Effect Of Aging Precipitation On The Corrosion Behavior Of Mg-Gd-Y-Zr Alloy

Because of the low density,high specific strength,easy processing and other excellent properties,magnesium alloy widely used in aerospace,military and national defense,racing cars and other fields.However,due to the low electrode potential?-2.37 V?vs.SHE?? and poor corrosion resistance,restricting the wider application of magnesium alloy in industrial production seriously.After years of research,researchers found that the addition of rare earth?RE? elements to magnesium alloys can improve the mechanical properties,heat resistance and corrosion resistance in room and elevated temperature of the alloy.Among them,Mg-Gd-Y alloy is one of the most promising and potential candidates in RE-Mg alloys.In this study,Mg-10Gd-3Y-0.4Zr?GW103K? alloy was chosenas the research object,and the effects of microstructure evolution on the corrosion behavior of GW103K alloy under different heat treatment conditions were investigated by various test methods.The influence of aging precipitation on the corrosion behavior of GW103K alloy was revealed.The morphology,composition and structure of precipitates in GW103K alloy after different heat treatments were studied by Optical Microscopy?OM?,Scanning Electron Microscopy?SEM?,Energy Dispersive Spectrometer?EDS? and High?angle Annular Dark?field Scanning Transmission Electron Microscopy?HAADF-STEM?.The results show that the main precipitates in as-cast alloys is skeletal Mg₂₄?Gd,Y?₅.After solid solution treatment?T4?,Mg₂₄?Gd,Y?₅ was basically dissolved into the matrix,and massive Mg₂?Gd,Y?and?Gd,Y? solid solutions were regenerated at the grain boundary.Some Mg₂?Gd,Y? and?Gd,Y? solid solutions were alternately arranged to form a chain-like structure.After T6 treatment?solid solution treatment+artificial aging treatment? for 0.5 h,smaller skeletal Mg₂₄?Gd,Y?₅ was precipitated and a new phase of?"was formed.When the T6 treatment time reaches 8 h,the?"phase was converted to?'phase,and the quantity and the size of Mg₂₄?Gd,Y?₅ increases.When the T6 treatment time reaches 16 h,the three variants of?'phase in different directions were connected to form a shape similar to"X".Mg₂₄?Gd,Y?₅ increases in size and decreases in quantity.After T6 treatment for 193 h,the coarse?'phase interlinked each other to forms a network structure,and the?₁ phase with diamond hexahedron structure forms at the neck point of the ?'phase.The number of Mg₂₄?Gd,Y?₅ decreased,but the morphology was coarser.When T6 treatment for 432 h and 500 h,the spacing of ?'-network became wider and sparser,and transformed into a more stable flake-like?phase continuously.The corrosion morphologies of GW103K alloy under different heat treatment conditions were observed by Laser Confocal Microscopy?LSCM?,SEM,Electrochemical Atomic Force Microscopy?EC-AFM? and HAADF-STEM.The results show that the Mg₂₄?Gd,Y?₅ dissolves itself during the corrosion process in as-cast alloy,so that the surface of the alloy was covered with large-scale corrosion pits.The main corrosion mode of the alloy after T4 treatment was also the dissolution of the residual Mg₂₄?Gd,Y?₅,but the amount of residual Mg₂₄?Gd,Y?₅ was small,and the corrosion caused by it was reduced.In the initial stage of T6 treatment,the corrosion was aggravated because the dissolution of both the re-precipitated skeletal Mg₂₄?Gd,Y?₅ and the aging nano-precipitated?'phase.But when T6 treatment for 16 h,the number of Mg₂₄?Gd,Y?₅ decreases,and?'phase becomes the dominant factor affecting the corrosion behavior.Although the?'phase dissolves preferentially,the corrosion rate was slow and the corrosion degree was reduced due to its small size and chemical dissolution mode.Especially when T6 treatment for 193 h,the corrosion resistance reaches optimal.When T6 treatment for 432 h and 500 h,partial?'phases were converted into lamellar Mg₂₄?Gd,Y?₅ and the volume fraction of which increased.The dissolution of Mg₂₄?Gd,Y?₅ results in large corrosion pits in the corrosion morphology of the alloy,and the corrosion resistance decreases.According to the hydrogen evolution curve,polarization test and electrochemical impedance test,the corrosion resistance of GW103K alloys under different heat treatment conditions is as follows:as-cast<T6 treatment 0.5 h<T4 treatment<T6 treatment 8 h<T6 treatment 500 h<T6treatment 432 h<T6 treatment 16 h<T6 treatment 193 h.Scanning Kelvin Probe Microscope?SKPFM? was used to measure the potential distribution in the micro-zone of GW103K alloy under different heat treatment conditions,and to further clarify the corrosion mechanism of GW103K alloy.The results show that the difference of potential between Mg₂₄?Gd,Y?₅ and matrix?or segregation bands rich in Gd and Y? was-150 mV under as-cast,T4 and T6 treatment.Mg₂₄?Gd,Y?₅ acts as the anode phase of electrochemical corrosion,thus occurs self-corrosion.The corrosion mechanism of the?'phase was related to its unique sandwich structure.The RE atoms in the?'phase preferentially undergo chemical dissolution.The difference of potential between Mg₂?Gd,Y? and matrix was 160²00 mV while the difference of potential between?Gd,Y? solid solution and matrix was 280³30 mV.Both of them were cathode phases,which can promote the corrosion of adjacent matrix.The Mg₂?Gd,Y?with smaller difference of potential has stronger ability to promote matrix corrosion,which may be due to the semi-conformal combination between Mg₂?Gd,Y? and matrix.The semi-conformal interface has higher energy,poor chemical stability,and corrosion is more likely to occur.