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Directional solidification of Ni–Ni3Si eutectic in situ composites by electron beam floating zone melting
Physica B 412 (2013) 70–73
Contents lists available at SciVerse ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Directional solidification of Ni–Ni3Si eutectic in situ composites by electron beam floating zone melting Chunjuan Cui a,n, Jun Zhang b, Kun Wu a, Dening Zou a, Youping Ma a, Lin Liu b, Hengzhi Fu b a b
School of Metallurgical Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 November 2012 Received in revised form 7 December 2012 Accepted 8 December 2012 Available online 22 December 2012
Combining the intermetallic compound with the ductile metal at the eutectic composition is one promising method to improve the ductility of the intermetallic compound. This paper reports the microstructure and the micro-hardness of the Ni–Ni3Si eutectic in situ composites prepared by electron beam floating zone melting technique. Ni–Ni3Si eutectic in situ composites display regular lamellar eutectic structure at the solidification rate R ¼0.3–4.0 mm/min. The lamellar spacing is decreased with the increase of the solidification rate. The phase composition of the Ni–Ni3Si eutectic in situ composites is also determined by X-ray diffraction. Ni–Ni3Si eutectic in situ composites present lower microhardness than pure Ni3Si, although a small quantity of metastable Ni31Si12 phase is formed during the directional solidification process. & 2012 Elsevier B.V. All rights reserved.
Keywords: Directional solidification Eutectic in situ composite Electron beam floating zone melting Micro-hardness
1. Introduction Intermetallics possess higher melting temperatures than the superalloys, and with metallic bonding, at least the possibility of better toughness than ceramics. Therefore, intermetallics have been paid more attentions. Ni3Si-based alloy has been considered to be a candidate material, which can be used as the basis of hightemperature structural materials and chemical parts because Ni3Si displays an increasing strength with increasing temperatures [1] and also shows excellent oxidation and corrosion resistance over a wide range of temperatures [2–4]. However, the major obstacles for the use of Ni3Si compound are its poor ductility at ambient temperatures and its bad fabricability at high temperatures [5,6]. Much work has been done to improve the ductility of the Ni3Si compound, for example, disordering treatment [7], alloying [1,8–10], grain refinement [11], etc. The incorporation of a ductile phase into the intermetallic materials is an attractive method to improve the ductility of the intermetallic materials. This can be achieved by directional solidification process of eutectic alloys, and eutectic in situ composites which are thermodynamically stable, chemically compatible, and well aligned can be obtained. Caram et al. [12,13] produced Ni–Ni3Si eutectic in situ composite with the Bridgman directional solidification technique at solidification rates R¼ 32–56 mm/s. Although many significant results have been achieved,
n
Corresponding author. Tel.: þ86 29 82205104; fax: þ 86 29 82202923. E-mail address: [email protected] (C. Cui).
0921-4526/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physb.2012.12.010
there are still a series of unsolved problems such as the microstructure control, crystal growth mechanism, and the phase composition of the Ni–Ni3Si composites. The preparation technique still needs to be improved as well. Electron beam floating zone melting (EBFZM) technique has many advantages, e.g. high energy density(10 thousand times arc), high vacuum degree (o10 5 mbar), high temperature gradient (350–500 K/cm) and no crucible pollution. These advantages can result in the improvement of the final structure, grain size and properties of the alloy. In the present paper, the EBFZM technique is adopted in order to obtain Ni–Ni3Si eutectic in situ composite which has high-aligned and uniformly-distributed Ni3Si compounds embedded into the Ni matrix. The solidification characteristics and phase composition are studied in detail. In general, the harder the metal material is, the worse its ductility is. Therefore, micro-hardness can be used to represent the ductility of the Ni–Si alloy. Micro-hardness of the Ni–Ni3Si eutectic composites at the different solidification rates are studied by micro-hardness tester.
