Experimental determination of the phase equilibria of the Mg–Nd–Zn system at 320 °C

Journal of Alloys and Compounds 603 (2014) 100–110

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom

Experimental determination of the phase equilibria of the Mg–Nd–Zn system at 320 °C Honghui Xu a,⇑, Jingjing Fan a, Han-Lin Chen b, Rainer Schmid-Fetzer c, Fan Zhang a, Yangbin Wang a, Qiannan Gao a, Tao Zhou a a b c

State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, PR China Thermo-Calc Software AB, Norra Stationsgatan 93, Plan 5, 11364 Stockholm, Sweden Institute of Metallurgy, Clausthal University of Technology, Robert-Koch-Str. 42, D-38678 Clausthal-Zellerfeld, Germany

a r t i c l e

i n f o

Article history: Received 28 January 2014 Received in revised form 21 February 2014 Accepted 21 February 2014 Available online 10 March 2014 Keywords: Mg–Nd–Zn system Phase diagrams EPMA X-ray diffraction Microstructure

a b s t r a c t The phase equilibria of the Mg–Nd–Zn system at 320 °C were investigated with an Mg–Nd–Zn diffusion couple and 34 equilibrated alloys, by means of X-ray diffraction technique and electron probe microanalyses. Eight ternary phases, denoted as s–s7 and T-NdMg12, respectively, were found at 320 °C. It was revealed that the T-NdMg12 phase is based on the metastable NdMg12 compound and stabilized by the substitution of Zn for Mg. The s1 phase with a C-centered orthorhombic crystal structure was determined to have a composition range of 7.4–7.7 at.% Nd and 25.8–40.1 at.% Zn. The s2 phase with a hexagonal crystal structure was measured to have a composition range of 6.3–7.8 at.% Nd and 61.0–64.0 at.% Zn. The crystal structures of the five ternary phases s3–s7 are still unknown. Four ternary phases, viz. T-NdMg12 and s5 to s7, can be regarded as the newly found ternary phases in the present work. According to the present work, the s3, s4 and s5 phases have the Zn composition ranges of 61.6–64.6 at.% Zn, 54.4–67.9 at.% Zn and 59.5–64.5 at.% Zn, and have the approximate Nd contents of 10.4, 13.5 and 17.2 at.% Nd, respectively. The s6 phase has a approximate chemical composition of Nd15.7Mg8.4Zn75.9 while the s7 phase has a composition range of 58.0–60.2 at.% Zn at about 21.3 at.% Nd. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Magnesium alloys are increasingly used for structural, aerospace and automotive applications due to its excellent properties such as high specific strength and stiffness, good castability, high recycle ability, etc. Over the past decades, much research work has been devoted to the development of the creep-resistant Magnesium alloys at elevated temperatures [1,2]. The Mg–Nd–Zn alloys with minor addition of Zr as the grain refiner are a promising candidate for the creep-resistant alloys [3,4]. Moreover, the quasicrystal strengthening phases found in Mg–RE–Zn alloys (RE = rare-earth elements including Nd) [5,6] have also caught great interest of condensed matter physicists and of many materials scientists. For the advanced development of the Mg–Nd–Zn alloys with high performance and better understanding the formation mechanism of the quasicrystals, knowledge of the phase equilibria in the Mg–Nd–Zn system and information on the interaction between Mg, Nd and Zn are of fundamental importance.

⇑ Corresponding author. E-mail addresses: [email protected], [email protected] (H. Xu). http://dx.doi.org/10.1016/j.jallcom.2014.02.131 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.

Though the phase equilibria in the Mg–Zn–Nd system have been investigated by several groups of researchers, the controversy on the existing ternary phases and their phase relationships is still unresolved. In the early 1970s, the phase equilibria at the Mg-rich region of the Mg–Nd–Zn ternary system were investigated by Drits et al. [7,8] using the X-ray diffraction (XRD) and thermal analysis. Three ternary phases (Nd4MgZn5, Nd2Mg6Zn7 and Nd2Mg2Zn9) were discovered. And the partial isothermal sections at 200 °C, 300 °C, 400 °C and 500 °C as well as three vertical sections at 10 wt.% Nd, 20 wt.% Nd and Mg–20 wt.% Nd to Mg–30 wt.% Zn were presented by Drits et al. [7,8]. Later on, a complete isothermal section at 300 °C was established by Kinzhibalo et al. [9] using XRD analysis and four ternary phases (NdMg7Zn12, Nd2Mg7Zn11, NdMg6Zn3 and Nd3Mg6Zn11), which are different from the ones reported by Drits et al. [7,8], were found at 300 °C. Recently, Huang et al. [10,11] investigated the phase equilibria of the Mg–Nd–Zn system at 300 °C, 350 °C and 400 °C by means of scanning electron microscopy (SEM), electron probe microanalysis (EPMA), XRD and transmission electron microscopy (TEM). Four ternary phases, which were designated as T1–T4, respectively, were found. The T2 phase with a C-centered orthorhombic crystal structure and an approximate chemical formula of Nd(Mg, Zn)11.5 was reported by

