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Lamellar microstructure alignment and fracture toughness in Ti–47Al alloy by electromagnetic confinement and directional solidification
Materials Science & Engineering A 621 (2015) 94–99
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Lamellar microstructure alignment and fracture toughness in Ti–47Al alloy by electromagnetic confinement and directional solidification Yujun Du, Jun Shen n, Yilong Xiong, Zhao Shang, Lei Wang, Hengzhi Fu State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China
art ic l e i nf o
a b s t r a c t
Article history: Received 23 July 2014 Received in revised form 21 October 2014 Accepted 23 October 2014 Available online 31 October 2014
Large-scale Ti–47Al sample without contamination is shaped by electromagnetic confinement and directional solidification. Controlled by a Ti–43Al–3Si seed, numerous columnar grains grow directionally in transition region and the lamellar microstructure within those grains is aligned parallel to the growth direction. Because the solid/liquid interface is convex towards the liquid and its h/r value is 0.25, one of the columnar grains in sample center grows divergently and then a near single (NS) crystal forms with crystal growth. Fracture toughness of the NS crystal with the desired lamellae is detected and the value is measured 34.7 MPa m1/2. The crack propagation path is checked after the fracture toughness testing and a schematic model is used to illustrate the relevant toughness mechanism. & 2014 Elsevier B.V. All rights reserved.
Keywords: Ti–47Al PST crystal Seed Fracture toughness
1. Introduction Polysynthetically twinned (PST) crystals of γ-TiAl based alloys have been studied extensively due to their good combination of strength and ductility at room temperature when the lamellar microstructure is aligned parallel to the tensile direction [1–3]. And these excellent mechanical properties make them prospective materials for aerospace and automobile industry. However, intense chemical reactions occur inevitably between the TiAl melt and various ceramic crucibles. As a consequence, numerous crucible fragments are produced during the directional solidification and are significantly detrimental to the mechanical properties of TiAl alloys [4]. Up to now, optical float zone (FZ) furnace has been employed to prepare non-polluting PST crystals by using a seeding method [5,6]. But this method is only useful for laboratory research to produce samples of small section size. It is mainly owing to the fact that the surface tension used to confine the molten zone would decrease with increasing sample diameter [7,8]. Consequently, the largest TiAl sample prepared by FZ furnace was no more than 14 mm in diameter [9]. To overcome this shortcoming, an additional electromagnetic force was employed to confine the molten zone in various electromagnetic processing techniques. Electromagnetic confinement and directional solidification (EMCDS), as a type of electromagnetic processing, has been widely researched to obtain non-contamination samples of stainless steel
n
Corresponding author. Tel.: þ 86 29 88494708; fax: þ86 29 88494080. E-mail address: [email protected] (J. Shen).
http://dx.doi.org/10.1016/j.msea.2014.10.066 0921-5093/& 2014 Elsevier B.V. All rights reserved.
and Ni-based superalloys over the years [10,11]. As the main force used to confine the molten zone is electromagnetic force rather than surface tension, samples with 18 mm in diameter were obtained successfully. Therefore, it is expected to obtain largescale samples of TiAl alloys with the desired lamellar microstructure by this technology. However, little work has been carried out at present. In our previous studies, 12.5 mm diameter samples of Ti–47Al alloy were obtained by EMCDS [12]. Besides, controlled by a T–43Al–3Si seed, columnar grains grew directionally and the lamellar microstructure within those grains was aligned well. In this paper, Ti–47Al sample with 20 mm in diameter is obtained and the lamellae in the sample are aligned well by using a seeding method. Besides, the crystal growth during EMCDS is studied by discussing the stability of the molten zone and the shape of the solid/liquid (S/L) interface. Additionally, the fracture toughness of the lamellar microstructure is detected and the relevant toughness mechanism is discussed by checking the crack propagation path.
2. Experiment Nominal chemical compositions of Ti–47Al and Ti–43Al–3Si alloys were used in EMCDS process. The purity of raw materials used in this study is Ti 99.96 (wt%), Al 99.99% (wt%) and Si 99.95% (wt%), respectively. A master bar with 20 mm in diameter was cut from a Ti–47Al ingot by electro-discharge machining (EDM). And a seed bar with the same diameter, in which the α2/γ lamellae are aligned parallel to the longitudinal direction, was cut from a
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Ti–43Al–3Si ingot. The preparation of the seed ingot was described elsewhere [13]. The EMCDS system is shown schematically in Fig. 1, which mainly consists of confinement coil, screen, feeding part, withdrawing part and liquid cooling metal. The experimental procedures were as follows: first, the master bar was fixed on the feeding part and the seed bar was fixed on the withdrawing part. The chamber was pumped to 6.6 10 3 Pa, and then filled with purified argon; second, the top part of the seed bar and the bottom part of the master bar were melted simultaneously and mixed together by induction heat when high-frequency alternating current traveled through the confinement coil. At the same time, the electromagnetic force, whose direction pointed to the center of melt, would act on the melt and confine it to be a molten zone with certain height; third, the sample began to be pulled downward into the liquid cooling metal at 10 μm/s after a holding period of 10 min; finally, the sample was quenched into the liquid cooling metal to observe the S/L interfaces. More details about EMCDS were provided in Ref. [10,11]. For optical microscopy, the directionally solidified (DS) sample was sectioned longitudinally by EDM. And the sections were mechanically polished using standard metallographic techniques and then etched with a solution of 10 ml HF, 5 ml HNO3 and 85 ml H2O. After metallographic preparation, the macrostructure was photographed by a digital camera and the microstructure was revealed by a Leica DM4000M optical microscope. To identify the fracture toughness of the sample, two three-point bend (3PB) specimens with dimension of 6 mm 3 mm 30 mm were cut from the DS sample. And the lamellar boundary within them was ensured parallel to the longitudinal direction. Prior to fracture testing, all the flat faces of bend specimens were ground on emery paper to a finish of 1000 grit. A narrow straight 3 mm-deep notch was also machined by EDM. But fatigue pre-crack was not initiated at the notch tip prior to 3PB testing. The scheme of the specimen is shown in Fig. 2. Fracture toughness testing was performed in air at room temperature in an Instron 3382 universal
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test machine and at a cross-head speed of 0.05 mm/min. Scanning electron microscopy (SEM) in back-scattered electron (BSE) and second electron (SE) mode was used to check the crack propagation path after fracture testing.
