Intergranular microstructure in magnesium based die cast AE alloys

A systematic investigation of the microstructures of a series of magnesium based die cast magnesium–aluminium–rare earth (RE) alloys has been performed using X-ray diffraction analysis, scanning electron microscopy and transmission electron microscopy. The alloys had an aluminium content of 3–4 wt- and RE content of between 0·6 and 3·5 wt-. The alloys were studied in the as cast condition and after aging at 200 and 250°C. Three kinds of binary aluminium–RE phases were found in the alloys existing intergranularly. Al₁₁RE₃ was the predominant intergranular phase. In addition, Al₃RE and Al₂RE phases were found in the alloys with high RE content. The Al₃RE phase became unstable when the alloy was aged at 200°C, while the Al₁₁RE₃ phase remained stable at 250°C. In alloys with a RE/aluminium weight ratio of <0·6, primary Al₁₂Mg₁₇ and aluminium enriched zones existed at the grain boundaries beside the Al₁₁RE₃ phase, both of which are considered to contribute to the reduction in creep resistance. The aluminium enriched zone transformed into Al₁₂Mg₁₇ precipitates when the alloys were aged. Therefore, to achieve improved creep resistance in Mg–Al–RE alloys, the RE/Al ratio should exceed 0·6.