| Rare earth alloying,as one of the important ways to strengthen magnesium alloy,has been widely concerned by related scholars in recent years.However,as-cast Mg-RE alloys often have the disadvantages,such as too large grain size,inhomogeneous microstructure and worse mechanical property,which cannot meet the needs of its application in industrial structural parts.Therefore,it is urgent to make a breakthrough in alloy composition design and forming technology.Based on this,a novel Mg-Gd-Nd-Zn-Zr alloy with multi-element RE has been designed and studied in this paper.Moreover,the deformation process of Mg-9Gd-3Nd-1Zn-1Sn-0.5Zr(GNZ931K-1Sn)alloy was confirmed,relations between the microstructure and mechanical properties in various heat treatment and deformation parameter is systematically studied.Research conclusions are as follow:The results show that Gd can not only refine the microstructure of Mg-3Nd-1Zn-0.5Zr alloy,but also enhance its mechanical properties.Meanwhile,it is revealed that the strengthening mechanism of Gd on Mg-xGd-3Nd-1Zn-0.5Zr(x=0,1,5,9 wt.%)alloy was mainly caused by the substitution of the Mg12Nd phase to the Mg5(Gd,Nd,Zn)phase inside the alloy,which helped to hinder the growth of alloy grains.Moreover,it is found that the addition of Sn can promote the formation of a high melting point Mg2(Sn,Nd)3Gd2 phase in the alloy and effectively refine microstructure.Comparing the tensile strength of Mg-9Gd-3Nd-1Zn-0.5Zr alloy with and without 1wt.%Sn,it can be seen that the effect of Sn on grain refinement and solution strengthening of as-cast Mg-9Gd-3Nd-1Zn-0.5Zr alloy is not obvious,but can obviously improve the aging strengthening effect.A constitutive model and hot processing map of the GNZ931K-1Sn alloy is constructed.It is concluded that the safe processing zone of the material was close to 350?/1s-1,395?/0.001s-1,450?/0.01s-1.Based on this,extrusion verification experiments were carried out,which confirmed that the predicted results of the hot processing map matched the experimental results.Besides,the numerical model and finite element simulation platform for high temperature forming of GNZ931K-1Sn alloy were established in DEFORM-3D.The research result indicates that the internal temperature field,stress and strain of the alloy were inhomogeneous during extrusion and rolling,and the strain value increases with the increase of deformation.Through the analysis of the forming process,the variation law of various fields of GNZ931K-1Sn alloy during forming is explored.It is found that the dynamic recrystallization(DRX)of GNZ931K-1Sn alloy is mainly concentrated on the narrow region during extrusion in the die with narrow zone.Extrusion helps to refine the grain size and promote the uniform distribution of eutectic phase.The peak aging treatment of extruded alloy found that the ?" phase produced by under-aging gradually changed into ellipsoid ?' phase,in addition,the amount of LPSO phases in the alloy increases obviously,and the interaction between?' phase and LPSO promotes to reach the maximum strength.Aging increased the ultimate tensile strength(UTS)and yield strength(YS)of the extruded alloy from 370MPa and 292MPa to 462MPa and 392MPa,respectively,but the elongation(EL)decreased from 11.44%to 4.19%.In the over-aged state,the spherical ?' phase in the alloy gradually transformed into a thicker ?1 phase and the number of LPSO phases also increased significantly.The formation of ?1 phase and the increase of LPSO phase consumed the RE in the ?' phase,which resulted in a significant decrease in the number of ?' phases,and resulting in a decrease in the strengthening effect of the alloy compared to the peak-aging state.In addition,the aged GNZ931K-1Sn bars have high tensile strength below 250?,while the alloy softens significantly above 250?,which may be caused by activating a new slip system or coarsening recrystallized grains near this temperature.After rolling with different reductions,there are obvious twins in the alloy,and the UTS,YS and EL of the rolled alloy with a reduction of 80%are 349MPa,303MPa and 6.68%,respectively.The number of ?' phase in the peak-aged alloy is larger,and its UTS and YS increase to 431 MPa and 372MPa,respectively,but its plasticity decreases to 3.11%.However,a small amount of LPSO phase perpendicular to the ?' phase appears in the grain of the over-aged alloy.At this time,the ?' phase in the peak-aged alloy becomes plate-like,and its size becomes larger but the number decreases,which reduces the alloy performance.After peak-aging treatment,the rolled sheet has excellent high temperature tensile properties below 250?.However,the strength of the alloy decreases significantly at 300?,but its elongation increases significantly. |
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