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An atomic-scale investigation on the initial precipitation behavior of nitrides in TiAl alloys
Intermetallics 121 (2020) 106777
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Intermetallics journal homepage: http://www.elsevier.com/locate/intermet
An atomic-scale investigation on the initial precipitation behavior of nitrides in TiAl alloys Pei Liu a, b, Jingpei Xie a, b, *, Aiqin Wang a, **, Douqin Ma a, b, Zhiping Mao a, b a
College of Materials Science and Engineering, Henan University of Science and Technology, Luoyang, 471023, China Provincial and Ministerial Co-construction of Collaborative Innovation Center for Non-ferrous Metal New Materials and Advanced Processing Technology, Luoyang, 471023, China
b
A R T I C L E I N F O
A B S T R A C T
Keywords: TiAl alloys Nitride precipitate Crystallographic orientation relationship Interface atomic structure
In the present work, the initial precipitation behavior of nitrides in TiAl alloys was investigated by monitoring the morphology, distribution, crystallographic orientation relationship and interface atomic structure of the nitride precipitates using TEM and HRTEM. It is found that there are two kinds of nitride precipitates (Ti2AlN and Ti3AlN) forming in the initial precipitation stage of Ti-49Al-1N alloys. The rod-like Ti2AlN precipitates tend to distribute at γ(TiAl)/α2(Ti3Al) interface with the crystallographic orientation relationship of [1120]Ti3Al// [1120]Ti2AlN//[101]TiAl, (0001)Ti3Al//(0001)Ti2AlN//(111)TiAl. Both the Ti3Al(0001)/Ti2AlN(0001) interface and Ti2AlN(0001)/TiAl(111) interface display a plane-to-plane matching with coherent atomic correspondence. The plate-like Ti3AlN precipitates tend to distribute in the γ(TiAl) matrix with the crystallographic orientation relationship of [110]Ti3AlN//[110]TiAl, (111)Ti3AlN//(111)TiAl. The Ti3AlN(111)/TiAl(111) interface displays an edge-to-edge matching with coherent atomic correspondence. The morphology, distribution, crystallographic orientation relationship and interface atomic structure of nitride precipitates are determined by the crystal structure and lattice misfit of nitride precipitates and matrix. The understanding here will be useful in guiding morphology design and dispersion controlling of the nitride precipitates in TiAl alloys, and will further be benefit for improving the mechanical properties of TiAl alloys.
1. Introduction TiAl alloys have been regarded as potential high-temperature structural materials in aerospace industry because of their excellent physical and mechanical properties at elevated temperature [1–6]. However, the inherent poor ductility of TiAl alloys at room temperature prohibits their wider applications. It has been found that alloying technique is a practical way to improve the properties of TiAl alloys. In recent years, numerous research studies have been devoted on the composition optimization and microstructure adjustment of TiAl alloys [7–12]. Among various alloy elements, interstitial atoms B, C, N, O are always impossible to be completely avoided during the melting, casting and forging process. These interstitial atoms have been proven to notably affect the microstructure and mechanical properties of TiAl al loys [13–16]. As a main component of the atmosphere, the effect of nitrogen atoms on the microstructure and mechanical properties of TiAl alloys has been
studied extensively. Tian et al. [17] and Liu et al. [18] have reported that two kinds of nitrides, Ti2AlN and Ti3AlN, could precipitate in the TiAl alloys containing nitrogen. Zhang et al. [19] have reported that the addition of nitrogen could lead to the refinement of lamellar structure and colony size. Nam et al. [20] have reported that the nitrogen could reduce the mean lamellar thickness of TiAl alloy due to the precipitation of Ti2AlN and Ti3AlN, which is beneficial for the fracture toughness. Yuki et al. [17] have reported that the addition of 0.3 at.% nitrogen could increase both the room-temperature ductility and strength due to the grain refinement and precipitation hardening. Base on the above analysis, it can be concluded that the precipitation of Ti2AlN and Ti3AlN has a significant influence on the strength and ductility of TiAl alloys. Thus proper control of initial precipitation behavior for Ti2AlN and Ti3AlN, especially the distribution, morphology and orientation of these precipitates, is crucial for the optimum microstructure and mechanical properties of TiAl alloys containing nitrogen. However, to the best knowledge of the authors, there is still a lack of detailed information
* Corresponding author. College of Materials Science and Engineering, Henan University of Science and Technology, Luoyang, 471023, China. ** Corresponding author. E-mail addresses: [email protected] (J. Xie), [email protected] (A. Wang). https://doi.org/10.1016/j.intermet.2020.106777 Received 11 September 2019; Received in revised form 2 March 2020; Accepted 16 March 2020 Available online 26 March 2020 0966-9795/© 2020 Elsevier Ltd. All rights reserved.
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Intermetallics 121 (2020) 106777
Fig. 1. The precipitation behavior of Ti2AlN in TiAl alloys. (a) TEM image of TiAl alloys; (b) TEM image of γ/α2 interface; (c) HRTEM image of γ/α2 interface; (d) Electronic diffraction pattern of α2–Ti3Al (area 1 in Fig. 1c); (e) Electronic diffraction pattern of Ti2AlN (area 2 in Fig. 1c); (f) Electronic diffraction pattern of γ–TiAl (area 3 in Fig. 1c).
regarding the initial precipitation behavior of nitrides in TiAl alloys. In this work, by monitoring the morphology, distribution, crystal lographic orientation relationship and interface atomic structure of the precipitates using TEM and HRTEM, we have investigated the initial precipitation behavior of nitrides in TiAl alloys. The understanding here will be useful in guiding morphology design and dispersion controlling of the nitride precipitates in TiAl alloys, and will further be benefit for improving the mechanical properties of TiAl alloys.
