Correlation Between Corrosion Behavior And Microstructure Of Intermetallic Phases In AZ91 Magnesium Alloy With Rare Earth Addition

Magnesium alloy is the lightest structural material and paid more attention because of many advantages. However, magnesium alloy’s further development was limited because of its’ worst corrosion resistance and resistance of elevated temperature. Appropriate rare earth addition is one of the most effective ways to improve magnesium alloy’s properties. In present work, ingredient,structure,potential difference of precipitated phase in AZ91 magnesium alloy and interface combination of precipitated phases with matrix was investigated by various testing methods. And then correlation of microstructure of precipated phases and effect of precipitated phases on corrosion behavior in AZ91 magnesium alloy was investigated.Microstructure,ingredient and compound form of precipitated phase in magnesium alloy was invesitgated by sacnning electron microscopy（SEM）, energy diffraction dispersive spectrometer（EDS） and transmission electron microscopy（TEM）. The results showed that the microstructure of AZ91 magnesium alloy without rare earth addition is consisted of matrix, second phase β-Mg₁₇Al₁₂ and AlMn phase. Microstructure of AZ91 with Y addition is consisted of matrix, β-Mg₁₇Al₁₂ and bulk rare earth phasewhich conclude Al₂Y、Al₂Y₃ and Al10Mn2 Y.There are needle-like Al₄（La,Ce） phase and rod-like Al₄（La,Ce） phase formed in magnesium alloy with（La,Ce） misch metal, and there are also bulk Al8Mn4 Ce phases formed in the alloy except for matrix and β-Mg₁₇Al₁₂.Corrosion behavior of kinds of precipitated phases was investigated by Laser Scanning Confocal Microscope（LSCM） and EC-AFM mode of Scanning Probe Microscope. It was found out that general corrosion happened on the surface of AZ91 magnesium alloy and corroede area was largest, corrosion around AlMn phase was more serious, corrosion of β phase which near AlMn phase was relatively lightly. Pitting corrosion occurred on the magnesium alloy with rare earth Y addition, matrix around rare earth phases was seriously corroded while β phase near rare earth phases was corroded lightly. Needle-like phases in AZ91 magnesium alloy after（La,Ce） misch metal addition didn’t promote corrosion of matrix. In-situ corrosion results indicate that AlMn phases and rare earth phases which contain rare earth Y have obviously micro-galvanic corrosion driving function, and promote corrosion of matrix around these phases, however, this phenomenon didn’t happened on neddle-like phases. Peak-Force KPFM mode of SPM was used to detect the potential difference of matrix and precipitated phases and results show, potential difference between matrix and β phase was between 140¹60mV; potential difference of AlMn phase and matrix was around 340mV; potential difference between rare earth phases which contain rare earth Y（Al₂Y,Al₂Y₃） and matrix was between 350⁴00 mV; potential difference between bulk phases and matrix in AZ91 magnesium alloy with（La,Ce） misch metal addition was around 400 mV, needle-like phases was around 410⁴50mV, rod-like phases was about 410 mV. According to the HRTEM results known that combination mechanism of matrix and β phase was semi-coherent, while the mechanism of bulk phases（AlMn、Al₂Y、Al₂Y₃、Al8Mn4Ce） and matrix was incoherent. However, needle-like phase in magnesium alloy with（La,Ce） misch metal addition combined matrix with semi-coherent mode. HRTEM results shown that effect of precipitated phases in corrosion process of AZ91 magnesium alloy not only depended on potential differences but also combination mechanism of phases: because of high potential difference between matrix and AlMn phases and incoherent combination mechanism which was unstable and contain high energy, matrix around AlMn phase was likely to corroded; although potential difference between matrix and needle-like phase was larger, combination mechanism was semi-coherent, interface was stable, interface energy was lower, matrix around needle-like phase was relatively difficult to corrosion.