6Zn alloys is related to the bulk LPSO structure phases,and the nucleation and propagation of cracks in the LPSO structure phases is due to the stress concentration caused by the incongruity of the deformation between the LPSO structure phase and the matrix as well as the dislocation accumulation.The peak aging time of as-extruded Mg-8Gd-4Y-xZn-0.5Zr(x=0.6,1.1,1.6)alloys is about 72h at 200?.After peak aging,the strengths of the three alloys are generally improved by above 100 MPa.With the increase of Zn content,the strength and plasticity of the peak-aged alloys are on the rise.1.6Zn alloy exhibits the best comprehensive properties with yield strength,tensile strength and elongation 395MPa,458MPa and 10.5%,respectively.The peak aged a-Mg matrix structure mainly consists of stacking faults parallel to the(0 0 0 1)plane and columnar P 'phase growing perpendicular to the(0 0 0 1)plane.In the process of plastic deformation,the non-basal slip is easy to occur in the lamellar structures.The evolution of microstructure and mechanical properties of 1.6Zn alloy during aging is further studied.With the increase of aging time,?' phase grows along the direction of<11 2 0>?-Mg obviously,and gradually presents ring distribution.Stacking faults appear in long time aging and grow up with time.Different from the relationship between strength and plasticity in the traditional strengthening process,there is a stage in which the strength and plasticity increase at the same time during aging.The 1.6Zn alloy exhibits stable properties after long time aging,and with tensile strength higher than 450 MPa and elongation higher than 8%even aging for 500 h.In comporison,after aging for 108h,1.6Zn alloy exhibits an excellent matching of strength and plasticity with tensile strength 451MPa and elongation close to 15%.The increasing strength and high plasticity results from the synergistic strengthening and toughening of ?' phase and lamellar structures including lamellar LPSO structure phase and stacking faults.Firstly,the growth of ?' phase along the direction of<11 2 0>a-Mg and the obvious ring distribution further enhance the blocking effect on dislocation slip,promoting the start-up of more slip systems in the ring body,reducing a large number of dislocation accumulation caused by less plane slip for dislocations,thus dispersing stress concentration and reduce the possibility of crack nucleation.Secondly,the lamellar structure increases with the increase of aging time,which promotes the non-basal slip and enhances the ability to coordinate plastic deformation. |
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