2. Experimental procedures The nominal chemical composition of experimental alloys in the present work is Ti-49Al-1N. The reason why the chemical composition is designed as Ti-49Al-1N is that we want to keep the same chemical composition with the previous investigation by Tian et al. [17] in order to compare the initial and final precipitation behaviors of nitrides in TiAl alloys. The experimental TiAl alloys were prepared in a vacuum arc-melting furnace under the protection of argon atmosphere using a non-consumable tungsten electrode. The raw materials used in the
Fig. 2. Crystallographic orientation relationship and atomic structure of Ti3Al/Ti2AlN/TiAl interface. (a) SAED pattern from the Ti3Al/Ti2AlN/TiAl interface; (b) Indexing of the SAED pattern in Fig. 2(a); (c) Enlarged HRTEM image of the Ti3Al/Ti2AlN/TiAl interface. 2
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Fig. 3. The precipitation behavior of Ti3AlN in TiAl alloys. (a) TEM image of Ti3AlN in TiAl alloys; (b) enlarged TEM image of the square area in Fig. 3(a); (c) Electronic diffraction pattern of Ti3AlN; (d) HRTEM image of Ti3AlN; (f) IFFT image of the square area in Fig. 3(d).
present work were pure Ti (99.9 wt%), pure Al (99.9 wt%) and TiN (99.9 wt%). The ingot was re-melted five times in order to avoid the segregation of alloying elements. Then the sample of 5 � 5 � 5 mm3 was solution annealed at 1573 K for 24 h, followed by aging at 1173 K for 0.5 h. After that, the thin plate about 0.5 mm thick was wire-electrode cut from the aged specimen and grounded to 50 μm by mechanical thinning, and then was cut into 3 mm diameter foil. Finally, the foil was prepared by argon ion milling using Gatan 691 Precision Ion Polishing System and the TEM/HRTEM observations were performed on Talos F200x field emission transmission electron microscopy. The atomic models in this study were mapped by the software of Materials Studio 7.0 [21].
3. Results and discussion 3.1. Precipitation behavior of Ti2AlN in TiAl alloys Fig. 1 shows the initial precipitation behavior of Ti2AlN in the TiAl alloys. It can be seen from Fig. 1(a) that the microstructure of TiAl alloys consists of fine γ(TiAl)þα2(Ti3Al) lamellar colony and a small number of γ(TiAl) phases distributed at colony boundaries. Fig. 1(b) shows the enlarged TEM image of red square area in Fig. 1(a). It can be seen from Fig. 1(b) that there is a rod-like nano-phase distributing at the γ(TiAl)/ α2(Ti3Al) interface. Fig. 1(c) is the HRTEM image of γ(TiAl)/α2(Ti3Al) interface in Fig. 1(b), and it can be clearly seen that the lattice arrangement in the γ(TiAl)/α2(Ti3Al) interface is different from that in
Fig. 4. Crystallographic orientation relationship and atomic structure of Ti3AlN/TiAl interface. (a) SAED pattern from the Ti3AlN/TiAl interface; (b) HRTEM image of the Ti3AlN/TiAl interface; (c) Enlarged HRTEM image of the square area in Fig. 4(b). 3
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display a plane-to-plane matching with good atomic correspondence, which means that these interfaces are coherent.
Table 1 Crystal structure and lattice parameters of Ti3Al, Ti2AlN, TiAl and Ti3AlN. Phase
Structure type
Space group
a (nm)
c (nm)
Ti3Al Ti2AlN TiAl Ti3AlN
Hexagonal Hexagonal Tetragonal cubic
P63/mmc (194) P63/mmc (194) P4/mmm (123) Pm-3m (221)
0.5780 0.2998 0.3984 0.4112
0.4647 1.3610 0.4065 0.4112
3.2. Precipitation behavior of Ti3AlN in TiAl alloys Apart from the rod-like Ti2AlN precipitates at γ(TiAl)/α2(Ti3Al) interface, as shown in Fig. 3(a) and (b), it is also observed that a platelike precipitate distribute in the γ(TiAl) matrix. Fig. 3(c) shows the electronic diffraction pattern of the plate-like precipitate. After cali bration, it can be determined that Fig. 3(c) is the electronic diffraction pattern of Ti3AlN along [110] zone axis. Fig. 3(d) shows the HRTEM image of the plate-like Ti3AlN precipitate along Ti3AlN[110] zone axis and Fig. 3(e) shows the IFFT image of the square area in Fig. 3(d). It can be seen from Fig. 3(e) that the atomic arrangement of Ti3AlN presents the characteristics of cubic crystal structure, the interplanar spacing of Ti3AlN(001) is 0.41 nm, while the interplanar spacing of Ti3AlN(110) is 0.29 nm. All these measuring results are consistent with the experi mental results of Ti3AlN (cubic crystal structure with the lattice parameter: a ¼ 0.4112 nm) [24,25], which further demonstrates that the plate-like precipitate in the γ(TiAl) matrix is Ti3AlN. Fig. 4 shows the crystallographic orientation relationship and atomic structure of Ti3AlN/TiAl interface. Fig. 4(a) shows the selected area electron diffraction (SAED) pattern from the Ti3AlN/TiAl interface, the red lines represent the reflections from TiAl, and the orange lines represent the reflections from Ti3AlN. As indexed in Fig. 4(a), the inci dent beam is parallel to [110]Ti3AlN and [110]TiAl, and the (111) plane of Ti3AlN is parallel to the (111) plane of TiAl. Thus, their orientation relationship could be determined as follows:
either γ(TiAl) or α2(Ti3Al) phase, which indicates that there must be a new phase distributing at the γ(TiAl)/α2(Ti3Al) interface. Fig. 1(d), (e) and 1(f) are the FFT patterns corresponding to red square area 1, 2 and 3 in Fig. 1(c), respectively. After calibration, it can be determined that Fig. 1(d) is the electronic diffraction pattern of α2(Ti3Al) along [1120] zone axis, Fig. 1(e) is the electronic diffraction pattern of Ti2AlN along [1120] zone axis, and Fig. 1(f) is the electronic diffraction pattern of γ(TiAl) along [101] zone axis. Thus, it can be concluded that Ti2AlN precipitates tend to distribute at γ(TiAl)/α2(Ti3Al) interface with rodlike morphology. Fig. 2 shows the crystallographic orientation relationship and atomic structure of Ti3Al/Ti2AlN/TiAl interface. Fig. 2(a) shows the selected area electron diffraction (SAED) pattern from the Ti3Al/Ti2AlN/TiAl interface. Fig. 2(b) is corresponding indexed pattern of Fig. 2(a), the red dots, orange dots and blue dots represent the reflections from Ti3Al, Ti2AlN and TiAl, respectively. As indexed in Fig. 2(b), the incident beam is parallel to [1120]Ti3Al, [1120]Ti2AlN and [101]TiAl, and the Ti3Al (0001) is parallel to Ti2AlN(0001) and TiAl(111). Thus, their orientation relationship could be determined as follows: ½1120�Ti3Al ==½1120�Ti2A1N = =½101�TiA1 ; ð0001ÞTi3A1 ==ð0001ÞTi2A1N ==ð111ÞTiA1
½1�Ti3A1N = = ½110�TiA1 ; ð111ÞTi3A1N = = ð111ÞTiA1
A enlarged HRTEM image along [1120]Ti3Al//[1120]Ti2AlN// [101]TiAl is shown in Fig. 2(c), the red dots represent the atoms of Ti3Al, the orange circles represent the atoms of Ti2AlN, and the blue dots represent the atoms of TiAl. The atomic stacking sequence of Ti2AlN is visible from the central part of Fig. 2(c) and is obviously different from that of Ti3Al and TiAl. The atomic stacking sequence of Ti2AlN can be
Fig. 4(b) shows the HRTEM image along [110]Ti3AlN//[110]TiAl and an enlarged HRTEM image of the square area in Fig. 4(b) is shown in Fig. 4(c), the red dots represent the atoms of TiAl and the blue dots represent the atoms of Ti3AlN. it can be seen clearly from Fig. 4(c) that the Ti3AlN/TiAl interface displays an edge-to-edge matching with good atomic correspondence, the Ti3AlN(111) is parallel to the TiAl(111) and the lattice misfit between them is 0.025, which means that this interface is coherent.