2. Experiments The master alloys are obtained by cutting the middle of the Ni–11.5 wt% Si alloy into |6 120 mm slices, which are produced with vacuum induction melting technique. The Ni–Ni3Si eutectic in situ composites are prepared by EBZM-20 directional solidification equipment at different solidification rates. The schematic diagram of the electron beam gun is shown in Fig. 1. The directionally solidified samples are treated with conventional
C. Cui et al. / Physica B 412 (2013) 70–73
Fig. 1. Schematic diagram of the electron beam gun.
metallographic technique and etched by the mixture of 5%HClþH2OþFe3Cl solution. Microstructure and phase distributions are observed with OLYMPUS GX51 optical microscope. Phase composition of the Ni–Ni3Si eutectic in situ composites are studied by X-ray diffraction (X Pert MPDPRO) technique. Micro-hardness of the Ni–Ni3Si eutectic composites at different solidification rates are studied by 40MVD micro-hardness tester.
3. Results and discussions 3.1. Microstructure of the Ni–Ni3Si eutectic in situ composites The longitudinal microstructure of the Ni–Si alloy prepared by vacuum induction melting technique is shown in Fig. 2. The longitudinal and transverse microstructures of Ni–Ni3Si eutectic in situ composites prepared by the EBFZM technique at the different solidification rates are shown in Figs. 3 and 4, respectively. On the optical micrographs, the dark phase is Ni matrix, and the light one is Ni3Si compound. It can be seen from Figs. 2 and 3 that the crystal growth direction of the master alloy is random and the crystal grain is coarser, while the directional solidification microstructures are regular lamellar eutectic structures at the solidification rates R¼0.3–4.0 mm/min and the crystal grain gets fined obviously. It can be seen from Fig. 4 that the lamellar spacing of the Ni–Ni3Si eutectic in situ composites is decreased with the increase of the solidification rate. In the process of the crystal growth, both the nucleation rate and the diffusion rate of solute in the liquid are the two important parameters. At low solidification rates, atomic diffusion is efficient enough and eutectic growth happened at near equilibrium conditions. This can result in well-aligned and a large lamellar spacing as shown in Figs. 3a and 4a. According to the theory of constitutional undercooling, an increase in the solidification rate leads to the increase of constitutional undercooling, the nucleation rate of eutectic is increased, whereas the atomic diffusion in the melt is not efficient. If the solidification rate is relatively low, the nucleation rate will play the main role, which can result in the refinement of the solidification microstructure [14]. This is in concordance with the crystal growth theory of the regular eutectic alloys [15], and the lamellar spacing of Ni–Ni3Si eutectic in situ composite is decreased with the increase of the solidification rate as shown in Fig. 4. As far as Ni–Ni3Si eutectic is concerned, Ni is a non-faceted phase, while the Ni3Si compound is a faceted phase. Li and Zhou (LZ) [16] found that the kinetic effect will significantly alter the eutectic growth behaviors, and maintain the coupled eutectic growth to higher undercoolings when crystallization products
71
Fig. 2. Microstructure of the Ni–Si alloy produced with vacuum induction melting technique.
contain intermetallic compounds or other topologically complex phases. With the increase of the solidification rate, the undercooling is increased, and the kinetic undercooling is increased as well. Therefore the regular lamellar eutectic could be obtained at the solidification rate R ¼0.3–4.0 mm/min. 3.2. Phase composition of the Ni–Ni3Si eutectic in situ composites Fig. 5 shows the X-ray diffraction pattern of Ni–Ni3Si eutectic in situ composite prepared by the EBFZM technique. The XRD pattern clearly indicates that a small amount of metastable Ni31Si12 phase is found in the Ni–Ni3Si eutectic in situ composite besides Ni phase and Ni3Si phase. Formation mechanism metastable Ni31Si12 phase is as same as discussed in our former paper [17]. According to Ni–Si phase diagram, at the eutectic composition at 1143 1C, there are three phases: a-Ni, liquid phase and b3Ni3Si