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ONLINE ISSN: 1347-5320
PRINT ISSN: 1345-9678

MATERIALS TRANSACTIONS Vol. 62 (2021), No. 3

  • PREFACE

    Ryuji Tamura, Hiroyuki Takakura, Kazuhiko Deguchi, Ryo Yoshida, Kaoru Kimura

    pp. 297-297

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    DOI:10.2320/matertrans.MPR2021901

  • Magnetism of Tsai-Type Quasicrystal Approximants

    Shintaro Suzuki, Asuka Ishikawa, Tsunetomo Yamada, Takanori Sugimoto, Akira Sakurai, Ryuji Tamura

    pp. 298-306

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    DOI:10.2320/matertrans.MT-MB2020014

    Abstract

    Magnetism of Tsai-type 1/1 approximants has been extensively investigated for the last decade. The experimental results on Tsai-type approximants are reviewed and the magnetic behaviors are classified and discussed in terms of the average electron-per-atom (e/a) ratio. The paramagnetic Curie-Weiss temperature Θp is found to oscillate in a universal manner as a function of the e/a ratio, and the magnetic order systematically changes with the variation in Θp. In addition, recent work shows that the 2/1 approximants also follow the trend observed for the 1/1 approximants, which implies that the magnetic phase diagram of the 1/1 approximants can be applicable to higher-order approximants including quasicrystals that are yet to be discovered.

  • High Dimensional Approach to Antiferromagnetic Aperiodic Spin Systems

    Sam Coates, Ryuji Tamura

    pp. 307-311

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    DOI:10.2320/matertrans.MT-MB2020010

    Abstract

    Monte Carlo approaches to aperiodic spin systems have found a range of antiferromagnetic ground states for both classical and quantum spins. In each case, the spin calculations have been undertaken on quasilattices in the appropriate dimension i.e. 1D for a Fibonacci chain, 2D for a Penrose tiling etc. Here we show that high dimension spin models can be calculated using the simplest of spin Hamiltonians, and that the projection of these high dimensional spins reproduces antiferromagnetic ground states in the ‘correct’ dimension. We also show a model for an atomistic antiferromagnetic quasicrystal, as derived using this method.

  • Adsorption of Pentacene on the 2–Fold Surface of the Icosahedral Ag–In–Yb Quasicrystal

    Amnah Alofi, Dominic Burnie, Sam Coates, Ronan McGrath, Hem Raj Sharma

    pp. 312-316

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    DOI:10.2320/matertrans.MT-MB2020013

    Abstract

    The adsorption of pentacene molecules on the 2–fold surface of the icosahedral (i)–Ag–In–Yb quasicrystal (QC) has been studied using scanning tunnelling microscopy (STM). The STM images show that the molecules are arranged in rows, and aligned predominantly along the high–symmetry 5–fold and 2–fold axes of the surface. The row separation perpendicular to the 5–fold symmetry axes is either short S = 0.79 ± 0.03 nm or long L = 1.24 ± 0.02 nm. The ratio between them is close to the golden mean (τ = 1.618…) and creates segments of the Fibonacci sequence. Fourier transform of the scanning tunnelling microscopy (STM) images reveal 2–fold, quasicrystalline, long–range order. A comparison of the STM features with the model structure of the substrate shows that Pn molecules adsorb at Yb sites.

  • Possibility of Semiconducting Electronic Structure on Al–Pd–Co 1/1 Cubic Quasicrystalline Approximant

    Yutaka Iwasaki, Tomoyuki Kashimura, Koichi Kitahara, Kaoru Kimura

    pp. 317-320

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    DOI:10.2320/matertrans.MT-MB2020016

    Abstract

    Search for high-order semiconducting quasicrystalline approximants can play an essential role in finding clues to the discovery of semiconducting quasicrystals. According to the previous theoretical work, a model of Al–Pd–Co 1/1 cubic quasicrystalline approximant was predicted to be semiconductor from a calculation based on the density functional theory. We noticed that the F phase in the Al–Pd–Co system is a 2 × 2 × 2 superlattice structure of the calculated model. To verify this prediction, we synthesized the F phase sample, and measured its thermoelectric properties. The measured electrical conductivity linearly increases with increasing temperature. The magnitude of measured Seebeck coefficient is smaller than the typical semiconductor. These properties indicate that the prepared sample of the F phase has a pseudogap rather than a finite band gap. To investigate this discrepancy between the theoretical prediction and experimental results, we calculated the electronic structure for the three structural models using density functional theory. The most energetically stable model has a semimetallic electronic structure.