described as the sequence of BABABA (the underlined letters refer to Al layers and the rest to Ti layers) along the [0001] direction, which is consistent with the hexagonal layered crystal structure of Ti2AlN [22, 23]. In addition, it can also be seen clearly from Fig. 2(c) that the Ti3Al/Ti2AlN interface is Ti3Al(0001)//Ti2AlN(0001), and the Ti2AlN/TiAl interface is Ti2AlN(0001)//TiAl(111). Both the Ti3Al (0001)/Ti2AlN(0001) interface and Ti2AlN(0001)/TiAl(111) interface
3.3. Discussion Based on the above analysis, it can be seen that there are two kinds of nitride precipitates (Ti2AlN and Ti3AlN) forming during the initial
Fig. 5. The atomic model of TiAl, Ti2AlN, Ti3Al surface and TiAl/Ti2AlN/Ti3Al interface (a) top view of TiAl(111) surface; (b) top view of Ti2AlN(0001) surface; (c) top view of Ti3Al(0001) surface; (d) side view of TiAl/Ti2AlN/Ti3Al interface (In Fig. 5(a) ~(c), small spheres are the atoms in the third layer, medium spheres are the atoms in the second layer, large spheres are the atoms in the atoms in the first layer). 4
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precipitation stage of Ti-49Al-1N alloys. The rod-like Ti2AlN precipitates tend to distribute at γ(TiAl)/α2(Ti3Al) interface, both the Ti3Al(0001)/ Ti2AlN(0001) interface and Ti2AlN(0001)/TiAl(111) interface display a plane-to-plane matching with coherent atomic correspondence. While the plate-like Ti3AlN precipitates tend to distribute in the γ(TiAl) matrix, the Ti3AlN(111)/TiAl(111) interface displays an edge-to-edge matching with coherent atomic correspondence. In this section, we tried to explain the difference of the morphology, distribution, crystallographic orientation relationship and interface atomic structure between the Ti2AlN and Ti3AlN precipitates in the TiAl alloys by analyzing the crystal structure and misfit of nitride precipitates and matrix. Table 1 shows the crystal structure and lattice parameters of Ti3Al, Ti2AlN, TiAl and Ti3AlN, and it can be seen from Table 1 that both the crystal structure of Ti3Al and Ti2AlN is based on hexagonal structure, while both the crystal structure of TiAl and Ti3AlN are based on the fcc lattice. As for the Ti2AlN precipitates, because it has a similar crystal structure with Ti3Al and the γ(TiAl)/α2(Ti3Al) interface could provide the preferred sites for the alloying element segregation, the nitrogen atoms tend to accumulate at the γ(TiAl)/α2(Ti3Al) interface and form the Ti2AlN precipitates during the initial precipitation stage of Ti-49Al-1N alloys. Fig. 5(a)~(c) shows the atomic model of close-packed plane TiAl(111), Ti2AlN(0001) and Ti3Al(0001), respectively. It can be seen from Fig. 5(a)~(c) that the atomic arrangement in the TiAl(111), Ti2AlN (0001) and Ti3Al(0001) are the same. The lattice parameters of TiAl (111) surface with u[110� and v[011] are: u ¼ 0.564 nm, v ¼ 0.571 nm, α ¼ 120� , the lattice parameters of Ti2AlN(0001) surface with u[1210� and v[2110� are: u ¼ 0.587 nm, v ¼ 0.587 nm, α ¼ 120� , while the lattice parameters of Ti3Al(0001) surface with u[1210� and v[2110�are: u ¼ 0.578 nm, v ¼ 0.578 nm, α ¼ 120� . It can be calculated that the lattice misfit along u and v direction of Ti2AlN/TiAl is 0.039 and 0.027, the lattice misfit along u and v direction of Ti2AlN/Ti3Al is 0.015. Therefore, as shown in Fig. 5(d), the Ti2AlN(0001) plane will preferential precip itate from TiAl(111) plane and Ti3Al(0001) plane, forming the Ti3Al (0001)/Ti2AlN(0001) and Ti2AlN(0001)/TiAl(111) plane-to-plane matching interface with coherent atomic correspondence in order to reduce the total energy of the system. As a member of MAX phases, Ti2AlN has a hexagonal crystalline structure with every two Ti6N octa hedral separated by an Al layer along the [0001] direction. The Ti2AlN (0001) plane is made up with the same kinds of atoms, while the Ti2AlN along the [0001] direction is made up with the different kinds of atoms. In this case, the growth rate of Ti2AlN along the [0001] direction is much lower than those parallel to the (0001) plane [26,27]. Therefore, when precipitated at the γ(TiAl)/α2(Ti3Al) interface, the Ti2AlN phase
grew quickly in radial directions parallel to the (0001) plane, i.e. along the γ(TiAl)/α2(Ti3Al)interface, forming the rod-like morphology with the long axis is along Ti2AlN(0001) plane. As for the Ti3AlN precipitate, we only observed it tends to precipitate in the γ(TiAl) matrix, which consistent with the research results of Liu et al. [18]. This is mainly because both the crystal structure of Ti3AlN and γ(TiAl) is cubic structure type, thus the Ti3AlN phase tends to pre cipitate in the γ(TiAl) matrix. The reason why the nitrogen atoms accumulated in the γ(TiAl)/α2(Ti3Al) interface would not form the Ti3AlN is that the Ti3AlN has a cubic crystal structure which is different from the hexagonal layered crystalline structure for Ti2AlN. Thus, the Ti3AlN could not grow along the γ(TiAl)/α2(Ti3Al) interface from the perspective of growth kinetics. In this case, as shown in Fig. 6(a), the close-packed plane TiAl(111) could be the habit plane of