TEM characterization of the precipitation reaction in Ti–48Al–10Nb alloy
H.W. Liu
∗, Y. Yuan, Z.G. Liu, J.-M. Liu, X.N. Zhao, Y.Y. Zhu
National Laboratory of Solid State Microstructure, Nanjing University, Nanjing 210093, People’s Republic of China Accepted 7 September 2005
Abstract
A detailed transmission electron microscopy (TEM) study has been made of the morphology and the crystallographic characteristics of pre- cipitation of the␥1-Ti4Nb3Al9 in the␥-TiAl matrix in Ti–Al–Nb system. It is revealed that the␥1-Ti4Nb3Al9 precipitates, which are formed by a normal quench at 1473 K and ageing treatment at 1073 K, take needle-like morphology with a growing axis parallel to [0 0 1] direction of the matrix. The needle-like precipitates act as barrier grid to inhibit dislocation moving on the slipping plane{1 1 1}of the␥-TiAl matrix, which helps to improve the strength of the alloy. Selected electron microscopy analysis shows that the orientation relationship between the␥1phase and the
␥-TiAl matrix is [0 0 1]␥1//[0 0 1]␥, (1 0 0)␥1//(1 1 0)␥and (0 1 0)␥1//(¯1 1 0)␥. The needle-like morphology of the precipitated phase is discussed with the coherency stress across the precipitate/matrix interface that is considered to be the main factor controlling the precipitate morphology.
© 2005 Elsevier B.V. All rights reserved.
PACS: 71.20.L; 68.37.Lp; 81.40
Keywords: ␥-TiAl intermetallics; Transmission electron microscopy; Precipitate; Crystallographic orientation relationship
1. Introduction
Owing to their ordered structure and partially covalent bonding, intermetallics show a high elastic modulus, retain a high strength at elevated temperature, and are resistant to creep, recrystallization and corrosion. TiAl-based alloys have been specially investigated for the applications of aerial materials and automotive engine components due to their attractive properties such as high strength, low density and excellent creep resistance [1,2]. Although ␥-TiAl-based alloys show these promising properties at high temperature, their wide application is hindered by the poor ductility and toughness at ambient temperature. Microstructural control and alloying are the main methods to improve the properties of these alloys. Adding transition metals with high melting temperatures such as Nb, V, Cr and Mn is generally beneficial to enhance the high temperature strength of TiAl–base alloy [3].
␥-TiAl alloys containing 5 at.% and 10 at.% Nb are regarded as the most perspective␥-TiAl alloys for high temperature inter- metallics materials. Carbide and silicide precipitation in C + Si
∗Corresponding author. Tel.: +86 25 8359 7060; fax: +86 25 8359 5535.
E-mail address:[email protected] (H.W. Liu).
alloyed␥-TiAl[4,5]and precipitation behavior in Ag-modified L12–Al3Ti and L10–TiAl (Ag)[6,7]have been investigated pre- viously. To the knowledge of the present authors, precipitation in high Nb-bearing␥-TiAl alloys has not been studied in detail.
Hellwig et al. [8] and Chen et al. [9] have reported the dia- gram of the 1273, 1473 and 1673 K isothermal section of the Ti–Al–Nb system.While the existence or non-existence of the phase NbTiAl3(␥1) is still not clear[10–12]. Recently, Chen et al.[13]gave a detailed report on the identification and crys- tallographic structure of the phase Ti4Nb3Al9(␥1). According to Chen et al.’s report[13], the chemical formula of ␥1phase is Ti4Nb3Al9. The unit cell of␥1-Ti4Nb3Al9phase contains 16 atoms and consists of four␥-TiAl unit cells. The␥1-Ti4Nb3Al9 phase is a tetragonal phase with lattice parameterain the range 0.558–0.584 nm andcin the range 0.815–0.845 nm. The space group of the lattice isP4/mmm. While it has not been reported that the␥1-Ti4Nb3Al9phase was found in Ti–48Al–10Nb alloy.