phase. With the decrease of the temperature, the b3-Ni3Si phase transforms to b2 phase, and finally transforms to b1 at 1035 1C. During these process, Silicon-rich b1-Ni3Si is also formed through the eutectoid decomposition b2-b1 þ g, where g has the formula Ni31Si12 and a complex hexagonal crystal structure [18]. Thus the metastable Ni31Si12 phase is formed. However, as compared with the X-ray diffraction pattern of Ni–Ni3Si eutectic in situ composite prepared by the Bridgman technique shown in Fig. 6 [17], the X-ray diffraction peak intensity of metastable Ni31Si12 phase in the Ni–Ni3Si eutectic in situ composite prepared by the EBFZM technique is greatly decreased. Which means the amount of the metastable Ni31Si12 phase is decreased during the EBFZM crystal growth process. The high temperature gradient (350–500 K/cm) and high solidification rates are the main reasons to decrease the amount of the metastable Ni31Si12 phase. 3.3. Micro-hardness of the Ni–Ni3Si eutectic in situ composites Table 1 shows the micro-hardness of the Ni–Ni3Si eutectic in situ composites prepared by the EBFZM technique at the different solidification rates. The relationship between solidification rate and micro-hardness of the Ni–Ni3Si eutectic in situ composites is shown in Fig. 7, which demonstrates that microhardness of the Ni–Ni3Si eutectic in situ composites are decreased as compared with the pure Ni3Si compound, i.e., the ductility of the composites has been improved when Ni3Si compound is combined with Ni matrix at the eutectic composition. Microhardness of the Ni–Ni3Si eutectic in situ composites is firstly decreased and then increased with the increase of the solidification rate. The minimum micro-hardness of the Ni–Ni3Si eutectic in situ composites is obtained when the solidification rate is R¼1.0 mm/min. As compared with our former data [17],
72
C. Cui et al. / Physica B 412 (2013) 70–73
Fig. 3. Longitudinal microstructures of the Ni–Ni3Si eutectic in situ composites at different solidification rates. (a) R¼ 0.3 mm/min; (b) R¼ 1.0 mm/min; (c) R¼ 2.0 mm/min; (d) R ¼4.0 mm/min.
Fig. 4. Transverse microstructures of the Ni–Ni3Si eutectic in situ composites at different solidification rates. (a) R ¼0.3 mm/min; (b) R¼ 1.0 mm/min; (c) R¼ 2.0 mm/min; (d) R ¼4.0 mm/min.
micro-hardness of the Ni–Ni3Si eutectic in situ composites prepared by the EBFZM technique are lower than that of the Ni–Ni3Si eutectic in situ composites prepared by the Bridgman technique and higher than that of the Ni–Ni3Si hypoeutectic in situ composites prepared by the Bridgman technique. The reason which results in the difference of the micro-hardness is the amount of
the metastable Ni31Si12 phase. There is a small amount of the metastable Ni31Si12 phase found in the Ni–Ni3Si eutectic in situ composites prepared by the EBFZM technique, a great deal of metastable Ni31Si12 phase is found in the Ni–Ni3Si eutectic in situ composites prepared by the Bridgman directional solidification technique, and no metastable Ni31Si12 phase is formed in the
C. Cui et al. / Physica B 412 (2013) 70–73
73
Fig. 5. XRD pattern for the Ni–Ni3Si eutectic in situ composite prepared by the EBFZM technique. Fig. 7. Relationship between solidification rate and micro-hardness of the Ni–Ni3Si eutectic in situ composites prepared by the EBFZM technique.
2. Ni–Ni3Si eutectic in situ composites prepared by the EBFZM technique are composed of Ni, Ni3Si phase, and a small amount of metastable Ni31Si12 phase. 3. In comparison with the pure Ni3Si compound, ductility of the Ni–Ni3Si eutectic in situ composites prepared by the EBFZM technique has been obviously improved.
Acknowledgments
Fig. 6. XRD pattern for the Ni–Ni3Si eutectic in situ composite prepared by the Bridgman technique [17].
Table 1 Micro-hardness of the Ni–Ni3Si eutectic in situ composites at the different solidification rates. Sample Ni–Ni3Si eutectic in situ Ni–Ni3Si eutectic in situ Ni–Ni3Si eutectic in situ Ni–Ni3Si eutectic in situ Pure Ni3Si compound
Micro-hardness (MPa) composite(R¼ 0.3 mm/min) composite(R¼ 1.0 mm/min) composite(R¼ 2.0 mm/min) composite(R¼ 4.0 mm/min)
708.5 579.4 643.1 641.2 716.0
Ni–Ni3Si hypoeutectic in situ composites prepared by the Bridgman technique [17].