  • Composition Effect of Kondo Behavior in Au–Al–Ce Quasicrystalline Approximants

    Yuji Muro, Tadashi Fukuhara, Takahiro Namiki, Tomohiko Kuwai, Akira Sakurai, Asuka Ishikawa, Shintaro Suzuki, Ryuji Tamura

    pp. 321-324

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    DOI:10.2320/matertrans.MT-MB2020008

    Abstract

    We have studied the magnetic and transport properties of Tsai-type quasicrystalline approximants, AuxAl84−xCe16 and AuyAl86−yCe14 by the measurements of magnetic susceptibility χ, specific heat Cp and electrical resistivity ρ. A Curie-Weiss behavior in χ for all compounds indicates the stable trivalent state of Ce ions in Au–Al–Ce. The effective magnetic moment increases with increasing the Au concentration whereas the Weiss temperature decreases. These behaviors suggest the Kondo effect in Au–Al–Ce suppresses with the increase of Au concentration. In Cp, all measured compounds show a broad peak due to the Kondo resonance at 0.8 K. In addition, Cp/T shows a peak below 0.6 K, which implies the existence of spin-glass transition. In all compounds, a pronounced −log T behavior due to Kondo effect are observed in ρ(T) below 10 K. Moreover, AuyAl86−yCe14 approximants are the first example displaying a precursor of coherent Kondo state at low temperatures because a broad maximum is observed in ρ(T) at 1.3 K.

  • Frank-Kasper σ and A-15 Phases Formed in Symmetry and Asymmetry Block Copolymer Blend System

    Katsuhiro Yamamoto, Hideaki Takagi

    pp. 325-328

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    DOI:10.2320/matertrans.MT-MB2020002

    Abstract

    Frank Kasper (FK) σ and A15 phases were found in the blend system of asymmetric and symmetric polystyrene-b-poly(methyl acrylate) block copolymers. Bates and co-workers first discovered these complex particle packings in the blend system where the diblock copolymers with a constant chain length of one component which consists of corona chains and variable chain lengths being micelle core. It is significantly important to have discover the formation of these complex phases in different block copolymer systems and also from a different research group, which strongly supports the prediction that the FK and A15 packing. Such complex phases had been found in limited conditions with a low molecular weight BCP with high χ (strongly segregated system) and conformationally asymmetric BCP system (flexibility asymmetry between constituent blocks). Discovery of the FK and A15 phases in a high molecular weight diblock copolymer without high χ or conformational asymmetry indicates a universal phenomenon of the formation of the complex particle packing in a substantially wide range of polymers.

  • A Unified Geometrical Framework for Face-Centered Icosahedral Approximants in Al–Pd–TM (TM = Transition Metal) Systems

    Nobuhisa Fujita, Makoto Ogashiwa

    pp. 329-337

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    DOI:10.2320/matertrans.MT-MB2020007

    Abstract

    The complex atomic structure of a high-order approximant to face-centered icosahedral quasicrystal in Al–Pd–TM (TM = transition metal) systems can be deconvoluted into two kinds of atomic clusters pinned at the vertices of a tiling composed of four basic polyhedra, called the canonical cells. As a result, thousands of atoms per unit cell can be registered in fifteen orbits associated with vertices, edges, faces, and cells of the relevant canonical-cell tiling. This geometrical framework facilitates a rational guess of an atomic jungle in an unknown approximant structure, could an underlying tiling be postulated. A novel approximant phase in the Al–Pd–(Mo–Fe) system is discussed within the present framework.

  • Pressure–Volume Relationship of a Au–Al–Yb Intermediate Valence Quasicrystal and Its Crystalline Approximant

    Tetsu Watanuki, Akihiko Machida, Tsutomu Ishimasa

    pp. 338-341

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    DOI:10.2320/matertrans.MT-MB2020006

    Abstract

    Synchrotron X-ray diffraction experiments of an icosahedral Au–Al–Yb quasicrystal and its 1/1 crystalline approximant were performed under quasi-hydrostatic pressure up to approximately 20 GPa at room temperature. Their pressure–volume compression curves are similar to each other, and show a monotonic decrease, which indicates the structural stability of these compounds under pressure. The bulk moduli were determined to be B0 = 105.1(8) GPa for the quasicrystal and B0 = 108.1(7) GPa for the crystalline approximant. These values are explained by the composition-weighted average of the bulk moduli of each constituent element.

