Effects Of Solid Solution And Aging Process On The Microstructure And Corrosion Resishiice Of Mg-7Y-1Nd Alloy

Mganesium alloy has huge application prospects in structural materials,functional materials and biological materials because it’s lightweight,which is called the most promising environmental engineering material in 21th century.However,poor corrosion resistance hinders its great properties to play a role.Forturnately,the addition of earth element and heat treatment are excellent choice to effectively improve the high temperature performance and corrosion resistance of magnesium alloys.In this paper,we selected the self-made Mg-7Y-1Nd alloy of WE series.Because of the large solid solubility of the Y and Nd element in the magnesium alloy,we can improve the corrosion resistance by proper heat treatment.The immersion weight loss test and the electrochemical behaviors,including polarization curves and electrochemical impedance spectroscopy tests,were studied to research the corrosion resistance and its mechanisim of Mg-7Y-1Nd alloy.The surface morphology,composition and phase of Mg-7Y-1Nd alloy were studied by scannning electron microscope(SEM),energy dispersive spectrometer(EDS)and X-ray diffraction(XRD).The SEM analyses results showed there were irregular skeletal Nd-rich phases along the grain boundaries and granular Y-rich phases scattered in the grains in the microstructure of as-cast Mg-7Y-1Nd alloy.The XRD patterns showed the second phases in the as-cast and T4 Mg-7Y-1Nd alloy mainly included Mg,Mg24Y5 and Mg41Nd5.As the solution time was prolonged,the skeletal phases were gradually dissolved into the matrix and little granular phases were left over.The second phases were dissolved more thoroughly when the solution time was 535℃ than 515℃.More importanyly,the most proper solid solution process parameters was 535℃ and 12 hours.The XRD patterns indicated that the precipitates of Mg-7Y-1Nd alloy were mostly Mg,Mg24Y5,Mg41Nd5 and a small amount of Mg12Nd,which were in the forms of granular,rod,needle and skeletons.The higher the aging temperature was,the more larger the precipitates were in the T6 Mg-7Y-1Nd alloy,however,it was easer of the precipitate to gather to grow up in the higher temperature.The extension of the aging time made the diffusion of rare earth elements more fully and the precipitation distribution was more even and dispersed.Finally,225℃ and 14 hours were determained the best aging process parameters for the morphology of Mg-7Y-1Nd alloy,the microstructures were finest and precipations were smaller and more dispersed in such conditions.It was found that the solid solution at 535℃ can improve the corrosion resistance effectively of Mg-7Y-1Nd alloy in the 3.5%NaCl solution.With the prolongation of the solution time,the corrosion resistance of Mg-7Y-1Nd alloy was firstly improved and then decreased.The aging temperature and time both had influence in the corrosion resistance of T6 Mg-7Y-1Nd alloy,and finally,the better heat treatment was that the solid solution temperature was at 535℃ for 12 hours,and then followed the aging at 225℃ for 14 hours according to the corrosion resistance analyses.The optomium heat treatment for improving corrosion resistance of Mg-7Y-1Nd alloy was solved at 535℃ for 12 hours,water quenching and then aged at 225℃ for 14 hours,air cooling.The corrosion weight loss rate,corrosion current density and balanced potential of as-cast Mg-7Y-1Nd alloy in the 3.5%NaCl solution were 0.080 g·cm-2-d-1 1.0732×10-3 A·cm-2 and-1.7526 V respectively.Finally,after the optimium heat treatment,the corresponding corrosion data were 0.0122 g·cm-2·d-1,2.8018×10-4 A·cm-2 and-1.6382 V.The corrosion model of T4 and T6 Mg-7Y-1Nd alloy were eventually established by analyses of electrochemical impedance spectroscopy and corrosion morphology.The corrosion types of T4 Mg-7Y-1Nd alloy was mainly pitting while the T6 Mg-7Y-1Nd alloy were mainly pitting and galvanic corrosion.The corrosion products of as-cast and T4 Mg-7Y-1Nd alloy included Mg(OIH)2 and MgO,besides which corrosion products of T6 alloy included Mg24Y5 and Mg41Nd5.Maybe the reason was that there were a large number of precipitations in the aged Mg-7Y-1Nd alloy,the precipatations cotainning rare earth elements were peeled off from the matrix when the serious corrosion occurred.