Furthermore, the ageing reaction in high-Nb bearing alloys has also not been investigated up to now. Now the question is whether this␥1-Ti4Nb3Al9phase could be identified in Ti–48Al–10Nb alloy in a composition range where the␥1-Ti4Nb3Al9 phase should be stable. Thus, the present study was undertaken in order to reveal the precipitation reaction and the nature and charac- teristics of the precipitated phase in Ti–48Al–10Nb alloy after ageing treatment.
0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.msea.2005.09.039
H.W. Liu et al. / Materials Science and Engineering A 412 (2005) 328–335 329 Table 1
The nominal and analytical composition of the Ti–48Al–10Nb alloy
Ti Al Nb Fe Total
Nominal 42 48 10 0 100
Analytical 43.28 45.10 10.71 0.81 100
2. Experimental procedure
The material used in this study has a nominal composition of Ti–48Al–10Nb (atom percentage). The alloy was prepared by levitation melting of high purity (99.9%) Ti, Al and Nb in copper crucible under argon atmosphere. Heart-button ingot of about 200 g was reversed and remelted for three times in order to reduce segregation. Since the weight loss of this melting method is very low, the final alloy composition verified by X-ray fluorescence spectrometer (XRF) is close to the nominal com- position (seeTable 1). Then, the alloy ingot was homogenized at 1473 K for 4 h and ageing at 1073 K for 34 h. X-ray diffrac- tion test was carried out on a Switzerland X’ TRA with Cu K␣ radiation.
The transmission electron microscopy (TEM) samples were cut into thin slices from the ingots followed by mechanical grinding to about 60m thickness. Preparation of discs was carried out using double-jet electropolishing technique with an electrolyte consisting of 65 vol.% methanol, 30 vol.% butanol and 5 vol.% perchloric acid at 30 V and 253 K to 243 K. The foils were examined in a Philips Tecnai F20 transmission elec- tron microscope equipped with an energy-dispersive X-ray analysis system. High-resolution TEM investigation was also performed.
3. Results and discussion
3.1. X-ray diffraction analysis of Ti–48Al–10Nb alloy
Fig. 1shows the X-ray diffraction pattern of Ti–48Al–10Nb alloy after annealing treatment. Identification result is shown inTable 2. It shows only␥-TiAl,␣2-Ti3Al and B2[14]phase existing and no other abnormal peaks were observed. The␣2- Ti3Al phase results from inadequate homogeneous heat treat- ment. There is no new peak after ageing treatment and intensity of some peaks of␥-TiAl becomes stronger after ageing treat-
Fig. 1. X-ray diffraction patterns of the Ti–48Al–10Nb alloy aged at 1073 K for various time.
Table 2
Phase identification of X-ray diffraction patterns of␥,␣2and␥1phases
Peak Ageing time (h) Identify
No. θ(◦) 0 10 18 34 ␣2 B2 ␥ ␥1
1 21.74 * * * * 0 0 1 0 0 2
2 31.56 * * * * 1 1 0 2 0 0
3 35.86 * * 2 0 0
4 38.68 * * * * 1 1 1 2 0 2
5 42.02 * * * * 1 1 0
6 42.88 * * * * 2 0 1
7 44.34 * * * 0 0 2 0 0 4
8 45.24 * * * 2 0 0 2 2 0
9 50.72 * * * 2 0 1 2 2 2
10 55.48 * * * 1 1 2 2 0 4
11 65.30 * * * 2 0 2 2 2 4
12 65.96 * * * 2 2 0 4 0 0
ment than after annealing treatment. The peaks of␥-TiAl phase and those of ␥1-Ti4Nb3Al9 phase are almost overlapped[13].
In order to make sure that the precipitated phase is really␥1- Ti4Nb3Al9phase, TEM and high-resolution electron transmis- sion microscopy (HRTEM) investigation on the alloy need to be carried out carefully.
