Elsevier

Intermetallics

Volume 24, May 2012, Pages 22-29
Intermetallics

Computational phase equilibria and experimental investigation of magnesium–aluminum–calcium alloys

https://doi.org/10.1016/j.intermet.2012.01.001Get rights and content

Abstract

Mg–Al–Ca alloys provide excellent creep resistance and castability for elevated temperature applications. Computational thermodynamics calculations and experimental investigation of the Mg–Al–Ca ternary system have validated the earlier development of creep-resistant AX52 (Mg–5Al–2Ca1) and AX53 (Mg–5Al–3Ca) alloys. The Scheil simulation of alloy solidification has suggested key guidelines of alloy composition design in promoting the thermally stable (Mg,Al)2Ca phase, while replacing the less stable Mg17Al12 in the microstructure. The suppression of the Mg17Al12 phase can increase the solidus temperature, reduce the freezing range, and increase the latent heat during solidification, all of which contribute to improved castability in the AX53 and AX52 alloys compared with the AX51 (Mg–5Al–1Ca) alloy. The quantitative phase equilibrium data, microstructure characterization, as well as the thermal physical properties of the Mg–Al–Ca alloy system generated from this study are important basis for further optimization of alloy composition and microstructure for elevated temperature applications.

Highlights

► New understanding in the phase equilibria of Mg–Al–Ca system. ► Clarified the role of ternary (Mg,Al)2Ca phase in creep of Mg–Al–Ca alloys. ► Calculated Ca/Al composition ranges suppressing the formation of Mg17Al12 phase. ► Scheil simulation results agree with high-pressure die casting microstructure.

Introduction

Magnesium alloys have significant mass saving potential compared with other automotive materials such as advanced high-strength steel (AHSS), aluminum alloys, polymers, and glass fiber-reinforced polymers (GFRP) on the basis of equal stiffness or strength [1]. Additionally, magnesium castings can offer part consolidation and better dimensional accuracy compared with fabricated and joined steel or aluminum components [2]. There are currently two major alloy systems, Mg–Al–Zn (AZ) and Mg–Al–Mn (AM), for automotive magnesium casting applications [3]. AZ91 (Mg-9%Al-1%Zn) is used for non-structural parts like brackets, covers, cases and housings that are strength-dominated and exposed to ambient temperature. For semi-structural applications in secondary load paths, e.g., instrument panels, steering components, radiator supports and closure, AM50 (Mg-6%Al-0.3%Mn) or AM60 (Mg-6%Al-0.3%Mn) alloys offer unique advantages due to their higher ductility (10–15% elongation) and higher impact strength compared with die cast A380 aluminum alloy [1].

Neither of these two alloys can be used for temperatures above about 125 °C because of their inadequate creep performance. Thus, applications such as automatic transmission cases which can operate at up to 175 °C, engine blocks; up to 200 °C, and engine pistons; up to 300 °C, have been designed with the AE (Mg–Al–RE) alloys that contain expensive rare earth (RE) elements [4]. As a result, the use of magnesium in applications subject to elevated temperatures has been limited.

Mg–Al–Ca alloys (hereafter referred to as AX alloys) were developed as a lower cost alternative to the AE alloys [5]. Subsequently, Sr-containing AX alloys (hereafter referred to as AXJ) were developed [6], [7] that demonstrated creep resistance and elevated temperature tensile properties approaching that for A380 aluminum alloy, which is currently used in many powertrain applications such as transmission cases. The specific mechanism that provided the creep resistance in the Mg–Al–Ca based alloys was attributed to the thermal stability of the Al2Ca [5] or (Mg,Al)2Ca [7] in the microstructure. This was unlike, for example, in the AZ alloys, where the low eutectic (437 °C) Mg17Al12 phase softens at elevated temperatures above 125 °C [4], resulting in a dramatic reduction in creep resistance by promoting grain boundary sliding [8]. The other mechanism, in addition to grain boundary sliding, that limits creep performance of magnesium alloys is diffusion-controlled dislocation climb as a result of accelerated diffusion of solute aluminum in the magnesium matrix and the self-diffusion of magnesium at these elevated temperatures [4]. Either or both of these mechanisms can be controlling depending on the alloy composition, microstructure and operating temperature and stress [7].

