Intergranular microstructure in magnesium based die cast AE alloys

Abstract

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.

Keywords:

• Die cast magnesium–aluminium–rare earth alloys
• Intergranular phase constitutions
• Al2RE
• Al3RE

This research was partly financially supported by TFR, Sweden, and Malmö University, Sweden. The authors thank Norsk Hydro for providing test bars of the studied alloys and G. Carpenter and L. Benkis, MTL/CANMET, Ottawa, Ont., Canada, for providing the ‘Difpat’ program for solving electron diffraction patterns.