Competitive Nucleation and Rapid Growth of Co-Si Intermetallic Compounds during Eutectic Solidification under Containerless Processing Condition

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1 J. Mater. Sci. Technol., 2011, 27(11, Competitive Nucleation and Rapid Growth of Co- Intermetallic Compounds during Eutectic Solidification under Containerless Processing Condition Wenjing Yao 1, Zipeng Ye 1, Nan Wang 1, Xiujun Han 2, Jianyuan Wang 1 and Xixing Wen 1 1 Department of Applied Physics, Northwestern Polytechnical University, Xi an , China 2 School of Materials Science, Shanghai Jiaotong University, Shanghai , China [Manuscript received January 28, 2011, in revised form March 10, 2011] The liquid-solid transitions of ( +Co and (Co+Co 2 eutectic alloys were realized in drop tube and the rapid eutectic growth mechanism of intermetallic compounds was examined. The experimental and calculated results indicate that with increasing Co content, the intermetallic compound prefers nucleating primarily. The eutectic microstructures experience the transitions of lamellar-anomalous-divorced eutectic with undercooling. In undercooled state, the growth of Co intermetallic compound always lags behind others, and no matter how large the undercooling is, this intermetallic compound grows under the solutal diffusion control. The calculated coupled zone demonstrates that ( +Co eutectic can form within certain undercooling regime, when the composition is in the range from 23.6% to 25.4%. And the calculated coupled zone of (Co+Co 2 covers a composition range from 40.8% to 43.8%. KEY WORDS: Liquid-solid transition; Competitive nucleation; Intermetallic compound; Undercooling 1. Introduction Intermetallic compounds have long been of interest for high-temperature technological applications. As an ordered material, it possesses many properties superior to traditional materials, such as low density, high melting point, outstanding antioxidation properties and corrosion resistance. Especially, -based intermetallic compounds with excellent and stable capability of electronic and magnetic characteristics have a wide potential application. The previous works have investigated the structures and various physical properties of the intermetallic compounds in Fe- [1 5], Ni- [6 9], Mo- [10 12], Al- [13 15], Ti- [16,17], Co- [18,19] alloys. However, most of them focused mainly on the electronic or magnetic properties of their intermatallics produced by the traditional preparing method. In recent years, some new man- Corresponding author. Prof.; Tel.: ; address: nan.wang@nwpu.edu.cn (N. Wang. ufacturing ways have been developed. One of them is drop tube processing method, which not only provides a technique for containerless solidification in microgravity environment but also has an advantage of combining rapid cooling with high undercooling. Therefore, it has attracted great research interest and is widely used to study the rapid solidification of alloys. Co- binary system is one typical sample of the -based alloys and four intermetallic compounds can be produced: Co 3,, Co and Co 2. From the equilibrium phase diagram of Co- system, evaluated by Ishida et al. [20], it is found that two eutectic alloys are composed of eutectic phases of three intermetallic compounds aforementioned: Co-23.9 wt% alloy with and Co, and Co-43.5 wt% alloy with Co and Co 2. In general, when solidification occurs, the nucleation is dominated by the primary phase at first and the other one nucleates dependently on it. Then the two intermetallic com-

2 1078 W.J. Yao et al.: J. Mater. Sci. Technol., 2011, 27(11, pounds keep growing cooperatively. If rapid solidification occurs during drop tube process, the growth characteristics of intermetallic compounds may have some differences from those at slow growth rate. Thus, examining the solidification microstructure and studying the competitive nucleation and rapid growth of the intermetallic compounds under rapid solidification condition are helpful to understand the mechanism of liquid to solid transition at high undercooling. The purpose of this work is to investigate the rapid solidification behaviors of ( +Co and (Co+Co 2 eutectics under free fall condition in the drop tube. The solidification microstructure was examined to show the rapid crystal growth characteristics of Co- intermetallic compounds at different undercoolings and the TMK [21] eutectic growth and LKT [22,23] dendritic growth theories were used to reveal the rapid growth mechanisms. 