H. Xu et al. / Journal of Alloys and Compounds 603 (2014) 100–110

Huang et al. [11] to have a homogeneity composition range of up to 26.9–41.3 at.%Zn, which is significantly different from the previous results [7–9]. All the ternary phases reported by Drits et al. [8], Kinzhibalo et al. [9], Huang et al. [11] and Zhang et al. [12] at 400 °C were summarized by Zhang et al. [12] in one plot for comparison. It was revealed that the ternary phases reported by the different groups have significant discrepancies. The Mg–Nd–Zn ternary system was thermodynamically evaluated in 2011 by Zhang et al. [12] and Qi et al. [13], respectively. Nevertheless, it should be noted that the isothermal sections accepted by the two groups in their modeling are different from each other. Zhang et al. [12] accepted the one at 300 °C by Huang et al. [11] while the latter [13] accepted the isothermal section at 300 °C established by Kinzhibalo et al. [9]. In order to resolve these existing controversies in the literature and to provide reliable data on the phase equilibria of the Mg–Nd–Zn system for future thermodynamic modeling, detailed investigation of the phase equilibria of the Mg–Nd–Zn system is necessary. So, the present work is intended to determine the phase equilibria of the Mg–Nd–Zn system at 320 °C via a combination of the diffusion couple technique and equilibrated alloy method. The alloys are prepared by melting the pure elements in the tantalum crucibles that were individually sealed in quartz tubes due to the high evaporability of Mg and the strong reactivity of the RE and Mg. 2. Experimental procedures Magnesium (99.99 wt.%), Zinc (99.999 wt.%) and Nd (99.9 wt.%) were used as starting materials. The ternary Mg–Nd/Zn couple was prepared with the previously described method in Ref. [14]. The binary Mg–Nd couples, which were bound together by Mo wires, were firstly made by annealing at 500 °C for 20 days and then at 320 °C for 12 days. Subsequently, the ternary Mg–Nd/Zn couple, which was bound together by Mo wires, was annealed at 320 °C for 21 days, followed by quenching into cold water. Mg–Nd–Zn ternary alloys were selected on the basis of the experimental results obtained with the EPMA measurement of the diffusion couples in the present work and of the information on the ternary system in the literature. The Mg–Nd–Zn alloys, the nominal compositions (in at.%) of which are listed in Table 1 and also plotted in Fig. 1, were prepared as described below. Neodymium was cut into small blocks and kept ready in acetone for use. Before weighting, the oxide layers on the surfaces of the magnesium, zinc and neodymium blocks were mechanically removed. The accurately weighed and cleaned materials of the Zn, Mg and Nd blocks were sealed in the Ta crucibles that were then separately sealed in quartz tubes. The quartz capsules were put in an L4514-type diffusion furnace (Qingdao Sunred Electronic Equipment Co., China) at 600–700 °C, and then heated to the temperatures between 850 °C and 1000 °C depending on the estimated liquidus temperatures of the alloys. After being held at the specific temperatures for 3–5 h, the furnace was slowly cooled down to 320 °C and the capsules were quenched in cold water. All the alloys prepared by the above process were annealed at 320 °C for 100 days, and then quenched in cold water. The metallographic samples of the diffusion couples and alloys were firstly examined by using optical microscopy and then analyzed with an electron probe microanalyser (EPMA) (JXA-8530, JEOL, Japan) employing pure Mg (99.99 wt.%), Zn (99.999 wt.%) and Nd (99.9 wt.%) as standard materials. The EPMA measurements were analyzed at 15 kV and 2  108 A. The X-ray powder diffraction measurement (XRD) of the annealed alloys was performed using a Cu Ka radiation on a Rigaku D-max/2550 VB+ X-ray diffractometer at 40 kV and 200 mA. Lattice parameters were calculated with the JADE software for most of the identified major phases.

3. Results and discussion Fig. 1 shows the determined tie-lies and tie-triangles besides the alloy positions with numbers for easy reading. All the compositions below are in atomic percent unless otherwise stated. The microstructures of the Mg–Nd/Zn diffusion couple annealed at 320 °C for 21 days were shown in Fig. 2, where Fig. 2b–e are the local magnification of the zones A, B, C and D in Fig. 2a. The XRD patterns and backscattered electron (BSE) images of the representative ternary alloys, which were annealed at 320 °C for 100 days, are presented in Figs. 3 and 4, respectively. The variation of the

101

lattice parameter a for the NdMg3 or T0 phase with its Zn content in certain alloys is displayed in Fig. 5, where the points are labeled with the alloy numbers. Error bars are not given in Fig. 5 since no internal standard was used for the 2h calibration during the XRD measurement. Table 1 gives the nominal compositions of the ternary alloys, the phases identified by a combination of XRD and EPMA, and the phase equilibrium data determined by EPMA. Fig. 6 presents the isothermal section at 320 °C of the Mg–Nd–Zn system, which was constructed according to the present work and the currently accepted binary phase diagrams in the literature [15–17]. Eight ternary phases, which were found at 320°°C, are denoted as s1–s7, and T-NdMg12, respectively. Solid lines are used for the tie-triangles determined by the present experimental data. The phase equilibria, which were deduced or not well determined, are indicated with dashed lines. 3.1. Ternary phases Table 2 summarizes the observed ternary phases in this study as well as the reported in the literature, together with their crystal structures, if known, to enable the unique identification of these phases. Different symbols used for the phases in the literature are also given. The phase Nd(Mg,Zn)3 is not a separate ternary phase but is also listed because it will be addressed in the discussion specifically as T0 at higher Zn-content. The most Mg-rich ternary phase, T-NdMg12, was determined to be based on the binary compound NdMg12. The Mnl2Th-type NdMgl2 has been considered as a metastable phase in the Mg–Nd system [15] and was reported to be observed only in samples quenched from the liquid state [18], which was confirmed by Gorsse et al. [19]. Its metastability was also corroborated by the present experimental observations. As shown in Figs. 3a and 4a, alloy #1 (Mg94.0Nd6.0) was determined to be in the two-phase equilibrium of (Mg) + Nd5Mg41, while alloy #5 (Mg91.5Nd6.5Nd2.0) mainly consists of (Mg) and a ternary phase T-Nd(Mg,Zn)12, which is isostructural to NdMg12. This ternary phase has the same Nd content as NdMg12 but contains about 2.4 at.% Zn. It is the Zn-stabilized NdMg12 phase. The T-NdMg12 phase was also observed in alloys #6 (Mg70.5Nd17.0Zn12.5) and #7 (Mg85.5Nd11.0Zn3.5). Drits et al. [7,8] had reported three ternary phases (i.e. Nd4Mg1Zn5, Nd2Mg6Zn7 and Nd2Mg2Zn9). The Nd4Mg1Zn5 phase is much richer in Nd than any of other ternary compounds. This compound is located in the presently determined three-phase region of NdZn + NdZn2 + T0 and was not detected in the present work. The second compound Nd2Mg6Zn7 has a very close Nd content to s4 but a significantly lower Zn content than s4. This compound is located in the three-phase region of s1 + T0 + s4 in the present work. And also, the third compound Nd2Mg2Zn9 has a close Nd content to s6 but its Zn content is obviously lower than s6. Therefore, these three ternary phases are not accepted in the presently constructed isothermal section. Kinzhibalo et al. [9] had reported four ternary compounds (i.e. NdMg6Zn3, NdMg7Zn12, Nd2Mg7Zn11 and Nd3Mg6Zn11). According to their chemical compositions, the first compound Nd1Mg6Zn3 is close to s1 and could be regarded as s1 though its Nd content is obviously higher than s1. The second compound Nd1Mg7Zn12 could be assumed to be identical to s2 though its Nd content is slightly lower than s2. The third compound Nd2Mg7Zn11 could be regarded to be s3 though its Zn content is slightly lower than s3. The fourth compound Nd3Mg6Zn11 could roughly be considered as s4 though its Nd content is slightly higher than s4. Huang et al. [11] had reported four ternary phases at 400 °C, which were designated to be T1–T4, respectively. The T2 phase with a C-centered orthorhombic crystal structure was reported to have a chemical formula of (Mg, Zn)11.5Nd and a Zn composition range of

