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JPEDAV (2007) 28:450–452 DOI: 10.1007/s11669-007-9137-3 1547-7037 ÓASM International

Al-Ca-Mg (Aluminum-Calcium-Magnesium) V. Raghavan

The early literature on this ternary system, which includes the work of [1958Cat], is summarized by [1995Vil], who presented partial isothermal sections for Mg-rich alloys at 400, 370, and 290 °C. A number of recent publications [1995Nin, 2001Ozt, 2003Ame, 2003Gro, 2003Ozt, 2003Tka, 2004Zho, 2005Isl, 2005Par, 2005Suz, 2005Zho, 2006Suz, 2007Alj] have appeared on this system with focus on Mg-rich alloys.

Binary Systems The Al-Ca system was experimentally reinvestigated by [2001Kev1] and assessed thermodynamically by [2001Kev2]. There are four intermediate phases in this system: Al4Ca (D13, Al4Ba-type tetragonal), Al2Ca (C15, MgCu2-type cubic), AlCa (or Al14Ca13; monoclinic, space group C2/m), and Al3Ca8 (Ca8In3-type triclinic, space group P 1). The Al-Mg phase diagram [2003Cze, Massalski2] has the following intermediate phases: Mg2Al3 (cubic, denoted b), e or R (rhombohedral), and Mg17Al12 (A12, aMn-type cubic, denoted c). The Ca-Mg phase diagram [1995Aga] has one intermediate phase Mg2Ca (C14, MgZn2-type hexagonal). A thermodynamic reassessment of this system was reported recently by [2006Zho].

Ternary Phases There are several reports on the existence of a C36, MgNi2-type hexagonal ternary phase along the Al2CaMg2Ca join [2003Ame, 2005Suz, 2005Zho, 2006Suz].

[2003Ame] found a homogeneity range from CaAl1.34 Mg0.66 to CaAl0.93Mg1.07 and the corresponding lattice parameter ranges of a = 0.5835-0.5935 nm and c = 1.88971.9258 nm. However, [2006Suz] reported a stoichiometry close to CaAl1.33Mg0.67. The existence of the C36 phase appears to be established and the absence of this ternary phase in the earlier reports is probably due to the difficulty of distinguishing the x-ray patterns of the C14 and C36 phases. The existence of a second ternary phase Al2(Mg, Ca) with Mg concentration range of 17.5-22.5 at.% was reported by [2005Zho]. This phase needs confirmation.

The Liquidus Surface Calculated liquidus projections for the entire composition range of this ternary system presented by [2003Gro] and [2005Isl] are in agreement, but both calculations ignored the existence of the ternary C36 phase. Partial experimental liquidus projections for Mg-rich alloys were presented by [2003Tka] and [2005Suz]. The primary crystallization of the C36 phase was not reported by [2003Tka]. The partial projection from [2005Suz] is shown in Fig. 1. The phases of primary crystallization in the Mg-rich alloys are (Mg), c, C14, and C36. Among the computed liquidus projections [2001Ozt, 2003Gro, 2003Ozt, 2004Zho, 2005Isl], only the projection by [2004Zho] shows a primary field of the C36 phase. Figure 2 and 3 show the partial isothermal sections at 500 and 400 °C determined by [2006Suz]. At 500 °C (Fig. 2), the C36 phase forms tie-lines with C14 (Mg2Ca), C15 (Al2Ca),

Fig. 1 Al-Ca-Mg partial liquidus projection [2005Suz]

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Journal of Phase Equilibria and Diffusion Vol. 28 No. 5 2007

Phase Diagram Evaluati