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Journal of Phase Equilibria and Diffusion

, Volume 39, Issue 5, pp 549–561 | Cite as

Phase Equilibria of the Ti-Al-Nb System at 1000, 1100 and 1150 °C

  • Lin Li
  • Libin LiuEmail author
  • Ligang ZhangEmail author
  • Lijun Zeng
  • Yun Zhao
  • Weimin Bai
  • Yurong Jiang
Article
  • 373 Downloads

Abstract

1000, 1100 and 1150 °C isothermal sections of the Ti-Al-Nb system were studied using x-ray diffraction, scanning electron microscopy and electron probe microanalysis. A small island-like region of single β0 is present at 1000, but absent at 1100 and 1150 °C. γ1 is not a stable phase at 1000 and 1150 °C. Three three-phase fields (α2 + β0 + σ, β0 + σ + γ and α2 + β0 + γ) are identified in the 1000 °C isothermal section (30-60 at.% Ti content). The 1100 °C isothermal section is firstly studied completely. It includes six three-phase and thirteen two-phase fields. Two three-phase fields β + α2 + γ and β + σ + γ are identified in the isothermal section (30-60 at.% Ti content) at 1150 °C. These data are helpful to the fabrication of the TiAl and Ti2AlNb intermetallics.

Keywords

intermetallics microstructure phase diagram TiAl alloy 

1 Introduction

Ti-Al intermetallic phases have excellent mechanical properties and good oxidation resistance up to 700 °C.[1, 2, 3, 4, 5, 6, 7, 8] The alloying element Nb is used to enhance their oxidation resistance, hot formability and creep resistance.[9, 10, 11, 12] The ternary Ti-Al-Nb system also contains many intermetallics which have a potential for producing alloys with various mechanical properties.[4] Recently, advanced materials based on the Ti-Al-Nb alloys that can be used at temperatures above 800 °C have been investigated.[13] The alloys normally show multiple-phase microstructures consisting of γ-TiAl, α2-Ti3Al, O-Ti2NbAl, σ-Nb2Al and β-ordered solid solution, i.e. β0.[4,14, 15, 16]

The fundamental knowledge, e.g. the information about phase equilibria in the Ti-Al-Nb related systems is indispensible for the design and fabrication of these intermallic based alloys.[17] Phase diagrams of the three constituent binary systems, i.e., Ti-Al,[18, 19, 20] Ti-Nb,[21,22] and Al-Nb,[23] have been well investigated and assessed.

Hellwig [24] studied partial isothermal section at 1000 and 1200 °C. Leonard[25] studied partial isothermal section at 1100 °C. Chen[26] and Ding[27] studied the isothermal section at 1000, 1150 and 1400 °C by diffusion couples. Jewett[28] commented Chen’s work,[26] and their results about γ1 phase are different. There are also some literature regarding the calculation of the Ti-Al-Nb ternary phase diagram using thermodynamic methods.[20,23,29, 30, 31, 32] Witusiewicz[23] and Cupid[29] independently optimized the Ti-Al-Nb ternary phase diagram using the CALPHAD (Calculation of Phase Diagrams) method in 2009. Witusiewicz re-evaluated the Al-Ti[19] and Al-Nb[23] binary systems. Crystal structures of phases in the Ti-Al-Nb ternary system are summarized in Table 1.
Table 1

The phase designations most often used in literature for the Al–Nb–Ti system along with crystal structure data[23]

Phase (designation)

Pearson symbol

Space group

Strukturbericht designation

Prototype

(Al)(αAl), fcc_A1

cF4

Fm-3m

A1

Cu

α, (αTi), hcp_A3

hp2

P63/mmc

A3

Mg

α2, Ti3Al

hP8

P63/mmc

D019

Ni3Sn

β, (βTi), bcc_A2

cI2

Im-3m

A2

W

β0, bcc_B2

cI2

Pm-3m

B2

CsCl

γ, γTiAl, TiAl

tP4

P4/mmm

L10

AuCu

δ, Nb3Al

cP8

Pm-3n

A15

Cr3Si

ε, (Ti1−xNbx)Al3, TiAl3(h), NbAl3

tI8

I4/mmm

D022

TiAl3(h)

ε(l), TiAl3(l)

tI32

I4/mmm

TiAl3(l)