Ti3AlN pre cipitate. Fig. 6(b) and (c) show the atomic model of TiAl(111) and Ti3AlN(111) surface, it can be seen that the atomic arrangement in the TiAl(111) and Ti3AlN(111) are similar. Therefore, the Ti3AlN(111) tends to nuclear in the TiAl(111) plane. Then the Ti3AlN grows parallel to the (111) plane and normal to the (111) plane with almost the same velocity due to its cubic crystal structure, and as shown in Fig. 6(d), the Ti3AlN(111) keeps to parallel to the TiAl(111) during the growth process because the Ti3AlN(111)/TiAl(111) edge-to-edge matching coherent interface could reduce the total energy of system. Therefore, plate-like Ti3AlN precipitates tend to distribute in the γ(TiAl) matrix, forming the Ti3AlN(111)/TiAl(111) edge-to-edge matching interface with coherent atomic correspondence. Because the aging time in the present work is very short and visual field of TEM is limited, we could not give a conclusion for which nitride precipitate (Ti2AlN or Ti3AlN) is formed more in the present Ti-49Al-1N alloys aged at 1173 K for 0.5 h. However, according to some previous work [17,18,28], it seems to achieve a consensus that the Ti3AlN would likely nucleate first during the precipitation stage. This is because the nitrogen has much greater solubility in α2(Ti3Al) phase compared with γ(TiAl) phase [29], and thus the nitrogen atoms in γ(TiAl) phase are easier to precipitate and form the Ti3AlN during the aging stage due to the crystal structure similarity between γ(TiAl) and Ti3AlN. It is worth pointing that the Ti3AlN is a metastable phase, and it can be split to form the Ti2AlN with the increase of nitrogen atomic content, aging tem perature and time [28]. How the morphology variants of split up nitride compounds influence the mechanical properties of TiAl alloys is an interesting point for investigation in further studies. 4. Conclusions In the present work, the initial precipitation behavior of nitrides in
Fig. 6. The schematic diagram of precipitation behavior of Ti3AlN in TiAl matrix (a) Ti3AlN(111) nucleate in the TiAl(111); (b) top view of TiAl(111) surface; (c) top view of Ti3AlN(111) surface; (d) side view of Ti3AlN(111)/TiAl(111) edge-toedge matching interface (In Fig. 6(b)~(c), large spheres are the atoms in the atoms in the first layer, small spheres are the atoms in the second and third layer). 5
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TiAl alloys was investigated by monitoring the morphology, distribu tion, crystallographic orientation relationship and interface atomic structure of the nitride precipitates using TEM and HRTEM. These conclusions could be obtained from the present work:
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(1) There are two kinds of nitride precipitates, i.e. rod-like Ti2AlN and plate-like Ti3AlN, forming in the initial precipitation stage of Ti-49Al-1N alloys. (2) The rod-like Ti2AlN precipitates tend to distribute at γ(TiAl)/ α2(Ti3Al) interface with the crystallographic orientation rela tionship of [1120]Ti3Al//[1120]Ti2AlN//[101]TiAl, (0001)Ti3Al// (0001)Ti2AlN//(111)TiAl. Both the Ti3Al(0001)/Ti2AlN(0001) interface and Ti2AlN(0001)/TiAl(111) interface display a planeto-plane matching with coherent atomic correspondence. (3) The plate-like Ti3AlN precipitates tend to distribute in the γ(TiAl) matrix with the crystallographic orientation relationship of [110]Ti3AlN//[110]TiAl, (111)Ti3AlN//(111)TiAl. The Ti3AlN(111)/ TiAl(111) interface displays an edge-to-edge matching with coherent atomic correspondence. (4) The morphology, distribution, crystallographic orientation rela tionship and interface atomic structure of nitride precipitates are determined by crystal structure and misfit of nitride precipitates and matrix. The results in this study will be useful in guiding morphology design and dispersion controlling of the precipitates, and will further be benefit for improving the mechanical properties of TiAl alloys. Declaration of competing interest No conflict of interest. CRediT authorship contribution statement Pei Liu: Formal analysis, Writing - original draft. Jingpei Xie: Conceptualization. Aiqin Wang: Formal analysis. Douqin Ma: Formal analysis. Zhiping Mao: Formal analysis. Acknowledgments The authors acknowledge the financial support from National Nat ural Science Foundation of China (grant nos.51771070, U1604251) and the China Postdoctoral Science Foundation (Grant No.2019M662491). References [1] G. Chen, Y.B. Peng, G. Zheng, Z.X. Qi, M.Z. Wang, H.C. Yu, C.L. Dong, C.T. Liu, Polysynthetic twinned TiAl single crystals for high-temperature applications, Nat. Mater. 15 (2016) 876–882. [2] H. Wu, G.H. Fan, L. Geng, X.P. Cui, M. Huang, Nanoscale origins of the oriented precipitation of Ti3Al in Ti-Al systems, Scripta Mater. 125 (2016) 34–38. [3] L. Song, X.G. Hu, T.B. Zhang, J.S. Li, Precipitation behaviors in a quenched high Nb-containing TiAl alloy during annealing, Intermetallics 89 (2017) 79–85. [4] F. Appel, H. Clemens, F.D. Fischer, Modeling concepts for intermetallic titanium aluminides, Prog. Mater. Sci. 81 (2016) 55–124.
