Research On The High Strength Mg-Zn-Dy Alloys With Long-Period Stacking Ordered Structures

The long-period ordered stacking reinforced magnesium alloy has achieved great developments since the materials scientists Y. Kawamura successfully prepared high strength Mg97Zn1Y2alloy firstly in Japan. But the magnesium alloy with long-period ordered stacking structure remains several key issues unresolved, the formation and transformation mechanism, strengthening mechanism of long-period ordered stacking structures have been studied but did not form a unified theoretical system. It is necessary to further study the long-period ordered stacking reinforced magnesium alloy.In this paper, the Mg-Zn-Dy based alloys with long-period stacking ordered structures were prepared successfully by the conventional permanent mould casting method. The long-period stacking ordered structures at different states were investigated by optical microscope, scanning electron microscope, X-ray diffraction and transmission electron microscopy. The influence of heat treatment and equal channel angular pressing (ECAP) on the microstructure evolution of long-period stacking ordered structures and mechanical properties of alloys were investigated. The results are as follows:In as-cast Mg-Zn-Dy alloys, when Dy/Zn=1, no long-period stacking ordered structures formed, when Dy/Zn=2, only a small amount of LPSO structures formed, when Dy/Zn>2, a large amount of LPSO structures began to appear, and with the increase of Zn and Dy content, the number of LPSO structures increased in as-cast alloys.It was found that Zr affects the formation of the Mg8ZnDy eutectic phases and suppresses the formation of LPSO phases in as-cast Mg-Zn-Dy-Zr alloy. With increase of Zr content, the area fraction of LPSO phases decreases, while the area fraction of Mg8ZnDy eutectic phases increases. When the content of Zr in Mg-Zn-Dy-Zr alloys is0.35at.%, the microstructure of cast alloy is composed of Mg8ZnDy eutectic phases and a-Mg phases without LPSO phase. Mg8ZnDy eutectic phase can transform into LPSO phase by heat treatment.The microstructure of as-cast Mg-Zn-Dy alloy can be refined by adding Zr, and the critical value of the Zr content is0.17at.%, the effect on grain refinement is obvious with the Zr content is no more than0.17at%, which is mainly attributed to the heterogeneous nucleation effect of the Zr dissolved in Mg matrix and the inhibition of grain growth.The morphology and distribution of LPSO phases in Mg93.83Zn1.5Dy4.5Zr0.17alloys had a dramatically change after different kind of treatments. The bulk LPSO phases distributed near the grain boundaries, connected into a continuous network structure, with Mg8ZnDy eutectic phases. After solid solution treatment, the bulk phases became coarse, and a large amount of acicular LPSO phases appeared and grew interior of grains. After aging treatment, the coarse bulk LPSO phases decreased, while the acicular LPSO in grains increased. After the ECAP treatment, LPSO phases got be refined obviously and emerged a large number of kinks, the LPSO phases distributed dispersed.The existence of LPSO phases can improve the performance of the alloys, but its morphology and distribution also plays a very crucial role in the mechanical properties of the alloys. The bulk LPSO phases can improve the ductility of the alloys, the sandwich structure of acicular LPSO phases and a-Mg matrix is helpful to the strength of alloys. The kinks of LPSO phases can coordinate the plastically deform of alloys. The dispersed LPSO phases can hinder dislocation motion effectively and increase the tensile strength.The LPSO structure can form in Mg-Ni-Dy alloy. The grain sizes of Mg-Ni-Dy alloys are finer than that of Mg-Zn-Dy alloys, and more LPSO structures formed in Mg-Ni-Dy alloys. In as-cast Mg100-2xNixDyx alloys, with increasing the contents of Ni and Dy, the number of LPSO phases increased and the grain sizes reduced.In heat-treated Mg100-2xNixDyx alloys, a large amount of LPSO phases distributed in grain boundaries and grain interior, with increase of Ni and Dy contents, the number of LPSO phases at grain boundaries increased and the LPSO phases in grain interior decreased.