  • The Local Structure of the Fibonacci Chain and the Penrose Tiling from X-Ray Fluorescence Holography

    Jens R. Stellhorn, Hiroyuki Takakura, Shinya Hosokawa, Kouichi Hayashi

    pp. 342-349

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    DOI:10.2320/matertrans.MT-MB2020005

    Abstract

    Structural investigations based on X-ray fluorescence holography can add a new perspective to the research of aperiodic systems. This technique can reconstruct the average 3-dimensional (3-D) local structure around specific elements directly from the experimental data. However, these structures can be difficult to disentangle for the complex arrangements found in quasicrystals. Therefore, we illustrate this point of view by considering appropriate reference models with a 1-D model of a Fibonacci chain and a 2-D Penrose tiling. It is demonstrated that the holographic reconstructions correspond to a projection of the average structure. The results from X-ray fluorescence holography can then be interpreted as statistical information on inter-atomic connections in the system.

  • Accuracy of Cluster Model Calculations for Quasicrystal Surface

    Masanori Sato, Takanobu Hiroto, Yoshitaka Matsushita, Kazuki Nozawa

    pp. 350-355

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    DOI:10.2320/matertrans.MT-MB2020015

    Abstract

    Accuracy of the adsorption energy on a quasicrystal surface calculated with the density functional theory and cluster models were discussed. Two types of cluster models of various sizes were tested to represent a fivefold surface of the Ag–In–Yb quasicrystal. Some cylindrical clusters caused unnaturally rippled potential energy surfaces regardless of the cluster thickness. However, it was confirmed that the ripples are disappeared with a cluster radius of 1.4 nm or larger. A similar trend was observed even in the hemispherical clusters, and also confirmed in their root mean square errors. It was concluded that both cluster models with a certain size are expected to give relative adsorption energy within an error of 0.15 eV.

  • High-Temperature Specific Heat of Al–Cu–Ru Icosahedral Quasicrystals and 1/1 Crystal Approximants

    Satoshi Tamura, Kento Fukushima, Yuki Tokumoto, Yoshiki Takagiwa, Keiichi Edagawa

    pp. 356-359

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    DOI:10.2320/matertrans.MT-MB2020004

    Abstract

    We report the results of specific heat measurements for Al–Cu–Ru icosahedral quasicrystals (i-QCs) and 1/1 crystal approximants (1/1-CAs) in the temperature range from 350 to 1250 K. The i-QCs and 1/1-CAs showed a marked upward deviation of the specific heat from the value of Dulong-Petit’s law above 600 K, reaching approximately 1.5 times the value at 1100 K. In addition, some i-QCs showed a large broad peak in the specific heat at approximately 1200 K. The origin of these excessive specific heat values above the Dulong-Petit value is discussed considering the high-dimensional nature of the structural order in the QCs and CAs.

  • Antiferromagnetically Ordered State in the Half-Filled Hubbard Model on the Socolar Dodecagonal Tiling

    Akihisa Koga

    pp. 360-366

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    DOI:10.2320/matertrans.MT-MB2020003

    Abstract

    We investigate the antiferromagnetically ordered state in the half-filled Hubbard model on the Socolar dodecagonal tiling. When the interaction is introduced, the staggered magnetizations suddenly appear, which results from the existence of the macroscopically degenerate states in the tightbinding model. The increase of the interaction strength monotonically increases the magnetizations although its magnitude depends on the local environments. Magnetization profile is discussed in the perpendicular space. The similarity and difference are also addressed in magnetic properties in the Hubbard model on the Penrose, Ammann-Beenker, and Socolar dodecagonal tilings.

  • Classical and Quantum Magnetic Ground States on an Icosahedral Cluster

    Shintaro Suzuki, Ryuji Tamura, Takanori Sugimoto

    pp. 367-373

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    DOI:10.2320/matertrans.MT-MB2020011

    Abstract

    Recent discovery of various magnetism in Tsai-type quasicrystal approximants, in whose alloys rare-earth ions located on icosahedral vertices are coupled with each other via the Ruderman-Kittel-Kasuya-Yosida interaction, indicates an avenue to find novel magnetism originating from the icosahedral symmetry. Here we investigate classical and quantum magnetic states on an icosahedral cluster within the Heisenberg interactions of all bonds. Simulated annealing and numerical diagonalization are performed to obtain the classical and quantum ground states. We obtain qualitative correspondence of classical and quantum phase diagrams. Our study gives a good starting point to understand the various magnetism in not only quasicrystal approximants but also quasicrystals.