ζ, Ti2+xAl5−x

tP28

P4/mmm

Ti2Al5

η, TiAl2

tI24

I41/amd

HfGa2

σ, Nb2Al

tP30

P42/mnm

D8b

σCrFe

Ti3Al5

tP32

P4/mbm

Ti3Al5

O1, O, O1(h), Ti2NbAl

oC16

Cmcm

NaHg

O2, O2(r), Ti2NbAl

oC16

Cmcm

NaHg

τ, Ti4NbAl3

hP6

P63/mmc

B82

Ni2In

γ1-Ti4Nb3Al9

tP16

P4/mmm

γ1-Ti4Nb3Al9

However, there are many differences between the experimental results. For example, Chen[26] and Ding[27] found that there is a γ1 phase existing in this ternary system. However, this phase has not been found by Hellwig. Hellwig found that a small island-like region of single β0 is a stable phase at 1000 °C, however it was not found by Chen[26] and Ding.[27] For this reason, the phase equilibria at 1000, 1100 and 1150 °C in the Ti-Al-Nb system was re-investigated using SEM (Scanning Electron Microscope), EPMA (Electron Probe Microanalysis) and XRD (x-ray diffraction) in this work.

2 Experimental

More than 35 samples have been prepared to measure the isothermal section of Ti-Al-Nb ternary system at 1000, 1100 and 1150 °C. The starting materials Ti, Al and Nb (purity of all 99.99%) supplied by China Jinyu Materials Technology Co., Ltd. were used to prepare the experimental alloys. The weight of each sample was limited to about 6 g. Predetermined amount of each raw material was weighted by analytical balance, and followed by arc-melting on a water-cooled copper crucible under argon atmosphere. The hot/liquid titanium has been used as oxygen getter. To ensure a good homogeneity of the samples, each obtained button was turned over and re-melted at least three times. The weight losses did not exceed 1%. The obtained button alloys were sealed in a silica capsule back-filled with high purity argon, and then annealed at 1000 °C for 1440 h, 1100 °C for 1080 h or 1150 °C for 360 h in diffusion furnace. After annealing, the alloys were quenched into ice water.

The annealed specimens were polished and their microstructure were investigated using electron probe microanalysis (EPMA) (JEOL JXA-8530F). Standard deviations of the measured concentration is ± 0.5 at.%. The total mass of Ti, Al and Nb in each phase is in the range of 97-103%. No silicon was found in the samples, so the effect of reactions between the samples and silica capsules could be neglected. XRD was also performed for phase identification in some selected annealed alloys using a Cu Kα radiation on a Rigaku D-max/2500 x-ray diffractometer operated at 40 kV and 200 mA. Phase identification and calculation of the lattice parameters of each phase were carried out using the Jade 6.0 program.

3 Results and Discussion

3.1 Isothermal Section at 1000 °C

The nominal chemical compositions of the alloys and the measured chemical compositions of individual phases by EPMA at 1000 °C samples are given in Table 2.
Table 2

Equilibrium compositions at 1000 °C measured with EPMA method

Number

Alloy, at.%

Phase equilibrium

Phase composition, at.%

Al

Ti

Nb

Phase 1/Phase 2/Phase 3

Phase 1

Phase 2

Phase 3

Al

Ti

Al

Ti

Al

Ti

#A1

23.7

32.5

43.8

β/δ/σ

24.8

58.7

21.0

31.2

29.5

29.8

#A2

26.8

45.8

27.4

α2/β/σ

25.0

59.1

22.5

49.3

30.1

33.3

#A3

34.3

50.0

15.7

α20

33.1

55.3

34.1

50.9

33.2

36.7

#A4

38.9

51.5

9.6

α20

34.4

56.2

34.5

53.0

44.1

46.6

#A5

18.0

22.0

60.0

β/δ

7.0

29.1

18.3

21.9

  

#A6

16.8

16.4

66.8

β/δ

5.7

21.4

18.2

17.5

  

#A7

13.0

16.0

71.0

β/δ

4.7

17.6

18.1

13.8

  

#A8

21.1

6.1

72.8

δ/σ

21.3

6.4

30.3

6.5

  

#A9

42.5

53.1

4.4

α2

35.6

60.1

45.9

49.5

  

#A10

36.5

33.5

30.0

γ/σ

45.3

40.9

33.9

30.7

  

#A11

46.0

35.0

19.0

γ/σ

47.1

37.8

34.1

27.3

  

#A12

50.0

28.0

22.0

γ/σ

51.9

25.3

35.9

13.9

  

#A13

40.7

17.6

41.7

γ/σ

51.4

23.1

35.5

13.4

  

#A14

63.0

11.5

25.5

γ/σ/ε

54.2

22.0

37.1

12.9

72.9

3.4

#A15

58.2

10.8

31.0

γ/σ/ε

54.6

21.7

36.0

11.6

73.1

2.7

#A16

63.0

20.0

17.0

ε/γ

72.5

6.1

55.3

30.4

  

