Physics Letters A 378 (2014) 1746–1750 Contents lists available at ScienceDirect Physics Letters A www.elsevier.com/locate/pla Persistent local chemical bonds in intermetallic phase formation Yanwen Bai a , Xiufang Bian a,∗ , Xubo Qin a , Shuo Zhang b , Yuying Huang b a b Key Laboratory for Liquid–Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Jinan 250061, China Shanghai Synchrotron Radiation Facilities, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China a r t i c l e i n f o Article history: Received 21 October 2013 Received in revised form 17 April 2014 Accepted 20 April 2014 Available online 24 April 2014 Communicated by R. Wu Keywords: Liquid structure High-temperature X-ray diffraction Intermetallics Rapid solidiﬁcation a b s t r a c t We found a direct evidence for the existence of the local chemical Bi–In bonds in the BiIn2 melt. These bonds are strong and prevail, dominating the structure evolution of the intermetallic clusters. From the local structure of the melt-quenched BiIn2 ribbon, the chemical Bi–In bonds strengthen compared with those in the equilibrium solidiﬁed alloy. The chemical bonds in BiIn2 melt retain to solid during a rapid quenching process. The results suggest that the intermetallic clusters in the melt evolve into the asquenched intermetallic phase, and the intermetallic phase originates from the chemical bonds between unlike atoms in the melt. The chemical bonds preserve the chemical ordered clusters and dominate the clusters evolution. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Known as important structural materials [1,2], the excellent physical properties, such as the superconductivity [3–5], the magnetic susceptibility [6,7], and the electrical resistivity , make the intermetallic compounds attract considerable attention. Recently, Wu and Li  reported the “intermetallic glasses” in the Cu–Zr system with near-intermetallic compositions. Then Wang et al.  explained the origin of these glass formers from the thermodynamics and kinetics points of view, using the melt fragility. Their research has explored new potential applications of the intermetallic alloys. There is no doubt that the formation of the intermetallic alloys is critical to their performance. In these studies, concerned with the intermetallic compounds, one of the core issues is how to synthesize them with enhanced performance. Among the various synthetic methods [10–15], the intermetallic compounds are mostly formed in the liquid-phase reaction. Therefore, the behavior of the intermetallic compounds in the melt is crucial to the formation and performance of the materials. For example, Singh  and Prasad  have demonstrated that the Al3 Mg2 intermetallic clusters in the liquid phase lead to the asymmetry in the properties of mixing of Al–Mg molten alloys. The large diversity of the physical properties of the solid Fe–Si system with different compositions is correlated with the changing volume fraction of the intermetallic clusters in the liquid . When describing the liquid * Corresponding author. Tel.: +86 531 88392748; fax: +86 531 88395011. E-mail address: [email protected] (X. Bian). http://dx.doi.org/10.1016/j.physleta.2014.04.037 0375-9601/© 2014 Elsevier B.V. All rights reserved. structure in the whole concentration range in the Fe–Si  and Fe–Al  systems, the phase equilibrium diagrams of which are very complex, the intermetallic clusters have also been used as the important structural model. Based on the above studies, since melt is the initial state when the intermetallic compounds form, the knowledge about the structure and the formation of the intermetallic phase in the melt can assist us in the synthesis and controlling of the intermetallic compounds in the materials. As one of the most typical intermetallic compounds, the BiIn2 alloy is very attractive. It is known that its solidiﬁcation behavior correlates with the structures of both the liquid and solid phases . The viscosity coeﬃcient shows the anomalous temperature dependence , which was supposed to be accompanied with concentration ﬂuctuations in melt. In addition, the magnetic susceptibilities, electrical resistivities, and thermoelectric powers display anomaly near the concentration range of BiIn2 . It is widely accepted that the anomalous properties of the BiIn2 are due to the self-associated atomic groups in the melt [21,23,24]. Moreover, the behaviors of these