3.2. Microstructure of the Ti–48Al–10Nb alloy
After annealing treatment, Ti–48Al–10Nb alloy is com- posed of equiaxed single␥-TiAl phase microstructure and small amount of ␣2-Ti3Al remained from as-cast state. Fig. 3(a) shows such an equiaxed single ␥-TiAl phase microstructure (see isothermal section of the Ti–Al–Nb alloy shown inFig. 2 [9]).Fig. 3(b)–(d) are microstructures taken at [0 1 0] axis of the alloy aged at 1073 K for 10 h, 18 h and 34 h, respectively.
Large quantity of the needle-like precipitates can be found to distribute homogenously in ␥-TiAl matrix. Contrast-free line can be observed in every precipitate.
Fig. 2. Isothermal section of the Ti–Al–Nb system at 1273 K.
Fig. 3. TEM microstructure of Ti–48Al–10Nb alloy: (a) annealing at 1473 K for 4 h; (b) aged at 1073 K for 10 h; (c) aged at 1073 K for 18 h; (d) aged at 1073 K for 34 h.
3.3. Identification and crystallography of the precipitates
Fig. 4(a) is a TEM bright field image showing the needle- like precipitates in Ti–48Al–10Nb alloy taken at [0 1 0]␥. Fig. 4(b) is a center dark field image with additional super- lattice (1 1 0)␥1. The morphology of the precipitated phase is needle-like. In order to determine the structure of the pre- cipitated phase, systematic tilting of the sample was pro- ceeded for various areas. Figs. 4(c), (e) and 5(a), (c), (e) show some of the electron diffraction patterns (EDPs) of the precipitated phase. These EDPs can be indexed accord- ing to the ␥1 phase with a lattice parameter of a= 0.56 nm, c= 0.82 nm by internal standard method with reference to TiAl matrix (the crystal structure and lattice constants are known) as shown in Figs. 4(d), (f) and 5(b), (d), (f). The experimental angles and theoretic angles between these zone axes are coincident well. All the experimental dhkl val- ues of the lattice plane distances obtained from the dis- tances Rhkl between the (h k l) reflection spots and the central
spot in EDPs are in good agreement with the calculated ones.
EDS from the needle-like precipitated phase is shown in Fig. 6. The result shows that the precipitated phase mainly con- sists of Ti, Al and Nb.
3.4. Orientation relationship and lattice correspondence between the precipitates and theγ-TiAl matrix
A set of EDPs patterns taken from the matrix/precipitate area and the index results (Figs. 4 and 5) allow the crys- tallography of the precipitated phase to be determined.
The incident beam directions of Fig. 4(c) and (e) are [0 1 0]␥//[1 1 0]␥1 and [0 1 3]␥//[1 1 3]␥1. The incident beam directions of Fig. 5(a), (c) and (e) are [0 0 1]␥//[0 0 1]␥1, [1 1 1]␥//[2 0 1]␥1and [1 1 0]␥//[1 0 0]␥1, respectively. Moreover, as can be seen fromTable 3, the angles between two zone axes [U1V1W1] and [U2V2W2] of␥phase are also in good agree- ment with those of␥1phase.
H.W. Liu et al. / Materials Science and Engineering A 412 (2005) 328–335 331
Fig. 4. The microstructure of the precipitates viewed at [0 1 0]␥and corresponding EDPs: (a) bright field image; (b) dark field image; (c) and (d) [0 1 0]␥//[1 1 0]␥1; (e) and (f) [0 1 3]␥//[1 1 3]␥1.