#A17

65.2

28.7

6.1

γ/η/ε

57.3

37.0

65.2

31.4

73.1

14.9

#A18

68.5

26.3

5.2

η/ε

65.6

32.8

74.3

14.7

  

#A19

37.8

43.5

18.7

β0/γ/σ

34.4

50.5

44.0

44.6

33.7

36.3

#A20

33.5

55

11.5

α20

32.6

57.6

33.5

53.0

  

The Nb concentration in each phase can be calculated as 100—(Al concentrationin at.% + Ti concentration in at.%)

Figure 1(a) shows the microstructure of alloy #A3, which contains α2 (gray phase), β0 (light gray phase) and σ (bright phase) based on the XRD result (see Fig. 1b). Whereas, the alloy #A4 is located in the α2 + β0 + γ three-phase area (see Fig. 1c and d). Figure 1(e) illustrates the EPMA micrograph of the alloy #A19, which featured a three-phase σ + γ + β0 that agrees with the XRD result shown in Fig. 1(f). Figure 1(g) shows the microstructure of alloy #A15, which contains ε (dark gray phase), γ (light gray phase) and σ (bright phase).
Fig. 1

EPMA images and XRD results of typical alloys after annealing at 1000 °C, which contain three phases after quenching: (a) the microstructure of alloy #A3, (b) the XRD result of alloy #A3, (c) the microstructure of alloy #A4, (d) the XRD result of alloy #A4, (e) the microstructure of alloy #A19, (f) the XRD result of alloy #A19, (g) the microstructure of alloy #A15 and (h) the XRD result ofalloy #A15

Figure 2(a, b) shows the EPMA micrograph and XRD result of the alloy #A13, which featured a two-phase equilibrium σ + γ.
Fig. 2

EPMA images and XRD results of typical alloys after annealing at 1000 °C, which contain two phases after quenching: (a) the microstructure of alloy #A13 and (b) the XRD result of alloy #A13

Based on the tie-lines and tie-triangles identified by the present work, the isothermal section at 1000 °C was thus constructed as illustrated in Fig. 3(a).
Fig. 3

The 1000 °C isothermal section of the Ti-Al-Nb system: (a) the tie-lines, the tie-triangles and the measured 1000 °C isothermal section in the present work, (b) the experimental results from Chen’s work,[26] (c) the experimental results from Ding’s work[27] and (d) the experimental results from Hellwig’s work[24]

Figure 3(b, c and d) shows the experimental results of the 1000 °C isothermal section by Chen,[26] Ding[27] and Hellwig[24]. Chen[26] and Ding[27] reported the existence of γ1 phase, while Hellwig[24] didn’t found it. The present result is consistent with Hellwig’s result,[24] and a two-phase σ + γ field in this area. In the Ti 30-60 at.% region, Chen[26] and Ding[27] reported two three-phase fields (β + γ1 + α2 and α2 + γ1 + γ) and one two-phase field (α2 + γ1). However in this composition range Hellwig[24] reported three three-phase fields (α2 + β0 + σ, β0 + σ + γ and α2 + β0 + γ), three two-phase fields (α2 + β0, β0 + σ and β0 + γ) and one single-phase field (β0). The phase relationship of the present work is consistent with Hellwig’s work.[24]

3.2 Isothermal Section at 1100 °C

The nominal compositions of the alloys investigated in this work and chemical compositions of the individual phases at 1100 °C samples as determined by EPMA are given in Table 3.
Table 3

Equilibrium compositions at 1100 °C measured with EPMA method

Number

Alloy, at.%

Phase equilibrium

Phase composition, at.%

Al

Ti

Nb

Phase 1/Phase 2/Phase 3

Phase 1

Phase 2

Phase 3

Al

Ti

Al

Ti

Al

Ti

#B1

23.7

32.5

43.8

β/δ/σ

18.4

43.5

21.0

28.8

28.9

27.3

#B2

26.6

48.6

24.8

α2/β/σ

25.7

58.7

24.3

48.5

30.1

32.3

#B3

34.3

51.2

14.5

α2

34.3

52.7

33.5

35.1

  

#B4

39.1

43.2

17.7

α2/γ/σ

37.4

48.5

45.2

40.0

34.5

30.6

#B5

42.5

53.1

4.4

α2

36.2

59.9

47.1

48.2

  

#B6

18.0

22.0

60.0

β/δ

9.7

29.9

18.5

21.8

  