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Intermetallics journal homepage: http://www.elsevier.com/locate/intermet
An atomic-scale investigation on the initial precipitation behavior of nitrides in TiAl alloys Pei Liu a, b, Jingpei Xie a, b, *, Aiqin Wang a, **, Douqin Ma a, b, Zhiping Mao a, b a
College of Materials Science and Engineering, Henan University of Science and Technology, Luoyang, 471023, China Provincial and Ministerial Co-construction of Collaborative Innovation Center for Non-ferrous Metal New Materials and Advanced Processing Technology, Luoyang, 471023, China
b
A R T I C L E I N F O
A B S T R A C T
Keywords: TiAl alloys Nitride precipitate Crystallographic orientation relationship Interface atomic structure
In the present work, the initial precipitation behavior of nitrides in TiAl alloys was investigated by monitoring the morphology, distribution, crystallographic orientation relationship and interface atomic structure of the nitride precipitates using TEM and HRTEM. It is found that there are two kinds of nitride precipitates (Ti2AlN and Ti3AlN) forming in the initial precipitation stage of Ti-49Al-1N alloys. The rod-like Ti2AlN precipitates tend to distribute at γ(TiAl)/α2(Ti3Al) interface with the crystallographic orientation relationship of [1120]Ti3Al// [1120]Ti2AlN//[101]TiAl, (0001)Ti3Al//(0001)Ti2AlN//(111)TiAl. Both the Ti3Al(0001)/Ti2AlN(0001) interface and Ti2AlN(0001)/TiAl(111) interface display a plane-to-plane matching with coherent atomic correspondence. The plate-like Ti3AlN precipitates tend to distribute in the γ(TiAl) matrix with the crystallographic orientation relationship of [110]Ti3AlN//[110]TiAl, (111)Ti3AlN//(111)TiAl. The Ti3AlN(111)/TiAl(111) interface displays an edge-to-edge matching with coherent atomic correspondence. The morphology, distribution, crystallographic orientation relationship and interface atomic structure of nitride precipitates are determined by the crystal structure and lattice misfit of nitride precipitates and matrix. The understanding here will be useful in guiding morphology design and dispersion controlling of the nitride precipitates in TiAl alloys, and will further be benefit for improving the mechanical properties of TiAl alloys.
1. Introduction TiAl alloys have been regarded as potential high-temperature structural materials in aerospace industry because of their excellent physical and mechanical properties at elevated temperature [1–6]. However, the inherent poor ductility of TiAl alloys at room temperature prohibits their wider applications. It has been found that alloying technique is a practical way to improve the properties of TiAl alloys. In recent years, numerous research studies have been devoted on the composition optimization and microstructure adjustment of TiAl alloys [7–12]. Among various alloy elements, interstitial atoms B, C, N, O are always impossible to be completely avoided during the melting, casting and forging process. These interstitial atoms have been proven to notably affect the microstructure and mechanical properties of TiAl al loys [13–16]. As a main component of the atmosphere, the effect of nitrogen atoms on the microstructure and mechanical properties of TiAl alloys has been
studied extensively. Tian et al. [17] and Liu et al. [18] have reported that two kinds of nitrides, Ti2AlN and Ti3AlN, could precipitate in the TiAl alloys containing nitrogen. Zhang et al. [19] have reported that the addition of nitrogen could lead to the refinement of lamellar structure and colony size. Nam et al. [20] have reported that the nitrogen could reduce the mean lamellar thickness of TiAl alloy due to the precipitation of Ti2AlN and Ti3AlN, which is beneficial for the fracture toughness. Yuki et al. [17] have reported that the addition of 0.3 at.% nitrogen could increase both the room-temperature ductility and strength due to the grain refinement and precipitation hardening. Base on the above analysis, it can be concluded that the precipitation of Ti2AlN and Ti3AlN has a significant influence on the strength and ductility of TiAl alloys. Thus proper control of initial precipitation behavior for Ti2AlN and Ti3AlN, especially the distribution, morphology and orientation of these precipitates, is crucial for the optimum microstructure and mechanical properties of TiAl alloys containing nitrogen. However, to the best knowledge of the authors, there is still a lack of detailed information
* Corresponding author. College of Materials Science and Engineering, Henan University of Science and Technology, Luoyang, 471023, China. ** Corresponding author. E-mail addresses: [email protected] (J. Xie), [email protected] (A. Wang). https://doi.org/10.1016/j.intermet.2020.106777 Received 11 September 2019; Received in revised form 2 March 2020; Accepted 16 March 2020 Available online 26 March 2020 0966-9795/© 2020 Elsevier Ltd. All rights reserved.
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Fig. 1. The precipitation behavior of Ti2AlN in TiAl alloys. (a) TEM image of TiAl alloys; (b) TEM image of γ/α2 interface; (c) HRTEM image of γ/α2 interface; (d) Electronic diffraction pattern of α2–Ti3Al (area 1 in Fig. 1c); (e) Electronic diffraction pattern of Ti2AlN (area 2 in Fig. 1c); (f) Electronic diffraction pattern of γ–TiAl (area 3 in Fig. 1c).
regarding the initial precipitation behavior of nitrides in TiAl alloys. In this work, by monitoring the morphology, distribution, crystal lographic orientation relationship and interface atomic structure of the precipitates using TEM and HRTEM, we have investigated the initial precipitation behavior of nitrides in TiAl alloys. The understanding here will be useful in guiding morphology design and dispersion controlling of the nitride precipitates in TiAl alloys, and will further be benefit for improving the mechanical properties of TiAl alloys.
