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Microstructure and properties of Ni–Ni3Si composites by directional solidification
Physica B 407 (2012) 3566–3569
Contents lists available at SciVerse ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Microstructure and properties of Ni–Ni3Si composites by directional solidification Chunjuan Cui a,n, Jun Zhang b, Kun Wu 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 13 April 2012 Received in revised form 8 May 2012 Accepted 9 May 2012 Available online 17 May 2012
Ni–Ni3Si composites are prepared by the Bridgman directional solidification technology under different growth conditions, aiming to improve the ductility of the Ni3Si compound and investigate the relationship between solidification microstructure and the properties. Microstructure of the Ni–Ni3Si hypoeutectic in situ composites transforms from regular lamellar eutectic to cellular structure then to dendritic crystal with the increase of the solidification rate. Ni–Ni3Si eutectic composites display regular lamellar eutectic structure at the solidification rate R ¼ 6.0–40.0 mm/s and the lamellar spacing is decreased with the increase of the solidification rate. Moreover, the Ni–Ni3Si hypoeutectic composites present lower micro-hardness than pure Ni3Si, which indicate Ni–Ni3Si hypoeutectic composites have higher ductility, whereas the ductility of the Ni–Ni3Si eutectic composites has scarcely been improved. This is caused by the formation of the metastable Ni31Si12 phase in the Ni–Ni3Si eutectic composites. & 2012 Elsevier B.V. All rights reserved.
Keywords: Directional solidification Eutectic in situ composite Micro-hardness
1. Introduction Due to high strength, low density and elevated thermal stability, intermetallic compound materials have the greatest potential to meet ever increasing requirements of the automobile, aeronautic and aerospace industry. Among those materials, the Ni3Si compound has been paid more attention, because it has many characters, e.g. high melting point, high strength, low density, excellent oxidation resistance at elevated temperatures, and magnificent corrosion resistance in acid environments, particularly in sulfuric acid solutions [1–6]. However, the engineering application of the Ni3Si compound has been limited by its poor ductility at ambient temperatures and its bad fabricability at high temperatures [6,7]. The grain boundaries in Ni3Si compound are also intrinsically brittle, like in the majority of ordered L12 intermetallics, resulting in a brittle intergranular fracture. Much work has been done to improve the ductility of the Ni3Si compound, for example, disordering treatment, alloying, microstructure control, and composition, etc. The incorporation of a ductile phase into the intermetallic materials is an attractive means to improve the ductility of the intermetallic materials. This can be achieved by directional solidification processing of eutectic alloys, and eutectic in situ
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.2012.05.026
composites which are thermodynamically stable, chemically compatible, and well aligned. Dyck et al. [8] reported Ni–Ni3Si eutectic in situ composite by powder metallurgy technique. Dutra et al. [6] and Milenkovic and Caram [9] obtained Ni–Ni3Si eutectic in situ composite with the Bridgman directional solidification technique at solidification rates R¼32–56 mm/s. Hui et al. [10] adopted the micro-denucleation technique of bulk melt to obtain 224 K undercooling of the Ni–Ni3Si composite and studied the growth mechanisms of the Ni3Si phase. Although many significant results have been achieved, the crystal growth mechanism and the relationship between the solidification microstructure and the properties have not been thoroughly studied. There are still a series of unsolved problems such as the microstructure control, the N–Ni3Si interface structure and the phase composition of the Ni–Ni3Si composites. This paper reports the preparation of the Ni–Ni3Si composites by the Bridgman directional solidification technique at sub-high rates. The eutectic microstructure is obtained at a wide composition range by increasing solidification rate and temperature gradient in front of the solid–liquid interface. Microstructures of the directionally solidified Ni–Ni3Si eutectic and Ni–Ni3Si hypoeutectic 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 and Ni–Ni3Si hypoeutectic composites at the different solidification rates are studied by micro-hardness tester. Finally, the relationship among solidification rate, alloying constituent and micro-hardness is obtained.