clusters can cause the structural changes of the melt . However, the studies on the formation of the intermetallic clusters and their local structures are scarce, which are important for understanding the behaviors of the melt structure and the anomalous properties. In our work, by means of the high temperature X-ray diffraction , the liquid structure of the BiIn2 intermetallic alloy has been detected. With the Extended X-ray Absorption Fine Structure (EXAFS) technique , the local structure around In atoms of the melt-quenched BiIn2 intermetallic alloy ribbon has been obtained. The formation and evolution of the intermetallic clusters from the melt to the as-quenched crystalline state have been Y. Bai et al. / Physics Letters A 378 (2014) 1746–1750 1747 Fig. 1. (a) The structure factors S ( Q ) of the pure Bi melt at 305 ◦ C , pure In melt at 280 ◦ C , and BiIn2 alloy melt from 90 to 300 ◦ C. (b) The pair distribution functions g (r ) of the pure Bi melt at 305 ◦ C , pure In melt at 280 ◦ C , and BiIn2 alloy melt from 90 to 300 ◦ C. revealed. The structural differences between the melt and the asquenched ribbon have been discussed, and the driven factors for the intermetallic alloy formation have been pointed out. 2. Experiments The ingots of the BiIn2 alloy were prepared by melting nominal amounts of pure Bi and In of 99.99% purity in a high frequency induction furnace under Ar atmosphere. X-ray diffraction measurements were carried out using a high temperature θ –θ type X-ray diffractometer. Mo Kα radiation (wavelength λ = 0.7089 Å) was reﬂected from the free surface of the specimen and reached the detector through a Zr monochromator in the diffraction beam. Experiments were performed at a high purity helium (99.999%) atmosphere of 2.0 × 10−5 Pa after the chamber was cleaned in vacuum of 1.0 × 10−3 Pa. The scanning voltage of the X-ray tube was 50 kV, the current was 40 mA, the exposure time was 5 s and the measured scattering angle 2θ was from 5◦ to 68◦ . The scanning step was 0.2◦ . The sample was held for 30 min to reach equilibrium every time before a new scattering scan started. The detailed data processing can be found in Ref. . The error is less than 2% for r1 by the adopted method. The BiIn2 intermetallic alloy ribbon was prepared by a singleroller melt-spinning technique with a circumferential speed of 27.5 m/s. The sample thickness is 40–50 μm. The In K-edge EXAFS spectra were measured at the beamline BL14W1 of Shanghai Synchrotron Radiation Faculty. The electron beam energy was 3.5 GeV and the maximum stored current was 300 mA. Data were collected by a ﬁxed-exit double-crystal Si(311) channel-cut monochromator. The anterior ion chamber was ﬁlled with Ar, and the other with a mixture of Ar and Kr gases. The gases were used to detect incident X-ray intensity and transmitted intensity simultaneously. The energy resolution was 0.5 × 10−4 . The EXAFS data were analyzed using standard procedures with IFEFFIT code . 3. Results and discussion 3.1. The structure of the BiIn2 intermetallic alloy melt The structure factors S ( Q ) of the pure In melt, pure Bi melt, and BiIn2 intermetallic alloy melt at different temperatures are shown in Fig. 1(a). The ﬁrst peaks in the S ( Q ) curves of the BiIn2 melt are asymmetrical. There is a shoulder on the high- Q side of the ﬁrst peak of the BiIn2 melt, which is similar to the S ( Q ) curves of the pure Bi melt in Fig. 1(a) as well as in Refs. [32–34]. The shoulder on the S ( Q ) curve is related to the interatomic potential  and the covalent bonds [35,36] in the melt. Comparatively, the intensity of the shoulder on the curve of the BiIn2 melt is much weaker than that of the pure Bi melt. This suggests that the Bi–Bi covalent bonds in the pure Bi melt are largely reduced in the BiIn2 melt. Fig. 1(b) shows the pair distribution functions g (r ) of the pure In melt, pure Bi melt, and BiIn2 alloy melt at different temperatures. There are apparent humps between the ﬁrst and the second peaks of the three melts, as marked in the dashed rectangle region. The shoulders on the high-r side of the ﬁrst peaks can be observed clearly in the g (r ) curves of the BiIn2 melt, while there is not any shoulder on the g (r ) curve of In. This indicates that the structure of the BiIn2 melt is more complex, compared with that of the In melt. In addition, the proﬁle of the g (r ) curve in the BiIn2 melt changes with decreasing