102

H. Xu et al. / Journal of Alloys and Compounds 603 (2014) 100–110

Table 1 Summary of the XRD and EPMA experimental results of the alloys annealed at 320 °C. No

Nominal composition Mg

Nd

Phases Zn

Mg

Nd

99.9 90.5 89.4 77.9

0.1 9.5 10.6 22.1

0.0 0.0 0.0 0.0

Tie-lie

0.0

NdMg3 Nd(Mg,Zn)

74.6 53.4

25.4 46.6

0.0 0.0

Tie-lie

61.0

0.0

Nd(Mg,Zn) (Nd)

49.3 6.2

50.7 93.8

0.0 0.0

Tie-lie

6.5

2.0

(Mg) T-NdMg12 Nd5Mg41 (Mg) T-NdMg12 NdMg3 Nd5Mg41 T-NdMg12 NdMg3

99.8 90.4 88.5 99.5 90.2 74.2 87.4 90.1 73.8

0.1 7.5 9.9 0.3 7.4 19.5 10.9 7.9 19.6

0.1 2.1 1.6 0.2 2.4 6.3 1.7 2.0 6.6

Tie-triangle

Nd5Mg41 T0 T-NdMg12 Nd5Mg41 T0 NdMg3 Nd5Mg41 T-NdMg12 NdMg3

86.7 58.8 89.5 86.9 57.7 71.7 86.9 89.5 71.0

10.8 22.6 7.7 10.7 22.8 20.8 10.7 7.8 20.6

2.5 18.6 2.8 2.4 19.5 7.5 2.4 2.7 8.4

Tie-triangleb

Nd5Mg41 T0 T-NdMg12

87.1 61.6 90.1

10.7 22.6 7.5

2.2 15.8 2.4

Tie-triangle

(Mg)

99.4 66.6 32.7

0.1 7.6 23.8

0.5 25.8 43.5

Tie-triangle

96.6 49.7 30.4

0.0 0.1 6.4

3.4 50.2 63.2

Tie-triangle

96.8 50.3 30.6 48.9 30.3 41.3

0.0 0.1 6.5 0.1 6.3 0.2

3.2 49.6 62.9 51.0 63.4 58.5

Tie-triangle

98.4 52.4 30.4 54.4 31.9 28.2 54.5 27.6 21.4

0.1 7.5 7.6 7.4 7.1 10.2 7.7 10.6 13.4

1.5 40.1 62.0 38.2 61.0 61.6 37.8 61.8 65.2

27.4 30.9 23.4 59.8 28.0 32.4

24.8 13.3 17.1 7.7 24.6 13.2

47.8 55.8 59.5 32.5 47.4 54.4

Tie-triangle

0.1 7.4 7.5 7.6 7.8 10.4 13.4 16.6 18.5 18.1

1.5 38.9 60.5 38.4 60.7 61.3 66.1 63.0 61.3 74.5

Tie-triangle

Nd13Zn58

98.4 53.7 31.9 54.0 31.5 28.3 20.5 20.4 20.2 7.4

MgZn2 Mg2Zn11

33.2 15.2

0.0 0.0

66.8 84.8

94.0

6.0

0.0

2

82.0

18.0

0.0

3

60.0

40.0

4

39.0

5

91.5

70.5

17.0

12.5

7

85.5

11.0

3.5

8

73.0

10.0

17.0

s1 T0 9

62.0

3.0

35.0

10

43.5

1.5

55.0

(Mg) MgZn

s2 (Mg) MgZn

s2 MgZn

s2 Mg2Zn3 11

33.7

10.3

56.0

(Mg)

s1 s2 s1 s2 s3 s1 s3 s4 12

43.0

12.0

45.0

T0

s4 s5 s1 T0

s4 13

27.5

12.5

60.0

(Mg)