Dr. Zhongwu Hu and Dr. Jing Zheng from Northwest Institute for Non-ferrous Metal Research are given special thanks for the preparation of the samples. The authors would like to thank the National Nature Science Foundation of China (51201121), the Specialized Research Fund for the Doctoral Program of Higher Education (20096120120017), the fund of the State Key Laboratory of Solidification Processing in NWPU (SKLSP200904), the Natural Science Foundation of Shaanxi Province of China (2012JQ6004), the Specialized Research Fund of Education Commission of Shaanxi Province of China (12JK0425) for financial support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
4. Conclusion 1. Microstructures of the Ni–Ni3Si eutectic in situ composites prepared by the EBFZM technique are regular lamellar eutectic structure at the solidification rate R¼0.3–4.0 mm/min. The lamellar spacing is decreased with the increase of the solidification rate.
[12] [13] [14] [15] [16] [17] [18]
T. Takasugi, H. Kawai, Y. Kaneno, Mater. Sci. Eng. A 329–331 (2002) 446. H.M. Wang, C.M. Wang, L.X. Cai, Surf. Coat. Technol. 168 (2003) 202. K. Ohira, Y. Kaneno, T. Takasugi, Mater. Sci. Eng. A 399 (2005) 332. J.H. Zhu, C.T. Liu, Intermetallics 10 (2002) 309. A.T. Dutra, P.L. Ferrandini, R. Caram, J. Alloys Compd. 432 (2007) 167. D.M. Dimiduk, M.G. Mendiratta, P.R. Subramaniam, Structural Intermetallics, TMS, Warrendale, 1993, p. 619. M.L. Bhatia, R.W. Cahn, Intermetallics 11 (2003) 673. T. Takasugi, M. Nagashima, O. Izumi, Acta Metall. Mater. 38 (1990) 747. M.Y. Niu, Q.L. Bi, J. Yang, W.M. Liu, Tribol. Int. 48 (2012) 216. C.T. Liu, E.P. George, W.C. Oliver, Intermetallics 4 (1996) 77. Y. Kaneno, M. Wada, H. Inone, et al. Effects of grain size and temperature on environmental embrittlement of Ni3(Si,Ti) alloys. Autumn Meetings of the Japan Institute of Metals, Nagoya, Japan, 2002. A.T. Dutra, P.L. Ferrandini, R. Caram, J. Alloys Compd. 432 (2007) 167. S. Milenkovic, R. Caram, J. Cryst. Growth 237–239 (2002) 95. J.J. Yu, J. Zhang, F. Wang, J.S. Li, H.Z. Fu, Mater. Sci. Eng. A 311 (2001) 200. J.D. Hunt, K.A. Jackson, Trans. AIME 236 (1966) 843. J.F. Li, Y.H. Zhou, Acta Mater. 53 (2005) 2351. C.J. Cui, J. Zhang, K. Wu, et al., Physica B 407 (2012) 3566. I. Baker, J. Yuan, E.M. Schulson, Metall. Trans. A 24A (1993) 283.
Contents lists available at SciVerse ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Directional solidification of Ni–Ni3Si eutectic in situ composites by electron beam floating zone melting Chunjuan Cui a,n, Jun Zhang b, Kun Wu a, Dening Zou a, Youping Ma a, Lin Liu b, Hengzhi Fu b a b
School of Metallurgical Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 November 2012 Received in revised form 7 December 2012 Accepted 8 December 2012 Available online 22 December 2012
Combining the intermetallic compound with the ductile metal at the eutectic composition is one promising method to improve the ductility of the intermetallic compound. This paper reports the microstructure and the micro-hardness of the Ni–Ni3Si eutectic in situ composites prepared by electron beam floating zone melting technique. Ni–Ni3Si eutectic in situ composites display regular lamellar eutectic structure at the solidification rate R ¼0.3–4.0 mm/min. The lamellar spacing is decreased with the increase of the solidification rate. The phase composition of the Ni–Ni3Si eutectic in situ composites is also determined by X-ray diffraction. Ni–Ni3Si eutectic in situ composites present lower microhardness than pure Ni3Si, although a small quantity of metastable Ni31Si12 phase is formed during the directional solidification process. & 2012 Elsevier B.V. All rights reserved.