Thus, the following orientation relationship between the pre- cipitated phase␥1and matrix␥-TiAl yields:
(0 0 1)␥//(0 0 1)␥1; [1 1 0]␥//[1 0 0]␥1;
[¯1 1 0]␥//[0 1 0]␥1 (1)
This orientation relationship is in agreement with the report by Chen et al. [13]. The growing axis of the needle-like ␥1
phase is [0 0 1]␥//[0 0 1]␥1and the smallest and the largest pro- jection length can be determined and viewed at [0 0 1]␥//[0 0 1]␥1 and [0 1 0]␥//[1 1 0]␥1directions, respectively. HRTEM observa-
Fig. 5. EDPs corresponding toFig. 4(a): (a) and (b) [0 0 1]␥//[0 0 1]␥1; (c) and (d) [1 1 1]␥//[2 0 1]␥1; (e) and (f) [1 1 0]␥//[1 0 0]␥1.
tion along the zone axis [0 0 1]␥//[0 0 1]␥1and [0 1 0]␥//[1 1 0]␥1 are shown in Figs. 7 and 8. InFig. 7, a quadratic precipitate taken from [0 0 1]␥//[0 0 1]␥1 direction indicates that the inter- face (¯1 1 0)␥//(0 1 0)␥1and (1 1 0)␥//(1 0 0)␥1matches well. In-
set (a) and (b) are fast Fourier transform (FFT) images taken from part of HRTEM image corresponding to matrix and precipitate, respectively. The arrows point out the additional superlattice spots of the␥1phase. In-set (c) is an inverse fast Fourier trans-
H.W. Liu et al. / Materials Science and Engineering A 412 (2005) 328–335 333
Fig. 6. EDS of the precipitate indicating the composites of Ti, Al and Nb.
Table 3
Angles between zone axes [U1V1W1] and [U2V2W2] of␥and␥1phase [U1V1W1]␥ [U2V2W2]␥ θ (␥)
(◦)
θc(␥1) (◦)
[U1V1W1]␥1 [U2V2W2]␥1
0 0 1 0 1 0 90 90 0 0 1 1 1 0
0 0 1 0 1 3 18.75 17.89 0 0 1 1 1 3
0 0 1 1 1 1 55.23 53.86 0 0 1 2 0 1
0 0 1 1 1 0 90 90 0 0 1 1 0 0
form (IFFT) image giving distinct interface matching figure. In Fig. 8, a needle-like precipitate taken from [0 1 0]␥//[1 1 0]␥1 direction indicates that the interface (0 0 1)␥//(0 0 2)␥1matches well. In-set (a) and (b) are FFT images taken from part of HRTEM image corresponding to matrix and precipitate, respec- tively. The arrows point out the additional superlattice spots of the␥1phase. So, all the lattice planes relevant to the precipitate have been confirmed by the HRTEM observation.
4. Discussion
4.1. Existence or non-existence ofγ1-Ti4Nb3Al9phase The existence or non-existence of the phase␥1was discussed for many years. Since the first investigation of the Ti–Al–Nb system the existence of a tetragonal phase, denoted as NiTiAl3 or␥1, has been claimed by several investigators[9,10,15]. Chen et al.[13]finally gave a detailed report about the phase␥1.
Fig. 7. HRTEM image of the precipitate with corresponding EDP viewed at [0 0 1] zone axis: in-set (a) and (b) are FFT images taken at part of HRTEM image corresponding to the matrix and the precipitate, respectively; in-set (c) is an IFFT image giving distinct interface matching figure.
Fig. 8. HRTEM image of the precipitate with corresponding EDP viewed at [0 1 0] zone axis. In-set (a) and (b) are FFT images taken at part of HRTEM image corresponding to the matrix and the precipitate, respectively.
The chemistry, structure, morphology, diffraction patterns and the crystallographic orientation relationship of the precip- itated phase found in this Ti–48Al–10Nb alloy aged at 1073 K for 34 h are revealed to be in agreement with the features of the phase␥1. The␥1phase acts as a precipitated phase in aged Ti–48Al–10Nb alloy, which has not been observed previously.