C. Cui et al. / Physica B 407 (2012) 3566–3569
2. Experiments The master alloys are obtained by cutting the middle of the Ni– 9 wt% Si hypoeutectic alloy and the Ni–11.5 wt% Si alloy into F6 30 mm slices, which are produced with the vacuum arc melting technique. The Ni–Ni3Si hypoeutectic (eutectic) in situ composites are prepared by the Bridgman directional solidification technique at sub-high rates. The directionally solidified samples are treated with a conventional metallographic technique and etched by the mixture of 5%HClþ H2OþFe3Cl solution, and the microstructure and phase distributions are observed with OLYMPUS GX51 optical microscope. Micro-hardness of the Ni–Ni3Si hypoeutectic (eutectic) composites at different solidification rates are studied by the 40MVD microhardness tester. Phase composition of the Ni–Ni3Si hypoeutectic (eutectic) in situ composites are studied by the X-ray diffraction (X Pert MPDPRO) technique.
3. Results and discussions 3.1. Microstructure and micro-hardness of the Ni–Ni3Si hypoeutectic in situ composites Microstructure of the Ni–Ni3Si hypoeutectic in situ composites on the longitudinal direction is shown in Fig. 1. It clearly shows the composite microstructure of Ni3Si compound (light phase) in Ni matrix (dark phase). It can be seen from Fig. 1 that the solidification microstructure is a regular lamellar eutectic structure at the solidification rate R¼3.0–8.0 mm/s as shown in Fig. 1a and b. The directional heat flow perpendicular to the solid–liquid interface and a high temperature gradient in front of the solid–liquid interface are the two important factors to guaranty the directional growth of crystals. In the present study, the directional heat flow and higher temperature gradient can be achieved by using Ga–In– Sn alloy as coolant and the thermal shield to separate the cool and the hot zones. Meanwhile, the positive temperature gradient maintains the planar solid–liquid interface. Therefore, the lamellar eutectic structure is obtained at the lower solidification rate as shown in Fig. 1a and b. In general, a higher solidification rate can lead to a bigger supercooling. With the increase of supercooling, the nucleation rate and growth rate of Ni3Si phase are increased, whereas the diffusion rate of solute in the liquid is decreased. If the solidification rate is relatively small, the nucleation rate will play the main role and result in the refinement of the solidification
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microstructure, which is why the lamellar spacing of Ni–Ni3Si hypoeutectic in situ composite is decreased with the increase of the solidification rate as shown in Fig. 1a and b. Constitutional supercooling at the solid/liquid interface is gradually increased with the increase of the solidification rate. When the constitutional supercooling is large enough to destroy the planar solid–liquid interface, the cellular crystal appears when the solidification rate reaches 25 mm/s as shown in Fig. 1c. With the further increase of the solidification rate, the solid–liquid interface becomes more unstable, the deep cellular structure appears, and those small cellulars then start to grow and eventually transform into dendrites as shown in Fig. 1d. Moreover, the lamellar spacing of Ni– Ni3Si hypoeutectic in situ composite is synchronously decreased with the increase of the solidification. Indentation pattern of the Ni–Ni3Si hypoeutectic in situ composite is shown in Fig. 2. Table 1 shows the micro-hardness of the Ni–Ni3Si hypoeutectic in situ composites at different solidification rates. The relationship between solidification rate and the micro-hardness of the Ni–Ni3Si hypoeutectic in situ composites is shown in Fig. 3, which demonstrates that the micro-hardness of the Ni–Ni3Si hypoeutectic in situ composites is first decreased and then increased with the increase of the solidification rate. This is caused by the change of microstructure of the Ni–Ni3Si hypoeutectic in situ composite. The optimal microstructure of the Ni–Ni3Si hypoeutectic in situ composite is obtained when the solidification rate is R¼8.0 mm/s and the microhardness of that is the minimum as a result. In comparison with the micro-hardness of pure Ni3Si compound [11], micro-hardness of the Ni–Ni3Si hypoeutectic in situ composite is greatly decreased. That is to say, the ductility of the Ni–Ni3Si hypoeutectic in situ compound is greatly improved when Ni3Si compound is combined with the Ni matrix. 3.2. Microstructure and micro-hardness of the Ni–Ni3Si eutectic in situ composites Microstructure of the Ni–Ni3Si eutectic in situ composites on the longitudinal direction is shown in Fig. 4. It clearly shows the
Fig. 2. Indentation pattern of the Ni–Ni3Si hypoeutectic in situ composite.
Table 1 Micro-hardness of the Ni–Ni3Si hypoeutectic in situ composite at different solidification rates.
Fig. 1. Longitudinal microstructure of the Ni–Ni3Si hypoeutectic in situ composites at different solidification rates. (a) R¼ 3 mm/s, (b) R ¼8 mm/s, (c) R ¼25 mm/s and (d) R ¼ 40 mm/s.
