Effect of Extrusion Direction on Microstructure and Corrosion Behavior of Rare Earth doped AZ91 Alloy

By studying the changes in the grain size of alpha-Mg matrix, the distribution and size of the intermetallic phases in the transverse and longitudinal sections of the extruded AZ91 alloy with 1.0% (La, Ce) mischmetal (MM) addition, in combination with the corrosion behavior on the transverse and longitudinal sections, the corrosion mechanism of them caused by the change of microstructure was revealed. The results showed that a grain refinement and size reducing in beta-Mg17Al12 and Al-4(La, Ce) phases had been found in extruded alloy. The relative potential difference of the intermetallic phases was influenced by extrusion process, the one between the Al4(La, Ce) phase and Mg matrix in the longitudinal section of the extruded alloy is the highest, but Al-4(La, Ce) phase was a weak cathode. The decrease in the size of beta-Mg17Al12 reduced the area of the cathode of the micro-galvanic coupling, the change of the size of Al-4(La, Ce) phase has no remarkable effect on the micro-galvanic corrosion behavior. Corrosion morphologies were varied with difference in extrusion direction, the corrosion morphology in the transverse section is mainly pitting corrosion caused by the cathode phases of beta-Mg17Al12 and individual Al-4(La, Ce). The corrosion morphology in the longitudinal section are characterized by corrosion streamline parallel to extrusion direction, the corrosion occurs at the grain boundaries of the recrystallized grain.

Automated summary

The study revealed the corrosion mechanism of extruded AZ91 alloy with 1.0% (La, Ce) mischmetal addition by investigating the microstructural changes and corrosion behavior. The decrease in the size of intermetallic phases after extrusion influenced the corrosion morphology, with transverse section exhibiting mainly pitting corrosion and longitudinal section showing corrosion along grain boundaries.