According to the isothermal section of the Ti–Al–Nb system at 1273 K by Chen et al.[12], shown asFig. 2, Ti–48Al–10Nb alloy treated at 1273 K consists of single phase ␥-TiAl. The
mono-phase region (␥) is conterminous with three-phase region (␥+␥1+). Although the isothermal section of the Ti–Al–Nb system at 1073 K (the ageing temperature used here) is not avail- able up to now, it is possible that Ti–48Al–10Nb alloy treated at 1073 K enters into the three-phase region (␥+␥1+) from mono-phase (␥) region shown in Fig. 2. To the knowledge of the present authors, the phase␥1-Ti4Nb3Al9existing as precipi- tate in Ti–48Al–10Nb alloy revealed here has not been reported earlier. So this work could be regarded as a strong support of
H.W. Liu et al. / Materials Science and Engineering A 412 (2005) 328–335 335 Table 4
Lattice parameters of␥and␥1phase and misfits,ε1,ε2andε3
Phase a(nm) c(nm) ε1(%) ε2(%) ε3(%)
␥ 0.3976 0.4049 – – –
␥1 0.56 0.82 0.025 1.821 1.821
the existence of the ternary phase␥1-Ti4Nb3Al9in Ti–Al–Nb system.
4.2. Needle-like precipitates formed in L12matrix (γ-TiAl) The morphology of a precipitate, which is formed by a nor- mal quench and ageing treatment, is generally determined by its boundary between the precipitate and the matrix. When the crystal structures of precipitates and the matrix are different, the structure of the interphase boundary becomes the decisive factor in the morphology of precipitates[16,17].
The structures of␥-TiAl and␥1-Ti4Nb3Al9are all based on fcc lattice. The lattice parameters of these two phases and the lattice misfit are tabulated in Table 4. The lattice misfits are estimated using the following equations:
ε1= d002␥1 −d001␥
d001␥ ; ε2= d100␥1−d110␥
d110␥ ; ε3= d010␥1 −d¯110␥
d¯110␥
(2) The lattice misfit in [0 0 1]␥1 directions is much smaller than that in the [1 0 0]␥1and [0 1 0]␥1directions for the precipitated
␥1phase in␥-TiAl matrix. Thus, the growth of the precipitates is restrained in both the [1 0 0]␥1and [0 1 0]␥1directions and the precipitates takes a needle-like shape in morphology parallel to the [0 0 1]␥1directions at initial ageing time.
4.3. The role of the precipitates playing in strength of the matrix
Precipitation strengthening works when the precipitates formed by a normal quench and ageing treatment hinder dis- location slipping on the slipping plane. The slipping plane of
␥-TiAl is{1 1 1}␥. The long-axis direction of the␥1precipitates is parallel to [0 0 1]␥. The angle between [1 1 1]␥and [0 0 1]␥is 54.7◦. The needle-like precipitates will act as a barrier grid to inhibit dislocation moving on the slipping plane{1 1 1}␥. Thus, the strength of the matrix can be improved. If we let the growth direction of the needle-like precipitates parallel to [1 1 1]␥, the strength of the matrix can be improved further.
5. Conclusions
1. Ageing at 1073 K for 34 h leads to needle-like␥1-Ti4Nb3Al9 phase precipitates from the Ti–48Al–10Nb alloy.
2. The precipitated phase has tetragonal structure with lattice constanta= 0.56 nm andc= 0.82 nm determined by system- atic tilting method in TEM and confirmed by HRTEM obser- vation.
3. The orientation relationship between the precipitate␥1phase and ␥-TiAl matrix is (0 0 1)␥//(0 0 1)␥1; [1 1 0]␥//[1 0 0]␥1; [¯1 1 0]␥//[0 1 0]␥1.
4. The interfaces of ␥-TiAl/␥1-Ti4Nb3Al9 system viewed at [0 0 1]␥//[0 0 1]␥1 direction, (1 1 0)␥//(0 1 0)␥ and (1 1 0)␥//(1 0 0)␥1, matches well, respectively.
5. Microstructure development during ageing in the present alloy system was found to be mainly controlled by coherency stress across the precipitate/matrix interface.
Acknowledgement
This work was financially supported by Chinesisch- Deutsches Zeutrum fuer Wissenschaftsfoerderung, Beijing and Jiangsu Planned Projects for Postdoctoral Research Funds, China.
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