3. Results 3.1. Directional solidification in EMCDS using a seed The image in Fig. 3(a) shows the macrostructure of Ti–47Al sample obtained by EMCDS with a Ti–43Al–3Si seed. Caused by the instability of power supply, the shape of the molten zone varies slightly and thus undulated sample surface is observed from the macrostructure. Nevertheless, columnar grains grow directionally and four regions could be distinguished: (I) annealing region, (II) transition region, (III) DS region and (IV) quenched zone. In the transition region, numerous columnar gains form and grow competitively, and most of them are eliminated with crystal growth. At the end of solidification, a near single (NS) crystal forms in sample center besides some other narrow grains in sample edges, as shown in Fig. 3(a). This phenomenon is largely attributed to that the S/L interface is convex slightly towards the liquid, and more details will be discussed later. Fig. 3(b) and (c) shows the microstructures of the NS crystal marked in Fig. 3(a), respectively. It can be seen that the NS crystal only consists of one single grain and the lamellae in the grain are aligned parallel to the growth direction. The max width of the NS crystal is measured 16 mm from Fig. 3(a), which suggests that Ti–47Al sample with the desired lamellar microstructure is successfully obtained by EMCDS. Note that, due to improper handling in the experiment, new grains nucleate in front of the NS crystal and then grow stably. And the 451 angle between the lamellae and the growth direction suggests that these grains are β phase [14]. According to phase selection map of binary Ti–Al alloy [15], either α or β cells could grow stably without nucleation of β or α phase. When α phase grows continuously in stable stage, it is difficult for new α or β phase to nucleate. For this case, maintaining a stable molten zone is the most critical factor for the continuous crystal growth. However, a sudden decrease of power volt occurs during the present experiment. This leads to a higher undercooling and thus new β grains nucleate in front of the α grain. As mentioned above, not only α but also β phase could grow stably along the growth direction. Therefore, once β phase nucleates ahead of the S/L interface, it will grow stably although the power returns to normal condition instantly. Afterwards, the growth of α grain is terminated and β grains dominate in the sample subsequently. 3.2. Microstructures
Fig. 1. Scheme for EMCDS system.
Fig. 2. Scheme of specimen for fracture testing.
The images in Fig. 4 show the microstructures in different regions of the sample. Fig. 4(a) shows the microstructure in the annealing region. Since the composition and microstructure of the seed satisfy the demand described in Ref. [16], the original lamellar orientation of the seed grains is retained well after heating to and cooling from the melting point. Besides, numerous rod-shaped and block-shaped Ti5Si3 particles are distributed within the seed grains while fine Ti5Si3 particles are observed in grain boundaries. The microstructure in the transition region is shown in Fig. 4 (b). Coarse α dendrites form in initial stage and fine eutectic Ti5Si3 particles are distributed in inter-dendritic region. Meanwhile, the rod-shaped and block-shaped Ti5Si3 particles almost disappear in this region. Notably, although the orientation of coarse dendrites is
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Fig. 3. Typical structures of Ti–47Al sample obtained by EMCDS using a seed: (a) longitudinal macrostructure of the sample, (b) and (c) microstructures of regions marked in Fig. 3(a), respectively.
Fig. 4. Microstructures of the Ti–47Al sample in (a) annealing region, (b) transition region, (c) DS region and (d) S/L interface.
difficult to be distinguished, the lamellae in these dendrites are still aligned parallel to the growth direction controlled by the seed. With crystal growth, the composition of the molten zone changes from Ti–43Al–3Si to Ti–47Al. So the interdendritic spacing decreases and the growth morphology of the S/L interface is transformed from dendritic to cellular, which is evidenced from the S/L interface shown in Fig. 4(d). Additionally, fine Ti5Si3 particles also reduce gradually and then disappear in this stage. Fig. 4(c) depicts the microstructure of NS crystal in the DS region, which shows that the lamellae are aligned parallel to the growth direction. For accuracy, the interlamellar spacing is detected from the transverse section and the value is measured 0.5 70.1 μm.
3.3. Fracture toughness The typical load–displacement curve is shown in Fig. 5(a). And the average value of Pmax and KQ of the specimens are 757 N and 34.7 MPa m1/2, respectively. The measured fracture toughness for present alloy is higher than that for Ti–49.3Al alloy reported by S. Yokoshima and M. Yamaguchi [17], Ti–43Al–3Si alloy reported by Kim et al. [18] and Ti–45Al–2Nb–1.5V–1Mo–0.3Y alloy reported by Chen et al. [19]. Note that, using standard method and equations for 3PB specimens, the KQ obtained in this study is validated as KIC. Fig. 5(b) shows the corresponding crack propagation of the specimen after fracture testing. By comparing two figures, it is
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Fig. 6(b) illustrates the magnified crack path from point 4 to 5 marked in Fig. 5(b). Translamellar and interlamellar cracks propagate alternately and thus a zigzag-like crack path is observed, although it appears a straight crack shown in Fig. 5(b). Besides, a common observation from Fig. 6(a) and (b) is that the crack path inclines to propagate slantingly to the lamellar boundary. The main reason for this phenomenon is that more energy is needed if the crack path propagates normally to the lamellar boundary. Consequently, the interlamellar crack, which provides the least toughness, usually occurs together with the translamellar crack. The image in Fig. 6(c) shows the magnified regions ahead of the crack tip. Besides the interlamellar microcracks, numerous translamellar microcracks and glide steps are observed in single γ phase. And most of them occur along 601 with respect to the lamellar boundary and are resisted by α2 lamellae. And it should be noted that some of the translamellar microcracks are likely formed by further cracking of those glide steps. With increasing load, some of the translamellar cracks will cross the α2 lamellae and then extend further. As marked by arrows in Fig. 6(d), the crack is initiated and then propagates ahead of the crack tip, and is likely to connect with the main crack if it propagates further. This can be used to explain the formation of second crack as well as crack bridging.
4. Discussion 4.1. Lamellar alignment in EMCDS
Fig. 5. Typical load–displacement curve (a) and the corresponding crack propagation (b) of the specimen after fracture testing.
found that the crack resistance of the lamellae has a close relationship with the angle between the lamellar boundary and the crack path. When the lamellae are normal to the crack path, the crack propagation is resisted significantly and the load– displacement curve declines slowly. But once the crack propagates along the lamellar boundary, the load–displacement curve declines sharply. In most regions, however, the crack propagation is neither normal nor parallel, but inclines to the lamellar boundary. For this case, the descent speed of the load–displacement curve is also between that of the above two cases.
