Phase equilibria in the Mg-Al-Ca ternary system at 773 and 673 K
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I. INTRODUCTION
MAGNESIUM alloys are attractive to the automotive industry, with their low densities compared with aluminum alloys and steel.[1] However, the application of magnesium alloys has been limited to the components that are not exposed to high-temperature environments, e.g., cross-car instrument panel beams and steering wheels. Recently, automotive and magnesium industries have been making great effort to develop creep-resistant magnesium die-cast alloys for structural powertrain components, such as engine blocks and transmission cases.[2,3,4] The Mg-Al–based die-cast alloys with additions of Ca, Sr, and RE (RE: rare earth mischmetal consisting of Ce, La, Nd, and Pr) have been reported to exhibit high creep-resistance, for example, AXJ530 (Mg-5Al-3Ca0.15Sr, wt pct), AJ62X (Mg-6Al-2Sr, wt pct), and AE44 (Mg-4Al-4RE, wt pct).[5–10] The as-cast microstructures of the previously described alloys consist of dendritic -Mg solid-solution phase and intermetallic compound(s) in the interdendritic near-grain boundary regions. For die-cast alloys that are to be used in the as-cast form, the microstructural stability during elevated temperature service is important, especially in the vicinity of grain boundaries, since the creep resistance is thought to be significantly affected by microstructural stability.[11–14] Therefore, a fundamental understanding of solidification paths and phase stability is essential for alloy development. Recently, we investigated the solidification paths in the Mg-Al-Ca ternary system and determined the liquidus projection.[15] In alloys in the -Mg solidification region, intermetallic compounds are formed by eutectic reactions during solidification. The eutectic compounds are Mg2Ca (C14, hexagonal), (Mg, Al)2Ca (C36, dihexagonal), and -Mg17Al12 (A12, cubic) phases in the order of increasing Al content. There are two invariant reactions, L : C14 C36 and L C36 : . In the experimentally determined liquidus projection, the C36 phase has a wide primary solidification region adjacent to the -Mg solidification region, and this phase is the main eutectic compound in Mg-5Al3Ca, which is a base composition of AXJ530. However, durA. SUZUKI, Research Fellow, N.D. SADDOCK, Graduate Student, and J.W. JONES and T.M. POLLOCK, Professors, are with the Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109. Contact e-mail: [email protected] Manuscript submitted September 2, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
ing annealing of AXJ530 and AX53 (Mg-5Al-3Ca) diecast alloys at 573 K, the C36 phase transforms to the Al2Ca phase with C15 structure.[15,16,17] Since there are few experimental reports on the existence of the C36 phase in the ternary system,[18] the range of compositional stability of the C36 phase and the causes for instability of the C36 phase at 573 K are problems that need clarification. Phase equilibria in the Mg-Al-Ca ternary system have been investigated by experimental methods and via thermodynamic modeling.[18–22] In t
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