Experimental investigation and thermodynamic calculation of phase relations in the Mg–Nd–Y ternary system

Experimental investigation and thermodynamic calculation of phase relations in the Mg–Nd–Y ternary system

Materials Science and Engineering A 454–455 (2007) 266–273 Experimental investigation and thermodynamic calculation of phase relations in the Mg–Nd–Y...

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Materials Science and Engineering A 454–455 (2007) 266–273

Experimental investigation and thermodynamic calculation of phase relations in the Mg–Nd–Y ternary system F.G. Meng, J. Wang, H.S. Liu, L.B. Liu, Z.P. Jin ∗ School of Materials Science and Engineering, Central South University, Changsha, Hunan 410083, People’s Republic of China Received 18 May 2006; received in revised form 3 November 2006; accepted 7 November 2006

Abstract Mg–Nd–Y alloys ingots were cast and annealed at 753 K, and then analyzed with scanning electron microscopy (SEM), X-ray diffraction (XRD) and electron-probe microanalysis (EPMA) to determine phase equilibria in the Mg-rich part of the Mg–Nd–Y system. Existence of the ternary compound ␤ with Mg5 Gd-type structure was verified, which is in the equilibrium with hcp A3 (Mg). Furthermore, thermodynamic description of the Mg–Nd–Y system was carried out on the basis of the present experimental results. The thermodynamic parameters of the boundary Mg–Nd and Nd–Y binary systems were directly cited from literatures, while most of the parameters for the Mg–Y binary system were taken from the latest literature with an exception of a minor revision for the bcc B2 phase in this paper. The ordered bcc B2 and disordered bcc A2 phases were described with a sublattice model (Mg,Nd,Y)0.5 (Mg,Nd,Y)0.5 and their Gibbs energies were expressed with same function. Most of the binary intermetallic phases, except for Mg3 Nd, were assumed to have no ternary solubility. The Mg3 Nd and ternary compound ␤ were treated as semistoichiometric compounds with mutual substitution between Nd and Y. Reasonable agreement of the phase equilibria in the Mg–Nd–Y ternary system between thermodynamic extrapolation and experiments was achieved. © 2006 Elsevier B.V. All rights reserved. Keywords: Experimental investigation; Mg–Nd–Y; Phase diagram; Thermodynamic calculation

1. Introduction Magnesium alloys have been attracting many attentions in recent years due to their properties such as low density, high specific strength, good castability, etc. [1]. As important elements, rare earths are often added in Mg–base alloys to further improve on corrosion-resistance and castability. For example, WE54 (Mg–5.0–5.5 wt% Y–1.5–2.0 wt% Nd–1.5–2.0 wt% heavy rare earths–0.4 wt% Zr) and WE43 (Mg–4 wt% Y–2.25 wt% Nd–0.6 wt% Zr) have become relatively successful magnesium alloys [2]. However, the mechanism why rare earths can improve the properties of Mg–base alloys has not been clearly identified. To improve our understanding about the precipitating process and design alloy compositions, knowledge of phase diagrams and thermodynamic data of the involved systems are crucially necessary. Investigations were made in the Mg-rich corner of the Mg–Nd–Y system at 773 and 573 K by Sviderskaya and Padezhnova [3] by optical micrograph. Three phases Mg9 Nd, Mg24 Y5 and (Mg) were claimed but their structures were not ∗

Corresponding author. Fax: +86 7318876692. E-mail address: [email protected] (Z.P. Jin).

0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.11.048

determined [3]. Recently, a stable ternary fcc phase ␤ isomorphous with the Mg5 Gd was found to be in equilibrium with the solid solution of (Mg) after aging for a long time aging at 250 ◦ C in the Mg–Nd–Y alloys and all commercial WE types alloys [4–8]. Until now there is little other information about phase equilibria in the Mg–Nd–Y ternary system. In this paper, experimental investigation of the phase relations in the Mg-rich part and a thermodynamic description of the Mg–Nd–Y system are to be presented. 2. Experimental investigation 2.1. Experimental procedure Ten ternary alloys (>70 at.% Mg) were prepared with starting materials Mg, Nd, and Y of purity (99.9%), (99.5%) and (99.5%), respectively. The samples, each with a total weight of approximately 20 g, were enclosed in a sealed steel crucible in pure argon and melted at 1053 K for 2 h. During melting, the crucible was shaken in order to improve composition homogenization of the melt. Subsequently, the crucible was dropped into cold water for rapid cooling. All the quenched alloys were annealed at

