Thermodynamics of phase formation in Mg-La-Ce-Nd alloys

摘要

Experimentally validated thermodynamic descriptions have beendeveloped for the ternary Mg-La-Ce, Mg-La-Nd, and Mg-Ce-Ndsystems by selecting key alloys in both systems and analyzing thephase formation in both the as-cast and heat treated state bySEM/EDS and DSC. These results were combined to form thevalidated thermodynamic Calphad-type description for quaternaryMg-La-Ce-Nd alloys. It is shown that for these light rare earthelements (La, Ce, Nd) the intermetallic phases with Mg exhibitsignificant mutual solid solubility in the ternary systems,extending into the quaternary alloy system. This is reflected byconsidering the shared crystal structures in the thermodynamicmodeling. Simulated solidification paths of three Mg-La-Ce-Ndalloys with different La:Ce:Nd ratios and a common total contentof 5 wt.% rare earth (RE) metals are evaluated usingcomputational thermodynamics. Unexpected and distinctlydifferent solidification behavior of these three alloys is revealed.The sequence La→Ce→Nd in the periodic table is not at allreflected in a monotonous solidification behavior. Thedemonstrated individual impact of each of these elements forbidstreating the RE additions as a mere wt.% sum of RE elements.