Formation of intermetallic phases during solidification in Al-Mg-Mn 5xxx alloys with various Mg levels

Department of Applied Science, University of Quebec at Chicoutimi, Saguenay (QC), Canada

^* Corresponding author: kun.liu@uqac.ca

Abstract

In the present study, four Al-Mg-Mn 5xxx alloys with different Mg levels (2-5 wt.%) were investigated for better understanding the evolution of intermetallic phases formed during solidification. Optical and scanning electron microscopes, electron backscattered diffraction and differential scanning calorimetry analyses in combination with thermodynamic calculation were used to identify various intermetallic phases. Results showed that the most dominant intermetallic phases are Al₆(Mn,Fe), α-Al(Fe,Mn)Si, Al₃Fe, Alm(Mn,Fe) and Mg₂Si in experimental Al-Mg-Mn alloys, which is greatly dependant on the Mg levels. It is found that Chinese script α-Al(Fe,Mn)Si is the dominant iron-rich intermetallic phase for the alloys containing 2-3 wt.% Mg, while blocky Al₆(Mn,Fe) and needle-like Al₃(Mn,Fe) become the major phases for the alloy containing 4 wt.% Mg. Further increasing Mg content to 5 wt. %, the dominant phase transfers to blocky Al₆(Mn,Fe) intermetallic. Meanwhile, the morphology of primary Mg2Si is changed from well-branched to plate-like with increasing Mg contents. In addition, β-Al₃Mg₂ and τ-Al₆CuMg₄ eutectic phases have been observed in the alloys with 3-5 wt. % Mg. A comparison on various intermetallic phases from the Scheil simulation and the actual as-cast microstructure is provided.