s1 s2 s1 s2 s3 s4 s5 s5 14

16.5

Remark

(Mg) Nd5Mg41 Nd5Mg41 NdMg3

1

6

Composition

1.5

82.0

Zn

Tie-lie

Tie-trianglea

Tie-trianglea

Tie-trianglea

Tie-trianglea

Tie-triangle

Tie-triangle

Tie-triangle

Tie-triangle

Tie-triangle

Tie-triangle

Tie-lie Tie-lie Tie-triangle

103

H. Xu et al. / Journal of Alloys and Compounds 603 (2014) 100–110 Table 1 (continued) No

Nominal composition Mg

Nd

Phases Zn NdZn11

15

22.5

1.5

76.0

Zn

0.2

8.1

91.7

33.5 15.4 0.1 33.3 0.2 0.4 15.6 0.2 0.4

0.1 0.1 8.4 0.2 8.6 10.8 0.2 8.3 10.9

66.4 85.4 91.5 66.5 91.2 88.8 84.2 91.5 88.7

Tie-triangle

MgZn2 Nd2Zn17

34.6 4.0 29.4

0.2 10.8 6.6

65.2 85.2 64.0

Tie-triangle

28.6 24.8 6.1

8.1 10.6 10.9

63.3 64.6 83.0

Tie-triangle

18.2 18.5 6.4

14.2 17.0 18.3

67.6 64.5 75.3

Tie-triangle

17.9 8.6 4.6

14.2 15.8 18.1

67.9 75.6 77.3

Tie-triangle

14.1 15.6 18.4

67.4 76.2 75.8

Tie-triangle

Tie-triangle

Tie-trianglea

16

5.0

10.0

85.0

17

26.5

8.5

65.0

s2 s3

18

19.0

15.0

66.0

s4 s5

19

11.0

14.0

75.0

s4 s6

20

8.5

15.5

76.0

s4 s6 Nd13Zn58

18.5 8.2 5.8

21

2.5

17.0

80.5

Nd3Zn22 Nd13Zn58

2.6 3.3

12.9 17.5

84.5 79.2

Tie-lie

22

1.0

25.0

74.0

NdZn3 NdZn2 Nd3Zn11

0.1 0.1 2.3

24.9 33.3 21.6

75.0 66.6 76.1

Tie-triangle

23

3.0

29.0

68.0

Nd3Zn11 T0 NdZn2

5.1 25.8 0.3

21.8 24.9 33.3

73.1 49.3 66.4

Tie-triangle

24

6.0

42.0

52.0

T0 NdZn2 Nd(Mg,Zn)

30.0 0.8 3.9

24.8 33.8 48.9

45.2 65.4 47.2

Tie-triangle

25

6.0

21.0

73.0

Nd3Zn11 Nd13Zn58

5.8 6.4 21.5

21.7 18.5 21.2

72.5 75.1 57.3

Tie-triangle

26

17.0

21.5

61.5

26.4 19.1 9.1 26.9 19.9 20.1 18.8 18.6 7.9

25.0 17.0 18.5 24.9 17.5 21.5 16.7 21.2 18.8

48.6 63.9 72.4 48.2 62.6 58.4 64.5 60.2 73.3

Tie-trianglea

Nd2Zn17

Nd13Zn58

Nd13Zn58

s7 T0

s5 Nd13Zn58 T0

s5 s7 s5 s7 Nd13Zn58

a

Remark Nd

MgZn2 Mg2Zn11 NdZn11 MgZn2 NdZn11 Nd2Zn17 Mg2Zn11 NdZn11 Nd2Zn17

s2

b

Composition Mg

Tie-triangle

Tie-triangle

27

0.0

45.0

55.0

NdZn2 Nd(Mg,Zn)

0.0 0.0

33.4 50.0

66.6 50.0

Tie-lie

28

0.0

75.0

25.0

Nd(Mg,Zn) (Nd)

0.0 0.0

50.5 99.4

49.5 0.6

Tie-lie

29

35.0

40.0

25.0

NdMg3 Nd(Mg,Zn)

55.9 26.5

25.2 48.3

18.8 25.2

Tie-lie

30

53.5

24.0

22.5

T0 NdMg3

58.4 71.1

23.2 20.7

18.4 8.2

Tie-lie

31

53.0

27.5

19.5

NdMg3 Nd(Mg,Zn)

56.9 25.1

24.9 48.8

18.2 26.1

Tie-lie

32

58.5

27.0

14.5

NdMg3 Nd(Mg,Zn)

63.0 32.2

24.1 49.2

12.9 18.6

Tie-lie

33

62.5

27.5

10.0

NdMg3 Nd(Mg,Zn)

65.1 36.9

25.1 48.8

9.8 14.3

Tie-lie

34

67.5

27.5

5.0

NdMg3 Nd(Mg,Zn)

69.7 44.8

24.9 48.5

5.4 6.7

Tie-lie

The corresponding phase equilibrium is not in agreement with the presently established isothermal section. T0 is treated as the same phase as NdMg3 in Fig. 6. But the symbol T0 is to be kept in Table 1 for the convenience of the discussion on the experimental results.

104

H. Xu et al. / Journal of Alloys and Compounds 603 (2014) 100–110

Fig. 1. Mg–Nd–Zn Alloys and the experimentally determined tie-triangles (/tielines) from Table 1.

26.9–41.3 at.%Zn. According to the present work, the s1 phase was measured to have a composition range of 7.4–7.7 at.% Nd and 25.8– 40.1 at.%Zn and was indexed to have a C-centered orthorhombic crystal structure as shown in Fig. 3b. The s1 phase in the present work can be assumed to be identical to the T2 phase reported by Huang et al. [11]. The T3 phase with a face-centered cubic structure was reported by Huang et al. [11] to be isostructural to NdMg3 and could be written as Nd(Mg,Zn)3. An identical phase to this T3 phase seems to be observed in the present work and is labeled as T0 . However, it should be noted that the doubtful miscibility gap between NdMg3 and T0 was only observed in the alloys #6 and #30 on the Mg-rich side (as shown in Figs. 4b and 4w) and was not detected in the alloys on the Nd-rich side. The alloys #6 and #30 were far from the equilibrium state. T0 is treated as the same phase as NdMg3 in Fig. 6, as the existing evidence is still insufficient to support the existence of the miscibility gap between NdMg3 and T0 . Nevertheless, the symbol T0 is to be kept in Table 1 and in the following text for the convenience of the further discussion on the experimental results in Section 3.3. s2 was measured to have a composition range of 6.3–7.8 at.%Nd and 61.0–64.0 at.%Zn. The s2 phase, which was indexed to have a

Fig. 2. Microstructures of the Mg–Nd/Zn diffusion couple annealed at 320 °C for 21 days: (a) Mg–Nd/Zn diffusion couple, (b) local magnification of the zone A, (c) local magnification of the zone B, (d) local magnification of the zone C, and (e) local magnification of the zone D.

