Experiments coupled with modeling to establish the Mg-rich phase equilibria of Mg–Al–Ca

Determining whether a four-phase equilibrium in the Mg-rich corner of the Mg–Al–Ca system is a type I (eutectic: L → α(Mg) + C14–Mg2Ca + C36–(Mg, Al)2Ca) or type II invariant (L + C14–Mg2Ca → α(Mg) + C36–(Mg, Al)2Ca) has been rather challenging. One recent experimental study proposed a type I eutectic invariant without a micrograph confirming the ternary eutectic. But another study using DTA/DSC coupled with thermodynamic modeling suggested a type II invariant. By modifying the C36 model of the second study, we obtained a thermodynamic description that not only showed the ternary eutectic but also accounted for experimental data in both studies. Based on the results of Scheil simulation for alloys with either α(Mg) or C36 as the primary solidification phase, we identified three alloys for directional solidification experiments and their microstructure confirmed the ternary eutectic of α(Mg), C14 and C36. These phases were identified by SEM, EPMA, and TEM.