Research On Aging Precipitation Behavior Of Mg-Zn-Nd Alloy

Magnesium alloys are considered as the lightest metal structural materials, and expected to be green engineering materials in21st century. These alloys have been extensively applied in aerospace, automotive,3C products and other fields, due to their good properties prior to other metal materials. However, the further application is limited because of their lower strength and creep resistance, compared with Fe or Al-based alloys. Moreover, Al-based alloys with similar melting point and elastic modulus of Mg-based alloys have higher strength up to1GPa because of the precipitation hardening effect, which is not well utilized for Mg-based alloys. Mg-Zn-based alloys are one of wrought Mg-based alloys with obvious aging precipitation effect. But, it is controversial that whether the rod-like precipitate strengthening phase is MgZn2or Mg4Zn7phase. Besides, the precipitation phase in the grain boundary was not extensively concerned, which plays an important role in pinning the movement of grain boundary, hindering the grain growth and improving high temperature mechanical properties of Mg-Zn-based Mg alloys. Therefore, in this paper, Mg-4wt.%Zn-(0,0.3,0.6,1.0wt.%) Nd alloys have been prepared by indirect hot extrusion and subsequent solid solution treatment, and the aging precipitation behavior of Mg-Zn binary alloy have been investigated at70～250℃by means of hardness test, field emission scanning electron microscope and transmission electron microscope, as well as the additions of Nd and loading stress. The results are as followed:After the Mg-4wt.%Zn-(0,0.3,0.6,1.0wt.%) Nd alloys were hot extruded at300℃, the α-Mg matrix grains appear equiaxed due to the complete dynamic recrystallization. The grain size is31.6μm、13.0μm、7.6μm and4.2μm, respectively, which indicates that the Nd element can obviously refine the grain size of extruded Mg-Zn-based Mg alloys. The optimal solid solution process is400℃/24h, where there appears single phase supersaturated solid solution in the Mg-4Zn binary alloy. However, there is residual Mg-Zn-Nd Ternary compound T2phase with high melting point after additions of Nd element. Nd is almost insoluble in the α-Mg matrix.The hardness of Mg-4Zn alloy reaches maximum when aged at150℃for200h. There are two types of precipitates:one is rod-like precipitate along [0002]Mg, the other is plate-like precipitate lying on (0001)Mg. The nucleation and growth of rod-like precipitate occur before aging peak. The coarsening of rod-like precipitate and the increasing of plate-like precipitate lead to over age. The peak-aged status of Mg-4Zn at150℃can sustain for more than2000h. The rod-like precipitate is MgZn2phase with hcp structure, and its lattice parameter:a=0.52nm, c=0.857nm. The orientation relationship between the rod-like precipitate and a-Mg matrix is (2110)β1’//(0002)Mg and [0001]β1’//[2110]Mg. The phase in the grain boundaries is precipitated after aging at150℃for50h, and its morphology and size is not essentially changed with the extension of aging time. And its structure is triclinic(a=1.724nm, b=1.445nm, c=0.520nm, α=96°,β=89°γ＝138°), which is the Mg2Zn3compound.There is no precipitation hardening with the extension of aging time to5000h at both70℃and100℃in the Mg-4Zn alloy, probably because the temperature is too low to precipitate the strengthening phase. There is also no precipitation hardening when the Mg-4Zn alloy is aged at250℃for500h, because supersaturated solid solution don’t form with the content of4wt.%Zn at250℃. The peak hardness is obtained when aged at200℃for50h, however, the hardness decreases rapidly with the extension of aging time.The aging precipitation at100℃can be improved by the additions of Nd element. The hardness of Mg-4Zn-0.3Nd alloy reaches maximum for5000h, and the rod-like precipitate is still the MgZn2phase with hcp structure, whose length and diameter is170nm and10nm, respectively. The orientation relationship between the MgZn2phase and the Mg matrix is:[0110]MgZn2//[0110]Mg,(2110)MgZn2//(0002)Mg。However, the aging hardness has been not improved by the additions of Nd element when aged at150℃and200℃. And, the peak time is extended with the increasing of Nd additions. Besides, the phase precipitated in the grain boundary is still the Mg2Zn3phase with triclinic structure. Moreover, the growth of the phase is restrained by the Nd additions when aged at the same temperature. The undissolved T2phase doesn’t change during aging process, which is a considerably stable Mg-Zn-Nd Ternary compound that can hinder the a-Mg matrix grain growth during aging. When the stress of75Mpa stress is imposed on the Mg-4Zn alloy aged at150℃, a lot of dislocations exist on the (0002)Mg crystal plane, which provides more nucleation sites of the precipitation phase in the α-Mg matrix. Therefore, the precipitates are smaller and dispersed. Meanwhile, the precipitation phase located in the grain boundary is also smaller, compared with the free stress.