Microstructural examination of directionally solidified Mg alloys confirming Mg-rich phase equilibria of Mg-Al-Ca

The main purpose of the present study is to further confirm our recent developed thermodynamic modeling in the Mg-rich corner of the Mg-Al-Ca ternary system. The microstructure of directionally solidified Mg-rich Mg-Al-Ca alloys, including Mg-4Al-4Ca, Mg- 8Al-12Ca and Mg-12A1-12Ca, was examined by using SEM, TEM and EPMA. A ternary eutectic structure consisting of a(Mg), C14-Mg ₂Ca and C36-(Mg,Al)₂Ca was clearly identified. Such a ternary eutectic agrees with the thermodynamic modeling, which shows the existence of a type I eutectic invariant, Liquid → α(Mg) + C14 + C36. The microstructural evidence of this ternary eutectic played a critical role in establishing the Mg-rich phase equilibria of Mg-Al-Ca for the following two reasons. First, such a eutectic invariant was proposed in a recent experimental study, but the microstructure of the ternary eutectic was not shown. Second, a type II invariant, Liquid + C14 → α(Mg) + C36, was proposed in another recent study using DTA/DSC coupled with thermodynamic modeling, which upon our modification of the C36 model changes into the type I invariant. The present report provides additional microstructural evidence beyond our recent thermodynamic description and microstructure characterization described elsewhere.