Microstructure And Mechanical Properties Of Die Casting Mg-RE-Zn Alloy

As the lightest constructional material, magnesium alloy has not distinctive advantages,while a series of shortcomings, such as active chemical property, poor deformation processing capacity and corrosion resistance and so on, which brings about that magnesium alloys have not been widely applied compared with aluminum alloys. The magnesium and rare earth resources are very abundant in China. Accompanied by purification of the alloy melt and reduction of grain size, microstructure, mechanical properties and corrosion resistance could be improved apparently by alloying with rare-earth elements in the magnesium alloys.Accordingly, studies about RE-containing magnesium alloys have become a very important subject with realistic significance. This paper focuses on the microstructure,particularly the thermal stability of intermetallic phase, and mechanical properties, corrosion resistance of the die-casting Mg-RE-Zn alloys. The strengthening mechanisms of the alloys were analyzed.Mg-4La-2Pr-1Zn alloy was prepared by high pressure die casting, and the microstructure and tensile properties, particularly the thermal stability of intermetallic phase were studied.The results show that the alloy which is prepared by die casting has a favorable performance in fluidity and mold-filling capacity, uniform microstructure and few defects. The microstructure of the alloy is fine equiaxed grains and the average grain size is about 10 ?m.The strengthening phases of the alloy are Mg12La, Mg12Pr and a small amount of(Mg,Zn)12(La,Pr) phase distributed in the grain boundary. After heat treatment at 200? for 400h, there was no obvious change in the microstructure, but the content of the second phase decrease slightly. After heat treatment at 400? for 400h, the microstructure of the alloy was changed obviously, and the network structure consisting of the second phases changed into discontinuous fragments,the chemical composition of the second phase was not changed.Rietveld whole-pattern fitting method was carried out to quantitatively analysis of all phases in alloy. According to the calculation, the content of a-Mg and second phases are 93.5% and 6.5%, respectively. Tensile test experiment results show that the UTS of the alloy is 215 MPa,the TYS is 172 MPa, the elongation is 2.7% at room temperature, and the UTS is 143 MPa,the TYS is 116 MPa, the elongation is 13.5 % at 200?.Mg-4Er-2Y-1Zn alloy was prepared by high pressure die casting, and the microstructure and tensile properties. The main strengthening phase of the alloy is Mg12(Er,Y)Zn characteristic with long-period stacking ordered structure (LPSO). Additionally,a very small amount of block phase, Mg24RE5, is distributed in the grain boundary. After heat treatment at 200? for 400h, the microstructure of alloy has been changed, The long strip second phase distributed in the grain boundary gradually turned into discrete small block phases. When the heat treatment temperature is increased to 400?, remarkable changes have taken place in the microstructure of the alloy, the long strip second phase distributed in the grain boundary decreased significantly. However, the Mg24RE5 phase increases relatively. The micro-hardness of the alloy is 65.9 Hv0.01, and the hardness value reduces to 53.0 Hv0.01 after heat treatment at 200? for 400h. Tensile test experiment results show that the UTS of the alloy is 225 MPa,the TYS is 136 MPa, the elongation is 3.7% at room temperature, and the UTS is 148 MPa,the TYS is 126 MPa, the elongation is 16.1 % at 200?.The corrosion resistance of the two alloys are also studied, the scanning electron microscope (SEM), energy spectrum analysis, X-ray diffraction and electrochemical impedance spectroscopy (EIS) are employed to investigate the open circuit potential,polarization curve, AC impedance, corrosion rate and corrosion morphology respectively. The corrosion rate of die casting Mg-4La-2Pr-1Zn alloy is the lowest, followed by the Mg-4Er-2Y-1Zn alloy, and the corrosion rate of AZ91D alloy is the fastest. Therefore, the corrosion resistance of two experimental alloys is better than that of AZ91D alloy.