H. Xu et al. / Journal of Alloys and Compounds 603 (2014) 100–110

105

Fig. 3. XRD patterns of the representative alloys annealed at 320 °C for 100 days (a) alloys #1 (Mg94.0Nd6.0) and #5 (Mg91.5Nd6.5Nd2.0); (b) alloy #8 (Mg73.0Nd10.0Zn17.0); (c) Alloy #6 (Mg70.5Nd17.0Zn12.5); (d) alloy #9 (Mg62.0Nd3.0Zn35.0); (e) alloy #16 (Mg5.0Nd10.0Zn85.0); (f) alloy #23 (Mg3.0Nd29.0Zn68.0); (g) alloys #29, #30 and #31; (h) Alloys #3 and #31–#34.

hexagonal crystal structure as shown in Fig. 3d and e, could be assumed to be identical to the T1 phase reported by Huang et al. [10]. Huang et al. [11] reported that T4 has a large Nd composition range but a narrow Zn composition range, which suggest that the T4 phase stretch approximately parallel to the Mg-Nd side. According to the present work, two ternary phases, s3 and s4, exist in the area belonging to the so-called T4 phase. Moreover, both the s3 and s4 phases with a narrow Nd composition range stretch parallel to the Mg–Zn side. The Zn composition ranges for the s3 and s4 phases were measured to be 61.6–64.6 at.%Zn and 54.4–67.9 at.%Zn,

respectively. The T4 phase reported by Huang et al. [11] is not accepted in the present work. In summary, four ternary phases, viz. T-NdMg12 and s5–s7 can be regarded as the newly found ternary phases in the present work. The crystal structures of the five ternary phases s3–s7 are still unknown. 3.2. Diffusion couple After interdiffusion at 320 °C for 21 days, as shown in Fig. 2, the thickness of the NdMg3 and Nd5Mg41 phases in the Mg–Nd/Zn

106

H. Xu et al. / Journal of Alloys and Compounds 603 (2014) 100–110

Fig. 4. BSE micrographs of the representative alloys annealed at 320 °C for 100 days: (a) alloy #5 (Mg91.5Nd6.5Nd2.0); (b) alloy #6 (Mg70.5Nd17.0Zn12.5); (c) alloy #7 (Mg85.5Nd11.0Zn3.5); (d) alloy #8 (Mg73.0Nd10.0Zn17.0); (e) alloy #9 (Mg62.0Nd3.0Zn35.0); (f) alloy #10 (Mg43.0Nd1.5Zn55.0); (g) alloy #11 (Mg33.7Nd10.3Zn56.0); (h) alloy #12 (Mg43.0Nd12.0Zn45.0); (i) alloy #13 (Mg27.5Nd12.5Zn60.0); (j) alloy #13 (Mg27.5Nd12.5Zn60.0); (k) alloy #14 (Mg16.5Nd1.5Zn82.0), (m) alloy #15 (Mg22.5Nd1.5Zn76.0); (n) alloy #16 (Mg5.0Nd10.0Zn85.0); (o) alloy #17 (Mg26.5Nd8.5Zn65.0); (p) alloy #18 (Mg19.0Nd15.0Zn66.0); (q) alloy #19 (Mg11.0Nd14.0Zn75.0); (r) alloy #22 (Mg1.0Nd25.0Zn74.0); (s) alloy #23 (Mg3.0Nd29.0Zn68.0); (t) alloy #24 (Mg6.0Nd42.0Zn52.0); (u) alloy #25 (Mg6.0Nd21.0Zn73.0); (v) alloy #26 (Mg17.0Nd21.5Zn61.5); (w) alloy #30 (Mg53.5Nd24.0Zn22.5).

H. Xu et al. / Journal of Alloys and Compounds 603 (2014) 100–110

107

Fig. 4 (continued)

diffusion couple were very large while the thickness of the NdMg phase was still too tiny to be detected though the Mg–Nd binary couple had been pre-treated at 500 °C for 20 days and at 320 °C for 12 days. The scale of the MgZn2 in the Mg–Zn system was much larger than that of the Mg2Zn11 and Mg2Zn3 phases while the thickness of the MgZn phase was too tiny to be observed. And also, the scales of the NdZn11 and Nd13Zn58 phases was much larger than that of the Nd3Zn11 phase in the Nd–Zn system while the remaining binary phases (viz. NdZn, NdZn2, NdZn3 and Nd3Zn22) were too thin to be observed. As seen in Fig. 2a, c and d, two of the ternary phases that were formed in the diffusion couple, s1 and s2, could be well determined though the scale of s2 is much larger than that of s1. The layered s2 phase, which was regularly embedded in the MgZn2 phase at an interval, took up a large area in the products0 zone. In addition, it should be mentioned that there appeared an area that is adjacent to s1, s2, NdMg3 and Nd5Mg41, where several phases were mingled together, as shown in Fig. 2d. Though certain ternary phases other than s1 and s2 were very likely to form in this area, they could not be well identified or definitely differentiated from each other due to the confounding microstructure or the low color contrast among them. These phases are not explicitly marked in Fig. 2. 3.3. Equilibrated alloys and isothermal section Alloy #5 (Mg91.5Nd6.5Nd2.0) is nearly composed of the two phases T-NdMg12 and (Mg). Fig. 4a highlights the special zone of

