RARE METALS Vol. 25, No. 5, Oct 2006, p . 441

Thermodynamic database of the phase diagrams in the Mg-Al-Zn-Y-Ce system

LIU Xingjun'), WANG Cuiping", WEN Mingzhong'), CHEN Xing", and PAN Fusheng" 1 ) Department of Materials Science and Engineering, Xiamen University, Xianien 361005, China 2) College of Materials Science and Engineering, Chongqing University, Chongqing 400045, China

(Received 2006-06-25)

Abstract: The Mg-Al-Zn-Y-Ce system is one of the key systems for designing high-strength Mg alloys. The purpose of the present article is to develop a thermodynamic database for the Mg-Al-Zn-Y-Ce multicomponent system to design Mg alloys using the calculation of phase diagrams (CALPHAD) method, where the Gibbs energies of solution phases such as liquid, fcc, bcc, and hcp phases were described by the subregular solution model, whereas those of all the compounds were de- scribed by the sublattice model. The thermodynamic parameters describing Gibbs energies of the different phases in this database were evaluated by fitting the experimental data for phase equilibria and thermodynamic properties. On the basis of this database, a lot of information concerning stable and metastable phase equilibria of isothermal and vertical sections, mo- lar fractions of constituent phases, the liquidus projection, etc., can be predicted. This database is expected to play an im- portant role in the design of Mg alloys.

Key words: thermodynamic database; CALPHAD method; phase diagram; Mg-Al-Zn-Y-Ce system; magnesium alloys

[This study was financially supported by the National Natural Science Foundation of China and Chongqing Science and Technology Commission.]

1. Introduction Magnesium alloys are potential candidates for

structural, automotive, and aerospace applications owing to their lowest density and remarkably high specific strength [l-31. Rare-earth elements Y and Ce are added to Mg alloys to improve the high tem- perature strength, creep resistance, and heat resis- tance by precipitation hardness [4]. The material de- sign is necessary because the precipitation sequence is almost always complex in multicomponent mag- nesium alloys.

The calculation of phase diagrams (CALPHAD) method is a powerful tool for designing materials because it helps not only to calculate the phase equilbria in multicomponent systems but also to simulate the phase consistency and solidification process of individual alloys [5]. The CALPHAD method provides a clear guideline for such selections

and helps to avoid long-term experiments. Thus, it could be a powerful tool that would help reduce cost and save time during the development of Mg alloys.

The purpose of the present article is to construct a thermodynamic database of the phase equilibria of the Mg-Al-Zn-Y-Ce system on the basis of the CALPHAD method.

2. Thermodynamic models The Gibbs energies of the liquid, fcc, bcc, hcp,

and other solid-solution phases are described by the subregular solution model using the Redlich-Kister formula [6]. For instance, the Gibbs energy of a phase in an A-B-C ternary system is expressed as G = C O G ? X , + R T C x , I ~ x , + L ~ ~ x ~ x ~ +

i = A , B C c = A . B , C

LtcxAxc + &xBxC + LkcxAxBxC (1) where "GI is the Gibbs energy of pure component i in the respective reference state, xi is the mole frac-

Corresponding author: LIU Xingjun E-mail: lxj@xrnu,edu.cn

442 RARE METALS, Vol. 25, No. 5, Oct 2006

tion of component i, and and the tempera- ture and composition-dependent interaction energies in the binary and ternary systems, respectively.

In the Mg-Al-Zn-Y-Ce system, there are many intermetallic compounds in the binary and ternary systems. Their Gibbs energies are described by the sublattice model [7]. The Gibbs energy of the pure component i, in its different phase states is obtained from the SGTE database [ 81.

On the basis of the experimental information in some ternary systems, solubilities of the third ele- ments in the intermetallic compounds are considered in the present assessment.

3. Thermodynamic database The present thermodynamic database contains the

thermodynamic parameters for calculating the phase equilibria in binary and ternary alloys in the Mg-Al-Zn-Y-Ce system. The thermodynamic pa- rameters for describing the Gibbs energy of each phase are evaluated by combining thermodynamic

models and the experimental data, including phase equilibria and thermodynamic properties. Regarding the present database, there are some important alloy systems for which there is little or no experimental data.

Experimental studies to determine phase equilib- ria were carried out to obtain a better estimation of the thermodynamic parameters, with good agree- ment between the calculated and experimental data. The thermodynamic parameters were evaluated by Thermo-Calc software, which was originally devel- oped by Sundman et al. [5]. The information on the thermodynamic assessments and experiments in the binary and ternary systems is summarized in Tables 1 and2.

The present thermodynamic database can provide a lot of information such as stable and metastable phase equilibria, phase fraction, and various thermo- dynamic quantities such as activity, mixing enthalpy, Gibbs energy of formation, driving forces for phase transformation, and simulation of solidification.

