Effect Of Ce,Nd Sr On The Microstructure And Corrosion Property Of Magnesium Alloy AZ91

The application for magnesium alloys castings in the automotive, electronics and communication industries has been expanding at a high rate due to their low density, high strength-to-weigh, rigidity-to-weigh ratio, good damping capacity and good electromagnetic shielding properties. However their commercial application has been limited because of poor corrosion resistance. In this thesis, Ce, Nd and Sr were added to AZ91 magnesium alloy respectively, which is the most widely used magnesium alloy, to study their effects on the microstructure and corrosion performance.The influence of Ce, Nd and Sr on the as-cast microstructure of AZ91 alloy was investigated by optical microscope(OM), scanning electron microscope(SEM), electron probe microscope analyzer(EPMA), differential thermal analyzer (DTA) and X-ray diffract analyzer(XRD), et al. The experimental results showed that the three elements all can refine the a-Mg grains of AZ91 alloy. In addition, some new phase Al4Ce, Al3Nd and Al4Sr were found. In the present work(the element content from 0.2% to 0.8%), 0.8%Ce, 0.5%Nd and 0.5%Sr added to AZ91 could get the finest grain respectively. At the same time, Al4Ce, Al3Nd and Al4Sr all have high thermal stability, which did not dissolve into a-Mg matrix when solution treatment was carried out for 8h at 410℃ followed by water quenching and delayed the ageing process. The grain-refined effect of Ce and Nd could be attributed to the constitutional supercooling and the reason for Sr is to decrease the undercooling degree of AZ91 alloy.The effect of Ce, Nd and Sr on the corrosion property and corrosion mechanism in NaCl solution were studied by the static weight-loss test and polarization curve measurement. The experiment found that Ce and Nd were benefit to improve the corrosion resistance, however when the content of Sr is from 0.2% to 0.5%, it had poor corrosion property; when 0.8%Sr was added, the corrosion performance was improved. In addition, 16 heat treatment can also enhance corrosion resistance. In this experiment, the alloy containing 0.8%Ce or 0.5%Nd had excellent corrosion performance.The relationship between microstructure and corrosion performance was discussed. The investigation showed that the corrosion resistant property had been improved by the addition of Ce and Nd. This could be attributed to the following reasons: fined grain is benefit for the improvement of corrosion property, in which the homogeneous distribution of Al element was also good to improve corrosion performance. The amount and distribution of β-Mg17Al12 phase has great effect on the corrosion performance, adding Ce or Nd could decrease the amount of effective cathode phase (Mg17Al12) and caused the dispersed distribution of β-Mg17Al12 phase; addition of Ce and Nd can enhance the alloy electrode voltage, On the contrary, Sr decrease the electrode voltage, perhaps which was the reason for their poor corrosion performance.