Phase equilibria and solidification of Mg-rich Mg–Al–Zn alloys

doi:10.1016/j.msea.2006.02.006

Materials Science and Engineering: A

Volume 421, Issues 1–2, 15 April 2006, Pages 328-337

https://doi.org/10.1016/j.msea.2006.02.006 Get rights and content

Abstract

Phase equilibria in Mg-rich corner of Mg–Al–Zn system are analyzed in detail. Thermodynamic calculations are compared with literature data and own key experimental results by means of DSC and DTA measurements. The detailed comparison strongly supports the reliability of the selected thermodynamic description. Furthermore, our focus is placed on proper interpretation of experimental results obtained by thermal analysis. Based on thermodynamic calculation, it is clarified that a signal observed in thermal analysis, which was interpreted as end of solidification in the literature, is related to the start of the monovariant eutectic reaction L + (Mg) + γ-Mg₁₇Al₁₂ under non-equilibrium condition and the solidification process ends at lower temperature. This fact is supported by our microstructural observation.

Introduction

Phase equilibria of the Mg–Al–Mn–Zn quaternary system provide crucial information in development and designing of Mg-alloys like the significant AZ and AM series. The main goal of the series of present studies is to establish the thermodynamic description for the Mg–Al–Mn–Zn system on the basis of well assessed and properly interpreted experimental data. To this end, the thermodynamic description for each sub-ternary system as well as sub-binary system needs to be obtained, scrutinized or improved to compute the phase equilibria with high precision. The thermodynamic description for the Mg-rich corner of the Al–Mg–Mn system has been recently improved by the present authors [1]. In this paper, we focus on the Mg–Al–Zn ternary system.

The thermodynamic description of Mg–Al–Zn system has been reported by Liang et al. [2]. They performed the experimental investigation on the ternary solubilities of the Al–Mg and Mg–Zn phases as well as the homogeneity ranges of ternary compounds τ and ϕ phases by means of X-ray diffraction (XRD), differential thermal analysis (DTA), differential scanning calorimetry (DSC) and electron probe microanalysis (EPMA). Then, the thermodynamic description was obtained based on their experimental data and available literature data. They showed the comparison between the calculated and experimental results concerning invariant reaction temperatures, isothermal section at 608 K and some vertical sections, and the satisfying agreement was presented.

Since the focus of Liang et al. [2] was placed on the entire composition range, the detailed comparison regarding Mg-rich corner was not demonstrated and the reliability of their thermodynamic description for Mg-rich corner has not been fully discussed. For the practical application of Calphad approach to development of Mg-alloys, the accuracy and precision of the calculated phase equilibria are significant to know. In the present paper, we firstly demonstrate the detailed comparison between results calculated using the thermodynamic description of Liang et al. [2] and the experimental data, with a view to proving the reliability of the calculated phase equilibria. Our focus is on the composition range of Mg-rich corner, 0–30 wt.% Al and 0–30 wt.% Zn. One will see that the calculated results are quite consistent with the experimental data.

In the present study, subsequently, a particular attention is directed to the composition range, 0–10 wt.% Al and 0–3 wt.% Zn, which is quite important range from the point of view of industrial applicability of Mg-alloys. Since highly precise analysis on the phase equilibria in this composition range has not been carefully performed by means of current sophisticated equipment, we carried out own key experiments of DSC and DTA measurements with Ta crucible. A proper interpretation of the experimental result is in the focus of this discussion. From our measurement, it is demonstrated that the equilibrium solidification process can hardly be realized in Mg-rich alloys even with the cooling rate of 1 K/min. More importantly, one will see that the thermal signal observed at low temperature, which has been interpreted as end of solidification in the literature, is not the true solidus temperature and solidification process finishes at lower temperature.

Section snippets

Experimental data in literature

The experimental results for the entire Mg–Al–Zn system have been assessed by Petrov [3]. Also, the phase diagrams drawn in the experimental works have been compiled by Villars et al. [4]. As mentioned in Section 1, we focus on the composition range 0–30 wt.% Al and 0–30 wt.% Zn. In the following, the experimental works relevant to this composition range are briefly summarized.

Liquidus surface for Mg-rich corner has been investigated by means of thermal analysis in several works [5], [6], [7], [8]

Experimental procedure and results

The chemical compositions of Mg-alloys investigated in the present work are shown in Table 1. The compositions for the elements of our concern, i.e., Al and Zn, are denoted in bold letters. These alloys were prepared using following pure materials: Mg bars, min. 99.9 wt.% provided by Norsk Hydro Magnesiumgesellschaft mbH, Bottrop; Al bars, 99.98 wt.% produced by Hydro Aluminium High Purity GmbH, Grevenbroich and Zn rods, 99.98 wt.% obtained from the Harzer Zink GmbH, Goslar, all purities referred

Liquidus surface

The software “Pandat”¹ [15] and the thermodynamic descriptions of Liang et al. [2] were employed for all the calculations in this work. It is emphasized that none of the comparisons and data specific to Mg-rich alloys shown below are given in their work [2].

Fig. 1 demonstrates the calculated polythermal projection of liquidus surface with the alloy compositions

Conclusion

In the present study, focusing on Mg-rich alloys, we have discussed the reliability of thermodynamic description of the Mg–Al–Zn system [2]. The good agreement between the experimental and the calculated results strongly supports the reliability of the thermodynamic description. Also, we investigated solidification and melting of alloys in the practically important composition range by means of DTA and DSC measurements and compared the experimental results with thermodynamic calculations. The

Acknowledgements

The present authors are grateful to Prof. R. Ferro at the University of Genova for performing the comparative DSC measurements (DSC2). This study is supported by the German Research Foundation (DFG) in the Priority Programme “DFG-SPP 1168: InnoMagTec” under grant no. Schm 588/29.

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Phase equilibria and solidification of Mg-rich Mg–Al–Zn alloys

Abstract

Introduction

Section snippets

Experimental data in literature

Experimental procedure and results

Liquidus surface

Conclusion

Acknowledgements

Therm. Acta

Mater. Sci. Eng. A

Z. Metallkd.

Ternary alloys

Int. Z. Metallogr.

Nippon Kogyo. Kaishi