Table 1. Information on thermodynamic assessment in binary systems

Table 2. Information on experimental data and thermodynamic assessment in ternary systems

Liu X.J. ef al., Thermodynamic database of the phase diagrams in the Mg-Al-Zn-Y-Ce system

Continued Table 2

4. Examples of calculation 4.1. The Mg-Al-Zn-Y system

In this quaternary system, the phase equilibria in the Mg-Al-Zn and Mg-Zn-Y ternary systems have been assessed by Liang et al. [2S] and Shao et al. [35], respectively. The thermodynamic assessments of the A1-Zn-Y and Mg-Al-Y systems were carried out by our group [22, 301. Figs. 1 and 2 show the assessed phase diagram and Gibbs energy of forma- tion of the compounds in the Zn-Y binary system, where the calculated results are in good agreement

A Mason et a/. [43

Y 20 40 60 80 Zn J at.%

Fig. 1. Calculated phase diagram of the Zn-Y system compared with experimental data.

443

with the experimental data [42-431. The thermody- namic parameters in the AI-Zn-Y system have been evaluated on the basis of the experimental data [40]. The isothermal section at SOOOC with a comparison of the experimental data are calculated and shown in Fig. 3, where no ternary compounds are found, and A1 is soluble in the YZn compound. On the basis of the assessed thermodynamic parameters, the liquid projection of the A1-Zn-Y system is predicated (Fig. 4), where there are three eutectic reactions and twelve peritectic reactions.

\ ~ 3 . Chi& e l ul. [42]

Y 20 40 60 80 Zn Zn i at.%

Fig. 2. Calculated Gibbs energy of formation at 773 K compared with experimental data.

444 RARE METALS, Vol. 25, No. 5, Oct 2006

Y B Three phase region Y A Three phase region 3 Two phase region

A1 J at.(?”

Fig. 3. The calculated isothermal-section of the Al-Zn-Y ternary system at 500OC with experimental data [271.

Y

Y

Z n h c p 20 40 60 80 fcc A1 Al I at.%

Fig. 4. Calculated liquidus-surface of the AI-Zn-Y ter- nary system.

In the Mg-Al-Y ternary system, the isothermal section at 400°C and the Mg2A13-A12Y and T-YA~, vertical sections were determined by differential thermal analysis, X-ray diffraction, and metallogra- phy [29]. Fig. 5 shows the calculated isothermal sec- tion at 400°C with the experimental data. It is seen that A1 has a higher solubility in the MgY and Mg2Y compounds, and Mg in the A12Y compound, but no solubility of Mg in the AlY compound, and there is a ternary compound (T phase, A4MgY) in the Al-rich region. The calculated vertical section at Mg&-Al2Y section is shown in Fig. 6, where the detailed phase equilibria are given at lower tem- peratures.

4.2. The Mg-Al-Zn-Ce system In the Mg-Al-Zn-Ce quaternary system, the

thermodynamic assessments of the Al-Zn-Ce and

o Two phase region

a 7 phase 80

1

Al I at.% Fig. 5. Calculated isothermal-section of the Mg-AI-Y ternary system at 4OOOC with experimental data [27].

2 - 900

E

3 g 600

300 Y 0.00 5 10 15 20 25 30 Y 33.3

Mg 40.0 Y I at.% Mg 0.00 Al60.0 A1 66.7

Fig. 6. Calculated vertical section at the Mg&-AI2Y region with experimental data [29].

Mg-Zn-Ce systems were carried out by this study group [23, 381. Figs. 7 and 8 show the assessed phase diagram and Gibbs energy of formation of the compounds in the Zn-Ce binary system, where the calculated results are in good agreement with the experimental data [44]. Fig. 9 shows the calculated isothermal section at 320°C in the Al-Zn-Ce ternary system compared with the experimental data [41]. In this system, the complete thermodynamic assess- ment cannot be carried out because only a part of the experimental data of the isothermal section at 320°C

Liu X.J. et al., Thermodynamic database of the phase diagrams in the Mg-Al-Zn-Y-Ce system 445

is available. Further experiments should be carried out in the future.

1100 r-----l

Ce 20 40 60 SO Zn Zn I at.%

Fig. 7. Calculated phase-diagram in the Zn-Ce sys- tem with experimental data [44].

Zn I at.%

Fig. 8. Calculated Gibbs energy of formation at 973 K for the Zn-Ce system compared with experimental data [441.

The thermodynamic parameters in the Mg-Zn-Ce system were evaluated on the basis of the experi- mental data including the isothermal vertical sec- tions [36, 371, where there are at least four ternary compounds and a higher solubility of Zn in the CeMg3 and CeMgl;? compounds. In addition, the continuous line compound from the CeZn com- pound to the CeMg compound because these two compounds have the same structure.

Ce o Single phase regon

Two phase region Ce, Three phase region

I A1 I at.%

Fig. 9. Calculated isothermal-section of the Al-Zn-Ce system at 32OOC with experimental data [41].

5. Summary

A thermodynamic database of the Mg-AI- Zn-Y-Ce system was basically developed on the ba- sis of the CALPHAD method. Necessary experi- ments should be carried out for the thermodynamic assessments of the A1-Y-Ce and Zn-Y-Ce systems. This database should prove to be a powerful tool for developing new Mg alloys.

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