Sample
Micro-hardness (Mpa)
Ni–Ni3Si hypoeutectic in situ composite (R¼ 3.0 mm/s) Ni–Ni3Si hypoeutectic in situ composite (R¼ 8.0 mm/s) Ni–Ni3Si hypoeutectic in situ composite (R¼ 25.0 mm/s) Ni–Ni3Si hypoeutectic in situ composite (R¼ 40.0 mm/s) Pure Ni3Si compound
267.5 226.8 262.8 285.1 716
3568
C. Cui et al. / Physica B 407 (2012) 3566–3569
Fig. 5. Indentation pattern of the Ni–Ni3Si eutectic in situ composite.
Table 2 Micro-hardness of the Ni–Ni3Si eutectic in situ composite at different solidification rates. Sample Fig. 3. Relationship between solidification rate and microhardness of the Ni–Ni3Si hypoeutectic in situ composites.
Fig. 4. Longitudinal microstructure of the Ni–Ni3Si eutectic in situ composites at different solidification rates. (a) R ¼6 mm/s, (b) R ¼9 mm/s, (c) R¼ 25 mm/s and (d) R¼ 40 mm/s.
composite microstructure of Ni3Si (light phase) in Ni matrix (dark phase). It can be seen from Fig. 4 that the solidification microstructure are regular lamellar eutectic structures at the solidification rate R¼6.0–40.0 mm/s. No cellular or dendritic structures are formed with the increase of the solidification rate. All grains parallel to the growth direction and regular lamellar eutectic are obtained. This means that high GL/R make the solid/liquid interface be a planar interface in the present study. The diffusion paths tend to be larger with the increase of the solidification rate, which result in the decrease of the lamellar spacing. Indentation pattern of the Ni–Ni3Si eutectic in situ composite is shown in Fig. 5. Table 2 shows the micro-hardness of the Ni– Ni3Si eutectic in situ composites at different solidification rates. The relationship between solidification rate and micro-hardness of the Ni–Ni3Si eutectic in situ composites is shown in Fig. 6, which demonstrates that micro-hardness of the Ni–Ni3Si eutectic in situ composites is first decreased and then increased with the increase of the solidification rate. The minimum micro-hardness
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 composite composite composite
(R ¼6.0 mm/s) (R ¼9.0 mm/s) (R ¼25.0 mm/s) (R ¼40.0 mm/s)
743.8 626.4 806.9 944.0 716
Fig. 6. Relationship between solidification rate and microhardness of the Ni–Ni3Si eutectic in situ composites.
of the Ni–Ni3Si eutectic in situ composites is obtained when the solidification rate is R¼9.0 mm/s. Unfortunately, micro-hardness of the Ni–Ni3Si eutectic in situ composites is not obviously decreased, even increased as compared with that of pure Ni3Si compound. That is to say, the ductility of the Ni3Si eutectic in situ compound is scarcely improved when Ni3Si compound is combined with Ni matrix at the eutectic composition. In order to study the reason that the micro-hardness of the Ni–Ni3Si eutectic in situ composites is greater than that of the Ni–Ni3Si hypoeutectic in situ composite, XRD is adopted to analyze the phase composition of the Ni–Ni3Si composites. Figs. 7 and 8 are the XRD patterns for the Ni–Ni3Si hypoeutectic in situ composite and the Ni–Ni3Si eutectic in situ composite, respectively, which clearly indicate that the directionally solidified Ni–Ni3Si hypoeutectic in situ composite only consists of Ni phase and Ni3Si phase. However, metastable Ni31Si12 phase is
C. Cui et al. / Physica B 407 (2012) 3566–3569
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composite firstly is decreased due to the refinement of the microstructure and then increased due to the metastable Ni31Si12 phase formation. As far as the Ni–Ni3Si hypoeutectic in situ composite is concerned, eutectoid decomposition does not happen due to Silicon-poor, and no metastable phase is formed in the Ni–Ni3Si hypoeutectic in situ composite. Therefore, micro-hardness of the Ni–Ni3Si hypoeutectic in situ composite is greatly decreased as compared with that of the Ni–Ni3Si eutectic in situ composites.
4. Conclusion
Fig. 7. XRD pattern for the Ni–Ni3Si hypoeutectic in situ composite.