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Table 1 Constituent phases and compositions of alloys annealed at 753 K for 2160 h Alloys

Nominal composition (at.%)

Phases identified by XRD

Phases distinguished by BSE and EPMA

Phase composition (at.%)–EPMA data Mg

Nd

Y

1

96Mg1Nd3Y

(Mg) ␤

(Mg) ␤

97.0 84.3

0.4 6.8

2.6 8.8

2

92Mg6Nd2Y

(Mg) Mg41 Nd5

(Mg) Mg41 Nd5

97.6 89.4

0.6 9.1

1.8 1.4

3

94Mg3Nd3Y

(Mg) ␤ Mg41 Nd5

97.4 84.6 89.2

0.5 8.5 9.2

2.0 6.7 1.6

4

91Mg5Nd4Y

(Mg) ␤ Mg41 Nd5

97.1 85.6 89.8

1.0 7.6 8.7

1.9 6.8 1.5

5

91Mg1Nd8Y

(Mg) ␤ Mg24 Y5

96.5 86.4 88.0

0.6 3.9 1.2

2.8 9.7 11.6

6

90Mg2Nd8Y

(Mg) ␤ Mg24 Y5

96.3 85.4 87.1

0.4 5.0 1.6

3.2 9.7 11.3

7

83Mg3Nd14Y

␤ Mg24 Y5

␤ Mg24 Y5

83.4 83.9

3.1 1.1

13.5 14.9

8

79Mg7Nd14Y

Mg3 Nd ␤ Mg2 Y

Mg3 Nd ␤ Mg2 Y

81.6 78.2 74.2

6.3 9.3 2.7

12.1 12.5 23.1

9

86Mg9Nd5Y

␤ Mg41 Nd5

84.6 89.5

9.5 9.3

5.9 1.2

10

75Mg13Nd12Y

␤ Mg3 Nd

83.0 79.2

9.7 14.5

7.3 6.3

␤ Mg3 Nd

753 K for 2160 h, and then water quenched. Phase identification was carried out by X-ray diffraction (XRD) (Dmax-2500VBX) with Cu K␣ diffraction, 40 kV, 250 mA, 0.02 space and 4◦ takeoff angle and electron-probe microanalysis (EPMA) on a JEOL JXA-8800R (Japan Electron Optics Ltd., Tokyo, Japan) microprobe using a 20 kV voltage and 20 nA current. 2.2. Experimental results and discussion The constituent phases and their compositions in the selected alloys after treated are listed in Table 1. Largely extended homogeneities along Nd to Y in the ternary compound ␤ and binary compound Mg3 Nd was detected. The microstructure of the selected alloys is discussed in more detail subsequently. 2.2.1. Alloy 96Mg1Nd3Y (alloy 1) Fig. 1(a and b) shows the BSE image and X-ray diffraction profile of alloy 1, respectively. It can be seen that the alloy contains two phase (Mg) and ternary compound ␤. In the BSE image, the gray phase is (Mg), and the bright phase is ␤. 2.2.2. Alloy 92Mg6Nd2Y (alloy 2) The BSE image and X-ray diffraction profile of alloy 2 are shown in Fig. 2(a and b). The BSE image (Fig. 2(a)) shows that the alloy consists of two phases. Based on X-ray diffraction

in Fig. 2(b) and the EPMA compositions in Table 1, it can be concluded that the dark phase in the BSE image is (Mg), and the dark-gray phase is Mg41 Nd5 . 2.2.3. Alloys 94Mg3Nd3Y (alloy 3) and 91Mg5Nd4Y (alloy 4) The microstructure of alloys 3 and 4, heat treated at 753 K, consists of three phases (Mg), Mg41 Nd5 and ␤. Fig. 3 shows the microstructure of alloy 4. Based on the EPMA data (Table 1), the dark phase is (Mg), the dark-gray phase is Mg41 Nd5 , and the light-gray phase is ␤. 2.2.4. Alloys 91Mg1Nd8Y (alloy 5) and 90Mg2Nd8Y (alloy 6) The microstructure of alloys 5 and 6 consists of three phases (Mg), Mg24 Y5 and ␤. Fig. 4 shows the BSE image of alloy 6 annealed at 783 K for 2160 h. These two alloys clearly define the (Mg) + Mg24 Y5 + ␤ three-phase region. 2.2.5. Alloy 83Mg3Nd14Y (alloy 7) The BSE image of alloy 7 is shown in Fig. 5(a). The corresponding X-ray diffraction profile is shown in Fig. 5(b). The reflections illustrate the presence of the Mg24 Y5 and ␤ phases. In the BSE image, the dark-gray phase is Mg24 Y5 and the light-gray phase is ␤.

