Are Transition Metal-Rich Metallic Glasses Made up of Distorted BCC Clusters?

cie al S nce Ghafari et al., J Material Sci Eng 2016, 5:6 ri s te & a E M n DOI: 10.4172/2169-0022.1000299

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Research Article Article Open Access Are Transition Metal-Rich Metallic Glasses Made Up of Distorted BCC Clusters?

Ghafari M1*, Gleiter H1,2, Feng T1, Ohara K3 and Hahn H1,2 1Herbert Gleiter Institute of Nanoscience, Nanjing University of Science and Technology, Nanjing, China 2Institute of Nanotechnology, Karlsruhe Institute of Technology, Karlsruhe, Germany 3Japan Synchrotron Radiation Research Institute (JASRI), Hyogo, Japan

Abstract A structural model of metal-rich amorphous alloys is proposed. This model differs from the known models of the atomic structure of amorphous metal-rich alloys consisting of transition metal-early transition metal components or of transition metal-metalloid components. The atomic structures of these kinds of alloys were studied by means of synchrotron high energy X-ray diffraction. The results obtained are presented in the form of radial distribution

functions suggesting that the atomic structures of amorphous transition metal-rich alloys consist of clusters with chemically disordered and structurally distorted bcc cells, irrespective of the crystal structure of the corresponding pure metallic elements.

Keywords: Amorphous alloys; Atomic structure; Radial distribution in external magnetic fields and at various temperatures have verified function; Transition metals; X-ray diffraction the existence of magnetic clusters in a-Fe90Sc10 with average cluster sizes close to the interatomic spacing as deduced from the Radial Introduction Distribution Function, RDF [17,18]. Numerous experimental studies [1] of metallic glasses indicate that Moreover, additional structural information may be obtained the arrangement of atoms in metallic glasses is aperiodic and isotropic. by studying the photon energy because the photon energy is directly The presence of Short Range Order (SRO) and Medium Range Order related to the vibrational dynamics of the atoms and hence to their (MRO) indicates that the atomic positions are correlated up to few structural arrangement. The shape and the position of the maximum interatomic distances. of the phonon DOS amorphous Fe-rich alloys [19] and amorphous Fe [20] were found to be similar to nano-crystalline-Fe [21] indicating that Since the discovery of amorphous metallic alloys the following the atomic environments of 57Fe-atoms in both materials are similar prominent structural models have been proposed: (a) The Dense Random Packing of Hard Spheres (DRPHS) [2-4], (b) the model Using genetic algorithm calculations, Nguyen et al., [22] have based on the formation of structural units, such as trigonal prisms or shown that by starting of random configurations, the structure of icosahedral arrangements [5-9], and (c) the Cluster Model (CM) of minimum-energy of the FeCo alloy is the bcc atomic arrangement. All glasses [10,11]. of these observations suggest a structural similarity between crystalline Fe and amorphous Fe-rich alloys. Based on this observation, it is the In fact, the analysis of the structure provides the suggestion that aim of the study reported in this paper to investigate of the following a uniform structure model of amorphous alloy is ineligible. In some two questions: cases, the chemical bonding is the decisive factor [12,13], then other factors such as metallic binding [13]. 1- Is the atomic arrangement in transition metal–rich amorphous alloys similar to the atomic arrangement in transition metal– Recent reports on the structure of metal-rich amorphous alloys rich crystalline metals? [1] as well as of the structure of isolated nanometer-sized Fe crystals [14] suggest that new insight into the structure of amorphous metallic 2- Does the atomic arrangement in transition metal-rich materials consisting of transition metal-early transition metal amorphous alloys depend on their chemical composition? components or of transition metal-metalloid components may be obtained by investigating (e.g., by means of the Radial Distribution Experimental Function, RDF of these materials as well as of some of their physical The materials systems chosen for the investigations that will be properties) the atomic arrangements in these materials. In fact, metal- reported in this paper were the following two families of alloys. (1) rich amorphous alloys consisting of transition metal-early transition metal components or of transition metal–metalloid components seem promising model systems for such studies. This view is supported by the *Corresponding author: Ghafari M, Herbert Gleiter Institute of Nanoscience, following observations reported in the literature. Nanjing University of Science and Technology, Nanjing 210094, China, Tel: 862584303409; E-mail: [email protected] The results of Mössbauer measurements show that the distribution of the internal magnetic hyperfine fields and the quadrupole splitting Received October 28, 2016; Accepted November 14, 2016; Published November 24, 2016 distribution of pure nanometer-sized Fe crystals [14] and amorphous Fe Sc [15] or Fe Zr [16] are similar if the Fe crystals have a size of Citation: Ghafari M, Gleiter H, Feng T, Ohara K, Hahn H (2016) Are Transition 90 10 90 10 Metal-Rich Metallic Glasses Made Up of Distorted BCC Clusters? J Material Sci 2 nm or less. These observations indicate that the atomic environments Eng 5: 299. doi: 10.4172/2169-0022.1000299 of 57Fe-atoms in both materials are similar because the quadrupole splitting, which is caused by the interaction of electrical field gradient Copyright: © 2016 Ghafari M, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted and the quadrupole moment of nucleus is directly related to the use, distribution, and reproduction in any medium, provided the original author and atomic environment. Furthermore, Mössbauer experiments measured source are credited.