alloy #5 containing NdMg3 and Nd5Mg41 besides (Mg) and T-NdMg12. Note that the amounts of the NdMg3 and Nd5Mg41 in the whole sample are actually very little and were not detected by XRD as shown in Fig. 3a. The micrograph in Fig. 4a virtually shows the four three-phase conjunctions of (Mg) + T-NdMg12 + Nd5Mg41, (Mg) + T-NdMg12 + NdMg3, (Mg)+Nd5Mg41 + NdMg3 and Nd5Mg41 + T-NdMg12 + NdMg3. Of course, it is impossible for all of them to be real three-phase equilibria. The NdMg3 grains have well-developed facets and some of the grains are embedded in the T-NdMg12 phase. The compositions were measured to be about Mg74Nd19.5Zn6.5 for the NdMg3 grains. It can be assumed that NdMg3 formed primarily in this region of the sample during the cooling and it should have been transformed into other phases if the heat treatment is long enough. Thus only the first three-phase equilibrium (Mg) + T-NdMg12 + Nd5Mg41 is accepted. Fig. 4b displays the BSE micrograph of alloy #6 while inserted in the upper left corner is the local magnification for the zone A. Four phases, viz. T0 , T-NdMg12, Nd5Mg41 and NdMg3, were observed in this alloy. Note that T0 is based on the NdMg3 compound in the Mg–Nd binary system on the crystallographic point of view. The compositions were measured to be about Mg71.7Nd20.8Zn7.5 for the NdMg3 phase compared to Mg58Nd23Zn19 (at.%) for the T0 phase in alloy #6. The majority of the microstructure in alloy #6, although it is heterogeneous, clearly indicates the three-phase equilibria among Nd5Mg41, T0 and T-NdMg12, which is further substantiated by the microstructure of alloy #7 (Mg85.5Nd11.0Zn3.5) as seen in Fig. 4c. Besides Nd5Mg41 + T0 + T-NdMg12, two additional

108

H. Xu et al. / Journal of Alloys and Compounds 603 (2014) 100–110

Fig. 4 (continued)

Fig. 5. Variation of the lattice parameter a for the NdMg3 and T0 phases with the Zn content.

three-phase equilibria of Nd5Mg41 + T0 + NdMg3 and Nd5Mg41 + T-NdMg12 + NdMg3 were detected in alloy #6. The individual well-developed grains of NdMg3 remaining, as seen in Fig. 4b, could be ascribed to far from the phase equilibria, similar to the analysis on alloy #5. This inference is necessary, as the direct contact of NdMg3 on T-NdMg12 is contrary to the three-phase equilibrium of Nd5Mg41 + T0 + T-NdMg12 that was observed in alloys #6

Fig. 6. Isothermal section at 320 °C of the Mg–Zn–Nd system.

and #7. Therefore, the three-phase equilibrium of Nd5Mg41 + T-NdMg12 + NdMg3 is not accepted. The three-phase equilibrium of Nd5Mg41 + T0 + NdMg3 is also doubtful because more evidences are needed to support the existence of the miscibility gap between NdMg3 and T0 .

109

H. Xu et al. / Journal of Alloys and Compounds 603 (2014) 100–110 Table 2 Summary of the ternary phases in the Mg–Nd–Zn system. Phase/confinement temperature range (°C)

Pearson symbol/space group/prototype

Nd7.7Mg52.2-66.5Zn25.8-40.1

s1

C-centered Orthorhombic

Nd7Mg29-32Zn61-64

Hexagonal

Lattice parameters (pm) a = 970, b = 1120, c = 950 a = 980, b = 1130, c = 960 a = 984, b = 1135, c = 963

a = 1470, c = 870

s2

Nd10.4Mg25.0-28.0Zn61.6-64.6

This work At Nd7.1Mg29.9Zn63.0 [10] ‘‘T1’’ in Ref. [10,11] Nd1Mg7Zn12 [9] This work Nd2Mg7Zn11 [9]

Unknown

This work Nd3Mg6Zn11 [9]

Unknown

This worka

Unknown

This worka

Unknown

This worka

s4 Nd17.2Mg18.3-23.3Zn59.5-64.5

This work At Nd7.8Mg53.3Zn38.9 [11] At Nd7.8Mg61.1Zn31.1 [11] At Nd7.9Mg65.2Zn26.9 [11] ‘‘T2’’ in Ref. [11] Nd1Mg6Zn3 [9]

Unknown

s3 Nd13.5Mg18.6-32.1Zn54.4-67.9

References/comments

s5 Nd15.7Mg8.4Zn75.9

s6 Nd21.3Mg18.5-20.7Zn58.0-60.2

s7

a b c d

Nd(Mg,Zn)3 (T0 )d

cF16 Fm3m BiF3

a = 741.3 a = 720 a = 680

At NdMg3 [18] At Nd23.2Mg58.4Zn18.4 [this work] At Nd25.0Mg31.9Zn43.1 [11] ‘‘T3’’ in Ref. [11]

T-Nd(Mg,Zn)12

tI26 I4/mmm Mn12Th

This worka

Nd4Mg1Zn5b

Unknown

[7,8]

Nd2Mg6Zn7c

Unknown

[7,8]

Nd2Mg2Zn9c

Unknown

[7,8]

New ternary phases observed in the present work. Not detected and thus not accepted in this work. Nd2Mg6Zn7 and Nd2Mg2Zn9 might be identical to other phases listed above and thus are not accepted. T0 is used to denote this phase at higher Zn-content.

Figs. 3b and 4d show the BSE micrograph and XRD pattern of alloy #8 (Mg73.0Nd10.0Zn17.0), respectively. It was revealed that (Mg) is in three-phase equilibrium with s1 and T0 . Figs. 3d and 4e are the XRD pattern and BSE micrograph of alloy #9 (Mg62.0Nd3.0Zn35.0) while the BSE micrograph of alloy #10 (Mg43.5Nd1.5Zn55.0) is shown in Fig. 4f. The three-phase equilibrium of (Mg)+MgZn + s2 was determined with alloy #9 and substantiated with alloy #10. Besides, additional three-phase equilibrium of MgZn + s2 + Mg2Zn3 was measured with alloy #10. Fig. 4g shows the BSE micrographs of alloys #11 (Mg33.7Nd10.3Zn56.0) while the local magnification of the zone A is inserted in the upper right corner. Fig. 4i–j present the BSE micrograph of alloy 13 (Mg27.5Nd12.5Zn60.0) and the local magnification of the zone A in Fig. 4i, respectively. As shown in Figs. 4g, 3 three-phase equilibria, i.e. s1 + s2+(Mg), s1 + s3 + s4 and s1 + s2 + s3, were observed in alloy #11, in which the former two ones could be accurately determined. Besides the five phases of s1–s4 and (Mg) that were observed in alloy #11, additional two phases, s5 and Nd13Zn58, were observed in alloy #13. Two three-phase equilibria, viz. s1 + s2+(Mg) and s1 + s2 + s3, which were observed in alloy #11, were confirmed and accurately determined with alloy #13, as shown in Fig. 4i–j and listed in Table 2. Fig. 4h is the BSE micrograph of alloy #12 (Mg43.0Nd12.0Zn45.0) and inserted in the upper left corner is the local magnification of the zone A. Two three-phase equilibria, i.e. s1 + T0 + s4 and T0 + s4 + s5, were determined with this alloy, as indicated in Fig. 3h. Fig. 4k and m are the microstructures of alloys #14 (Mg16.5Nd1.5Zn82.0) and #15 (Mg22.5Nd1.5Zn76.0), respectively. The