#B7

16.8

16.4

66.8

β/δ

7.9

18.8

18.2

15.1

  

#B8

11.3

16.4

72.3

β/δ

6.3

16.2

17.7

13.4

  

#B9

24.0

16.0

60.0

δ/σ

20.9

17.0

30.0

15.7

  

#B10

46.0

35.0

19.0

γ/σ

46.4

37.1

35.0

27.5

  

#B11

50.0

28.0

22.0

γ/σ

50.2

28.9

35.9

18.8

  

#B12

49.7

24.1

26.2

γ/σ

52.0

24.9

36.9

15.1

  

#B13

42.0

14.5

43.5

γ/σ

52.9

23.1

37.3

13.3

  

#B14

58.2

10.8

31.0

γ/σ/ε

53.4

23.9

38.2

12.1

72.2

4.0

#B15

50.0

6.3

43.7

σ/ε

36.2

8.9

70.3

2.7

  

#B16

63.0

20.0

17.0

γ/ε

55.7

28.7

72.1

7.1

  

#B17

70.2

24.4

5.4

η/ε

65.7

32.3

73.8

16.7

  

The Nb concentration in each phase can be calculated as 100—(Al concentrationin at.% + Ti concentration in at.%)

Figure 4(a) shows the three-phase microstructure of β + σ + δ for alloy #B1, and the XRD result is showed in Fig. 4(b). The existence of the three-phase field β + α2 + σ and its location are also established based on the SEM, EPMA and XRD data of alloy #B2 in Fig. 4(c, d). Based on the XRD and EPMA results (see Fig. 4e and f), the σ + γ + α2 equilibrium is observed.
Fig. 4

EPMA images and XRD results of typical alloys after annealing at 1100 °C, which contain three phases after quenching: (a) the microstructure of alloy #B1, (b) the XRD result of alloy #B1, (c) the microstructure of alloy #B2, (d) the XRD result of alloy #B2, (e) the microstructure of alloy #B4 and (f) the XRD result of alloy #B4

Figure 5(a) shows the two-phase α2 + σ microstructure for alloy #B3 that agrees with the XRD result presented in Fig. 5(b). Figure 5(c) illustrates the γ + σ microstructure for alloy #B10, and the XRD result is shown in Fig. 5(d).
Fig. 5

EPMA images and XRD results of typical alloys, which contain two phases after quenching: (a) the microstructure of alloy #B3, (b) the XRD result of alloy #B3, (c) the microstructure of alloy #B10 and (d) the XRD result of alloy #B10

Figure 6(a) shows the tie-lines and tie-triangles for isothermal section at 1100 °C identified by the present work. Six three-phase and thirteen two-phase fields exit in the isothermal section (0-70 at.% Al content). Figure 6(b) shows the experimental isotherm of Leonard’s work[25] accomplished by experimental data of Kattner.[33]
Fig. 6

The 1100 °C isothermal section of phase diagrams: (a) the tie-lines, the tie-triangles and the 1100 °C isothermal section of phase diagram constructed by the present work and (b) Experimental isotherm of Leonard’s work[25] accomplished with experimental data of Kattner[33]

3.3 Isothermal Section at 1150 °C

Phases occurring in various samples annealed at 1150 °C are summarized in Table 4. Figure 7(a) exhibits microstructure image of alloy #C1, where three phases can be observed, including dark β, dark gray σ and white δ that agrees with the XRD results shown in Fig. 7(b). Two three-phase β + σ + γ and α2 + β + γ equilibria are found in the alloy #C2 and alloy #C3 (Fig. 7c and e) and the XRD patterns of them are shown in Fig. 7(d, f).
Table 4

Equilibrium compositions at 1150 °C measured with EPMA method

Number

Alloy (at.%)

Phase equilibrium

Phase composition (at.%)

Al

Ti

Nb

Phase 1/Phase 2/Phase 3

Phase 1

Phase 2

Phase 3

Al

Ti

Al

Ti

Al

Ti

#C1

23.7

32.5

43.8

β/δ/σ

18.7

39.5

20.9

25.8

29.1

24.5

#C2

39.1

43.2

17.7

β/γ/σ

36.1

45.7

44.8

40.6

35.1

32.9

#C3

38.9

51.5

9.6

α2/β/γ

37.5

53.1

36.0

52.6

44.7

46.1

#C4

42.5

53.1

4.4

α2

37.8

57.9

45.6

50.2

  

#C5

13.0

23.0

63.0

β/δ

8.6

23.1

18.6

17.3

  

#C6

13.0

16.0

71.0

β/δ

6.7

16.5

18.0

13.2

  