2. Experimental procedures The nominal chemical composition of experimental alloys in the present work is Ti-49Al-1N. The reason why the chemical composition is designed as Ti-49Al-1N is that we want to keep the same chemical composition with the previous investigation by Tian et al. [17] in order to compare the initial and final precipitation behaviors of nitrides in TiAl alloys. The experimental TiAl alloys were prepared in a vacuum arc-melting furnace under the protection of argon atmosphere using a non-consumable tungsten electrode. The raw materials used in the
Fig. 2. Crystallographic orientation relationship and atomic structure of Ti3Al/Ti2AlN/TiAl interface. (a) SAED pattern from the Ti3Al/Ti2AlN/TiAl interface; (b) Indexing of the SAED pattern in Fig. 2(a); (c) Enlarged HRTEM image of the Ti3Al/Ti2AlN/TiAl interface. 2
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Fig. 3. The precipitation behavior of Ti3AlN in TiAl alloys. (a) TEM image of Ti3AlN in TiAl alloys; (b) enlarged TEM image of the square area in Fig. 3(a); (c) Electronic diffraction pattern of Ti3AlN; (d) HRTEM image of Ti3AlN; (f) IFFT image of the square area in Fig. 3(d).
present work were pure Ti (99.9 wt%), pure Al (99.9 wt%) and TiN (99.9 wt%). The ingot was re-melted five times in order to avoid the segregation of alloying elements. Then the sample of 5 � 5 � 5 mm3 was solution annealed at 1573 K for 24 h, followed by aging at 1173 K for 0.5 h. After that, the thin plate about 0.5 mm thick was wire-electrode cut from the aged specimen and grounded to 50 μm by mechanical thinning, and then was cut into 3 mm diameter foil. Finally, the foil was prepared by argon ion milling using Gatan 691 Precision Ion Polishing System and the TEM/HRTEM observations were performed on Talos F200x field emission transmission electron microscopy. The atomic models in this study were mapped by the software of Materials Studio 7.0 [21].
3. Results and discussion 3.1. Precipitation behavior of Ti2AlN in TiAl alloys Fig. 1 shows the initial precipitation behavior of Ti2AlN in the TiAl alloys. It can be seen from Fig. 1(a) that the microstructure of TiAl alloys consists of fine γ(TiAl)þα2(Ti3Al) lamellar colony and a small number of γ(TiAl) phases distributed at colony boundaries. Fig. 1(b) shows the enlarged TEM image of red square area in Fig. 1(a). It can be seen from Fig. 1(b) that there is a rod-like nano-phase distributing at the γ(TiAl)/ α2(Ti3Al) interface. Fig. 1(c) is the HRTEM image of γ(TiAl)/α2(Ti3Al) interface in Fig. 1(b), and it can be clearly seen that the lattice arrangement in the γ(TiAl)/α2(Ti3Al) interface is different from that in
Fig. 4. Crystallographic orientation relationship and atomic structure of Ti3AlN/TiAl interface. (a) SAED pattern from the Ti3AlN/TiAl interface; (b) HRTEM image of the Ti3AlN/TiAl interface; (c) Enlarged HRTEM image of the square area in Fig. 4(b). 3
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display a plane-to-plane matching with good atomic correspondence, which means that these interfaces are coherent.
Table 1 Crystal structure and lattice parameters of Ti3Al, Ti2AlN, TiAl and Ti3AlN. Phase
Structure type
Space group
a (nm)
c (nm)
Ti3Al Ti2AlN TiAl Ti3AlN
Hexagonal Hexagonal Tetragonal cubic
P63/mmc (194) P63/mmc (194) P4/mmm (123) Pm-3m (221)
0.5780 0.2998 0.3984 0.4112
0.4647 1.3610 0.4065 0.4112
3.2. Precipitation behavior of Ti3AlN in TiAl alloys Apart from the rod-like Ti2AlN precipitates at γ(TiAl)/α2(Ti3Al) interface, as shown in Fig. 3(a) and (b), it is also observed that a platelike precipitate distribute in the γ(TiAl) matrix. Fig. 3(c) shows the electronic diffraction pattern of the plate-like precipitate. After cali bration, it can be determined that Fig. 3(c) is the electronic diffraction pattern of Ti3AlN along [110] zone axis. Fig. 3(d) shows the HRTEM image of the plate-like Ti3AlN precipitate along Ti3AlN[110] zone axis and Fig. 3(e) shows the IFFT image of the square area in Fig. 3(d). It can be seen from Fig. 3(e) that the atomic arrangement of Ti3AlN presents the characteristics of cubic crystal structure, the interplanar spacing of Ti3AlN(001) is 0.41 nm, while the interplanar spacing of Ti3AlN(110) is 0.29 nm. All these measuring results are consistent with the experi mental results of Ti3AlN (cubic crystal structure with the lattice parameter: a ¼ 0.4112 nm) [24,25], which further demonstrates that the plate-like precipitate in the γ(TiAl) matrix is Ti3AlN. Fig. 4 shows the crystallographic orientation relationship and atomic structure of Ti3AlN/TiAl interface. Fig. 4(a) shows the selected area electron diffraction (SAED) pattern from the Ti3AlN/TiAl interface, the red lines represent the reflections from TiAl, and the orange lines represent the reflections from Ti3AlN. As indexed in Fig. 4(a), the inci dent beam is parallel to [110]Ti3AlN and [110]TiAl, and the (111) plane of Ti3AlN is parallel to the (111) plane of TiAl. Thus, their orientation relationship could be determined as follows:
either γ(TiAl) or α2(Ti3Al) phase, which indicates that there must be a new phase distributing at the γ(TiAl)/α2(Ti3Al) interface. Fig. 1(d), (e) and 1(f) are the FFT patterns corresponding to red square area 1, 2 and 3 in Fig. 1(c), respectively. After calibration, it can be determined that Fig. 1(d) is the electronic diffraction pattern of α2(Ti3Al) along [1120] zone axis, Fig. 1(e) is the electronic diffraction pattern of Ti2AlN along [1120] zone axis, and Fig. 1(f) is the electronic diffraction pattern of γ(TiAl) along [101] zone axis. Thus, it can be concluded that Ti2AlN precipitates tend to distribute at γ(TiAl)/α2(Ti3Al) interface with rodlike morphology. Fig. 2 shows the crystallographic orientation relationship and atomic structure of Ti3Al/Ti2AlN/TiAl interface. Fig. 2(a) shows the selected area electron diffraction (SAED) pattern from the Ti3Al/Ti2AlN/TiAl interface. Fig. 2(b) is corresponding indexed pattern of Fig. 2(a), the red dots, orange dots and blue dots represent the reflections from Ti3Al, Ti2AlN and TiAl, respectively. As indexed in Fig. 2(b), the incident beam is parallel to [1120]Ti3Al, [1120]Ti2AlN and [101]TiAl, and the Ti3Al (0001) is parallel to Ti2AlN(0001) and TiAl(111). Thus, their orientation relationship could be determined as follows: ½1120�Ti3Al ==½1120�Ti2A1N = =½101�TiA1 ; ð0001ÞTi3A1 ==ð0001ÞTi2A1N ==ð111ÞTiA1
½1�Ti3A1N = = ½110�TiA1 ; ð111ÞTi3A1N = = ð111ÞTiA1
A enlarged HRTEM image along [1120]Ti3Al//[1120]Ti2AlN// [101]TiAl is shown in Fig. 2(c), the red dots represent the atoms of Ti3Al, the orange circles represent the atoms of Ti2AlN, and the blue dots represent the atoms of TiAl. The atomic stacking sequence of Ti2AlN is visible from the central part of Fig. 2(c) and is obviously different from that of Ti3Al and TiAl. The atomic stacking sequence of Ti2AlN can be
Fig. 4(b) shows the HRTEM image along [110]Ti3AlN//[110]TiAl and an enlarged HRTEM image of the square area in Fig. 4(b) is shown in Fig. 4(c), the red dots represent the atoms of TiAl and the blue dots represent the atoms of Ti3AlN. it can be seen clearly from Fig. 4(c) that the Ti3AlN/TiAl interface displays an edge-to-edge matching with good atomic correspondence, the Ti3AlN(111) is parallel to the TiAl(111) and the lattice misfit between them is 0.025, which means that this interface is coherent.