3.4. Fracture behavior To identify the toughness mechanism further, the crack propagation path and its interaction with the lamellar microstructure on the specimen surfaces are checked by SEM. And the typical crack propagation is depicted in Fig. 6(a). Besides the main crack, numerous interlamellar and translamellar cracks form during the crack propagation. Caused by the intersection of these microcracks, crack bridging is also found from the specimen surfaces. These microcracks and crack bridging can release much energy and then promote the fracture toughness pronouncedly.
Up to now, it is well accepted that γ-TiAl alloys could be solidified through different solidification paths, and the solidification microstructure highly depends on the solidification path [20,21]. If primary phase is α phase, the perpendicular lamellae with respect to the growth direction will form after directional solidification, whereas 451 lamellae are usually obtained if the primary phase is β phase [14]. Although the parallel lamellae were obtained by controlling the solidification path in some cases [22], it is difficult to obtain the desired lamellar microstructure by general directional solidification. Thus, seeding process is still the most efficient way to align the lamellar microstructure and has been studied extensively [23]. During the preparation of single crystal of rutile and YAG by FZ furnace, it is indispensable to maintain a stable molten zone and the shape of the S/L interface plays an important role for single crystal growth [24,25]. Similarly, it is also found that the experiment parameters have a great effect on the preparation of NS crystal at the present study, as observed in Fig. 3(a). It suggests that not only the seeding process but also the experiment parameters affect the crystal growth significantly during EMCDS. As observed in Fig. 3, Ti–47Al sample with 20 mm in diameter is shaped by EMCDS and larger-scale sample is endeavored to be obtained further. It is largely attributed to that electromagnetic force is the main force used to confine the molten zone [11]. Fig. 7 illustrates the relationship between electromagnetic pressure (Pm) and static pressure (Pg) schematically. Affected by the confinement coil and the screen, the Pm used to confine the molten zone decreases lineally from lower S/L interface to upper S/L interface. Such a distribution of Pm is well in accord with the variation of Pg. When a dynamic balance among the Pm, surface tension (Pr) and Pg of the melt reaches, the molten zone with certain height will be held. The relationship among the Pm, Pr and Pg in EMCDS could be simply expressed as Pm þPr ¼Pg. Theoretically, with the increase of sample diameter, the Pg keeps constant whereas the Pr decreases. This is just the reason why largerscale sample cannot be obtained by FZ furnace. For EMCDS [26], the intensity and curve slope of Pm shown in Fig. 7(b) could be adjusted by varying the experiment parameters, such as the volt of power
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Fig. 6. Typical crack propagation path of main crack (a, b) and the magnified region ahead of the crack tip (c, d). Figures a, c and d were taken in BSE mode and b was taken in SE mode.
simultaneously. Nevertheless, stray grains are still observed on sample edges and appear difficult to be eliminated with further crystal growth, which is probably attributed to the undulated molten zone as well as the intense fluid flow in the molten zone. Yet, little research has been performed and more investigations need to be carried out further. 4.2. Toughness mechanism
Fig. 7. Illustrations of the molten zone (a) and the distributions of Pm and Pg (b) in EMCDS.
supply and the shape of induction coil, and so on. Consequently, the shape of the molten zone and its stability can be modified correspondingly, which ensures that a stable molten zone with largerscale diameter can be obtained further. As mentioned before, the crystal growth in directional solidification depends not only on the shape and stability of the molten zone but also on the shape of the S/L interface. Extensive studies performed in FZ furnace suggest that the shape of the S/L surface plays an important role in single crystal growth [24]. If the interface is concave towards the melt, stray grains that nucleate on sample edges will grow towards sample center and then disturb the growth of single crystal. Besides, numerous defects, such as inclusions and cracks, are usually observed in sample center. By contrast, if the interface is convex slightly towards the melt, stray grains will be eliminated with crystal growth and single crystal can be well aligned. Notably, the convexity of interface is also restricted because thermal cracks will occur with increasing convexity [25]. To further evaluate the effect of the S/L interface shape on crystal growth, a value of h/r is used to describe the shape of the S/ L interface in FZ zone furnace. And the study performed by Kimura et al. [27] suggests that the interface with h/r value of 0.25–0.35 is beneficial to the growth of YIG single crystal and a slight higher value is detrimental to the single crystal growth. In our study, the S/L interface is convex and the h/r value is measured 0.25, which is favorable for single crystal growth. Therefore, a NS crystal grows divergently and most of the grains on sample edges are eliminated
The fracture toughness of PST crystal with different lamellar orientations was studied by Yokoshima and Yamaguchi [17]. The results show that the normal lamellae with respect to the crack tip exhibit higher fracture toughness than the parallel lamellae, which is because the crack tip is usually blunted by delamination when it meets the normal lamellae. Similar results were also reported by Kim et al. [9] and the effect of α2–α2 spacing on the fracture toughness of normal lamellae was discussed. And they suggest that the lamellae with small α2–α2 spacing have high toughness while that with high α2–α2 spacing have low toughness. It is due to the fact that the small α2–α2 spacing promotes the delamination in adjacent α2/γ lamellar boundary, whereas high α2–α2 spacing leads to the crack in middle γ phase rather than the occurrence of delamination. Notably, the above discussions assume that the stress field ahead of the crack tip is small and is only distributed in a region no larger than high α2–α2 spacing. But the numerous glide steps and microcracks shown in Fig. 6(c) and (d) suggest that the stress field ahead of the crack tip is distributed in a far larger region. In general, the more toughness behaviors occur during the crack propagation, the higher crack resistance will be obtained. In the study performed by Chan et al. [28], it is found that stress field ahead of the crack tip expands with the increase of delamination length. Considering that numerous interlamellar cracks with long delamination length are observed in our study, it is believed that the crack tip is blunted significantly and the stress field is distributed in a larger region. It can also be evidenced by the numerous translamellar cracks and glide steps within single γ phase shown in Fig. 6. Note that, according to the study performed by Zambaldi and Raabe, these translamellar cracks and glide steps occur on {111} planes [29].
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Fig. 8. Schematic illustration of crack propagation in the Ti–47Al specimen: (a) tip blunting, (b) microcracks ahead of the crack tip and (c) zigzag crack path.