Keywords: Directional solidification Eutectic in situ composite Electron beam floating zone melting Micro-hardness
1. Introduction Intermetallics possess higher melting temperatures than the superalloys, and with metallic bonding, at least the possibility of better toughness than ceramics. Therefore, intermetallics have been paid more attentions. Ni3Si-based alloy has been considered to be a candidate material, which can be used as the basis of hightemperature structural materials and chemical parts because Ni3Si displays an increasing strength with increasing temperatures [1] and also shows excellent oxidation and corrosion resistance over a wide range of temperatures [2–4]. However, the major obstacles for the use of Ni3Si compound are its poor ductility at ambient temperatures and its bad fabricability at high temperatures [5,6]. Much work has been done to improve the ductility of the Ni3Si compound, for example, disordering treatment [7], alloying [1,8–10], grain refinement [11], etc. The incorporation of a ductile phase into the intermetallic materials is an attractive method to improve the ductility of the intermetallic materials. This can be achieved by directional solidification process of eutectic alloys, and eutectic in situ composites which are thermodynamically stable, chemically compatible, and well aligned can be obtained. Caram et al. [12,13] produced Ni–Ni3Si eutectic in situ composite with the Bridgman directional solidification technique at solidification rates R¼ 32–56 mm/s. Although many significant results have been achieved,
n
Corresponding author. Tel.: þ86 29 82205104; fax: þ 86 29 82202923. E-mail address: [email protected] (C. Cui).
0921-4526/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physb.2012.12.010
there are still a series of unsolved problems such as the microstructure control, crystal growth mechanism, and the phase composition of the Ni–Ni3Si composites. The preparation technique still needs to be improved as well. Electron beam floating zone melting (EBFZM) technique has many advantages, e.g. high energy density(10 thousand times arc), high vacuum degree (o10 5 mbar), high temperature gradient (350–500 K/cm) and no crucible pollution. These advantages can result in the improvement of the final structure, grain size and properties of the alloy. In the present paper, the EBFZM technique is adopted in order to obtain Ni–Ni3Si eutectic in situ composite which has high-aligned and uniformly-distributed Ni3Si compounds embedded into the Ni matrix. The solidification characteristics and phase composition are studied in detail. In general, the harder the metal material is, the worse its ductility is. Therefore, micro-hardness can be used to represent the ductility of the Ni–Si alloy. Micro-hardness of the Ni–Ni3Si eutectic composites at the different solidification rates are studied by micro-hardness tester.
2. Experiments The master alloys are obtained by cutting the middle of the Ni–11.5 wt% Si alloy into |6 120 mm slices, which are produced with vacuum induction melting technique. The Ni–Ni3Si eutectic in situ composites are prepared by EBZM-20 directional solidification equipment at different solidification rates. The schematic diagram of the electron beam gun is shown in Fig. 1. The directionally solidified samples are treated with conventional
C. Cui et al. / Physica B 412 (2013) 70–73
Fig. 1. Schematic diagram of the electron beam gun.
metallographic technique and etched by the mixture of 5%HClþH2OþFe3Cl solution. Microstructure and phase distributions are observed with OLYMPUS GX51 optical microscope. Phase composition of the Ni–Ni3Si eutectic in situ composites are studied by X-ray diffraction (X Pert MPDPRO) technique. Micro-hardness of the Ni–Ni3Si eutectic composites at different solidification rates are studied by 40MVD micro-hardness tester.