1. Under the condition of undercooling, microstructure of the Ni– Ni3Si hypoeutectic composites transforms from regular lamellar eutectic to cellular structure then to dendritic crystal with the increase of the solidification rate. 2. Microstructure of the Ni–Ni3Si eutectic composites are regular lamellar eutectic structure at the solidification rate R¼6.0– 40.0 mm/s. The lamellar spacing is decreased with the increase of the solidification rate. 3. In comparison with pure Ni3Si compound, ductility of the Ni– Ni3Si hypoeutectic in situ composites have been greatly improved, whereas that of Ni–Ni3Si eutectic in situ composites scarcely have been improved due to the formation of the metastable Ni31Si12 phase at the eutectic composition. Acknowledgments
Fig. 8. XRD pattern for the Ni–Ni3Si eutectic in situ composite.
found in the Ni–Ni3Si eutectic in situ composite, besides Ni phase and Ni3Si phase. Formation of the metastable Ni31Si12 results in the increase of micro-hardness of the Ni–Ni3Si eutectic in situ composites. According to Ni–Si phase diagram, at the eutectic composition at 1143 1C, there are three phases: a-Ni, liquid phase and b3-Ni3Si phase. With the decrease of the temperature, the b3Ni3Si phase transforms to b2-phase, and finally transforms to b1 at 1035 1C. During this processing, 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 [12]. This is the formation of the metastable Ni31Si12 phase at the eutectic composition. Moreover, the formation of the metastable Ni31Si12 phase is increased with the increase of the solidification rate. Micro-hardness of the Ni–Ni3Si eutectic in situ
The authors would like to thank 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) and the Science Plan Project of Xi’an University of Architecture and Technology (RC0907; JC1109) for financial support.
References [1] S. Oukassi, J.S. Moulet, S. Lay, F. Hodaj, Microelectron. Eng. 86 (2009) 397. [2] H.M. Wang, C.M. Wang, L.X. Cai, Surf. Coat. Technol. 168 (2003) 202. [3] H. Amekura, H. Kitazawa, N. Umeda, Y. Takeda, N. Kishimoto, Methods Phys. Res. B 222 (2004) 114. [4] J.H. Jia, J.J. Lu, H.D. Zhou, J.M. Chen, Mater Sci Eng A 381 (2004) 80. [5] R. Caram, S. Milenkovic, J. Cryst. Growth 198/199 (1999) 844. [6] A.T. Dutra, P.L. Ferrandini, R. Caram, J. Alloys Compd. 432 (2007) 167. [7] D.M. Dimiduk, M.G. Mendiratta, P.R. Subramaniam, Structural Intermetallics, TMS, Warrendale, 1993, p. 619. [8] S.V. Dyck, L. Delaey, L. Froyen, L. Buekenhout, Intermetallics 3 (1995) 309. [9] S. Milenkovic, R. Caram, J. Cryst. Growth 237–239 (2002) 95. [10] Z.Z. Hui, G.C. Yang, Y.H. Zhou, Prog. Nat. Sci. 7 (1997) 475. [11] X. Guo, X.F. Lu, Q. Ma, et al., New Technol. New Process 12 (2009) 96. [12] 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
Microstructure and properties of Ni–Ni3Si composites by directional solidification Chunjuan Cui a,n, Jun Zhang b, Kun Wu 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 13 April 2012 Received in revised form 8 May 2012 Accepted 9 May 2012 Available online 17 May 2012
Ni–Ni3Si composites are prepared by the Bridgman directional solidification technology under different growth conditions, aiming to improve the ductility of the Ni3Si compound and investigate the relationship between solidification microstructure and the properties. Microstructure of the Ni–Ni3Si hypoeutectic in situ composites transforms from regular lamellar eutectic to cellular structure then to dendritic crystal with the increase of the solidification rate. Ni–Ni3Si eutectic composites display regular lamellar eutectic structure at the solidification rate R ¼ 6.0–40.0 mm/s and the lamellar spacing is decreased with the increase of the solidification rate. Moreover, the Ni–Ni3Si hypoeutectic composites present lower micro-hardness than pure Ni3Si, which indicate Ni–Ni3Si hypoeutectic composites have higher ductility, whereas the ductility of the Ni–Ni3Si eutectic composites has scarcely been improved. This is caused by the formation of the metastable Ni31Si12 phase in the Ni–Ni3Si eutectic composites. & 2012 Elsevier B.V. All rights reserved.
