Thermodynamic Analysis of Al-RE Phase Formation in AZ91-RE (Ce, Y, Gd) Magnesium Alloy

The Al-RE intermetallic compounds formed by adding rare earth elements into AZ magnesium alloy have important effects on the microstructure and properties of alloy. Based on classical thermodynamic theory and Miedema model and Toop model, the heat of formation of compounds in Mg-Al-RE (Ce, Y, Gd) ternary alloy and Gibbs free energy of Al-RE (Al4Ce, Al2Y, and Al2Gd) compounds are calculated theoretically, the order of formation of compounds in alloys and the formation process of Al-RE compounds are analyzed. The results show that, Al-RE (Al4Ce, Al2Y, and Al2Gd) compounds are preferentially formed in Mg-Al-RE (Ce, Y, Gd) alloys, Al-RE compounds are mainly formed before the crystallization of alpha-Mg solid solution; meanwhile, with the crystallization of alpha-Mg solid solution, Al-RE compounds can still be formed sustainably. The order of Al-RE compounds formation ability is Al4Ce, Al2Gd, and Al2Y, but the order of formation ability is Al2Gd, Al2Y, and Al4Ce, when adding the same mass fraction of rare earth (Ce, Y, Gd). In as-cast structure of AZ91-0.9 wt%; RE (Ce, Y, Gd) experimental alloys, Al-RE compounds mainly distributes at grain boundaries of alloys at two different scales, and the amount of beta-Mg17Al12 phase in alloys decreases relative to that of alloys without RE.