| So far, as the structural material, magnesium has the lowest density and hence hasbeen widely applied to fields such as automobile,aviation and so on. Meanwhile, as theincreasing needs for environment, more attention and interest was paid to thelightweight car. But its intrinsic structure has determined the poor property of thepresent ZK and AZ commercial magnesium alloys. For example, poor high temperaturestrength and high temperature resistant performance which seriously restricted itsapplication in car, especially in the key components where the working temperature ishigher than100℃, such as the automatic transmission shell, the engine block and so on.Therefore, to research and develop low cost, excellent room temperature and hightemperature property magnesium alloys are one of the main directions for thedevelopment and research.of magnesium alloys.Appeared in1980s, Mg-Zn-Cu alloys own perfect room-temperature mechanicalproperties, high temperature resistant performance, low cost, easy casting molding andother perfect property. The development Mg-Zn based heat-resistant magnesium alloysbecome an important point.At present, however, the research about Mg-Zn-Cu alloy is mainly focus on theinfluence of Cu element on the size, distrubution, type and quantities of the agingprecipitation phases. There is no systematic research about the structures, room and hightemperature mechanical properties, creep behavior, creep organization and other aspectof this alloy, which has prevented its further research and application.This paper by addition of Ce rare earth element to Mg-Zn-Cu alloy investigated theeffect of different mass ratio Cu and Zn contents on the microstructure and the secondphases. The optical microscope, X-ray diffracmeter, scanning electron microscope wereintroduced in the experiement. The microhardening testing, room and high temperaturetensile test and creep properties test were done to study the effect of Cu content on theproperty of this alloy, such as the evolution rule of its room and high temperaturemechanical property, hot deformation behavior, stengthening mechanism and resistantcreep behavior.The results indicate that the as-casting microstructure of the Mg-Zn-Cu-Ce alloysis the typical dendritic structure. As the content of the Cu element increasing, thedendritic structures grow larger and coarsening obviously. At the sanme time the grains were refined. The addition of Cu element generates the α-Mg+MgZnCu eutecticstructures. When the mass ratio of the Zn, Cu equals1, the Mg2Cu phases emerged. Theeutectic structure evolution process is acicular-lnamellation-needlelike chape-cellularand thin layers. The ardness of the as-cast alloys increase with the content of Cuelement. After homogenization treatment, the1#alloy’s organization changed greatly,dendrites strcuture decreases and its distribution transfered from continuious intointermittent. The dendrites of other alloys changed a little. The hardness of this allloydeclined coMPared to the as-cast alloys.After extrusion, the alloy undergone large deformation at high temperature will behighly refined and fully recrystallized, so it has perfect room temperature. The3#alloyperformation best room temperature mechanical property. The tensile strength and yieldstrengh is320MPa and291MPa respective. The strength of the alloy decreased at hightemperature, but behave higher elongation rate. Even the temperature is as high as150℃, its strength is still higher than200MPa. The alloy behaves same mechanicalproperty evolution rule at room and150℃temperature, that is, the mechanical propertyincreasing first and then declining as the mass ratio of the Zn/Cu decreases. When thetemperature ranges from175℃to200℃, the tensile strength and the yield strengthincreased as the Cu content increasiing, but the enlongation declined. The alloysfracture in the form of britttle fracture and the large quantity of second phases in theboundary are the main source of the crack. When tested under the condition of200℃/50MPa for100h, the results show that the alloy containing large percentage of Znbehaved poor creep property, and it cracked when tested for19h. Meanwhile, when themass ratio of Zn/Cu is1, the alloy has best creep property; steady creep speed rate is1.21x10-8s-1with the creep strain is only0.562%.At the same deformation temperature, the true stress level of the as-homogenized2#alloy increasese with increasing of the strain rate. The results indicate that the alloyis strain rate sensitive material. While at the same strain rate, the true stress level of thealloy declined with the temperature inceased. Therse is no obvious dynamicrecrystallization and the softening effects are maily result from the dynamicrecovery.The constitutive equation of2#alloy with Z parameter is：... |
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