Study On Ho Modification Of As-Cast Mg-xZn-yAl Magnesium Alloy

Magnesium alloys are the lightest commercially available structural materials because of their particularly high specific strength and specific stiffness,excellent damping capacity and good dimensional stability,so Magnesiumalloys are becoming increasingly attractive for engineering applications. Early2000the average components per vehicle quality has reached12kg magnesium alloy was used to one car at the beginning of2000. However, they have poor creep resistance above125℃, which makes them inadequate for major powertrain applications. Automatic transmission cases can operate up to175℃, engine blocks up to200℃, and engine pistons, even higher than300℃. At present, developing a high strength, good toughness, low cost, has good creep resistance cast magnesium alloys become the most urgent task.Rare Earth are important alloy elements in magnesium alloy They played a very significant role for improving the casting properties of magnesium alloy and heat resistance.In this paper, the effects of Ho content on microstructure and mechanical properties of two typical magnesium alloys of Mg-xZn-yAl have been investigated by using X-ray diffraction (XRD), optical microscopy (OM), and scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and various testing and impact testing machines.And it have explored that how rare earth modified magnesium alloy and what the alloy system strengthening and toughening mechanism is. The results show that:(1) ZA52and AZ61magnesium alloy:the matrix structure of different Zn/Al ratio in the two alloys are composed of a-Mg matrix and mesh phase. And the network phases are τ-Mg32(Al,Zn)49, β-Mg17Al12respectively.(2) The results indicate that addition of Ho modifies microstructure of them, new phases are formed in the forms of petal-like and graininess shape, which are verified as Al2Ho phase in ZA52.Furthermore, new phases are formed in the forms of petal-like,which are verified as Al-Mn-Ho phase in AZ61.Therefore, the combination of Al and Ho is not affected by different content of the rare earth Ho addition under different Zn/Al ratio.(3) Mg-xZn-yAl-zHo mechanical properties of alloys are subject to number,size,shape of network eutectic phase and rare earth distribution morphology.Ho addition of amount of1.0%to1.5%suppresses the formation of network phase and modifies the network phase.meanwhile,the second phase distributed within the grain boundaries uniformly and slow down micro-crack propagation.the alloy exhibits good strength and ductility.(4)With the increase of Ho content,the mechanical properties of ZA52, AZ61two experimental alloys have been significantly improved. Because a lot of hard and brittle phase formation, the macro-hardness increased significantly, which has caused the decline of toughness;The tensile strength and elongation of hard and brittle phase formation, the macro-hardness increased significantly, which has caused the decline of toughness;The tensile strength and elongation of ZA52and AZ61alloy achieved optimal results in content of1.5%,1.0%respectively at room temperature.Their values were243.3MPa,13.6%and207.88MPa,10.22%respectively; At200℃,the their tensile strength and elongation at1.5%to achieve the best and the values are117.5MPa,18.7%and128MPa,13.2%respectively.