results from alloy #14 indicated that MgZn2, Mg2Zn11 and NdZn11 are in a three-phase equilibrium, as shown in Fig. 4k. The solubility of Nd in MgZn2 and Mg2Zn11, and that of Mg in NdZn11 were measured to be negligibly little. Besides the three-phase equilibrium of MgZn2 + Mg2Zn11 + NdZn11 observed in alloy #14, however, additional two three-phase equilibria of MgZn2 + NdZn11 + Nd2Zn17 and Mg2Zn11 + Nd2Zn17 + NdZn11 were observed in alloy #15, as seen in Fig. 4m. It should be noted that one of the three-phase equilibria, i.e. Mg2Zn11 + Nd2Zn17 + NdZn11, is not compatible with the remaining two ones and is not accepted in the presently established isothermal section. Figs. 3e and 4n display the XRD pattern and BSE micrograph of alloy #16 (Mg5.0Nd10.0Zn85.0), respectively while Fig. 4o–p shows the BSE micrographs of alloys #17 (Mg26.5Nd8.5Zn65.0) and #18 (Mg19.0Nd15.0Zn66.0). The XRD and EPMA results on alloy #16 revealed that MgZn2, Nd2Zn17 and s2 are in a three-phase equilibrium. The three-phase equilibria of s2 + s3 + Nd2Zn17 and s4 + s5 + Nd13Zn58 were determined with alloy #17 and 18, respectively, as listed in Table 1. Both alloy #19 (Mg11.0Nd14.0Zn75.0) and alloy #20 (Mg8.5Nd15.5Zn76.0) involved the ternary phase of s6. The threephase equilibrium of s4 + s6 + Nd13Zn58 was determined with alloys #19 and confirmed with alloy #20, as shown in Fig. 4q and listed in Table 1. Figs. 3f and 4s exhibit the XRD pattern and BSE micrograph of alloy #23 (Mg3.0Nd29.0Zn68.0) while the BSE micrographs of alloys # 22 (Mg1.0Nd25.0Zn74.0) and #24 (Mg6.0Nd42.0Zn52.0) are shown in Fig. 4r and t, respectively. The EPMA and XRD results on alloy

110

H. Xu et al. / Journal of Alloys and Compounds 603 (2014) 100–110

#23 suggested that Nd3Zn11, T0 and NdZn2 are in a three-phase equilibrium. As shown in Fig. 4r and t and listed in Table 1, Alloy #22 and 24 are located in the three-phase equilibria of NdZn3 + NdZn2 + Nd3Zn11 and T0 + NdZn2 + Nd(Mg,Zn), respectively. The ternary phase of s7 was found in alloys #25 (Mg6.0Nd21.0Zn73.0) and #26 (Mg17.0Nd21.5Zn61.5). The microstructures of alloys #25 and #26 are presented in Fig. 4u and v, respectively. Note that another view on alloy #26 is also inserted in the upper left corner in Fig. 4v. The results on alloy #25 revealed that Nd3Zn11, Nd13Zn58 and s7 are in a three-phase equilibrium. As shown in Fig. 4v and listed in Table 1, nevertheless, 3 three-phase conjunctions of T0 + s5 + s7, s5 + s7 + Nd13Zn58 and s5 + T0 + Nd13Zn58 were observed in alloy #26. The direct contact of T0 on Nd13Zn58 was observed in both of the alloys #25 and #26, which is contradictory to the above-mentioned three-phase equilibrium of Nd3Zn11 + Nd13Zn58 + s7 that was observed in the alloys #25 and #26. So, the three-phase equilibrium for the last one (i.e. s5 + T0 + Nd13Zn58), which is inconsistent with the presently assessed isothermal section in Fig. 6, is not accepted in the present work. The simultaneous appearance of the incompatible phase equilibria in the same alloys could be ascribed to the fact that these two alloys were still far from the equilibrium state after annealing at 320 °C for 100 days. Fig. 3g and h show the XRD patterns of alloys #29 to 31 and of alloys #3 + #31 to 34 together for comparison, respectively, while Fig. 4w presents the BSE image of alloy #30. Though the microstructure and EPMA results for alloy #30 seems to indicate the phase separation of the T0 phase from the NdMg3 phase as observed in alloy #6 (see Fig. 4b), it is very difficult to differentiate them from each other via the XRD patterns since both the T0 phase and the NdMg3 phase have identical crystal structure, as seen in Fig. 3g. The microstructure and EPMA results of alloy #30 suggested that this alloy was still far from the equilibrium state after annealing at 320 °C for 100 days, as shown in Fig. 4w. The area around NdMg3 is obviously darker than the other area regarding the so-called T0 phase. If the annealing time is long enough, the composition gradient and color contrast in alloy #30 would be significantly lowered and this alloy would become of a single phase. The slight difference was revealed between the variation of the lattice parameter a with the Zn content for NdMg3 and that for T0 as seen in Fig. 5, but the difference could be regarded to be within the experimental error. In fact, the homogeneity range of T3 was not determined by Huang et al. [11], as the phase equilibria between T3 and NdMg3 was not studied by them. In contrast to Huang et al. [11], Kinzhibalo et al. [9] reported that NdMg3 extends into the ternary system up to 46 at.% Zn. As to the present work, the miscibility gap between NdMg3 and T0 was only observed in the alloys #6 and #30 on the Mg-rich side (as shown in Fig. 4b and w) and was not detected in the alloys on the Nd-rich side. Just as stated above, incompatible phase equilibria occurred simultaneously in alloy #6, suggesting that this alloy was far from the equilibrium state. The well-developed NdMg3 grains should have been transformed into other phases if the annealing time is long enough. Therefore, the miscibility gap between NdMg3 and T0 , which is doubtful, is not accepted in Fig. 6. More experimental work is still needed to ascertain if it really exist. Though many of the alloys (#5, #6, #10–13, #15, #25 and #26) in the present work contained more than three phases and the global equilibria had not been reached in those alloys, it is believed that most of the alloys after annealing for 100 days at 320 °C can still provide reliable information on the phase equilibria, according to the local phase equilibrium principle, which is the basis of the diffusion couple technique. As mentioned by Zhang et al. [12], the reported ternary phases by different groups [7–11] have significant discrepancies. This confusing issue could be mainly ascribed to the inadequacy or inaccuracy of the available experimental data, as well as the fact that