  • A Toy Model Approach to Fractal Nature: Thermodynamics on a Cantor-Lattice Ising Model

    Yoichiro Hashizume, Takanori Sugimoto

    pp. 374-379

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    DOI:10.2320/matertrans.MT-MB2020012

    Abstract

    Thermodynamics on a Cantor-lattice Ising model is studied to clarify effects of fractal structure. Exact solutions based on the transfer matrix are investigated for finite size systems, and it is found that there is non-trivial relationship between entropy and fractal structure. In order to understand the nature in the thermodynamic limit, the renormalization method is applied. The results suggest a possibility of residual entropy due to the competition between non-uniformity of the fractal structure and uniform external field. These pave a simple way to approach general behaviors of non-uniform systems including fractal structures, such as quasicrystals.

  • Effect of Electron-Electron Interactions on Metallic State in Quasicrystals

    Shiro Sakai, Akihisa Koga

    pp. 380-385

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    DOI:10.2320/matertrans.MT-MB2020001

    Abstract

    We theoretically study the effect of electron-electron interactions on the metallic state of quasicrystals. To address the problem, we introduce the extended Hubbard model on the Ammann-Beenker tiling as a simple theoretical model. The model is numerically solved within an inhomogeneous mean-field theory. Because of the lack of periodicity, the metallic state is nonuniform in the electron density even in the noninteracting limit. We clarify how this charge distribution pattern changes with electron-electron interactions. We find that the intersite interactions change the distribution substantially and in an electron-hole asymmetric way. We clarify the origin of these changes through the analyses in the real and perpendicular spaces. Our results offer a fundamental basis to understand the electronic states in quasicrystalline metals.

  • A Comparison between Tungsten Inert Gas Welded Joints Welded by Commercial ER5183 Filler and Al–Mg–Zn–Sc–Zr–Mn Filler on Microstructure and Properties in 7075-T651 Aluminum Alloys

    Zixin Qu, Tianru Han, Hongbo Cui, Xin Tang

    pp. 386-395

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    DOI:10.2320/matertrans.MT-M2020295

    Abstract

    A 7075-T651 aluminum alloy was successfully welded by tungsten inert gas welding using commercial ER5183 wire and Al–5Mg–1Zn–0.3Sc–0.15Zr welding wire containing different contents of Mn. The microstructure, corresponding mechanical properties, and the corrosion behavior of the welded joint were investigated by microscopy methods, tensile tests, and corrosion tests. The results revealed that Al3(Sc,Zr) particles were distributed in a small amount in the welds and refined the primary α-Al grain. The addition of Mn gives a strong interaction of improving fracture strength, micro-hardness, and corrosion resistance reducing the harmful influence of iron, as a result, the properties of the welded joint were enhanced. And the increase of Mn content improves the tensile strength and hardness of welds, which could be attributed to the refinement of eutectic T phase by Mn. Potentiodynamic polarization tests revealed that the corrosion resistance of welds increased with the increase of Mn content. Compared with the welded joint using commercial filler, the welded joint with Al–Mg–Zn–Sc–Zr–Mn filler exhibited a higher resistance to stress corrosion cracking (SCC). In conclusion, Al–Mg–Zn–Sc–Zr–Mn filler can be using for welding 7XXX aluminum alloys.

  • Characterization on Ti35Nb7Zr Based Hydroxyapatite Composites Prepared via Rapid Sintering for Biomedical Applications

    ZhengYuan He, ChangBo Yi, WenRui Shan, Lei Zhang, Jun Tan, YeHua Jiang

    pp. 396-402

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    DOI:10.2320/matertrans.MT-M2020198

    Abstract

    The TiNbZr/HA (hydroxyapatite) composites evoked widespread concern in biomedical application. However, their properties were affected by processing parameters, such as milling time, sintering temperature and pressure, etc., during sintering process. The Ti35Nb7Zr/10HA (hydroxyapatite) composites were herein synthesized via spark plasma sintering (SPS) at a range of temperatures (800, 900, 1000, 1100 and 1250°C). Results show that the composites are mainly composed of β-Ti, α-Ti, a little of HA and ceramic interphases, like Ti2O, CaTiO3, CaZrO3, CaO and TixPy. With the increase of sintering temperature, the the compressive strength of the composites increases from 960 to 2279 MPa, and the elastic modulus from 45 to 61 GPa, due to the increase of ceramic interphases. The vitro tests reveal that the composite sintered at 1000°C possesses excellent ability to induce the formation of bone-like apatite and cell proliferation. Therefore, the composite prepared at 1000°C with low modulus (52 GPa) and obvious bioactivity exhibits immense potential for biomedical applications.