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Fig. 3. BSE image of alloy 4.

2.2.6. Alloy 79Mg7Nd14Y (alloy 8) The BSE image and X-ray diffraction profile of alloy 8 are shown in Fig. 6(a and b), respectively. The equilibrium microstructure of alloy 8 contains three phases. Based on the EPMA compositions, the light phase in the BSE image is Mg3 Nd, the light-gray phase is Mg2 Y and the dark-gray phase is ␤. Fig. 1. BSE image (a) and X-ray diffraction profile (b) of alloy 1.

2.2.7. Alloy 86Mg9Nd5Y (alloy 9) Fig. 7 shows the BSE images of alloy 9, which consists of two phases. The dark-gray phase is Mg41 Nd5 and the light-gray phase is ␤. 2.2.8. Alloy 75Mg13Nd12Y (alloy 10) The BSE image and X-ray diffraction profile of alloy 10 are shown in Fig. 8(a and b), respectively. The microstructure of the alloy consists of two phases, ␤ and Mg3 Nd. Based on the EPMA compositions, we can conclude that the dark-gray phase in the BSE image is ␤ and the light-gray phase is Mg3 Nd.

Fig. 2. BSE image (a) and X-ray diffraction profile (b) of alloy 2.

Fig. 4. BSE image of alloy 6.

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Fig. 5. BSE image (a) and X-ray diffraction profile (b) of alloy 7.

269

Fig. 6. BSE image (a) and X-ray diffraction profile (b) of alloy 8.

3.1. Substitutional solutions According to the analysis above, the isothermal section of the Mg–Nd–Y ternary system at 753 K in the magnesium-rich side can be determined as shown in Fig. 9. 3. Thermodynamic models The liquid, hcp and dhcp phases are treated as substitutional solutions, and the bcc A2 and bcc B2 phases are described using a two-sublattice model. The intermetallic compounds Mg24 Y5 and Mg2 Y described by sublattice models in the assessed Mg–Y binary system [9] and the parameters for the two phases were accepted in this paper. Since the homogeneity ranges of Mg41 Nd5 , Mg3 Nd and Mg2 Nd were not well established experimentally, these phases were treated as stoichiometric phases in the Mg–Nd binary system [10]. In the present work, all binary intermetallic compounds, except Mg3 Nd, are treated to have no solubility of the third element due to the limited experimental data. As detailed in Section 3, the binary phase Mg3 Nd has an extensive solubility of the third element, so it is formulated as Mg0.75 (Nd,Y)0.25 . Similarly, the ternary compound ␤ is modeled as Mg0.83333 (Nd,Y)0.16667 . In addition, the bcc B2 and bcc A2 phases are modeled with (Mg,Nd,Y)0.5 (Mg,Nd,Y)0.5 . Gibbs energies of various phases in the Mg–Nd–Y system are calculated as following according to the above respective models.

The Gibbs energies of solution phases, liquid, hcp and dhcp, are described with a substitutional solution model based on random mixing of the metal atoms. They are expressed as follows:   G xi 0 G  xi ln xi + E G (1) m = i + RT i=Mg,Nd,Y

i=Mg,Nd,Y

Fig. 7. BSE image of alloy 9.

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where 0 G i is the molar Gibbs energy of pure element of  0  phase. 0 G Mg and GNd are cited from the work of Dinsdale [11] except that 0 GMg and 0 GNd are taken from Ref. [12]. 0 G Y is E  from Ref. [9]. G is the excess Gibbs energy, expressed by the Redlich–Kister polynomial:  E  j  G = xMg xNd LMg,Nd (xMg − xNd )j dhcp

hcp

j=0,1...

+ xMg xY



j

j L Mg,Y (xMg − xY )

j=0,1...

+ xNd xY



j

j L Nd,Y (xNd − xY )

(2)

j=0,1...

where j L is the interaction parameter, and it takes the general form as: L = Aj + Bj T

(3)

where Aj and Bj are parameters to be optimized. 3.2. Semistoichiometric phases The Gibbs energies of the phases Mgp (Nd,Y)q , formed by mixing two binary stoichiometric compounds Mgp Ndq and Mgp Ndq , are expressed as: Fig. 8. BSE image (a) and X-ray diffraction profile (b) of alloy 10.

G = yNd GMg:Nd + yY GMg:Y + ⎛ + yY ln yY ) + yNd yY ⎝

q RT (yNd ln yNd p+q



j



j⎠ L Mg:Nd,Y (yNd − yY )

j=0,1...

