The solidification behaviour of Mg-Al-rare earth alloys

Solidification paths and as-cast microstructures were determined for the Mg-Al-MM (misch metal) alloys: AE21, AE41 and AE91. Two types of interdendritic phase were identified using analytical electron microscopy. These were Al4MM and Al12Mg17. Only Al4MM was found in AE21 while both phases were found in the other two alloys. Most of the solidification in AE21 takes place in a rather narrow solidification gap (∼40°C), while this gap is much greater (∼150°C) in the higher Al alloys. This combined with the large volume fraction of interdendritic eutectic phases explains the good castability of AE91.