  • Silicon Refining by Solidification from Liquid Si–Zn Alloy and Floating Zone Method

    Yuanjia Ma, Kouji Yasuda, Akifumi Ido, Takeyuki Shimao, Ming Zhong, Rika Hagiwara, Toshiyuki Nohira

    pp. 403-411

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    DOI:10.2320/matertrans.M-M2020872

    Abstract

    This study evaluated the refining ability of a proposed production process for solar-grade silicon utilizing the electrolytic reduction of SiO2 on a liquid zinc electrode in molten salt. The distribution behaviors of impurity elements during the precipitation of solid silicon from a liquid Si–Zn alloy were studied by thermodynamic calculations at 923 K. In the precipitation experiment, silicon granules were recovered from a liquid Si–Zn alloy, which was prepared from metallurgical-grade silicon. The impurity removal ratios exceeded 99% for C, Al, and Ca, and 90% for Fe. High removal ratios were attained for B and O as well. As the post-processing, a silicon ingot was produced from the precipitated silicon granules by the floating zone method. The Zn residue in the precipitated silicon was completely evaporated during the floating zone refining. The total content of metallic elements (Al, Ca, Fe, Ti, and Zn) was lower than 0.2 ppmw, even though metallurgical-grade silicon was used as the starting material.

  • Observation of Pit Initiation and Growth of Stainless Steel under a Chloride Solution Droplet —Effect of S Content on Pit Initiation, Growth, and Repassivation—

    Azusa Ooi, Yaoki Ise, Eiji Tada, Atsushi Nishikata

    pp. 412-419

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    DOI:10.2320/matertrans.C-M2020869

    Abstract

    A system was developed based on which the pit growth automatically stops any time after the pit initiation under a chloride solution droplet. The atmospheric pitting corrosion of austenitic stainless steels with various sulfur (S) concentrations was investigated using this system. The results confirm that the initiation sites of pitting corrosion are manganese sulfide inclusions under the droplets regardless of the S concentration. In addition, the growth behavior of the active dissolution area is independent of the S concentration. When these specimens are subjected to wet–dry cycle tests, the probability of pitting corrosion increases with increasing S concentration due to the increase in initiation sites. The onset of pitting corrosion is independent of the chloride concentration. On the other hand, repassivation strongly depends on the S concentration. This Paper was Originally Published in Japanese in Zairyo-to-Kankyo 68 (2019) 347–354.

  • Polarization Curves of Carbon Steel in Concentrated LiBr Solutions Containing LiOH and Li2MoO4 at Different Temperatures after Short Immersion

    Tomoyoshi Irie, Daiki Morihashi, Youhei Hirohata, Takumi Haruna

    pp. 420-426

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    DOI:10.2320/matertrans.MT-M2020298

    Abstract

    We have tried to measure polarization curves of SS400 carbon steel in concentrated LiBr solutions over 393 K, and investigated the influence of temperature as well as concentrations of LiOH and Li2MoO4 on its corrosion behavior. Test solutions were 65 mass% (mass% is replaced by % hereafter) LiBr solutions containing 0 to 0.2% LiOH and 0 to 0.03% Li2MoO4. Test temperatures were 393 and 438 K. The test solutions were deaerated. The specimen was immersed in the test solution for a short time of 0.3 ks and was subjected to measurement of polarization curve in the same solution. As a result, the followings were obtained: The anodic current density in 65% LiBr solution without LiOH increased monotonically with a rise in a potential, and the relation was maintained regardless of Li2MoO4 addition and temperature change. In 65% LiBr solutions with LiOH up to 0.2% at 393 and 438 K, the anodic polarization curves showed active dissolution and passivation. When 0.05% LiOH was added to the 65% LiBr solution, the corrosion potential negatively shifted, and the potential was maintained regardless of more addition of LiOH. As a LiOH concentration increased, a pitting potential was raised. The polarization curves at 393 and 438 K showed almost no change regardless of addition of 0.03% Li2MoO4, meaning that Li2MoO4 had almost no effect on corrosion inhibition to the specimen for the short immersion. The corrosion rate at 393 K was approximately 0.3 A·m−2 regardless of the addition of LiOH nor Li2MoO4. Whereas, the corrosion rate at 438 K slightly decreased with increasing LiOH concentration, regardless of the addition of Li2MoO4. Cathodic current density in the solution with 0.2% LiOH and 0.03% Li2MoO4 increased with a rise in a temperature on the basis of Arrhenius relation. It is thought that insufficient effect of LiOH and Li2MoO4 on corrosion inhibition was observed because of a short immersion time of 0.3 ks before measurement of polarization curves.