described as the sequence of BABABA (the underlined letters refer to Al layers and the rest to Ti layers) along the [0001] direction, which is consistent with the hexagonal layered crystal structure of Ti2AlN [22, 23]. In addition, it can also be seen clearly from Fig. 2(c) that the Ti3Al/Ti2AlN interface is Ti3Al(0001)//Ti2AlN(0001), and the Ti2AlN/TiAl interface is Ti2AlN(0001)//TiAl(111). Both the Ti3Al (0001)/Ti2AlN(0001) interface and Ti2AlN(0001)/TiAl(111) interface
3.3. Discussion Based on the above analysis, it can be seen that there are two kinds of nitride precipitates (Ti2AlN and Ti3AlN) forming during the initial
Fig. 5. The atomic model of TiAl, Ti2AlN, Ti3Al surface and TiAl/Ti2AlN/Ti3Al interface (a) top view of TiAl(111) surface; (b) top view of Ti2AlN(0001) surface; (c) top view of Ti3Al(0001) surface; (d) side view of TiAl/Ti2AlN/Ti3Al interface (In Fig. 5(a) ~(c), small spheres are the atoms in the third layer, medium spheres are the atoms in the second layer, large spheres are the atoms in the atoms in the first layer). 4
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precipitation stage of Ti-49Al-1N alloys. The rod-like Ti2AlN precipitates tend to distribute at γ(TiAl)/α2(Ti3Al) interface, both the Ti3Al(0001)/ Ti2AlN(0001) interface and Ti2AlN(0001)/TiAl(111) interface display a plane-to-plane matching with coherent atomic correspondence. While the plate-like Ti3AlN precipitates tend to distribute in the γ(TiAl) matrix, the Ti3AlN(111)/TiAl(111) interface displays an edge-to-edge matching with coherent atomic correspondence. In this section, we tried to explain the difference of the morphology, distribution, crystallographic orientation relationship and interface atomic structure between the Ti2AlN and Ti3AlN precipitates in the TiAl alloys by analyzing the crystal structure and misfit of nitride precipitates and matrix. Table 1 shows the crystal structure and lattice parameters of Ti3Al, Ti2AlN, TiAl and Ti3AlN, and it can be seen from Table 1 that both the crystal structure of Ti3Al and Ti2AlN is based on hexagonal structure, while both the crystal structure of TiAl and Ti3AlN are based on the fcc lattice. As for the Ti2AlN precipitates, because it has a similar crystal structure with Ti3Al and the γ(TiAl)/α2(Ti3Al) interface could provide the preferred sites for the alloying element segregation, the nitrogen atoms tend to accumulate at the γ(TiAl)/α2(Ti3Al) interface and form the Ti2AlN precipitates during the initial precipitation stage of Ti-49Al-1N alloys. Fig. 5(a)~(c) shows the atomic model of close-packed plane TiAl(111), Ti2AlN(0001) and Ti3Al(0001), respectively. It can be seen from Fig. 5(a)~(c) that the atomic arrangement in the TiAl(111), Ti2AlN (0001) and Ti3Al(0001) are the same. The lattice parameters of TiAl (111) surface with u[110� and v[011] are: u ¼ 0.564 nm, v ¼ 0.571 nm, α ¼ 120� , the lattice parameters of Ti2AlN(0001) surface with u[1210� and v[2110� are: u ¼ 0.587 nm, v ¼ 0.587 nm, α ¼ 120� , while the lattice parameters of Ti3Al(0001) surface with u[1210� and v[2110�are: u ¼ 0.578 nm, v ¼ 0.578 nm, α ¼ 120� . It can be calculated that the lattice misfit along u and v direction of Ti2AlN/TiAl is 0.039 and 0.027, the lattice misfit along u and v direction of Ti2AlN/Ti3Al is 0.015. Therefore, as shown in Fig. 5(d), the Ti2AlN(0001) plane will preferential precip itate from TiAl(111) plane and Ti3Al(0001) plane, forming the Ti3Al (0001)/Ti2AlN(0001) and Ti2AlN(0001)/TiAl(111) plane-to-plane matching interface with coherent atomic correspondence in order to reduce the total energy of the system. As a member of MAX phases, Ti2AlN has a hexagonal crystalline structure with every two Ti6N octa hedral separated by an Al layer along the [0001] direction. The Ti2AlN (0001) plane is made up with the same kinds of atoms, while the Ti2AlN along the [0001] direction is made up with the different kinds of atoms. In this case, the growth rate of Ti2AlN along the [0001] direction is much lower than those parallel to the (0001) plane [26,27]. Therefore, when precipitated at the γ(TiAl)/α2(Ti3Al) interface, the Ti2AlN phase
grew quickly in radial directions parallel to the (0001) plane, i.e. along the γ(TiAl)/α2(Ti3Al)interface, forming the rod-like morphology with the long axis is along Ti2AlN(0001) plane. As for the Ti3AlN precipitate, we only observed it tends to precipitate in the γ(TiAl) matrix, which consistent with the research results of Liu et al. [18]. This is mainly because both the crystal structure of Ti3AlN and γ(TiAl) is cubic structure type, thus the Ti3AlN phase tends to pre cipitate in the γ(TiAl) matrix. The reason why the nitrogen atoms accumulated in the γ(TiAl)/α2(Ti3Al) interface would not form the Ti3AlN is that the Ti3AlN has a cubic crystal structure which is different from the hexagonal layered crystalline structure for Ti2AlN. Thus, the Ti3AlN could not grow along the γ(TiAl)/α2(Ti3Al) interface from the perspective of growth kinetics. In this case, as shown in Fig. 6(a), the close-packed plane TiAl(111) could be the habit plane of Ti3AlN pre cipitate. Fig. 