The image in Fig. 8 illustrates the crack sequence and paths in the specimen to understand the fracture behavior in our study. When the crack tip meets the normal α2/γ boundary, it is difficult to cross the α2 lamella and delamination occurs at the crack tip. For this case, the load cannot be concentrated at the crack tip and the stress ahead of the crack tip is relaxed and expands to a larger region. As a result, dislocations on {111} planes along interlamellar and translamellar direction occurs within single γ phase in this region, as shown in Fig. 8(a). Once the critical stress reaches, some of these dislocations will crack and then interlamellar and translamellar microcracks form. These new microcracks can dissipate the energy largely and then the stress will redistribute in a far larger region, as illustrated in Fig. 8(b). With load increasing, either the microcracks or the crack tip, or both of them will cross the α2 lamellae and then propagate further until connect with each other, as shown in Fig. 8(c). This can well explain the fracture behavior observed in Fig. 6. 5. Conclusion Non-contamination Ti–47Al sample with 20 mm in diameter is obtained by EMCDS at 10 μm/s. Controlled by a Ti–43Al–3Si seed, a NS crystal with the desired lamellar microstructure is obtained, and the fracture toughness is measured 34.7 MPa m1/2. The results can be summarized as follows: 1. Besides the seeding process, it is indispensable to maintain a stable molten zone and a proper S/L interface shape for lamellar alignment during EMCDS. Attributed to the convex S/L interface towards the liquid with 0.25 h/r value, a NS crystal, in which the lamellae are aligned parallel to the growth direction, is obtained with crystal growth. 2. The crack propagation path is checked after the fracture toughness testing and numerous interlamellar and translamellar microcracks as well as glide steps are observed on the specimen surfaces. When the crack tip meets the normal α2 lamella, delamination occurs to blunt the crack tip and the stress field expands to a larger region. For this case, numerous dislocations will occur in the region, and then glide steps as well as microcracks will form with increasing load. And some of these microcracks will extend further together with the propagation of the crack tip.
Acknowledgments This work was supported by the National Natural Science Foundation of China under Grant no. 51174167, and the Research Fund of State Key Laboratory of Solidification Processing (NWPU), China under Grant no. 63-TP-2011. It is also supported by the Doctorate Foundation of Northwestern Polytechnical University under Contract no. CX201308. References [1] M. Yamaguchi, H. Inui, K. Ito, Acta Mater. 48 (2000) 307–322. [2] X.H. Wu, Intermetallics 14 (2006) 1114–1122. [3] J.P. Lin, L.L. Zhao, G.Y. Li, L.Q. Zhang, X.P. Song, F. Ye, G.L. Chen, Intermetallics 19 (2011) 131–136. [4] X.F. Ding, L.Q. Zhang, J.P. Lin, J.P. He, J. Yin, G.L. Chen, Trans. Nonferr. Met. Soc. 22 (2012) 747–753. [5] H.N. Lee, D.R. Johnson, H. Inui, M.H. Oh, D.M. Wee, M. Yamaguchi, Acta Mater. 48 (2000) 3221–3233. [6] M. Takeyama, Y. Yamamoto, H. Morishima, K. Koike, S.Y. Chang, T. Matsuo, Mater. Sci. Eng. A 329 (2002) 7–12. [7] S.R. Coriell, M.R. Cordes, J. Cryst. Growth 42 (1977) 466–472. [8] C.W. Lan, S. Kou, J. Cryst. Growth 119 (1992) 281–291. [9] S.W. Kim, K.S. Kumar, M.H. Oh, D.M. Wee, Intermetallics 15 (2007) 976–984. [10] J. Shen, J.G. Li, H.Z. Fu, J. Mater. Process. Technol. 102 (2000) 109–114. [11] H.Z. Fu, J. Shen, L. Liu, Q.T. Hao, S.M. Li, J.S. Li, J. Mater. Process. Technol. 148 (2004) 25–29. [12] Y.J. Du, J. Shen, Y.L. Xiong, Z.W. Liu, Q. Zhao, H.Z. Fu, JOM 66 (2014) 1914–1922. [13] X.Q. Zheng, J. Shen, H.S. Ding, R.R. Chen, X. Xu, Mater. Rev. 19 (2005) 118–119. [14] M.C. Kim, M.H. Oh, J.H. Lee, H. Inui, M. Yamaguchi, D.M. Wee, Mater. Sci. Eng. A 239 (1997) 570–576. [15] X.Z. Li, T. Sun, C.X. Yu, Y.Q. Su, Y.Z. Cao, J.J. Guo, H.Z. Fu, Acta Metall. Sin. (Engl. Lett.) 45 (2009) 1479–1486. [16] M. Yamaguchi, D.R. Johnson, H.N. Lee, H. Inui, Intermetallics 8 (2000) 511–517. [17] S. Yokoshima, M. Yamaguchi, Acta Mater. 44 (1996) 873–883. [18] S.E. Kim, Y.T. Lee, M.H. Oh, H. Inui, M. Yamaguchi, Intermetallics 8 (2000) 399–405. [19] Y.Y. Chen, H.Z. Niu, F.T. Kong, S.L. Xiao, Intermetallics 19 (2011) 1405–1410. [20] M. Oehring, V. Küstner, F. Appel, U. Lorenz, Mater. Sci. Forum 539–543 (2007) 1475–1480. [21] Y.Q. Su, C. Liu, X.Z. Li, J.J. Guo, B.S. Li, J. Jia, H.Z. Fu, Intermetallics 13 (2005) 267–274. [22] D.R. Johnson, K. Chihara, H. Inui, M. Yamaguchi, Acta Mater. 46 (1998) 6529–6540. [23] D.R. Johnson, H. Inui, S. Muto, Y. Omiya, T. Yamanaka, Acta Mater. 54 (2006) 1077–1085. [24] K. Kitamura, S. Kimura, K. Watanabe, J. Cryst. Growth 57 (1982) 475–481. [25] M. Abdur Razzaque Sarker, S. Watauchi, M. Nagao, T. Watanabe, I. Shindo, I. Tanaka, J. Cryst. Growth 312 (2010) 2008–2011. [26] C.J. Song, G.F. Liang, Z.M. Xu, J. Shen, J.G. Li, J. Mater. Process. Technol. 180 (2006) 179–184. [27] S. Kimura, K. Kitamura, I. Shindo, J. Cryst. Growth 65 (1983) 543–548. [28] K.S. Chan, M.Y. He, J.W. Hutchinson, Mater. Sci. Eng. A 167 (1993) 57–64. [29] C. Zambaldi, D. Raabe, Acta Mater. 58 (2010) 3516–3530.