2. Experimental The experiments were performed in a 3 m drop tube. The master alloy was prepared by induction melting from pure Co (99.95% and pure (99.99%, and each sample had a mass of about 0.5 g. When an experiment began, the sample was placed in a 13 mm inner diamater 15 mm outside diamter 160 mm quartz tube, the bottom of which had a small orifice about 0.2 mm in diameter. The quartz tube was then installed on the top of the drop tube. Afterwards the drop tube was evacuated to Pa before backfilled with a mixture of He and Ar gas to about 0.1 MPa. The sample was melted by induction heating and further overheated to about 100 K above its liquidus temperature for several minutes. The temperature was measured by an infrared pyrometer, and then the melt was ejected out from the orifice and dispersed into many fine droplets after being blown by a gas flow of highly purified He or Ar into the quartz tube. After experiments, the solidified droplets were collected at the bottom of the drop tube and sieved into several groups according to their sizes. The solidified particles were mounted in epoxy, sectioned, polished, and etched with a mixture of 30 ml hydrofluoric acid + 10 ml HNO ml H 2 O. The microstructures of the solidified particles were analyzed by an XJG- 05 optical microscope and an S2700 scanning electronic microscope (SEM. The chemical compositions of the identified constituent phases were determined with an energy dispersive X-ray analyzer (EDX fitted in SEM. 3. Results and Discussion Figure 1 presents part of Co- binary phase diagram. Under equilibrium condition, the Co-23.9% eutectic is composed of and Co intermetallic compounds, while the Co-43.5% eutectic consists of T / K % L+Co 1559 K T=22 K Coupled Zone +Co T max =385 K (0.24T E L Co L+Co Coupled Zone 1583 K Co+Co 2 T max =444 K (0.28T E 43.5% T=29 K Co / wt% Fig. 1 Maximum undercoolings and calculated coupled zones of (+Co and (Co+Co 2 eutectic alloys shown in the Co- binary phase diagram Co and Co 2 intermetallic compounds. 3.1 Rapid eutectic growth of Co- intermetallic compounds For binary eutectic alloy, according to equilibrium phase diagram, two phases should nucleate competitively and grow cooperatively to form a lamellar or rod eutectic structure. However, the cooperative growth of two eutectic phases may be restrained under rapid solidification conditions Eutectic microstructures Figure 2 illustrates the microstructural morphologies of Co-23.9% and Co-43.5% eutectic alloy droplets with different sizes. The lamellar eutectic structures in the master alloys are shown in Fig. 2(a and 2(a. After the rapid solidification by containerless processing, the growth morphology shows different characteristics when droplet size decreases. In large droplet, chrysanthemum-like anomalous eutectic grains replace lamellar eutectic grains. An enlarged view of anomalous eutectic is presented in Fig. 2(b and 2(b. Experimental observation reveals that, with decreasing droplet size, the morphology of anomalous eutectic becomes more and more distinct. In some small droplets, the morphology is completely characterized by divorced eutectic. Figure 2(c is the enlarged view of divorced eutectic formed in Co-23.9% droplet with the diameter of 100 µm. According to the EDS (energy dispersive X-ray spectrometer analysis, the dark phase is and the bright one is Co. Obviously, phase grows into a dendrite, on which Co nucleates and grows dependently. Figure 2(c presents an enlarged view of a microstructure in the center of a Co-43.5% droplet of 100 µm diameter, where the bright phase is Co 2 and the dark one is Co by EDS analysis. In contrast to Fig. 2(c, the competitive nucleation of Co and Co 2 seems more intensive than that of and Co. The primary nucleation and growth phase of Co-43.5% eutectic Co2

3 W.J. Yao et al.: J. Mater. Sci. Technol., 2011, 27(11, Fig. 2 Rapid growth morphologies of (+Co and (Co+Co 2 eutectic at different size droplets: (a (c Co-23.9 wt% eutectic alloy, (a to (c Co-43.5 wt% eutectic alloy alloy cannot be judged from the microstructure of small droplet, as in the case of Co-23.9% eutectic alloy. It can be concluded from Fig. 2 that, with decreasing droplet diameter D, the eutectic microstructures experience the transitions from lamellar to anomalous eutectic, even to divorced eutectic. During free fall, small droplets usually obtain high undercoolings. A heat transfer model was applied to estimate the droplet undercooling [24] and the relationship between the undercooling ( T and the droplet diameter (D were fitted as follows: T = e D (1 T = e D (2 The larger the droplet size, the higher the undercooling. For the droplets of 100 µm in diameter of the two alloys, the undercooling are 385 K (0.25T E and 444 K (0.28T E, respectively. Thus, the experiences of the growth morphology transitions of ( +Co and (Co+Co 2 eutectics from lamellar to anomalous eutectic, even to divorced eutectic is owing to the increase of the undercooling Competitive nucleation and rapid growth By the classical nucleation theory, the nucleation rate can be described as [25] ( 16πσ 3 (T L 2 I = I 0 exp 3 ( H m 2 ( T 2 kt f(θ exp (3 where