6(b) and (c) show the atomic model of TiAl(111) and Ti3AlN(111) surface, it can be seen that the atomic arrangement in the TiAl(111) and Ti3AlN(111) are similar. Therefore, the Ti3AlN(111) tends to nuclear in the TiAl(111) plane. Then the Ti3AlN grows parallel to the (111) plane and normal to the (111) plane with almost the same velocity due to its cubic crystal structure, and as shown in Fig. 6(d), the Ti3AlN(111) keeps to parallel to the TiAl(111) during the growth process because the Ti3AlN(111)/TiAl(111) edge-to-edge matching coherent interface could reduce the total energy of system. Therefore, plate-like Ti3AlN precipitates tend to distribute in the γ(TiAl) matrix, forming the Ti3AlN(111)/TiAl(111) edge-to-edge matching interface with coherent atomic correspondence. Because the aging time in the present work is very short and visual field of TEM is limited, we could not give a conclusion for which nitride precipitate (Ti2AlN or Ti3AlN) is formed more in the present Ti-49Al-1N alloys aged at 1173 K for 0.5 h. However, according to some previous work [17,18,28], it seems to achieve a consensus that the Ti3AlN would likely nucleate first during the precipitation stage. This is because the nitrogen has much greater solubility in α2(Ti3Al) phase compared with γ(TiAl) phase [29], and thus the nitrogen atoms in γ(TiAl) phase are easier to precipitate and form the Ti3AlN during the aging stage due to the crystal structure similarity between γ(TiAl) and Ti3AlN. It is worth pointing that the Ti3AlN is a metastable phase, and it can be split to form the Ti2AlN with the increase of nitrogen atomic content, aging tem perature and time [28]. How the morphology variants of split up nitride compounds influence the mechanical properties of TiAl alloys is an interesting point for investigation in further studies. 4. Conclusions In the present work, the initial precipitation behavior of nitrides in
Fig. 6. The schematic diagram of precipitation behavior of Ti3AlN in TiAl matrix (a) Ti3AlN(111) nucleate in the TiAl(111); (b) top view of TiAl(111) surface; (c) top view of Ti3AlN(111) surface; (d) side view of Ti3AlN(111)/TiAl(111) edge-toedge matching interface (In Fig. 6(b)~(c), large spheres are the atoms in the atoms in the first layer, small spheres are the atoms in the second and third layer). 5
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TiAl alloys was investigated by monitoring the morphology, distribu tion, crystallographic orientation relationship and interface atomic structure of the nitride precipitates using TEM and HRTEM. These conclusions could be obtained from the present work:
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(1) There are two kinds of nitride precipitates, i.e. rod-like Ti2AlN and plate-like Ti3AlN, forming in the initial precipitation stage of Ti-49Al-1N alloys. (2) The rod-like Ti2AlN precipitates tend to distribute at γ(TiAl)/ α2(Ti3Al) interface with the crystallographic orientation rela tionship of [1120]Ti3Al//[1120]Ti2AlN//[101]TiAl, (0001)Ti3Al// (0001)Ti2AlN//(111)TiAl. Both the Ti3Al(0001)/Ti2AlN(0001) interface and Ti2AlN(0001)/TiAl(111) interface display a planeto-plane matching with coherent atomic correspondence. (3) The plate-like Ti3AlN precipitates tend to distribute in the γ(TiAl) matrix with the crystallographic orientation relationship of [110]Ti3AlN//[110]TiAl, (111)Ti3AlN//(111)TiAl. The Ti3AlN(111)/ TiAl(111) interface displays an edge-to-edge matching with coherent atomic correspondence. (4) The morphology, distribution, crystallographic orientation rela tionship and interface atomic structure of nitride precipitates are determined by crystal structure and misfit of nitride precipitates and matrix. The results in this study will be useful in guiding morphology design and dispersion controlling of the precipitates, and will further be benefit for improving the mechanical properties of TiAl alloys. Declaration of competing interest No conflict of interest. CRediT authorship contribution statement Pei Liu: Formal analysis, Writing - original draft. Jingpei Xie: Conceptualization. Aiqin Wang: Formal analysis. Douqin Ma: Formal analysis. Zhiping Mao: Formal analysis. Acknowledgments The authors acknowledge the financial support from National Nat ural Science Foundation of China (grant nos.51771070, U1604251) and the China Postdoctoral Science Foundation (Grant No.2019M662491). References [1] G. Chen, Y.B. Peng, G. Zheng, Z.X. Qi, M.Z. Wang, H.C. Yu, C.L. Dong, C.T. Liu, Polysynthetic twinned TiAl single crystals for high-temperature applications, Nat. Mater. 15 (2016) 876–882. [2] H. Wu, G.H. Fan, L. Geng, X.P. Cui, M. Huang, Nanoscale origins of the oriented precipitation of Ti3Al in Ti-Al systems, Scripta Mater. 125 (2016) 34–38. [3] L. Song, X.G. Hu, T.B. Zhang, J.S. Li, Precipitation behaviors in a quenched high Nb-containing TiAl alloy during annealing, Intermetallics 89 (2017) 79–85. [4] F. Appel, H. Clemens, F.D. Fischer, Modeling concepts for intermetallic titanium aluminides, Prog. Mater. Sci. 81 (2016) 55–124.
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