J Material Sci Eng, an open access journal Volume 5 • Issue 6 • 1000299 ISSN: 2169-0022 Citation: Ghafari M, Gleiter H, Feng T, Ohara K, Hahn H (2016) Are Transition Metal-Rich Metallic Glasses Made Up of Distorted BCC Clusters? J Material Sci Eng 5: 299. doi: 10.4172/2169-0022.1000299

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Transition metals-early transition alloys and (2) Transition metal-rich of the peak positions. In pure bcc-Fe, the nearest neighbor (nn) and and metalloid alloys (produced by Hitachi Metals Ltd., Vacuumschmelze the next nearest neighbor (nnn) atoms are located at 2.48 Å and GmbH & Co. KG and in our laboratory by electrochemical deposition 2.87 Å, respectively, with 8 atoms and 6 atoms as nn and nnn in the methods). Early transitions metals (such as Sc) and metalloids (such as corresponding distances. Due to the small difference between the P, B, and Si) are known as glass forming element. nn and nnn shells in bcc-Fe, the nn and nnn shells overlap in the amorphous alloy so that the nn and nnn shells become - in amorphous All the metallic glasses studied by us - except of amorphous Ni P 86 14 metal-rich alloy - indistinguishable. and Co80P20 alloys - were prepared by melt spinning. The preparation of As may be seen from Figures 2 and 3, the RDF’s of crystalline Ni90Sc10, Co90Sc10 and Fe90Sc10 alloys was performed in a high vacuum bcc-Fe and the RDF’s of amorphous Co Sc and Ni Sc alloys yields chamber. The amorphous Ni86P14 and Co80P20 alloys were prepared by 90 10 90 10 the electrochemical deposition methods [23,24]. the same result as for Fe90Sc10 alloy. The following two main features of the peak pattern of all three alloys are as follows: (1) All the peak The atomic structures of the metallic glass were studied using high- patterns of the RDFs are identical; (2) The major differences between energy XRD (HE-XRD) with a photon wavelength of 0.21 Å at the them are the enhanced peak widths and the shifts of peaks of the beamline BL04B2 at SPring-8. The beamline BL04B2 is designed for individual amorphous metal-rich alloys relative to the peaks of the diffraction and scattering experiments at photon energies larger than bcc-Fe. In other words, the SRO and MRO of metal-rich alloys can 37 keV. The main advantage of the beamline is the ability to structurally be described to consist of chemically disordered and structurally characterize disordered samples by employing a wide range of scattering distorted clusters with a bcc atomic arrangement. Figure 4 shows a vectors Q and a low instrumental background. Further advantages schematic drawing of two-dimensional atomic arrangements of the are small correction terms (especially for absorption correction) and nn and the nnn in a bcc cluster and the corresponding arrangements reduction of truncation errors. The analysis of High-Energy X-Ray in a distorted bcc cluster with a Sc-atom in the center. Because of Diffraction (HE-XRD) data and the transformation from k space to the the different electronegativity of Fe and Sc, the Sc-atoms have the real space is described elsewhere [18]. tendency to attract the Fe- atoms. Results On the other hand, it may be noted that according to the results reported above, the bcc clusters making up the amorphous structures Figure 1 shows the radial distribution function of crystalline bcc-Fe, seem not to be correlated with the crystal structure of main transition fcc-Ni and hcp-Co and of the amorphous transition metal-rich Fe Sc 90 10 metal, i.e., bcc for Fe, fcc for Ni and hcp for Co. alloy. As may be seen with the similarity exists between pattern of the peaks of the RDF of amorphous Fe90Sc10 alloy and crystalline Fe with a bcc structure. However, the peaks of the amorphous alloy (Figure 1) are broader than the ones of the Fe bcc crystals and the positions of the peaks are shifted which indicates different interatomic spacing in the amorphous Fe90Sc10 alloy and crystalline Fe. The similarity between pattern of the peaks of the RDF of amorphous Fe90Sc10 alloy and crystalline Fe with a bcc structure indicates that the atomic structure of the amorphous alloy can be described as arrangements of distorted cluster with a bcc structure (the peaks of the amorphous alloy were broader than the ones of the Fe bcc crystals) and a distribution of interatomic spacing centered on those of bcc-Fe with a certain shift