(4) in which yNd and yY are the site fractions of Nd and Y on the second sublattice. GMg:Nd and GMg:Y are the Gibbs energies of the compounds Mgp Ndq and Mgp Yq in per mole of atoms, respectively. j L Mg:Nd,Y represent the parameters to be optimized, standing for the interactions between atoms Nd and Y on the second sublattice. In the present work, p + q is set to 1. 3.3. bcc A2 and bcc B2 The molar Gibbs energy of the disordered bcc A2 and ordered bcc B2 phases are modeled with a single function based on the two-sublattice model (Mg,Nd,Y)0.5 (Mg,Nd,Y)0.5 , i.e., ord I II ord Gm = Gdis m (xi ) + Gm (yi , yi ) − Gm (xi , xi )

Fig. 9. The isothermal section in the magnesium-rich part of the Mg–Nd–Y system at 753 K. The asterisk (*) represents the investigated sample compositions. The solid triangles indicate well-defined three-phase equilibria (measured results from the present work). The dashed triangles are estimated three-phase equilibria. The measured tie-lines are shown with dotted lines.

(5)

where xi represents the alloy mole fraction of constituent i, and yiI and yiII are the so-called site fractions, i.e., the mole fractions in the first and second sublattices, respectively. The first term, Gdis m (xi ), represents the Gibbs energy of the disordered phase bcc A2 and is expressed by Eq. (1). The second term, I II Gord m (yi , yi ), is the Gibbs energy of the ordered phase bcc B2 as described by the sublattice model and contains implicitly a

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Table 2 Thermodynamic parameters assessed in the present work Phases

Parameters GMg:Y = GY:Mg = −10236.5 + 3.32225·T

bcc B2

0L Mg:Mg,Y

= 0 LMg,Y:Mg = −34012.9

1L Mg:Mg,Y

= 1 LMg,Y:Mg = −19857.3

0L Y:Mg,Y

= 0 LMg,Y:Y = 49935.3

1L Y:Mg,Y

= 1 LMg,Y:Y = −4051.5

0L Mg:Nd,Y

= 0 LNd,Y:Mg = −4000 hcp

hcp

GMg:Y = −12400 + 9.5·T + 0.75·0 GMg + 0.25·0 GY

Mg3 Nd

0L Mg:Nd,Y

= −9250

1L Mg:Nd,Y

= 5000 hcp



dhcp

GMg:Nd = −17257.6 + 15.0266·T + 0.83333·0 GMg + 0.16667·0 GNd hcp

hcp

GMg:Y = −8809.60 + 5.79184·T + 0.83333·0 GMg + 0.16667·0 GY 0L Mg:Nd,Y

= −26644.4+11.7040·T

contribution of the disordered state. It is expressed as follows:  I II yiI yjII Gi:j + 0.5RT Gord m (yi , yi ) = i

×

j

 i

×



(yiI ln yiI +yiII ln yiII ) +



v

v=0,1...

×



v=0,1...

v

Li,j:k (yiI − yjI )v + Lk:i,j (yiII − yjII )v

i j>i k

 i j>i k

yiI yjI ykII

ykI yiII yjII (6)

The parameters v L have the form shown in Eq. (3). Due to the crystallographical symmetry, the following relations are introduced in Ref. [13]: Gj:i = Gi:j v

et al. [9], a minor revision of the description of the ordered bcc B2 phase was introduced. As shown in Table 2, only 10 parameters, GMg:Y , GY:Mg , 0 LMg:Mg,Y , 0 LMg,Y:Mg , 1 LMg:Mg,Y , 1L 0 0 1 1 Mg,Y:Mg , LY:Mg,Y , LMg,Y:Y , LY:Mg,Y , LMg,Y:Y have been used for the description. The calculated phase diagram from our description for the bcc B2 phase is shown in Fig. 10. It can be seen that the calculated results are in good agreement with experimental data. Compared with Fabrichnaya’s work [9], the higher order parameters, 2 LMg:Mg,Y , 2 LMg,Y:Mg , 2 LY:Mg,Y , 2L 0 Mg,Y:Y , LMg,Y:Mg,Y , were not used in the present work. It is clear that the present description for bcc B2 is much simpler than that in Fabrichnaya’s work [9]. Thermodynamic assessment of the Mg–Nd binary system was recently done in our group [10]. Due to the lack of experimental data, the Nd–Y binary system was not optimized in this