  • Mechanism for Local Corrosion of Solid B2O3 at Water–Gallium Interface Induced by Branched Flow

    Itaru Hasegawa, Takuya Koizumi, Kazuhiko Kita, Masanori Suzuki, Toshihiro Tanaka

    pp. 427-435

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    DOI:10.2320/matertrans.MT-M2020275

    Abstract

    A fundamental study using a water model has been carried out to propose the mechanism for the local corrosion of a refractory near the slag–metal interface in terms of the bulk flow of the slag and liquid metal. The present model consists of water, liquid gallium and solid B2O3, whose physical properties have a similar relationship to high-temperature systems. It is found that the local corrosion of the refractory near the water–gallium interface is induced by a branched flow that separates from the main stream of the water. The branched flow is considered to be generated by the interaction between the capillary pressure within a gap near the gallium meniscus and the bulk water pressure. After the branched flow is generated, an eddy occurs near the water–gallium interface, and thus a drag force generated by the water is locally applied to the B2O3 surface, resulting in local corrosion. In corrosion tests using water–glycerin solutions, which have the same order of kinematic viscosity as slags at high temperatures, both the branched flow and the local corrosion are observed. The proposed mechanism can be applied to systems in which the slag–metal interfacial tension does not change significantly as well as to systems in which the interfacial tension changes when the refractory dissolves into the slag.

  • Development of Grain Growth Promotion Technique of Ni-Based Superalloy IN713 Fabricated by Metal Injection Molding

    Shinya Hibino, Kazushige Fujimitsu, Ryutaro Okada, Yoshimichi Nomura, Kenichiroh Igashira

    pp. 436-441

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    DOI:10.2320/matertrans.Y-M2020871

    Abstract

    High temperature strength is one of the critical technical issues to apply metal injection molding (MIM) to Nickel-based superalloy IN713. In this study, through investigating microstructure and high temperature strength of IN713-MIM materials in detail, we developed the technique to promote grain growth. In the experiments using IN713C-MIM materials, the high temperature tensile strength and the creep strength were remarkably inferior to the castings. The creep deformation mechanism of IN713C-MIM materials is revealed that grain boundary diffusion creep was dominant as the stress exponent was determined as n = 2.72. Therefore, we found out that it was important to reduce the amount of grain boundary by promoting grain growth. In the examinations aiming to reduce carbides on grain boundary, which inhibited grain boundary migration, the carbon content of IN713LC-MIM-sintered material could be suppressed to C = 0.05%. Subsequently, when IN713LC-MIM-HIP-ed material was subjected to additional heat treatment at 1280°C for 12 hr, significant grain growth was observed. This Paper was Originally Published in Japanese in J. Jpn. Soc. Powder Powder Metallurgy 66 (2019) 17–22.

  • Microstructural Observation of Dendrite and Its Surrounding Regions Formed during Solidification of Gray Cast Iron

    Yoichi Kishi, Hideharu Mochizuki, Zenjiro Yajima

    pp. 442-447

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    DOI:10.2320/matertrans.F-M2020867

    Abstract

    Molten metal for hypoeutectic gray cast irons was poured into a top-open-type sand mold with furan resin. Unsolidified molten metal was discharged from the mold before the molten metal completely solidified. This resulted in the formation of artificial defects on the solid–liquid phase boundary. Dendrites and other microstructures were observed to be formed near the artificial defects. Sharp dendrite stalks with periodic protrusions around the periphery were observed in macroscopic images. This observation confirmed that the dendrites were growing toward the center of the hole formed by the discharge of the unsolidified molten metal. These results corresponded to the observations made using a scanning electron microscope. Graphite flakes formed parallel to the dendrite branches were observed on the cross-sectional microstructures near the solid–liquid phase boundary. Scanning electron microscopy showed that perlite colonies segmented the dendrite stalk. The area around the dendrite, namely the region formed by eutectic solidification, was composed of perlite, ferrite, and graphite flakes. This Paper was Originally Published in Japanese in J. JFS 92 (2020) 333–338.

