Effect Of Rare Earth Elements On Microstructure And Corrosion Resistance Of ZMT613 Magnesium Alloy

As an environmental protection engineering material in the 21st century,magnesium alloys have many advantages such as excellent comprehensive mechanical properties,good biocompatibility,electromagnetic shielding properties,and recyclability,making them widely used in aerospace,biomedical,electronic devices and other fields.But it is easily corroded due to its active chemical properties and loose the surface oxide film,so that its application range is greatly limited.At present,alloying is one of the effective ways to improve the comprehensive mechanical properties of magnesium alloys.In particular,rare earth elements have good impact to the strength and plasticity of magnesium alloys.However,the research on the effect of different trace rare earth elements on the corrosion behavior of magnesium alloys is poor.This paper researched,by optical microscope,scanning electron microscope,energy spectrum analysis,X-ray diffraction analysis,immersion hydrogen evolution and weight loss,open circuit potential curve,potentiodynamic polarization curve,electrochemical impedance,the effects of different trace rare earth elements（holmium（Ho）,ytterbium（Yb）,dysprosium（Dy）,samarium（Sm））on the microstructure and corrosion properties of as-cast,extruded and two-stage aging Mg-6Zn-Mn-Sn（ZMT613,wt.%）wrought magnesium alloys.The main research results are as follows.（1）The addition of trace rare earth elements to the as-cast ZMT613 alloy significantly refined the secondary dendrite arm spacing of the alloy,and formed various rare earth second phase Mg_xSn RE（RE=Ho,Yb,Dy,Sm）.Further corrosion resistance research found that the addition of different trace rare earth elements improved the corrosion resistance of as-cast alloys:ZMT613-0.5Ho,ZMT613-0.5Dy,ZMT613-0.5Yb,ZMT613-0.5Sm,ZMT613（with the corrosion performance from high to low）.On the one hand,the addition of rare earth elements played a role in refining grains and inhibited the expansion of corrosion.On the other hand,rare earth elements are precipitated in the form of Mg_xSn RE phase,which resumes Sn elements so that inhabited the existence of the Mg₂Sn phase with higher corrosion susceptibility,thereby reducing the micro-galvanic corrosion caused by Mg₂Sn phase.（2）The addition of rare earth elements has different effects on the corrosion behavior of as-extruded ZMT613 alloy.The corrosion resistance of Ho-containing alloys and Yb-containing alloys increased by 18.5 and 40.4%,respectively,while the corrosion resistance of alloys containing Dy and Sm are decreased by 14.7%and 38.3%,respectively.The grains of Ho-containing alloys and Yb-containing alloys are finer,and the corrosion development process is more inclined to develop laterally parallel to the surface,which is relatively more uniform and the corrosion depth is shallower.However,the grains of Dy-containing alloy and Sm-containing alloy are relatively coarse,the corrosion process of longitudinal development is more obvious,and the local corrosion phenomenon is serious.（3）The corrosion resistance of all alloys was improved after solution+two-stage aging treatment.Compared with as-cast and as-extruded alloys,after solution-aging treatment,the second phase in the alloys was fine and dispersed.Therefore,local"small cathode"can effectively weaken the micro-galvanic corrosion process.The effect of rare earth elements on the corrosion properties of the alloys is basically the same as that in the as-extruded state:the addition of Ho and Yb can improve the corrosion resistance of the alloys,while the additions of Sm and Dy reduce the corrosion resistance of the alloys.