Figure 2: RDF of crystalline bcc-Fe, and amorphous Co90Sc10 alloy.

Figure 1: RDF of crystalline hcp-Co, fcc-Ni. bcc-Fe, and amorphous Fe90Sc10 alloy. The values of RDF in y-axis are valid for bcc-Fe and hcp-Co. In order to

see the different RDFs, the values for amorphous alloy and fcc-Ni are shifted Figure 3: RDF of bcc-Fe and distorted bcc structure of crystalline Ni90Sc10 alloy with a constant. with long-range order.

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In summary, the similarities of the RDFs of amorphous transition of an undercooled melt of Fe at T=1750 K, 1730 K and 1670 K. As may metal-rich alloys of Fe, Ni and Co, indicate that the structure of all of be seen, the position of peaks in the S(Q) plots (up to about 1 Å) and these alloys consist of distorted bcc clusters. For amorphous alloys with the number of the nn are similar in the amorphous Fe90Sc10, in the melt transition metal compositions larger ≥ 78 at percentage, the structure and in the undercooled Fe melt. This experimental observation suggests seems independent of the second components of these alloys with that in rapidly quenched melts bcc clusters are the preferred structural such as Sc, B, Si or P as presented in Figure 5. Finally, let’s address the units. question: Why are the distorted bcc clusters the preferred structural Acknowledgements units of the amorphous metal–rich alloys. In order to discuss this question, the structure factor, S(Q), of amorphous Fe Sc alloy as a This work is supported by the National Natural Science Foundation of China 90 10 (Grant no. 51571119, 51520105001), the Natural Science Foundation of Jiangsu function of Q=4Π⁄λ was compared, Figure 6 [25] by means of the pair Province and the Fundamental Research Funds for the Central Universities. T. F. correlation functions with the S(Q) of molten Fe at T=1870 K and at acknowledges the support by the innovation project and the specially-appointed professor project of Jiangsu province. The synchrotron radiation experiment 1830 K (the melting temperature of Fe is Tm=1811 K) and with the S(Q) was approved by the Japan Synchrotron Radiation Research Institute (proposal. 2015A1890).

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J Material Sci Eng, an open access journal Volume 5 • Issue 6 • 1000299 ISSN: 2169-0022