Corrosion Behavior Of Extruded Mg-Zn-Y Alloys

Magnesium alloys have grate application prospect in aerospace, transportation and electronic communications because of high specific strength and specific stiffness, good damping performance and machining property. However, poor corrosion resistance have limited the wide application of magnesium alloys. In recent years, Mg-Zn-Y alloys has become a reserch hotpot due to its excellent mechanical properties at room and elevated temperature. Scholars in many countries have reserched the corrosion behavior of Mg-Zn-Y alloys, but the corrosion mechanical properties and stress corrosion of large deformation Mg-Zn-Y alloys have been reserched rarely. So this paper has prepared extruded Mg-6xZn-xY (x=0.5,0.75,1, at.%) alloys and extruded Mg-xZn-0.5Y(x=1,2,3, at.%) alloys. The effect of Zn and Y content on microstructures, mechanical properties and corrosion properties of Mg-Zn-Y alloys was studied, also the corrosion mechanical properties and stress corrosion mechanism of Mg-Zn-Y alloys was studied respectively. The conclusions of this paper are as follows:All the extruded Mg-6xZn-xY alloys comprise a-Mg matrix and I-phase particle, the volume fraction of I-phase increased with the increasing of Zn and Y contents, but the corrosion resistant of the alloys decreased. The a-Mg matrix surrounding the I-phase to be corroded preferentially, then on the one hand, corrosion extended toward the Mg matrix and formed corrosion pits. On the other hand, corrosion extended along the interface of I-phase and a-Mg matrix, until the formation of crack loop at the interface which caused the I-phase particles fall off from the substrate surface and formed corrosion pits.The corrosion residual strength of extruded Mg-6xZn-xY alloys present negative linearly attenuation law with the increasing of corrosion time in 5wt.% NaCl salt atmosphere. Because of the I-phase along the extrusion direction of Mg-6Zn-1Y alloys has better continuity, it could stop the extension of pitting corrosion and effectively reduce the maximum corrosion pit depth, slow down the decay rate of corrosion residual strength and elongation. Further analysis indicates that the corrosion residual strength of extruded Mg-6xZn-xY alloys is linearly dependent on the extreme depth of corrosion pit.The extruded Mg-3Zn-0.5Y alloys occurred stress corrosion cracking both in distilled water and 0.1mol/L Na2SO4 solution. Crack propagation in distilled water was intergranular,but occurred transgranular stress corrosion cracking in 0.1mol/L Na2SO4 solution. In both cases, the crack propagation mechanism was a form of hydrogen embrittlement.The second phases of extruded Mg-1Zn-0.5Y, Mg-2Zn-0.75Y and Mg-3Zn-0.5Y are W-phase, W-phase+I-phase, I-phase, respectively. The grain size and the second phase contents decreased with the decreasing of Zn contents, and the corrosion resistant strengthening correspondingly. The Mg-1Zn-0.5Y alloys owns senior yield strength of 268MPa and superior elongation of 20%.