Investigating On Corrosion Sensitivity Of Mg-RE Alloys Induced By Stress And Precipitated Phase Microstructure

Magnesium rare earth(Mg-RE)alloys have excellent high temperature strength and creep resistance,which are widely used in aerospace,national defense and military fields.With the further development of magnesium rare earth alloys,higher requirements are put forward for the corrosion resistance of the alloys.Precipitates as the most important internal factor affecting the corrosion sensitivity of magnesium rare earth alloy,it is of great theoretical significance to explore its influence and mechanism for optimizing the corrosion performance of magnesium rare earth alloy.Meanwhile,the components of magnesium rare earth alloy are affected by stress and corrosion environment in the service process.The interaction of mechanics and chemistry greatly affects the reliability of magnesium rare earth alloy.Therefore,it is of great value for engineering application to explore the mechanism of stress effect on corrosion of magnesium rare earth alloy.In this paper,Mg-9Gd-3Y-0.6Zn-0.5Zr alloy is taken as the research object.The morphology,composition and structure of precipitated phase and different heat treatment states are studied.The influence of potential difference between precipitated phase and matrix,interface microstructure and thermodynamic state of precipitated phase on the corrosion of Mg-RE alloy is explored,there by reveal the mechanism of corrosion induced by precipitated phase.At the same time,a self-designed compressive stress loading device was designed to explore the influence of compressive stress on the corrosion sensitivity of magnesium rare earth alloy.The morphology,composition and structure of precipitated phases in different Mg-RE alloys were studied by means of scanning electron microscopy(SEM),transmission electron microscopy(TEM)and X-ray diffraction(XRD).The results show that the Mg-RE alloy is mainly composed of a-Mg,reticular Mg24Y5,a small amount of massive MgsGd and granular Zr.After T4 treatment,the grains of Mg-RE alloy grow up,the reticulated Mg24Ys phase dissolves,and the nanoscale rods appears in the solid solution in the grains.A large number of metastable ?'precipitates are formed in the grains of the peak aged Mg-RE alloy,and the precipitates in the grains of the overaged Mg-RE alloy are mainly the equilibrium ?1 phase.SEM was used to observe the corrosion morphology of precipitates in Mg-RE alloys of different states.Scanning Kelvin probe force microscope(SKPFM),high resolution transmission electron microscopy(HRTEM)were used to investigate the potential difference between precipitates and matrix and microstructure of different crystal surfaces.The results show that the potential of Mg24Y5 and MgsGd is 30 mV and 70 mV higher than that of a-Mg respectively.Both phases have the potential driving force of Mg matrix corrosion.However,in the corrosion process of Mg-RE alloy,the Mg24Ys phase with noncoherent interface induces matrix corrosion,while the Mg5Gd phase with coherent interface does not cause adjacent matrix corrosion.the crystal lattice distortion of ?-Mg in solid solution Mg-RE alloy is mainly caused by rare earth solute atoms,and there is multilayer atom interface distortion between nanoscale rods and parent phase,The higher lattice strain can lead to the petal corrosion pits surrounding shaft in the grains;in the aged Mg-RE alloy,the metastable ?' phase is easy to induce corrosion,while the equilibrium ?1 phase is difficult to induce corrosion.The effect of precipitation on the corrosion behavior of Mg-Gd-Y alloy is not only related to the interface potential difference and interface bonding mode,but also to the different orientations of crystal faces.Based on Density Functional Thoery(DFT),the electronic work functions of ?-Mg and Zr phases are calculated by materials studio.The results show that the electronic work functions of different crystal faces arc different,and the difference between Zr and ?-Mg exposed crystal faces in Mg-RE alloy will significantly affect the intrinsic potential difference between the two phases.In the process of corrosion,the Zr phase in as cast Mg-RE alloy did not cause Mg matrix corrosion,and the Zr phase in Mg-RE alloy after heat treatment always preferentially caused matrix corrosion.Through the characterization of corrosion morphology,weight loss and electrochemical test.In terms of corrosion thermodynamics,the Ecoor of cast Mg-RE alloy is the highest,the corrosion tendency is the smallest,the Ecoor of solid solution Mg-RE alloy is the lowest,and the corrosion tendency is the largest.With the increase of aging time,the Ecoor of Mg-RE alloy rises gradually,and the corrosion tendency decreases gradually.In terms of corrosion kinetics,the cast Mg-RE alloy Rt is the smallest,EIS low-frequency inductive resistance circuit marks the desorption of corrosion product film,so it shows the maximum corrosion rate in the weight-loss test.Rt of Mg-RE alloy increases gradually in solution,peak aging and aging state,and the product film on the surface of EIS low frequency second capacitive resistance circuit is relatively complete,while the corrosion rate decreases gradually in weight-loss test.In this paper,Mg-Gd-Y alloy was loaded with elastic and plastic compressive stress by a self-made compressive stress loading device.The effect of elastic-plastic compressive stress on the corrosion behavior of Mg alloy was studied in the collaborative environment with corrosion medium.The corrosion morphology of Mg-RE alloy under stress loading was observed by SEM,and the effect of elastic compressive stress on metal activity was studied by thermodynamic formula and DFT calculation.The results show that,under the action of elastic compressive stress,the corrosion in the grain of Mg-RE alloy was intensified,and the corrosion degree of Mg matrix around Mg24Y5 phase showed a decreasing trend.The electrochemical corrosion of the alloy did not increase monotonously with the increase of elastic compressive stress.As the elastic compressive stress reduces the potential difference between phase and phase of Mg24Y5 and Mg,the driving force for the microcouple corrosion induced by phase Mg24Y5 and ?-Mg is reduced.On the other hand,the elastic compressive stress reduces the electron work function of Mg,and the Mg matrix is more prone to self-occurring chemical corrosion.Under the action of plastic compressive stress,the corrosion morphology of Mg-RE alloy changed from the reticular corrosion extending along grain boundary to the discontinuous spot corrosion separated from grain boundary in the state of no stress.TEM results showed that a large number of dislocations and twins appeared in the microstructure of Mg-RE alloy under plastic compressive stress,and these internal defects led to the aggravation of corrosion degree of Mg-RE alloy.In this paper,the effect and mechanism of precipitation induced corrosion of Mg-RE alloy arc studied,and the effect mechanism of stress on the corrosion sensitivity of Mg-RE alloy is explored,so as to further expand the research field of corrosion mechanism of Mg-RE alloy,and provide theoretical guidance for improving the corrosion performance of Mg-RE alloy.