certain ternary phases have large homogeneity ranges. The latter could further increase the difficulty for us to identify the ternary phases and to establish the accurate phase relationships. 4. Conclusions The isothermal section at 320 °C of the Mg–Zn–Nd system was determined with a Mg–Nd–Zn diffusion couple and 34 equilibrated alloys. Eight ternary phases, denoted as s1–s7 and T-NdMg12, respectively, were found at 320 °C. Among them, the crystal structures of the five ternary phases s3–s7 are still unknown. It was revealed that the T-NdMg12 phase is the Nd-stabilized NdMg12 phase. The s1 phase with a C-centered orthorhombic crystal structure was determined to have a composition range of 7.4–7.7 at.% Nd and 25.8– 40.1 at.%Zn. The s2 phase with a hexagonal crystal structure was measured to have a composition range of 6.3–7.8 at.%Nd and 61.0–64.0 at.%Zn. The ternary phases s3, s4 and s5 have approximately constant Nd contents of 10.4, 13.5 and 17.2 at.%Nd, but have the Zn composition ranges of 61.6–64.6 at.%Zn, 54.4–67.9 at.%Zn and 59.5–64.5 at.%Zn, respectively. The s6 phase has a approximate chemical composition of Nd15.7Mg8.4Zn75.9 while the s7 phase has a composition range of 58.0–60.2 at.%Zn at about 21.3 at.%Nd. The ternary phases s1 and s2 in the present work was identified to be identical to the three phases T2 and T1 that were previously reported by Huang et al. [11], respectively. The T3 phase reported by Huang et al. [11] is not accepted because more convincing evidence is still needed to support the existence of the miscibility gap between NdMg3 and T3 (T0 ). Four of the ternary phases, viz. TNdMg12 and s5–s7, can be regarded as the newly found ternary phases in the present work. Acknowledgements This work is supported by the National Natural Science Foundation of China (NSFC) under No. 50971135, the Sino-German Center for Promotion of Science (GZ591) and the National Basic Research Program of China (2011CB610401). References [1] B.L. Mordike, Mater. Sci. Eng. A 324 (2002) 103–112. [2] M.O. Pekguleryuz, A.A. Kaya, Adv. Eng. Mater. 5 (12) (2003) 866–878. [3] Z.M. Li, P.H. Fu, L.M. Peng, Y.X. Wang, H.Y. Jiang, G.H. Wu, Mater. Sci. Eng. A 579 (2013) 170–179. [4] P.H. Fu, L.M. Peng, H.Y. Jiang, J.W. Chang, C.Q. Zhai, Mater. Sci. Eng. A 486 (2008) 183–192. [5] A. Niikura, A.P. Tsai, A. Inoue, T. Masumoto, Jpn. J. Appl. Phys. 33 (1994) L1538. [6] D.K. Xu, E.H. Han, Prog. Nat. Sci. 22 (5) (2012) 364–385. [7] M.E. Drits, E.M. Padezhnova, N.V. Miklina, Izvest. VUZ Tsvetn. Metall. 4 (1971) 104–107. [8] M.E. Drits, E.M. Padezhnova, N.V. Miklina, Izv. Akad. Nauk SSSR. Met. 3 (1974) 225–229. [9] V.V. Kinzhibalo, A.T. Tyvanchuk, E.V. Melnik, Stable and Metastable Phase Equilibria in Metallic Systems, Nauka, Moscow, USSR, 1985, pp. 70–74. [10] M.L. Huang, H.X. Li, J.Y. Yang, Y.P. Yu, H. Ding, S.M. Hao, Acta Metall. Sin. 44 (4) (2008) 385–390. [11] M.L. Huang, H.X. Li, H. Ding, Z.Y. Tang, R.B. Mei, H.T. Zhou, R.P. Ren, S.M. Hao, J. Alloys Comp. 489 (2) (2010) 620–625. [12] C. Zhang, A.A. Luo, L.M. Peng, D.S. Stone, Y.A. Chang, Intermetallics 19 (2011) 1720–1726. [13] H.Y. Qi, G.X. Huang, H. Bo, G.L. Xu, L.B. Liu, Z.P. Jin, J. Alloys Comp. 509 (7) (2011) 3274–3281. [14] H.H. Xu, Y. Du, C.Y. Tang, Y.H. He, B.Y. Huang, S.T. Li, Mater. Sci. Eng. A 412 (1– 2) (2005) 336–341. [15] H. Okamoto, J. Phase Equilib. 12 (1991) 249–250. [16] J.B. Clark, L. Zabdyr, Z. Moser, Phase Diagrams of Binary Magnesium Alloys, ASM International, Metals Park, OH, 1988. [17] J.T. Mason, P. Chiotti, Metall. Trans. 3 (1972) 2851–2855. [18] S. Delfino, A. Saccone, R. Ferro, Metall. Trans. A 21 (1990) 2109–2114. [19] S. Gorsse, C.R. Hutchinson, B. Chevalier, J.F. Nie, J Alloys Comp. 392 (2005) 253–262.