#C7

21.1

6.1

72.8

δ/σ

20.0

5.2

29.9

4.9

  

#C8

46.0

35.0

19.0

γ/σ

47.3

35.8

35.2

26.7

  

#C9

50.0

28.0

22.0

γ/σ

50.6

28.2

36.6

19.2

  

#C10

42.0

14.5

43.5

γ/σ

53.2

22.6

37.8

13.3

  

#C11

63.5

13.0

23.5

γ/ε

54.8

21.1

72.0

4.5

  

#C12

63.0

20.0

17.0

γ/ε

56.9

27.7

72.1

7.4

  

#C13

68.5

26.3

5.2

γ/η/ε

62.4

41.4

64.8

40.5

72.7

18.6

#C14

31.9

58.1

10.0

α2

31.5

60.0

32.3

56.2

  

#C15

37.9

47.7

14.4

β/γ

35.8

49.5

44.3

43.7

  

#C16

34.6

42.5

22.9

β/σ

34.7

47.4

33.9

34.2

  

#C17

57.2

11.3

31.5

γ/σ/ε

54.1

21.0

37.7

12.5

72.0

4.4

The Nb concentration in each phase can be calculated as 100—(Al concentrationin at.% + Ti concentration in at.%)

Fig. 7

EPMA images and XRD results of typical alloys, which contain three phases after quenching: (a) the microstructure of alloy #C1, (b) the XRD result of alloy #C1, (c) the microstructure of alloy #C2, (d) the XRD result of alloy #C2, (e) the microstructure of alloy #C3 and (f) the XRD result of alloy #C3

Two-phase equilibrium of σ + γ was found in the alloy #C10 (Fig. 8a) and XRD spectrum is shown in Fig. 8(b).
Fig. 8

EPMA images and XRD results of typical alloys, which contains three phases after quenching: (a) the microstructure of alloy #C10 and (b) the XRD result of alloy #C10

The identified tie-lines, tie-triangles, and the 1150 °C isothermal section of phase diagram constructed in the present work are illustrated in Fig. 9(a). Figure 9(b and c) shows the experimental results of the 1150 °C isothermal section from Chen’s work[26] and Ding’s work.[27] Figure 9(d) shows the experimental result of the 1200 °C isothermal section from Zhao’s work.[34] The data in the previous work of isothermal section at 1150 °C is limited. Chen’s[26] and Ding’s[27] experimental results support existence of γ1 phase, while according to the present work exits one two-phase field (σ + γ) in this area. Two three-phase fields (β + α2 + γ, β + σ + γ) are identified in Ti 30-60 at.% in the present work. The three-phase field β + δ + σ is also identified in the present work. Phase relationships between Zhao’s work[34] and present wok is quite similar.
Fig. 9

The of phase diagrams: (a) the tie-lines, the tie-trianges and the 1150 °C isothermal section of phase diagram constructed by the present work, (b) the experimental 1150 °C isothermal section from Chen’s work,[26] (c) the experimental 1150 °C isothermal section from Ding’s work[27] and (d) the experimental 1200 °C isothermal section from Zhao’s work[34]

4 Conclusions

The isothermal sections of the Ti-Al-Nb ternary system at 1000, 1100 and 1150 °C were determined using electron probe microanalysis and x-ray diffraction.
  1. 1.

    A small island-like region of single β0 present at 1000 °C, but absent at 1100 and 1150 °C. γ1 is not a stable phase at 1000 and 1150 °C.

     
  2. 2.

    Three three-phase fields (α2 + β0 + σ, β0 + σ + γ and α2 + β0 + γ) are identified in the 1000 °C isothermal section (30-60 at.% Ti content).

     
  3. 3.

    The 1100 °C isothermal section is firstly studied completely. Six three-phase and thirteen two-phase fields exit in the isothermal section (0-70 at.% Al content).

     
  4. 4.

    Two three-phase fields (β + α2 + γ, β + σ + γ) are identified in the isothermal section (30-60 at.% Ti content) at 1150 °C.

     

Notes

Acknowledgments

The work was financially supported by National Key Technologies R&D Program of China (Grant No. 2016YFB0701301), National Natural Science Foundation of China (Grant Numbers 51671218, 51501229). National Key Basic Research Program of China (973 Program) (Grant No. 2014CB644000).

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© ASM International 2018

Authors and Affiliations

  1. 1.School of Materials Science and EngineeringCentral South UniversityChangshaPeople’s Republic of China
  2. 2.State Key Laboratory of Powder MetallurgyCentral South UniversityChangshaPeople’s Republic of China

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