Study On Impression Creep Behavior Of AZ Series Magnesium Alloy

The application of magnesium alloy was restricted in industry because of the poor creepresistance. Improving high temperature properties is always the hotspot in magnesium alloyresearch field. In this paper, AZ81+0.5%RE alloy, AZ81+1.5%RE alloy and AZ81+2.5%REalloy can been acquired by adding Cerium-rich mischmetal into a typical alloy, AZ81alloy.The impression creep property of the magnesium alloys was investigated by a specialapparatus. The behavior property of creep was analyzed by constructing constitutiveequations of the impression creep steady-state, and the microstructure and compositions of thealloys before and after creep experiment was analyzed by scanning electron microscope andX-ray diffractometry. Finally, the impression creep mechanism of the alloys under theexperiment condition can be concluded. The results are as follows:⑴Under the experimental condition of120~180℃and125~175MPa, the stressexponents of AZ81alloy, AZ81+0.5%RE alloy, AZ81+1.5%RE alloy and AZ81+2.5%REalloy are respectively2.33,2.20,1.98and1.62. The creep activation energy of the alloys isrespectively49.0kJ/mol,52.9kJ/mol,56.0kJ/mol and59.6kJ/mol. The material structureconstants of the alloys are respectively2360,2080,471and130.⑵The as-cast microstructure of AZ81alloy is α-Mg and β-Mg17Al12, and there are nonew phases appeared after creep experiment in AZ81alloy. Meanwhile, great amount ofcoarse Mg17Al12is precipitated in the style of non-continuous, which leads to the creepresistance becoming poor sharply.⑶Acicular and punctiformAl4Ce phase is precipitated at grain boundary during thesolidification process of AZ81E alloy. Al4Ce phase can inhibit the grain growth and refinegrain. The as-cast microstructures of AZ81+0.5%RE alloy, AZ81+1.5%RE alloy andAZ81+2.5%RE alloy are α-Mg, β-Mg17Al12and Al4Ce phase. What’s more, the morphologyof Al4Ce phase in AZ81+2.5%RE alloy is coarse.⑷No new phases appear after creep experiment in AZ81E alloy. The larger theaddition amount of rare earth elements is, the better the creep resistance property of the alloywould be. Al4Ce phase, which has a high melting point, is generated owing to the strongbinding capacity of Ce and Al. Al4Ce phase constrains the Mg17Al12phase precipitating in thestyle of non-continuous. Furthermore, the capacity of the fixation of Al4Ce phase to grainboundary is strong which improves creep resistance of the alloys.⑸Grain boundary sliding plays an important role during the impression creep processof AZ81alloy and AZ81+0.5%RE alloy under this experiment condition but dislocation glidehas little effect on the impression creep. Similarly, grain boundary sliding has a significant effect on the impression creep of AZ81+1.5%RE alloy and AZ81+2.5%RE alloy, and thediffusion creep which appears during grain boundary sliding, affects the creep not veryseriously.⑹The creep resistance is better when the addition amount of rare earth elements islarge. However, multiple factors, such as the comprehensive mechanical properties at roomtemperature, tensile strength, yield strength, toughness at high temperature and economiccosts, should be considered comprehensively. Finally, it can be concluded the optimumcontent of cerium-rich mischmetal in AZ81alloy is1.5%.