I 0 is prefactor of nucleation; σ the interfacial energy, which can be approximated by the interfacial model of Thompson and Spaepen [26] ; R gas con- ( Q RT stant; k Boltzman constant; Q activation energy for diffusion; θ contact angle; and f(θp=(2+cos θ(1 cos θ 2 /4, catalytic factor. Based on Eq. (3, the homogeneous nucleation rates of Co- intermetallic compounds in Co-23.9% and Co-43.5% eutectic alloys were calculated and the used physical parameters are listed in Table 1. The calculated results show that even when undercooling exceeds the maximum undercooling obtained during experiment, the homogeneous nucleation rate will be far less than 1 m 3 s 1. It indicates that heterogeneous nucleation takes place. For heterogeneous nucleation, the catalytic factor f(θ should be determined. However, it is very difficult to find the data of contact angle. In order to obtain half quantitative analysis, f(θ=0.1 is assumed for the two eutectic phases in each alloy in the calculation. Under this condition, the calculated heterogeneous nucleation rates are illustrated in Fig. 3(a. Clearly, with increasing undercooling, the nucleation rate of intermetallic compound is always larger than that of Co intermetallic compound in Co-23.9% alloy melt, whereas the nucleation rate of Co intermetallic compound is always larger than that of Co 2 intermetallic compound in Co-43.5% alloy melt. It indicates that the intermetallic compound with higher Co content prefers to nucleate primarily. This has been verified experimentally and shown in Fig. 2(c, where the phase acts as a primary phase. The growth velocities of Co- intermetallic compounds at different undercoolings were calculated by using LKT dendritic growth theory [22,23]. The physical parameters of the two eutectic alloy used in

4 1080 W.J. Yao et al.: J. Mater. Sci. Technol., 2011, 27(11, Table 1 Physical parameters of Co- alloys used in calculations Parameter Symbol (unit Value Co-23.9% Co-43.5% Alloy composition C E (wt% Liquidius temperature T E (K Heat of fusion H (J/mol Specific heat of liquid C P(J/mol K Equilibrium liquidius slope m Lα (K/wt% Equilibrium liquidius slope m Lβ (K/wt% Diffusion coefficient D L (m 2 /s exp( /RT exp( /RT Characteristic length of diffusion a 0 (m Gibbs-Thomson coefficient Γ α (K m Gibbs-Thomson coefficient Γ β (K m Sound velocity V 0 (m/s Solute partition coefficient k eα Solute partition coefficient k eβ Interface energy of α phase σ Lα/(J/m Interface energy of β phase σ Lβ /(J/m log(i/m -3 s -1 Velocity / (m/s (a (b Co in Co-23.9% Co in Co-43.5% Co Undercooling / K Co 2 Co in Co-23.9% Co in Co-43.5% Undercooling / K Fig. 3 Calculated nucleation rates (a and growth velocities (b of Co- intermetallic compounds in Co- 23.9% and Co-43.5% eutectic alloy melts versus undercooling the calculation are listed in Table 1 and the calculated results are shown in Fig. 3(b. Obviously, as the undercooling rises up, all of growth velocities increase very quickly. It is demonstrated that intermetallic compound always grows primarily over Co intermetallic compound in Co-23.9% alloy melt. In the similar way, the growth of Co 2 intermetallic compound is primary over the growth of Co intermetallic compound in Co-43.5% alloy melt. Figure 3 indicates that, during the rapid growth of ( +Co eutectic, phase is not only the primary nucleation phase but also has high growth velocity. In this case, the primary nucleation phase is just the primary growth phase, so that one eutectic phase of the divorced eutectic grows in a manner as dendrite. However, in the growth of (Co+Co 2 eutectic, Co phase nucleates firstly but Co 2 phase grows faster, which means that the primary nucleation phase and the fast growth phase are not the same. Therefore, a divorced eutectic is not apparent in Co-43.5% alloy even though the undercooling is very large. Figure 4 is the calculated results of the ratio (τ T c / T t of solutal undercooling ( T c to thermal undercooling ( T t vs the bulk undercooling ( T. When τ>1, the solid phase grows under the control of the solutal diffusion. As τ <1, the thermal diffusion controls the growth. Apparently, when undercooling becomes greater, the growth of and Co 2 intermetallic compounds transforms from the solutal diffusion control to the thermal diffusion control. The critical undercoolings are 115 and 320 K, respectively. For Co phase in Co-23.9% or Co-43.5% alloy melt, the growth is always under the control of solutal diffusion. 3.2 Coupled zone The coupled zone is defined as the range of alloy compositions and interface temperatures, in which the lamellar eutectic morphology dominates the growth front [27]. The boundaries of the zone can be determined by comparing the lamellar eutectic growth velocity with dendrite growth velocity of the corresponding constituent phases. In a free growth, the structure, eutectic or dendrite, which has a higher