(7a)

v

Lk:i,j = Li,j:k

(7b) Gord m (xi , xi ),

represents the energy The last term in Eq. (5), contribution of the disordered state to the ordered phase. The last two terms cancel each other when the site fractions are equal, thus corresponding to a disordered phase. Hence, the parameters of both ordered and disordered phases can be evaluated independently. 4. Thermodynamic assessment The model parameters are evaluated using the Parrot module in Thermo-Calc [14] based on the experimental data. All parameters obtained in present work are listed in Table 2. The computational procedure and results are discussed in details in the following. 4.1. Boundary binary systems For the Mg–Y binary system, by using the same experimental information [15–17] as in the previous study by Fabrichnaya

Fig. 10. Calculated Mg–Y phase diagram compared with most of the phase diagram experimental data [15–17]. Solid lines: present revision. Dotted lines: the results of Fabrichnaya et al. [9].

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Fig. 11. Isothermal section at 753 K of the Mg–Nd–Y system.

work, and only a simple treatment was performed, where all phases in this system were regarded as ideal solutions.

Fig. 13. The liquidus projection with isotherms (dotted lines). Table 3 Calculated temperatures and compositions of the liquid at the invariant equilibria Reaction

T (K)

4.2. The Mg–Nd–Y ternary system The calculated isothermal sections at 753 and 523 K are shown in Figs. 11 and 12, respectively. In comparison with the experimental isothermal section (Fig. 9), the calculated phase relationships at 753 K in Mg-rich side are in good accord with the experimental results. In light of the present calculation, ␤ is in the equilibrium with the solid solution (Mg) at 523 K. This is in accordance with the reports [4–8]. No available experimental data can be used to compare with other part of the calculated Mg–Nd–Y phase diagram. However, considering the agreement between the calculated phase equilibria and experimental results in the Mg-rich side, our description will be helpful for related materials research, and the extrapolation of the description over the entire range of composition of this ternary system is hoped to be helpful in planning the

Liq + Mg2 Nd → Mg3 Nd + bcc B2 Liq + bcc B2 → Mg3 Nd + Mg2 Y Liq + Mg3 Nd + Mg2 Y → ␤ Liq + Mg2 Y → Mg24 Y5 + ␤ Liq + Mg3Nd → Mg41 Nd5 + ␤ Liq → Mg41 Nd5 + hcp A3 + ␤ Liq + Mg24 Y5 → hcp A3 + ␤

1011 942 883 871 830 819 836

Liquid X (Mg)

X (Nd)

X (Y)

0.646 0.748 0.833 0.854 0.914 0.932 0.915

0.330 0.075 0.038 0.020 0.078 0.060 0.010

0.021 0.177 0.129 0.126 0.007 0.008 0.075

desirable experiments in the region which is not yet sufficiently known. Due to lack of enough experimental information, additional experimental investigations are needed before we will have definitive information on the phase equilibria for this system over wide ranges of composition and temperature. The liquidus projection with isotherms is further calculated as shown in Fig. 13 and the calculated temperatures and liquid compositions for the invariant reactions involved liquid in the Mg–Nd–Y ternary system are summarized in Table 3. 5. Conclusions

Fig. 12. Isothermal section at 523 K of the Mg–Nd–Y system.

Based on the results of SEM, XRD and EPMA from the alloys annealed at 753 K, the phase relationships of Mg–Nd–Y ternary system in Mg-rich side at 753 K were well established. Taking into account those experimental information, a thermodynamic description of the ternary system Mg–Nd–Y was obtained. The calculated isothermal sections are in good agreement with experimental results. However, this description is based on the limited experimental information. It is clear that additional experimental investigations are needed before the phase equilibria for this system is well established over wide ranges of composition and temperature. However, in the absence of additional experimental data, various calculations of practical interest

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from this description will be helpful for Mg alloys research and for the phase diagram investigations away from the Mg corner. Acknowledgements The authors gratefully acknowledge associate professor, Zhengqing Ma, for his help in making alloy samples. One of the authors, H.S. Liu, would like to thank for the program for New Century Excellent Talents in Universities, China. References [1] B.L. Mordike, T. Ebert, Mater. Sci. Eng. A 302 (2001) 37–45. [2] J.F. Nie, B.C. Muddle, Scripta Mater. 40 (1999) 1089–1094. [3] Z.A. Sviderskaya, E.M. Padezhnova, Izv. Akad. Nauk SSSR, Met. (1971) 200–204. [4] J.F. Nie, B.C. Muddle, Acta Mater. 48 (2000) 1691–1703.

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