temperature, indicating the structure evolution of the intermetallic melt. The structural parameters of the BiIn2 alloy melt are shown in Fig. 2(a) and (b). From Fig. 2(a), the nearest neighbor distance r1 (i.e., the position of the ﬁrst peak in the g (r )) displays an increasing trend with decreasing temperature. This is due to the thermal contraction characteristic of In . The values of r1 are from 3.18 Å to 3.20 Å. Notice that, the atomic radii of In and Bi atoms are 1.67 Å and 1.63 Å, respectively. The r1 are smaller than the sum of the atomic radii of In and Bi. Furthermore, r1 of the alloy melt is smaller than that of pure In [31,33] and pure Bi [30,33] melts. The abnormally short bond length in the intermetallic alloy melt directly conﬁrms the unexpected strong interactions between In and Bi atoms. Considering the nearest neighbor distances of Bi23 In77 and Bi13 In87 in Ref.  (which are all larger than that of the BiIn2 melt), the increasing atomic fraction of Bi enhances the interactions between Bi–In bonds. However, the Bi23 In77 and Bi13 In87 alloys are not in the intermetallic compositions of the Bi–In system. This suggests that the Bi–In chemical bonds are tighter in the clusters with the intermetallic compound composition. Thus in the BiIn2 melt, these chemical Bi–In bonds maintain the chemically ordered intermetallic clusters. The atom packing of the clusters with intermetallic compounds BiIn2 in the melt deviates from the hard sphere random packing model [32,33]. 1748 Y. Bai et al. / Physics Letters A 378 (2014) 1746–1750 Fig. 2. (a) The nearest neighbor distance r1 and the coordination numbers N m of the ﬁrst coordination shell of BiIn2 alloy melt. (b) The correlation radius rc of the cluster and the ratio (r2 /r1 ) of the ﬁrst and second peak positions of the pair distribution function g (r ) of BiIn2 alloy melt. The coordination numbers N m ﬂuctuate around 10 in the temperature range. Therefore, the ﬁrst coordination shell in the BiIn2 melt is looser than that in the crystalline counterpart, which has 12 coordination atoms with a hexagonal structure. This is due to the open, low-coordinated structures of the polyvalent element Bi . There is an abnormal phenomenon that N m shows a decreasing trend as r1 increases when the temperature falls, although the variation of the values is weak. This may be caused by many comprehensive factors, such as the discontinuous change of density in liquid Bi as the temperature decreases , and the directional Bi–In bonds in the melt. One of the most important factors is that the evolution of the intermetallic clusters is different from the clusters with the hard sphere random packing model. Therefore, the relation between N m and r1 seems to be abnormal. Fig. 2(b) shows the evolution process of the correlation radius rc and r2 /r1 versus temperature. Interestingly, there is an unstable stage during this evolution process. From 300 ◦ C to 220 ◦ C, the value of rc keeps almost the same. During 220 ◦ C to 200 ◦ C, rc has a decreasing trend, indicating the small structure change. The similar phenomenon can also be found in the ratio of r2 /r1 , which has an increasing trend except at the temperature of 220 ◦ C. In fact, the r1 and the N m also experience the abrupt change in this temperature range. It has been reported that [20,21,25] in the temperature range from 410 to 220 ◦ C, the SRO clusters are composed of In and Bi atoms similar to the structure of crystalline BiIn2 . Below 220 ◦ C, some clusters remain the same, others form the “quasi-eutectic” structure . From these structural parameters, the nearest neighbor distance r1 can be observed as a dominant factor, i.e., the change in r1 leads to the changes in N m , rc , and r2 /r1 . Therefore, the chemical Bi–In bonds can help maintaining the intermetallic clusters in the melt, dominate the structure change as temperature decreases, and assist the intermetallic compound formation. The Bi–In bonds play a decisive role in the structural evolutions of the intermetallic clusters. 