Keywords: Directional solidification Eutectic in situ composite Micro-hardness
1. Introduction Due to high strength, low density and elevated thermal stability, intermetallic compound materials have the greatest potential to meet ever increasing requirements of the automobile, aeronautic and aerospace industry. Among those materials, the Ni3Si compound has been paid more attention, because it has many characters, e.g. high melting point, high strength, low density, excellent oxidation resistance at elevated temperatures, and magnificent corrosion resistance in acid environments, particularly in sulfuric acid solutions [1–6]. However, the engineering application of the Ni3Si compound has been limited by its poor ductility at ambient temperatures and its bad fabricability at high temperatures [6,7]. The grain boundaries in Ni3Si compound are also intrinsically brittle, like in the majority of ordered L12 intermetallics, resulting in a brittle intergranular fracture. Much work has been done to improve the ductility of the Ni3Si compound, for example, disordering treatment, alloying, microstructure control, and composition, etc. The incorporation of a ductile phase into the intermetallic materials is an attractive means to improve the ductility of the intermetallic materials. This can be achieved by directional solidification processing of eutectic alloys, and eutectic in situ
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.2012.05.026
composites which are thermodynamically stable, chemically compatible, and well aligned. Dyck et al. [8] reported Ni–Ni3Si eutectic in situ composite by powder metallurgy technique. Dutra et al. [6] and Milenkovic and Caram [9] obtained Ni–Ni3Si eutectic in situ composite with the Bridgman directional solidification technique at solidification rates R¼32–56 mm/s. Hui et al. [10] adopted the micro-denucleation technique of bulk melt to obtain 224 K undercooling of the Ni–Ni3Si composite and studied the growth mechanisms of the Ni3Si phase. Although many significant results have been achieved, the crystal growth mechanism and the relationship between the solidification microstructure and the properties have not been thoroughly studied. There are still a series of unsolved problems such as the microstructure control, the N–Ni3Si interface structure and the phase composition of the Ni–Ni3Si composites. This paper reports the preparation of the Ni–Ni3Si composites by the Bridgman directional solidification technique at sub-high rates. The eutectic microstructure is obtained at a wide composition range by increasing solidification rate and temperature gradient in front of the solid–liquid interface. Microstructures of the directionally solidified Ni–Ni3Si eutectic and Ni–Ni3Si hypoeutectic 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 and Ni–Ni3Si hypoeutectic composites at the different solidification rates are studied by micro-hardness tester. Finally, the relationship among solidification rate, alloying constituent and micro-hardness is obtained.
C. Cui et al. / Physica B 407 (2012) 3566–3569
2. Experiments The master alloys are obtained by cutting the middle of the Ni– 9 wt% Si hypoeutectic alloy and the Ni–11.5 wt% Si alloy into F6 30 mm slices, which are produced with the vacuum arc melting technique. The Ni–Ni3Si hypoeutectic (eutectic) in situ composites are prepared by the Bridgman directional solidification technique at sub-high rates. The directionally solidified samples are treated with a conventional metallographic technique and etched by the mixture of 5%HClþ H2OþFe3Cl solution, and the microstructure and phase distributions are observed with OLYMPUS GX51 optical microscope. Micro-hardness of the Ni–Ni3Si hypoeutectic (eutectic) composites at different solidification rates are studied by the 40MVD microhardness tester. Phase composition of the Ni–Ni3Si hypoeutectic (eutectic) in situ composites are studied by the X-ray diffraction (X Pert MPDPRO) technique.