3. Results and discussions 3.1. Microstructure of the Ni–Ni3Si eutectic in situ composites The longitudinal microstructure of the Ni–Si alloy prepared by vacuum induction melting technique is shown in Fig. 2. The longitudinal and transverse microstructures of Ni–Ni3Si eutectic in situ composites prepared by the EBFZM technique at the different solidification rates are shown in Figs. 3 and 4, respectively. On the optical micrographs, the dark phase is Ni matrix, and the light one is Ni3Si compound. It can be seen from Figs. 2 and 3 that the crystal growth direction of the master alloy is random and the crystal grain is coarser, while the directional solidification microstructures are regular lamellar eutectic structures at the solidification rates R¼0.3–4.0 mm/min and the crystal grain gets fined obviously. It can be seen from Fig. 4 that the lamellar spacing of the Ni–Ni3Si eutectic in situ composites is decreased with the increase of the solidification rate. In the process of the crystal growth, both the nucleation rate and the diffusion rate of solute in the liquid are the two important parameters. At low solidification rates, atomic diffusion is efficient enough and eutectic growth happened at near equilibrium conditions. This can result in well-aligned and a large lamellar spacing as shown in Figs. 3a and 4a. According to the theory of constitutional undercooling, an increase in the solidification rate leads to the increase of constitutional undercooling, the nucleation rate of eutectic is increased, whereas the atomic diffusion in the melt is not efficient. If the solidification rate is relatively low, the nucleation rate will play the main role, which can result in the refinement of the solidification microstructure [14]. This is in concordance with the crystal growth theory of the regular eutectic alloys [15], and the lamellar spacing of Ni–Ni3Si eutectic in situ composite is decreased with the increase of the solidification rate as shown in Fig. 4. As far as Ni–Ni3Si eutectic is concerned, Ni is a non-faceted phase, while the Ni3Si compound is a faceted phase. Li and Zhou (LZ) [16] found that the kinetic effect will significantly alter the eutectic growth behaviors, and maintain the coupled eutectic growth to higher undercoolings when crystallization products
71
Fig. 2. Microstructure of the Ni–Si alloy produced with vacuum induction melting technique.
contain intermetallic compounds or other topologically complex phases. With the increase of the solidification rate, the undercooling is increased, and the kinetic undercooling is increased as well. Therefore the regular lamellar eutectic could be obtained at the solidification rate R ¼0.3–4.0 mm/min. 3.2. Phase composition of the Ni–Ni3Si eutectic in situ composites Fig. 5 shows the X-ray diffraction pattern of Ni–Ni3Si eutectic in situ composite prepared by the EBFZM technique. The XRD pattern clearly indicates that a small amount of metastable Ni31Si12 phase is found in the Ni–Ni3Si eutectic in situ composite besides Ni phase and Ni3Si phase. Formation mechanism metastable Ni31Si12 phase is as same as discussed in our former paper [17]. According to Ni–Si phase diagram, at the eutectic composition at 1143 1C, there are three phases: a-Ni, liquid phase and b3Ni3Si phase. With the decrease of the temperature, the b3-Ni3Si phase transforms to b2 phase, and finally transforms to b1 at 1035 1C. During these process, Silicon-rich b1-Ni3Si is also formed through the eutectoid decomposition b2-b1 þ g, where g has the formula Ni31Si12 and a complex hexagonal crystal structure [18]. Thus the metastable Ni31Si12 phase is formed. However, as compared with the X-ray diffraction pattern of Ni–Ni3Si eutectic in situ composite prepared by the Bridgman technique shown in Fig. 6 [17], the X-ray diffraction peak intensity of metastable Ni31Si12 phase in the Ni–Ni3Si eutectic in situ composite prepared by the EBFZM technique is greatly decreased. Which means the amount of the metastable Ni31Si12 phase is decreased during the EBFZM crystal growth process. The high temperature gradient (350–500 K/cm) and high solidification rates are the main reasons to decrease the amount of the metastable Ni31Si12 phase. 3.3. Micro-hardness of the Ni–Ni3Si eutectic in situ composites Table 1 shows the micro-hardness of the Ni–Ni3Si eutectic in situ composites prepared by the EBFZM technique at the different solidification rates. The relationship between solidification rate and micro-hardness of the Ni–Ni3Si eutectic in situ composites is shown in Fig. 7, which demonstrates that microhardness of the Ni–Ni3Si eutectic in situ composites are decreased as compared with the pure Ni3Si compound, i.e., the ductility of the composites has been improved when Ni3Si compound is combined with Ni matrix at the eutectic composition. Microhardness of the Ni–Ni3Si eutectic in situ composites is firstly decreased and then increased with the increase of the solidification rate. The minimum micro-hardness of the Ni–Ni3Si eutectic in situ composites is obtained when the solidification rate is R¼1.0 mm/min. As compared with our former data [17],
72
C. Cui et al. / Physica B 412 (2013) 70–73
Fig. 3. Longitudinal microstructures of the Ni–Ni3Si eutectic in situ composites at different solidification rates. (a) R¼ 0.3 mm/min; (b) R¼ 1.0 mm/min; (c) R¼ 2.0 mm/min; (d) R ¼4.0 mm/min.