3.2. The structure of the melt-quenched BiIn2 ribbon In order to ﬁnd out how the intermetallic compound forms during the rapid quenching process, we obtained the XRD pattern in Fig. 3(a) and EXAFS spectra for the In K-edge in Fig. 3(b) of the as-quenched BiIn2 ribbon. From Fig. 3(a), it is known that the asquenched ribbon are totally formed into the intermetallic phase. Fig. 3(c) displays the Fourier transform (FT) of the k2 -weighted EXAFS signals χ (k) and the corresponding ﬁtting curve. The k-range of the FT is from 2.7 Å−1 to 10 Å−1 , and the r-range of the ﬁtting is from 2.2 Å to 5 Å. The ﬁtting is based on the model of normal BiIn2 compound  from Fig. 3(a), with a P63 /mmc crystal structure. Table 1 lists the parameters from the model as well as obtained from the ﬁtting. The results reveal the local structure differences between the normal crystal BiIn2 intermetallic compound model and the rapidly quenched BiIn2 intermetallic alloy ribbon. In the model, there are 2 In atoms closed to central In atom; while in the as-quenched ribbon, 2 Bi atoms with a distance of 3.10 Å become the nearest atoms around In (within the ﬁtting uncertainties). In addition, there are 2 In atoms with a distance of 3.31 Å, and 4 Bi atoms of 3.32 Å. Compared with the model, the Bi–In bond lengths decrease from 3.57 Å to 3.10 Å and 3.32 Å respectively; while most of the In–In bond lengths increase. It is known that during the rapid solidiﬁcation process, the chemical bond between the unlike Bi–In atoms strengthens. If we deﬁne 3.57 Å as a boundary of the nearest coordination distance around central In atom, the coordination numbers are about 10 (except for the 4 In atoms with a distance of 3.64 Å from the central In). The N m in the BiIn2 alloy melt is 9–10 as well. It can be seen that the rapid quenching process freezes the chemical ordered cluster structure of the melt. The diffraction intensity results of both the melt and the as-quenched ribbon of BiIn2 alloy shown in Fig. 3(d) directly conﬁrm this. The shoulder on the high- Q side of the alloy melt corresponds to the BiIn2 intermetallic phase. It is indicated that the intermetallic clusters with the chemical short-range order structure persistently exist at well above the melting, and the clusters are composed of the stoichiometry of the solid intermetallic. During the quenching process, these intermetallic clusters evolve into the BiIn2 intermetallic phase of the alloy ribbon. There is no doubt that the intermetallic clusters play a very important role in the alloy melt . It is known that the clusters in the melt can be served as the crystal seeds for the formation of the intermetallic phase in the solid state. Notice that the average Bi–In bond length in the ribbon is similar to the distance between the nearest Bi and In atoms in the melt. Combined the local structures of the BiIn2 melt with the as-quenched ribbon, one of the key factors keeping the intermetallic clusters is the persistent local chemical bonds from the melt to the solid. It is suggested that the Bi–In bonds are preserved during the quenching process. In other word, the strong and persistent chemical bonds dominate the whole solidiﬁcation process, which ﬁnally leads to the formation of the solid intermetallic BiIn2 phase. Y. Bai et al. / Physics Letters A 378 (2014) 1746–1750 1749 Fig. 3. (a) XRD pattern of the as-quenched intermetallic BiIn2 ribbon. Inset: the enlarged region from 10◦ to 36◦ . (b) EXAFS spetra of the pure In metal and the BiIn2 ribbon for the In K-edge. Inset: the corresponding k2 -weighted EXAFS spectra. (c) Fourier transform (FT) of the k2 -weighted EXAFS signals χ (k) and the ﬁtting curve. Inset: the EXAFS signals χ (k) and the ﬁtting curve. (d) Diffraction intensity curves of the BiIn2 alloy melt and the as-quenched alloy ribbon. Table 1 Parameters from the ﬁtting data and the normal compound model. Bond pair Sample Coordination numbers N r (Å) σ2 In–In ribbon model ribbon model ribbon model ribbon model ribbon model 1.96 ± 0.05 2 1.96 ± 0.05 2 3.92 ± 0.05 4 1.96 ± 0.05 2 3.92 ± 0.05 4 3.31 ± 0.02 3.29 3.42 ± 0.03 3.57 3.64 ± 0.03 3.57 3.10 ± 0.02 3.57 3.32 ± 0.03 3.57 0.01 – 0.01 – 0.01 – 0.003 ± 0.001 – 0.007 ± 0.001 – In–In In–In In–Bi In–Bi 4. Conclusions In this work, the structures of both the intermetallic alloy BiIn2 melt and the as-quenched ribbon have been detected. The nearest neighbor distance r1 directly revealed the existence of chemical Bi–In bonds in the melt, which maintain the chemically ordered intermetallic clusters and dominate the structure evolution process. The intermetallic clusters in the melt evolve into the as-quenched intermetallic phase. Through the EXAFS spectrum analysis, a local structure model has been proposed. 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