Study On The Microstructures And Mechanical Properties Of Mg-Zn-Y-Zr Alloys

In this paper,Mg-Zn-Y-Zr alloys were studied.The influence of cooling rate(mould and wall thickness),Y(Yttrium)contents and heat treatments on the microstructures and the mechanical properties of as-cast alloys were investigated deeply by means of optical microscopy(OM),scanning electron microscopy(SEM),mechanical property testing and so on.The effects of mold on the microstructures and mechanical properties of as-cast Mg-6Zn-1.6Y-0.18Zr alloys were studied.The experimental results show that the microstructures mainly includedα-Mg matrix and interdendrit -ic networked eutectic pockets.With the increasing of cooling rate,the micro -structures have refinement and the volume fraction of precipitation phases in grain boundaries increases remarkably,the mechanical properties also increase tremendously.The failure mode of the fracture surface all belong to quasi-cleavage plus intergranular fracture.With the decreasing of the wall thickness,the volume fraction of precipitation phases in interdendritic grain boundaries between graphite mou -ld and sand mould changed from 7.57%,4.41% to 6.65%,5.15%,respective -ly. The mechanical properties of as-cast Mg-6Zn-1.6Y-0.18Zr alloys increase tremendously with the decreasing of the wall thickness.With the increasing of Y content,the microstructure in the as-cast Mg-Zn-Y-Zr alloys have refinement, when the yttrium content reached 5.0wt.%,the phase composition of the alloys changed fromα-Mg+W-Mg3Y2Zn3(fishbone shape)+I-Mg3Zn6Y phase toα-Mg+W+Z-Mg12ZnY (stripe shape)phase.The volume fraction of precipitation phases in interdendritic grain boundaries and the mechanical properties of the alloys increase remarkably with the increasing of Y content.The as-cast Mg-6Zn-1.6Y-0.18Zr alloys were subject to T4,T5,T6 heat treatment,respectively.The results showed that,After T4 and T6 states, the continuous fishbone shape microstructures were substituted for interrupted fine globular-shape in grain boundaries,and the phases are both composed ofα-Mg+ W phase+Zr0.82Y0.18O1.91.T5 state alloys have the same phase composition comparing to as-cast alloys.The mechanical properties of T4 state have a slightly falling down but the T5 state alloys have been improved obviously comparing to as-cast alloys.T6 state alloys have the best mechanical properties.The mechanical properties of T6 state alloys were firstly decreased and then increased with the increasing of Y content,and the 1.6wt.%Y alloys have the optimum mechanical properties. When the yttrium content reached 3.0wt.%,the failure mode of the fracture surface changed from mixed mode fracture(brittle and ductile dimple)to quasi-cleavage fracture.