Since the discovery of the AX alloys, there have been sustained efforts to enhance the creep performance of these lower cost alloys by controlling the amount and morphology of the various desirable phases that resist the operating creep mechanisms at the service temperatures. Possible approaches for improving creep resistance in magnesium alloys that are also castable include: (1) suppressing the formation of the Mg17Al12 phase that is susceptible to softening at elevated temperatures; (2) pinning grain boundary sliding via the formation of thermally stable intermetallic phases; and (3) slowing solute diffusion in the magnesium matrix by introducing solid solution elements [4].

Recent research [9], [10], [11], [12], [13] has generated some new understanding in the phase equilibria and microstructure in the AX alloy system. However, it is still not clear what intermetallic phases, Mg2Ca, Al2Ca, (Mg,Al)2Ca and Mg17Al12, dominate the as-cast microstructure and mechanical properties of Mg–Al–Ca alloys, in relationship to the alloy composition and casting conditions. Particularly, no quantitative information has been reported on these intermetallic phases, which is critical to further optimization of AX or AXJ alloys. In this study, the ternary Mg–Al–Ca system is revisited using a CALPHAD (CALculation of PHAse Diagrams) software Pandat 8.1 by CompuTherm (Madison, WI) [14] and the latest magnesium database PanMg8 developed by Clausthal University of Technology (Clausthal, Germany) over the last decade [15]. The calculated phase diagrams will elucidate quantitatively the phase constituents produced for the various solidification paths, and allow correlations to mechanical properties, castability and corrosion resistance of these Mg–Al–Ca alloys. These fundamental correlations will provide a better metallurgical understanding of the Mg–Al–Ca alloy system, which will be used to further optimize the alloy composition and microstructure for the various engineering attributes mentioned above.

Section snippets

Materials and casting

Commercial magnesium alloy AM50 (Mg-5%Al-0.3%Mn) was used as the base alloy. Three Mg–Al–Ca alloys, designated as AX51 (Mg-5%Al-1%Ca), AX52 (Mg-5%Al-2%Ca) and AX53 (Mg-5%Al-3%Ca), respectively, were prepared in a steel crucible by adding pure calcium (99% purity) to the AM50 melt. Tensile specimens (6 mm diameter and 25 mm long) were die cast at 677 °C from these four alloys in a 700-ton Lester cold-chamber machine. Chemical analysis using ICP/AES (Inductively-Coupled Plasma/Atomic Emission

Thermodynamic calculations

The CALPHAD approach is based on the thermodynamic description of an alloy system, which denotes a set of thermodynamic parameters for all phases in the system. In a ternary alloy system, the phases of interest are solid, liquid and intermetallic phases. The Gibbs energy per mole of a liquid or a substitutional solid solution is [16],Gmϕ=ixiGiϕ+RTixilnxi+ΔxsGmϕ

The first term on the right-hand side (RHS) of Eq. (1) is the Gibbs energy of the component elements in the reference state at a

Microstructure

Fig. 7 shows typical optical as-cast microstructure of die cast samples of Mg–Al–Ca alloys (AX51, AX52 and AX53), compared with the baseline alloy AM50. All samples show a large amount (about 1–2% based on image analysis) of gas porosity (large black pores in the micrographs), which is due to gas entrapment during the high-speed injection in the die casting process. However, it was impossible to distinguish the second phases in the microstructure in image analysis, so only the total fractions

Conclusions

  • 1.

    Computational thermodynamics calculations and experimental investigation of the Mg–Al–Ca ternary system have confirmed the earlier findings for the Mg–Al–Ca based alloys containing about 5% Al and 2–3% Ca, i.e., AX52 (Mg-5Al-2Ca) and AX53 (Mg-5Al-3Ca) alloys. The excellent creep resistance of these alloys is due to the formation of the hexagonal (Mg,Al)2Ca phase in the casting microstructure. The (Mg,Al)2Ca phase has both a high eutectic temperature (517 °C) and melting point (710 °C),

Acknowledgments

The authors acknowledge Dr. Chuan Zhang of CompuTherm (Madison, Wisconsin) for many helpful discussions and Mr. M. Balogh of GM R&D for TEM analyses.

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    All compositions in wt.% except otherwise stated.

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