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Lamellar microstructure alignment and fracture toughness in Ti–47Al alloy by electromagnetic confinement and directional solidification Yujun Du, Jun Shen n, Yilong Xiong, Zhao Shang, Lei Wang, Hengzhi Fu State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China
art ic l e i nf o
a b s t r a c t
Article history: Received 23 July 2014 Received in revised form 21 October 2014 Accepted 23 October 2014 Available online 31 October 2014
Large-scale Ti–47Al sample without contamination is shaped by electromagnetic confinement and directional solidification. Controlled by a Ti–43Al–3Si seed, numerous columnar grains grow directionally in transition region and the lamellar microstructure within those grains is aligned parallel to the growth direction. Because the solid/liquid interface is convex towards the liquid and its h/r value is 0.25, one of the columnar grains in sample center grows divergently and then a near single (NS) crystal forms with crystal growth. Fracture toughness of the NS crystal with the desired lamellae is detected and the value is measured 34.7 MPa m1/2. The crack propagation path is checked after the fracture toughness testing and a schematic model is used to illustrate the relevant toughness mechanism. & 2014 Elsevier B.V. All rights reserved.
Keywords: Ti–47Al PST crystal Seed Fracture toughness
1. Introduction Polysynthetically twinned (PST) crystals of γ-TiAl based alloys have been studied extensively due to their good combination of strength and ductility at room temperature when the lamellar microstructure is aligned parallel to the tensile direction [1–3]. And these excellent mechanical properties make them prospective materials for aerospace and automobile industry. However, intense chemical reactions occur inevitably between the TiAl melt and various ceramic crucibles. As a consequence, numerous crucible fragments are produced during the directional solidification and are significantly detrimental to the mechanical properties of TiAl alloys [4]. Up to now, optical float zone (FZ) furnace has been employed to prepare non-polluting PST crystals by using a seeding method [5,6]. But this method is only useful for laboratory research to produce samples of small section size. It is mainly owing to the fact that the surface tension used to confine the molten zone would decrease with increasing sample diameter [7,8]. Consequently, the largest TiAl sample prepared by FZ furnace was no more than 14 mm in diameter [9]. To overcome this shortcoming, an additional electromagnetic force was employed to confine the molten zone in various electromagnetic processing techniques. Electromagnetic confinement and directional solidification (EMCDS), as a type of electromagnetic processing, has been widely researched to obtain non-contamination samples of stainless steel
n
Corresponding author. Tel.: þ 86 29 88494708; fax: þ86 29 88494080. E-mail address: [email protected] (J. Shen).
http://dx.doi.org/10.1016/j.msea.2014.10.066 0921-5093/& 2014 Elsevier B.V. All rights reserved.
and Ni-based superalloys over the years [10,11]. As the main force used to confine the molten zone is electromagnetic force rather than surface tension, samples with 18 mm in diameter were obtained successfully. Therefore, it is expected to obtain largescale samples of TiAl alloys with the desired lamellar microstructure by this technology. However, little work has been carried out at present. In our previous studies, 12.5 mm diameter samples of Ti–47Al alloy were obtained by EMCDS [12]. Besides, controlled by a T–43Al–3Si seed, columnar grains grew directionally and the lamellar microstructure within those grains was aligned well. In this paper, Ti–47Al sample with 20 mm in diameter is obtained and the lamellae in the sample are aligned well by using a seeding method. Besides, the crystal growth during EMCDS is studied by discussing the stability of the molten zone and the shape of the solid/liquid (S/L) interface. Additionally, the fracture toughness of the lamellar microstructure is detected and the relevant toughness mechanism is discussed by checking the crack propagation path.
2. Experiment Nominal chemical compositions of Ti–47Al and Ti–43Al–3Si alloys were used in EMCDS process. The purity of raw materials used in this study is Ti 99.96 (wt%), Al 99.99% (wt%) and Si 99.95% (wt%), respectively. A master bar with 20 mm in diameter was cut from a Ti–47Al ingot by electro-discharge machining (EDM). And a seed bar with the same diameter, in which the α2/γ lamellae are aligned parallel to the longitudinal direction, was cut from a
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Ti–43Al–3Si ingot. The preparation of the seed ingot was described elsewhere [13]. The EMCDS system is shown schematically in Fig. 1, which mainly consists of confinement coil, screen, feeding part, withdrawing part and liquid cooling metal. The experimental procedures were as follows: first, the master bar was fixed on the feeding part and the seed bar was fixed on the withdrawing part. The chamber was pumped to 6.6 10 3 Pa, and then filled with purified argon; second, the top part of the seed bar and the bottom part of the master bar were melted simultaneously and mixed together by induction heat when high-frequency alternating current traveled through the confinement coil. At the same time, the electromagnetic force, whose direction pointed to the center of melt, would act on the melt and confine it to be a molten zone with certain height; third, the sample began to be pulled downward into the liquid cooling metal at 10 μm/s after a holding period of 10 min; finally, the sample was quenched into the liquid cooling metal to observe the S/L interfaces. More details about EMCDS were provided in Ref. [10,11]. For optical microscopy, the directionally solidified (DS) sample was sectioned longitudinally by EDM. And the sections were mechanically polished using standard metallographic techniques and then etched with a solution of 10 ml HF, 5 ml HNO3 and 85 ml H2O. After metallographic preparation, the macrostructure was photographed by a digital camera and the microstructure was revealed by a Leica DM4000M optical microscope. To identify the fracture toughness of the sample, two three-point bend (3PB) specimens with dimension of 6 mm 3 mm 30 mm were cut from the DS sample. And the lamellar boundary within them was ensured parallel to the longitudinal direction. Prior to fracture testing, all the flat faces of bend specimens were ground on emery paper to a finish of 1000 grit. A narrow straight 3 mm-deep notch was also machined by EDM. But fatigue pre-crack was not initiated at the notch tip prior to 3PB testing. The scheme of the specimen is shown in Fig. 2. Fracture toughness testing was performed in air at room temperature in an Instron 3382 universal
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test machine and at a cross-head speed of 0.05 mm/min. Scanning electron microscopy (SEM) in back-scattered electron (BSE) and second electron (SE) mode was used to check the crack propagation path after fracture testing.