5 W.J. Yao et al.: J. Mater. Sci. Technol., 2011, 27(11, Conclusion Ratio, ( T c / T t T=115 K Co in Co-43.5% =1 Co in Co-23.9% Co 2 T=320 K Undercooling / K Fig. 4 Ratio of solutal undercooling to thermal undercooling vs the bulk undercooling growth velocity at the same undercooling, will grow. The growth velocity of eutectic can be computed by using TMK model [21] and that of dendrite can be calculated by using LKT theory [22,23]. The coupled zone of Co-23.9% eutectic alloy is obtained on the basis of the above two models and it is also shown in Fig. 1. It leans extremely to the rich side and covers a composition range from 23.6% to 25.4%. Judged from the calculated zone, the offeutectic alloy, the composition of which ranges from 23.6% to 25.4%, can form a eutectic microstructure when the undercooling exceeds a certain value. This calculated result of coupled zone can also explain the microstructure transition of Co-23.9% eutectic alloy. If the undercooling exceeds 22 K, the cooperative growth of two eutectic phases tends to be substituted by the divorced growth. Under large undercooling, the location of undercooled alloy melt is much lower than the boundary of coupled zone. Meanwhile, the growth velocity of intermetallic compound is much higher than that of Co intermetallic compound. Therefore, two intermetallic compounds grow independently in undercooled alloy melt to form the divorced eutectic. The higher undercooling is an essential factor to result in the microstructural transitions from lamellar to anomalous eutectic, even to divorced eutectic. The coupled zone of Co-43.5% eutectic alloy has been also illustrated in Fig. 1. It is obvious that the coupled zone leans strongly to Corich side and covers a composition ranging from 40.8% to 43.8%. For Co-43.5% eutectic alloy, when undercooling goes beyond 29 K, the cooperative growth of eutectic phases is destroyed. The critical undercooling is so small that the droplets can form anomalous eutectic very easily. In other words, when the undercooling is smaller than 29 K, the morphologies show lamellar eutectic structure, which is observed just in the master alloy. If the undercooling exceeds 29 K, the droplets show a mixture of anomalous and lamellar eutectic. When the undercooling is higher than 29 K, the complete anomalous eutectic forms in the droplet. In summary, the growth mechanisms of ( +Co and (Co+Co 2 eutectics during rapid solidification have been examined. Co-rich compound leads the nucleation during eutectic growth. In Co-23.9% eutectic alloy droplets, is the primary nucleation phase. When the undercooling is larger than 22 K, intermetallic compound will replace lamellar eutectic and grow dominantly. At high undercooling, intermetallic compound is not only the primary nucleation phase but also the primary growth phase. The primary nucleation phase in Co-43.5% eutectic alloy droplets is Co intermetallic compound, while Co 2 phase grows faster in high undercooled alloy melt. A morphology transition from lamellar to anomalous eutectic, even to divorced eutectic with increasing undercooling has been observed. With increasing undercooling, the growth of intermetallic compound in undercooled Co- 23.9% eutectic alloy melt and the growth of Co 2 intermetallic compound in undercooled Co-43.5% eutectic alloy melt experience the kinetic transition from the solutal-diffusion-controlled growth to the thermal-diffusion-controlled growth. However, for the Co intermetallic compounds in the two alloys, such a growth kinetic transition does not occur. Acknowledgements This work was financially supported by the National Natural Science Foundations of China (Grant Nos , and and the NPU Foundations for Fundamental Research (JC Authors are grateful to the financial support from the National Aerospace Science Foundation of China (Grant Nos. 2008ZF53052 and 2010ZF REFERENCES [1 ] D.H. Pi, J.M. Park, G.A. Song, J.H. Han, K.R. Lim, S. Yi, S.H. Yi, D.H. Kim, N.S. Lee, Y. Seo and K.B. Kim: Intermetallics, 2010, 18(10, [2 ] S.F. Lomayeva and A.N. Marathanova: Intermetallics, 2009, 17(9, 714. [3 ] S.W. Kim, M.K. Cho, Y. Mishima and D.C. Choi: Intermetallics, 2003, 11(5, 399. [4 ] N.I. Kulikov, D. Fristot, J. Hugel and A.V. Postnikov: Phys. Rev. B, 2002, 66(14, [5 ] C. Gras, N. Bernsten, F. Bernard and E. Gaffet: Intermetallics, 2002, 10(3, 271. [6 ] J.H. Zhu and C.T. Liu: Intermetallics, 2005, 13(6, 620. [7 ] R.A. Varin and Y.K. Song: Intermetallics, 2001, 9(8, 647. [8 ] C.T. Liu, E.P. George and W.C. Oliver: Intermetallics, 1996, 4(1, 77. [9 ] Y.P. Lu, N. Liu, T. Shi, D.W. Luo, W.P. Xu and T.J. Li: J. Alloy. Compd., 2010, 190(1-2, L1. [10] K. Ito, T. Hayashi, M. Yokobayashi and H. Numakura: Intermetallics, 2004, 12(4, 407.

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