Fig. 4. Transverse microstructures of the Ni–Ni3Si eutectic in situ composites at different solidification rates. (a) R ¼0.3 mm/min; (b) R¼ 1.0 mm/min; (c) R¼ 2.0 mm/min; (d) R ¼4.0 mm/min.
micro-hardness of the Ni–Ni3Si eutectic in situ composites prepared by the EBFZM technique are lower than that of the Ni–Ni3Si eutectic in situ composites prepared by the Bridgman technique and higher than that of the Ni–Ni3Si hypoeutectic in situ composites prepared by the Bridgman technique. The reason which results in the difference of the micro-hardness is the amount of
the metastable Ni31Si12 phase. There is a small amount of the metastable Ni31Si12 phase found in the Ni–Ni3Si eutectic in situ composites prepared by the EBFZM technique, a great deal of metastable Ni31Si12 phase is found in the Ni–Ni3Si eutectic in situ composites prepared by the Bridgman directional solidification technique, and no metastable Ni31Si12 phase is formed in the
C. Cui et al. / Physica B 412 (2013) 70–73
73
Fig. 5. XRD pattern for the Ni–Ni3Si eutectic in situ composite prepared by the EBFZM technique. Fig. 7. Relationship between solidification rate and micro-hardness of the Ni–Ni3Si eutectic in situ composites prepared by the EBFZM technique.
2. Ni–Ni3Si eutectic in situ composites prepared by the EBFZM technique are composed of Ni, Ni3Si phase, and a small amount of metastable Ni31Si12 phase. 3. In comparison with the pure Ni3Si compound, ductility of the Ni–Ni3Si eutectic in situ composites prepared by the EBFZM technique has been obviously improved.
Acknowledgments
Fig. 6. XRD pattern for the Ni–Ni3Si eutectic in situ composite prepared by the Bridgman technique [17].
Table 1 Micro-hardness of the Ni–Ni3Si eutectic in situ composites at the different solidification rates. Sample Ni–Ni3Si eutectic in situ Ni–Ni3Si eutectic in situ Ni–Ni3Si eutectic in situ Ni–Ni3Si eutectic in situ Pure Ni3Si compound
Micro-hardness (MPa) composite(R¼ 0.3 mm/min) composite(R¼ 1.0 mm/min) composite(R¼ 2.0 mm/min) composite(R¼ 4.0 mm/min)
708.5 579.4 643.1 641.2 716.0
Ni–Ni3Si hypoeutectic in situ composites prepared by the Bridgman technique [17].
Dr. Zhongwu Hu and Dr. Jing Zheng from Northwest Institute for Non-ferrous Metal Research are given special thanks for the preparation of the samples. The authors would like to thank the National Nature Science Foundation of China (51201121), the Specialized Research Fund for the Doctoral Program of Higher Education (20096120120017), the fund of the State Key Laboratory of Solidification Processing in NWPU (SKLSP200904), the Natural Science Foundation of Shaanxi Province of China (2012JQ6004), the Specialized Research Fund of Education Commission of Shaanxi Province of China (12JK0425) for financial support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
4. Conclusion 1. Microstructures of the Ni–Ni3Si eutectic in situ composites prepared by the EBFZM technique are regular lamellar eutectic structure at the solidification rate R¼0.3–4.0 mm/min. The lamellar spacing is decreased with the increase of the solidification rate.
[12] [13] [14] [15] [16] [17] [18]
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