3. Results and discussions 3.1. Microstructure and micro-hardness of the Ni–Ni3Si hypoeutectic in situ composites Microstructure of the Ni–Ni3Si hypoeutectic in situ composites on the longitudinal direction is shown in Fig. 1. It clearly shows the composite microstructure of Ni3Si compound (light phase) in Ni matrix (dark phase). It can be seen from Fig. 1 that the solidification microstructure is a regular lamellar eutectic structure at the solidification rate R¼3.0–8.0 mm/s as shown in Fig. 1a and b. The directional heat flow perpendicular to the solid–liquid interface and a high temperature gradient in front of the solid–liquid interface are the two important factors to guaranty the directional growth of crystals. In the present study, the directional heat flow and higher temperature gradient can be achieved by using Ga–In– Sn alloy as coolant and the thermal shield to separate the cool and the hot zones. Meanwhile, the positive temperature gradient maintains the planar solid–liquid interface. Therefore, the lamellar eutectic structure is obtained at the lower solidification rate as shown in Fig. 1a and b. In general, a higher solidification rate can lead to a bigger supercooling. With the increase of supercooling, the nucleation rate and growth rate of Ni3Si phase are increased, whereas the diffusion rate of solute in the liquid is decreased. If the solidification rate is relatively small, the nucleation rate will play the main role and result in the refinement of the solidification
3567
microstructure, which is why the lamellar spacing of Ni–Ni3Si hypoeutectic in situ composite is decreased with the increase of the solidification rate as shown in Fig. 1a and b. Constitutional supercooling at the solid/liquid interface is gradually increased with the increase of the solidification rate. When the constitutional supercooling is large enough to destroy the planar solid–liquid interface, the cellular crystal appears when the solidification rate reaches 25 mm/s as shown in Fig. 1c. With the further increase of the solidification rate, the solid–liquid interface becomes more unstable, the deep cellular structure appears, and those small cellulars then start to grow and eventually transform into dendrites as shown in Fig. 1d. Moreover, the lamellar spacing of Ni– Ni3Si hypoeutectic in situ composite is synchronously decreased with the increase of the solidification. Indentation pattern of the Ni–Ni3Si hypoeutectic in situ composite is shown in Fig. 2. Table 1 shows the micro-hardness of the Ni–Ni3Si hypoeutectic in situ composites at different solidification rates. The relationship between solidification rate and the micro-hardness of the Ni–Ni3Si hypoeutectic in situ composites is shown in Fig. 3, which demonstrates that the micro-hardness of the Ni–Ni3Si hypoeutectic in situ composites is first decreased and then increased with the increase of the solidification rate. This is caused by the change of microstructure of the Ni–Ni3Si hypoeutectic in situ composite. The optimal microstructure of the Ni–Ni3Si hypoeutectic in situ composite is obtained when the solidification rate is R¼8.0 mm/s and the microhardness of that is the minimum as a result. In comparison with the micro-hardness of pure Ni3Si compound [11], micro-hardness of the Ni–Ni3Si hypoeutectic in situ composite is greatly decreased. That is to say, the ductility of the Ni–Ni3Si hypoeutectic in situ compound is greatly improved when Ni3Si compound is combined with the Ni matrix. 3.2. Microstructure and micro-hardness of the Ni–Ni3Si eutectic in situ composites Microstructure of the Ni–Ni3Si eutectic in situ composites on the longitudinal direction is shown in Fig. 4. It clearly shows the
Fig. 2. Indentation pattern of the Ni–Ni3Si hypoeutectic in situ composite.
Table 1 Micro-hardness of the Ni–Ni3Si hypoeutectic in situ composite at different solidification rates.
Fig. 1. Longitudinal microstructure of the Ni–Ni3Si hypoeutectic in situ composites at different solidification rates. (a) R¼ 3 mm/s, (b) R ¼8 mm/s, (c) R ¼25 mm/s and (d) R ¼ 40 mm/s.
Sample
Micro-hardness (Mpa)
Ni–Ni3Si hypoeutectic in situ composite (R¼ 3.0 mm/s) Ni–Ni3Si hypoeutectic in situ composite (R¼ 8.0 mm/s) Ni–Ni3Si hypoeutectic in situ composite (R¼ 25.0 mm/s) Ni–Ni3Si hypoeutectic in situ composite (R¼ 40.0 mm/s) Pure Ni3Si compound
267.5 226.8 262.8 285.1 716
3568
C. Cui et al. / Physica B 407 (2012) 3566–3569
Fig. 5. Indentation pattern of the Ni–Ni3Si eutectic in situ composite.
Table 2 Micro-hardness of the Ni–Ni3Si eutectic in situ composite at different solidification rates. Sample Fig. 3. Relationship between solidification rate and microhardness of the Ni–Ni3Si hypoeutectic in situ composites.
Fig. 4. Longitudinal microstructure of the Ni–Ni3Si eutectic in situ composites at different solidification rates. (a) R ¼6 mm/s, (b) R ¼9 mm/s, (c) R¼ 25 mm/s and (d) R¼ 40 mm/s.