3. Results 3.1. Directional solidification in EMCDS using a seed The image in Fig. 3(a) shows the macrostructure of Ti–47Al sample obtained by EMCDS with a Ti–43Al–3Si seed. Caused by the instability of power supply, the shape of the molten zone varies slightly and thus undulated sample surface is observed from the macrostructure. Nevertheless, columnar grains grow directionally and four regions could be distinguished: (I) annealing region, (II) transition region, (III) DS region and (IV) quenched zone. In the transition region, numerous columnar gains form and grow competitively, and most of them are eliminated with crystal growth. At the end of solidification, a near single (NS) crystal forms in sample center besides some other narrow grains in sample edges, as shown in Fig. 3(a). This phenomenon is largely attributed to that the S/L interface is convex slightly towards the liquid, and more details will be discussed later. Fig. 3(b) and (c) shows the microstructures of the NS crystal marked in Fig. 3(a), respectively. It can be seen that the NS crystal only consists of one single grain and the lamellae in the grain are aligned parallel to the growth direction. The max width of the NS crystal is measured 16 mm from Fig. 3(a), which suggests that Ti–47Al sample with the desired lamellar microstructure is successfully obtained by EMCDS. Note that, due to improper handling in the experiment, new grains nucleate in front of the NS crystal and then grow stably. And the 451 angle between the lamellae and the growth direction suggests that these grains are β phase [14]. According to phase selection map of binary Ti–Al alloy [15], either α or β cells could grow stably without nucleation of β or α phase. When α phase grows continuously in stable stage, it is difficult for new α or β phase to nucleate. For this case, maintaining a stable molten zone is the most critical factor for the continuous crystal growth. However, a sudden decrease of power volt occurs during the present experiment. This leads to a higher undercooling and thus new β grains nucleate in front of the α grain. As mentioned above, not only α but also β phase could grow stably along the growth direction. Therefore, once β phase nucleates ahead of the S/L interface, it will grow stably although the power returns to normal condition instantly. Afterwards, the growth of α grain is terminated and β grains dominate in the sample subsequently. 3.2. Microstructures
Fig. 1. Scheme for EMCDS system.
Fig. 2. Scheme of specimen for fracture testing.
The images in Fig. 4 show the microstructures in different regions of the sample. Fig. 4(a) shows the microstructure in the annealing region. Since the composition and microstructure of the seed satisfy the demand described in Ref. [16], the original lamellar orientation of the seed grains is retained well after heating to and cooling from the melting point. Besides, numerous rod-shaped and block-shaped Ti5Si3 particles are distributed within the seed grains while fine Ti5Si3 particles are observed in grain boundaries. The microstructure in the transition region is shown in Fig. 4 (b). Coarse α dendrites form in initial stage and fine eutectic Ti5Si3 particles are distributed in inter-dendritic region. Meanwhile, the rod-shaped and block-shaped Ti5Si3 particles almost disappear in this region. Notably, although the orientation of coarse dendrites is
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Fig. 3. Typical structures of Ti–47Al sample obtained by EMCDS using a seed: (a) longitudinal macrostructure of the sample, (b) and (c) microstructures of regions marked in Fig. 3(a), respectively.
Fig. 4. Microstructures of the Ti–47Al sample in (a) annealing region, (b) transition region, (c) DS region and (d) S/L interface.
difficult to be distinguished, the lamellae in these dendrites are still aligned parallel to the growth direction controlled by the seed. With crystal growth, the composition of the molten zone changes from Ti–43Al–3Si to Ti–47Al. So the interdendritic spacing decreases and the growth morphology of the S/L interface is transformed from dendritic to cellular, which is evidenced from the S/L interface shown in Fig. 4(d). Additionally, fine Ti5Si3 particles also reduce gradually and then disappear in this stage. Fig. 4(c) depicts the microstructure of NS crystal in the DS region, which shows that the lamellae are aligned parallel to the growth direction. For accuracy, the interlamellar spacing is detected from the transverse section and the value is measured 0.5 70.1 μm.
3.3. Fracture toughness The typical load–displacement curve is shown in Fig. 5(a). And the average value of Pmax and KQ of the specimens are 757 N and 34.7 MPa m1/2, respectively. The measured fracture toughness for present alloy is higher than that for Ti–49.3Al alloy reported by S. Yokoshima and M. Yamaguchi [17], Ti–43Al–3Si alloy reported by Kim et al. [18] and Ti–45Al–2Nb–1.5V–1Mo–0.3Y alloy reported by Chen et al. [19]. Note that, using standard method and equations for 3PB specimens, the KQ obtained in this study is validated as KIC. Fig. 5(b) shows the corresponding crack propagation of the specimen after fracture testing. By comparing two figures, it is
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Fig. 6(b) illustrates the magnified crack path from point 4 to 5 marked in Fig. 5(b). Translamellar and interlamellar cracks propagate alternately and thus a zigzag-like crack path is observed, although it appears a straight crack shown in Fig. 5(b). Besides, a common observation from Fig. 6(a) and (b) is that the crack path inclines to propagate slantingly to the lamellar boundary. The main reason for this phenomenon is that more energy is needed if the crack path propagates normally to the lamellar boundary. Consequently, the interlamellar crack, which provides the least toughness, usually occurs together with the translamellar crack. The image in Fig. 6(c) shows the magnified regions ahead of the crack tip. Besides the interlamellar microcracks, numerous translamellar microcracks and glide steps are observed in single γ phase. And most of them occur along 601 with respect to the lamellar boundary and are resisted by α2 lamellae. And it should be noted that some of the translamellar microcracks are likely formed by further cracking of those glide steps. With increasing load, some of the translamellar cracks will cross the α2 lamellae and then extend further. As marked by arrows in Fig. 6(d), the crack is initiated and then propagates ahead of the crack tip, and is likely to connect with the main crack if it propagates further. This can be used to explain the formation of second crack as well as crack bridging.
4. Discussion 4.1. Lamellar alignment in EMCDS
Fig. 5. Typical load–displacement curve (a) and the corresponding crack propagation (b) of the specimen after fracture testing.
found that the crack resistance of the lamellae has a close relationship with the angle between the lamellar boundary and the crack path. When the lamellae are normal to the crack path, the crack propagation is resisted significantly and the load– displacement curve declines slowly. But once the crack propagates along the lamellar boundary, the load–displacement curve declines sharply. In most regions, however, the crack propagation is neither normal nor parallel, but inclines to the lamellar boundary. For this case, the descent speed of the load–displacement curve is also between that of the above two cases.
3.4. Fracture behavior To identify the toughness mechanism further, the crack propagation path and its interaction with the lamellar microstructure on the specimen surfaces are checked by SEM. And the typical crack propagation is depicted in Fig. 6(a). Besides the main crack, numerous interlamellar and translamellar cracks form during the crack propagation. Caused by the intersection of these microcracks, crack bridging is also found from the specimen surfaces. These microcracks and crack bridging can release much energy and then promote the fracture toughness pronouncedly.
Up to now, it is well accepted that γ-TiAl alloys could be solidified through different solidification paths, and the solidification microstructure highly depends on the solidification path [20,21]. If primary phase is α phase, the perpendicular lamellae with respect to the growth direction will form after directional solidification, whereas 451 lamellae are usually obtained if the primary phase is β phase [14]. Although the parallel lamellae were obtained by controlling the solidification path in some cases [22], it is difficult to obtain the desired lamellar microstructure by general directional solidification. Thus, seeding process is still the most efficient way to align the lamellar microstructure and has been studied extensively [23]. During the preparation of single crystal of rutile and YAG by FZ furnace, it is indispensable to maintain a stable molten zone and the shape of the S/L interface plays an important role for single crystal growth [24,25]. Similarly, it is also found that the experiment parameters have a great effect on the preparation of NS crystal at the present study, as observed in Fig. 3(a). It suggests that not only the seeding process but also the experiment parameters affect the crystal growth significantly during EMCDS. As observed in Fig. 3, Ti–47Al sample with 20 mm in diameter is shaped by EMCDS and larger-scale sample is endeavored to be obtained further. It is largely attributed to that electromagnetic force is the main force used to confine the molten zone [11]. Fig. 7 illustrates the relationship between electromagnetic pressure (Pm) and static pressure (Pg) schematically. Affected by the confinement coil and the screen, the Pm used to confine the molten zone decreases lineally from lower S/L interface to upper S/L interface. Such a distribution of Pm is well in accord with the variation of Pg. When a dynamic balance among the Pm, surface tension (Pr) and Pg of the melt reaches, the molten zone with certain height will be held. The relationship among the Pm, Pr and Pg in EMCDS could be simply expressed as Pm þPr ¼Pg. Theoretically, with the increase of sample diameter, the Pg keeps constant whereas the Pr decreases. This is just the reason why largerscale sample cannot be obtained by FZ furnace. For EMCDS [26], the intensity and curve slope of Pm shown in Fig. 7(b) could be adjusted by varying the experiment parameters, such as the volt of power
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Fig. 6. Typical crack propagation path of main crack (a, b) and the magnified region ahead of the crack tip (c, d). Figures a, c and d were taken in BSE mode and b was taken in SE mode.
simultaneously. Nevertheless, stray grains are still observed on sample edges and appear difficult to be eliminated with further crystal growth, which is probably attributed to the undulated molten zone as well as the intense fluid flow in the molten zone. Yet, little research has been performed and more investigations need to be carried out further. 4.2. Toughness mechanism
Fig. 7. Illustrations of the molten zone (a) and the distributions of Pm and Pg (b) in EMCDS.
supply and the shape of induction coil, and so on. Consequently, the shape of the molten zone and its stability can be modified correspondingly, which ensures that a stable molten zone with largerscale diameter can be obtained further. As mentioned before, the crystal growth in directional solidification depends not only on the shape and stability of the molten zone but also on the shape of the S/L interface. Extensive studies performed in FZ furnace suggest that the shape of the S/L surface plays an important role in single crystal growth [24]. If the interface is concave towards the melt, stray grains that nucleate on sample edges will grow towards sample center and then disturb the growth of single crystal. Besides, numerous defects, such as inclusions and cracks, are usually observed in sample center. By contrast, if the interface is convex slightly towards the melt, stray grains will be eliminated with crystal growth and single crystal can be well aligned. Notably, the convexity of interface is also restricted because thermal cracks will occur with increasing convexity [25]. To further evaluate the effect of the S/L interface shape on crystal growth, a value of h/r is used to describe the shape of the S/ L interface in FZ zone furnace. And the study performed by Kimura et al. [27] suggests that the interface with h/r value of 0.25–0.35 is beneficial to the growth of YIG single crystal and a slight higher value is detrimental to the single crystal growth. In our study, the S/L interface is convex and the h/r value is measured 0.25, which is favorable for single crystal growth. Therefore, a NS crystal grows divergently and most of the grains on sample edges are eliminated
The fracture toughness of PST crystal with different lamellar orientations was studied by Yokoshima and Yamaguchi [17]. The results show that the normal lamellae with respect to the crack tip exhibit higher fracture toughness than the parallel lamellae, which is because the crack tip is usually blunted by delamination when it meets the normal lamellae. Similar results were also reported by Kim et al. [9] and the effect of α2–α2 spacing on the fracture toughness of normal lamellae was discussed. And they suggest that the lamellae with small α2–α2 spacing have high toughness while that with high α2–α2 spacing have low toughness. It is due to the fact that the small α2–α2 spacing promotes the delamination in adjacent α2/γ lamellar boundary, whereas high α2–α2 spacing leads to the crack in middle γ phase rather than the occurrence of delamination. Notably, the above discussions assume that the stress field ahead of the crack tip is small and is only distributed in a region no larger than high α2–α2 spacing. But the numerous glide steps and microcracks shown in Fig. 6(c) and (d) suggest that the stress field ahead of the crack tip is distributed in a far larger region. In general, the more toughness behaviors occur during the crack propagation, the higher crack resistance will be obtained. In the study performed by Chan et al. [28], it is found that stress field ahead of the crack tip expands with the increase of delamination length. Considering that numerous interlamellar cracks with long delamination length are observed in our study, it is believed that the crack tip is blunted significantly and the stress field is distributed in a larger region. It can also be evidenced by the numerous translamellar cracks and glide steps within single γ phase shown in Fig. 6. Note that, according to the study performed by Zambaldi and Raabe, these translamellar cracks and glide steps occur on {111} planes [29].
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Fig. 8. Schematic illustration of crack propagation in the Ti–47Al specimen: (a) tip blunting, (b) microcracks ahead of the crack tip and (c) zigzag crack path.