composite microstructure of Ni3Si (light phase) in Ni matrix (dark phase). It can be seen from Fig. 4 that the solidification microstructure are regular lamellar eutectic structures at the solidification rate R¼6.0–40.0 mm/s. No cellular or dendritic structures are formed with the increase of the solidification rate. All grains parallel to the growth direction and regular lamellar eutectic are obtained. This means that high GL/R make the solid/liquid interface be a planar interface in the present study. The diffusion paths tend to be larger with the increase of the solidification rate, which result in the decrease of the lamellar spacing. Indentation pattern of the Ni–Ni3Si eutectic in situ composite is shown in Fig. 5. Table 2 shows the micro-hardness of the Ni– Ni3Si eutectic in situ composites at different solidification rates. The relationship between solidification rate and micro-hardness of the Ni–Ni3Si eutectic in situ composites is shown in Fig. 6, which demonstrates that micro-hardness of the Ni–Ni3Si eutectic in situ composites is first decreased and then increased with the increase of the solidification rate. The minimum micro-hardness
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 composite composite composite
(R ¼6.0 mm/s) (R ¼9.0 mm/s) (R ¼25.0 mm/s) (R ¼40.0 mm/s)
743.8 626.4 806.9 944.0 716
Fig. 6. Relationship between solidification rate and microhardness of the Ni–Ni3Si eutectic in situ composites.
of the Ni–Ni3Si eutectic in situ composites is obtained when the solidification rate is R¼9.0 mm/s. Unfortunately, micro-hardness of the Ni–Ni3Si eutectic in situ composites is not obviously decreased, even increased as compared with that of pure Ni3Si compound. That is to say, the ductility of the Ni3Si eutectic in situ compound is scarcely improved when Ni3Si compound is combined with Ni matrix at the eutectic composition. In order to study the reason that the micro-hardness of the Ni–Ni3Si eutectic in situ composites is greater than that of the Ni–Ni3Si hypoeutectic in situ composite, XRD is adopted to analyze the phase composition of the Ni–Ni3Si composites. Figs. 7 and 8 are the XRD patterns for the Ni–Ni3Si hypoeutectic in situ composite and the Ni–Ni3Si eutectic in situ composite, respectively, which clearly indicate that the directionally solidified Ni–Ni3Si hypoeutectic in situ composite only consists of Ni phase and Ni3Si phase. However, metastable Ni31Si12 phase is
C. Cui et al. / Physica B 407 (2012) 3566–3569
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composite firstly is decreased due to the refinement of the microstructure and then increased due to the metastable Ni31Si12 phase formation. As far as the Ni–Ni3Si hypoeutectic in situ composite is concerned, eutectoid decomposition does not happen due to Silicon-poor, and no metastable phase is formed in the Ni–Ni3Si hypoeutectic in situ composite. Therefore, micro-hardness of the Ni–Ni3Si hypoeutectic in situ composite is greatly decreased as compared with that of the Ni–Ni3Si eutectic in situ composites.
4. Conclusion
Fig. 7. XRD pattern for the Ni–Ni3Si hypoeutectic in situ composite.
1. Under the condition of undercooling, microstructure of the Ni– Ni3Si hypoeutectic composites transforms from regular lamellar eutectic to cellular structure then to dendritic crystal with the increase of the solidification rate. 2. Microstructure of the Ni–Ni3Si eutectic composites are regular lamellar eutectic structure at the solidification rate R¼6.0– 40.0 mm/s. The lamellar spacing is decreased with the increase of the solidification rate. 3. In comparison with pure Ni3Si compound, ductility of the Ni– Ni3Si hypoeutectic in situ composites have been greatly improved, whereas that of Ni–Ni3Si eutectic in situ composites scarcely have been improved due to the formation of the metastable Ni31Si12 phase at the eutectic composition. Acknowledgments
Fig. 8. XRD pattern for the Ni–Ni3Si eutectic in situ composite.
found in the Ni–Ni3Si eutectic in situ composite, besides Ni phase and Ni3Si phase. Formation of the metastable Ni31Si12 results in the increase of micro-hardness of the Ni–Ni3Si eutectic in situ composites. According to Ni–Si phase diagram, at the eutectic composition at 1143 1C, there are three phases: a-Ni, liquid phase and b3-Ni3Si phase. With the decrease of the temperature, the b3Ni3Si phase transforms to b2-phase, and finally transforms to b1 at 1035 1C. During this processing, 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 [12]. This is the formation of the metastable Ni31Si12 phase at the eutectic composition. Moreover, the formation of the metastable Ni31Si12 phase is increased with the increase of the solidification rate. Micro-hardness of the Ni–Ni3Si eutectic in situ
The authors would like to thank 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) and the Science Plan Project of Xi’an University of Architecture and Technology (RC0907; JC1109) for financial support.
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