The image in Fig. 8 illustrates the crack sequence and paths in the specimen to understand the fracture behavior in our study. When the crack tip meets the normal α2/γ boundary, it is difficult to cross the α2 lamella and delamination occurs at the crack tip. For this case, the load cannot be concentrated at the crack tip and the stress ahead of the crack tip is relaxed and expands to a larger region. As a result, dislocations on {111} planes along interlamellar and translamellar direction occurs within single γ phase in this region, as shown in Fig. 8(a). Once the critical stress reaches, some of these dislocations will crack and then interlamellar and translamellar microcracks form. These new microcracks can dissipate the energy largely and then the stress will redistribute in a far larger region, as illustrated in Fig. 8(b). With load increasing, either the microcracks or the crack tip, or both of them will cross the α2 lamellae and then propagate further until connect with each other, as shown in Fig. 8(c). This can well explain the fracture behavior observed in Fig. 6. 5. Conclusion Non-contamination Ti–47Al sample with 20 mm in diameter is obtained by EMCDS at 10 μm/s. Controlled by a Ti–43Al–3Si seed, a NS crystal with the desired lamellar microstructure is obtained, and the fracture toughness is measured 34.7 MPa m1/2. The results can be summarized as follows: 1. Besides the seeding process, it is indispensable to maintain a stable molten zone and a proper S/L interface shape for lamellar alignment during EMCDS. Attributed to the convex S/L interface towards the liquid with 0.25 h/r value, a NS crystal, in which the lamellae are aligned parallel to the growth direction, is obtained with crystal growth. 2. The crack propagation path is checked after the fracture toughness testing and numerous interlamellar and translamellar microcracks as well as glide steps are observed on the specimen surfaces. When the crack tip meets the normal α2 lamella, delamination occurs to blunt the crack tip and the stress field expands to a larger region. For this case, numerous dislocations will occur in the region, and then glide steps as well as microcracks will form with increasing load. And some of these microcracks will extend further together with the propagation of the crack tip.
Acknowledgments This work was supported by the National Natural Science Foundation of China under Grant no. 51174167, and the Research Fund of State Key Laboratory of Solidification Processing (NWPU), China under Grant no. 63-TP-2011. It is also supported by the Doctorate Foundation of Northwestern Polytechnical University under Contract no. CX201308. References [1] M. Yamaguchi, H. Inui, K. Ito, Acta Mater. 48 (2000) 307–322. [2] X.H. Wu, Intermetallics 14 (2006) 1114–1122. [3] J.P. Lin, L.L. Zhao, G.Y. Li, L.Q. Zhang, X.P. Song, F. Ye, G.L. Chen, Intermetallics 19 (2011) 131–136. [4] X.F. Ding, L.Q. Zhang, J.P. Lin, J.P. He, J. Yin, G.L. Chen, Trans. Nonferr. Met. Soc. 22 (2012) 747–753. [5] H.N. Lee, D.R. Johnson, H. Inui, M.H. Oh, D.M. Wee, M. Yamaguchi, Acta Mater. 48 (2000) 3221–3233. [6] M. Takeyama, Y. Yamamoto, H. Morishima, K. Koike, S.Y. Chang, T. Matsuo, Mater. Sci. Eng. A 329 (2002) 7–12. [7] S.R. Coriell, M.R. Cordes, J. Cryst. Growth 42 (1977) 466–472. [8] C.W. Lan, S. Kou, J. Cryst. Growth 119 (1992) 281–291. [9] S.W. Kim, K.S. Kumar, M.H. Oh, D.M. Wee, Intermetallics 15 (2007) 976–984. [10] J. Shen, J.G. Li, H.Z. Fu, J. Mater. Process. Technol. 102 (2000) 109–114. [11] H.Z. Fu, J. Shen, L. Liu, Q.T. Hao, S.M. Li, J.S. Li, J. Mater. Process. Technol. 148 (2004) 25–29. [12] Y.J. Du, J. Shen, Y.L. Xiong, Z.W. Liu, Q. Zhao, H.Z. Fu, JOM 66 (2014) 1914–1922. [13] X.Q. Zheng, J. Shen, H.S. Ding, R.R. Chen, X. Xu, Mater. Rev. 19 (2005) 118–119. [14] M.C. Kim, M.H. Oh, J.H. Lee, H. Inui, M. Yamaguchi, D.M. Wee, Mater. Sci. Eng. A 239 (1997) 570–576. [15] X.Z. Li, T. Sun, C.X. Yu, Y.Q. Su, Y.Z. Cao, J.J. Guo, H.Z. Fu, Acta Metall. Sin. (Engl. Lett.) 45 (2009) 1479–1486. [16] M. Yamaguchi, D.R. Johnson, H.N. Lee, H. Inui, Intermetallics 8 (2000) 511–517. [17] S. Yokoshima, M. Yamaguchi, Acta Mater. 44 (1996) 873–883. [18] S.E. Kim, Y.T. Lee, M.H. Oh, H. Inui, M. Yamaguchi, Intermetallics 8 (2000) 399–405. [19] Y.Y. Chen, H.Z. Niu, F.T. Kong, S.L. Xiao, Intermetallics 19 (2011) 1405–1410. [20] M. Oehring, V. Küstner, F. Appel, U. Lorenz, Mater. Sci. Forum 539–543 (2007) 1475–1480. [21] Y.Q. Su, C. Liu, X.Z. Li, J.J. Guo, B.S. Li, J. Jia, H.Z. Fu, Intermetallics 13 (2005) 267–274. [22] D.R. Johnson, K. Chihara, H. Inui, M. Yamaguchi, Acta Mater. 46 (1998) 6529–6540. [23] D.R. Johnson, H. Inui, S. Muto, Y. Omiya, T. Yamanaka, Acta Mater. 54 (2006) 1077–1085. [24] K. Kitamura, S. Kimura, K. Watanabe, J. Cryst. Growth 57 (1982) 475–481. [25] M. Abdur Razzaque Sarker, S. Watauchi, M. Nagao, T. Watanabe, I. Shindo, I. Tanaka, J. Cryst. Growth 312 (2010) 2008–2011. [26] C.J. Song, G.F. Liang, Z.M. Xu, J. Shen, J.G. Li, J. Mater. Process. Technol. 180 (2006) 179–184. [27] S. Kimura, K. Kitamura, I. Shindo, J. Cryst. Growth 65 (1983) 543–548. [28] K.S. Chan, M.Y. He, J.W. Hutchinson, Mater. Sci. Eng. A 167 (1993) 57–64. [29] C. Zambaldi, D. Raabe, Acta Mater. 58 (2010) 3516–3530.