  • Pulsed-Electric-Current Bonding of Oxygen-Free Copper and Austenitic Stainless Steel

    Hayato Nakao, Akio Nishimoto

    pp. 448-452

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    DOI:10.2320/matertrans.MT-M2020277

    Abstract

    Although pure copper has good electrical and thermal conductivity, it is often used in combination with other metals due to its poor strength. An inexpensive austenitic stainless steel that has excellent corrosion resistance is a good joining material for pure copper, and the joining of these dissimilar metals enables the fabrication of parts for a wide range of applications. However, it is difficult to weld pure copper and stainless steel due to the large differences between their thermal properties. Therefore, in this study, the pulsed-electric-current bonding was applied to achieve solid state bonding of oxygen-free copper (OFC) and SUS304 austenitic stainless steel, and the bonding characteristics were investigated. The results show that the joint tensile strength improves with the increase in the bonding temperature and the applied pressure, and the sample bonded at 973 K with an applied pressure of 20 MPa exhibited a high strength of greater than 200 MPa, which caused a fracture in the OFC base material. This Paper was Originally Published in Japanese in J. Jpn. Inst. Copper 59 (2020) 91–95.

  • Effect of Rotation Speed on the Tribological and Conductive Behaviors of Rolling Current-Carrying Cu Pairs

    Jiawei Li, Chenfei Song, Yanyan Zhang, Yixiang Sun, Tianhua Chen, Li Wang, Zili Liu, Shuai Wang, Yongzhen Zhang

    pp. 453-460

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    DOI:10.2320/matertrans.MT-M2020267

    Abstract

    The tribological and conductive performance of the rolling current-carrying pairs were studied at different rotation speeds based on the disc-disc Cu pairs. Whether the current was applied or not, the friction coefficient decreased with the increasing of rotation speed. The average contact resistance increased with the rotation speed and the applied current. The sharp fluctuation of the real-time current was proposed as the failure criterion for the rolling current-carrying pairs, and both the rotation speed and the current accelerated the failure process. Erosion pits and melting could be observed by SEM, and the surface oxidation could be detected by EDS on the failure surface. It was speculated that arc discharge appeared under high speed and large current conditions, which encouraged the transformation of damage mechanism from mechanical wear to arc erosion. These results may provide some useful suggestions on the failure mechanism and life tests of the electro-conductive rotary joints.

  • Prediction of Residual Mg Contents in Ladle and Product after Graphite Spheroidizing Treatment by Using Artificial Neural Network

    Kazuki Akiyama, Ilgoo Kang, Toshitake Kanno, Nozomu Uchida

    pp. 461-467

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    DOI:10.2320/matertrans.F-M2020868

    Abstract

    Mg Cored wire method is a type of spheroidization treatment method. It has been introduced by many foundries from the viewpoints of improving the environment safety of factory workers, automation and productivity. However, the prediction of residual Mg content depends on experience because various factors complexly affect each other and the effects of each factor on Mg yield rate have not been clarified theoretically.In this research, we regard the relationship between these factors of spheroidization and residual Mg content as a nonlinear optimization problem. Therefore, we attempted to predict residual Mg content in the ladle and the product after pouring, which was spheroidized by Mg Cored wire method using the layered neural network (ANN). In ANN learning, the method of using the actual measured data as it is, the method of utilizing engineering knowledge, and the method of correcting data noise were examined. In addition, we constructed ANN for the relationship between the operation conditions of the spheroidizing treatment and residual Mg content in the ladle, and the relationship between the residual Mg content of the ladle and pouring conditions and residual Mg content of the product.As a result, the predicted residual Mg content in the ladle using ANN could be estimated with the correction coefficient R = 0.91. According to the constructed ANN, the higher the residual Mg content in the ladle, the lower is the yield rate of the Mg in the product. The yield rate of Mg could be improved to 54% by reducing the residual Mg content in the ladle to 0.042%. It was also clarified that the residual Mg content is the same in the ladle and the product. When this result was verified in an actual plant using 25% Mg wire, the yield rate of Mg improved by 53% at a monthly average and to about 65% at the highest value. This Paper was Originally Published in Japanese in J. JFS 92 (2020) 408–415. The captions of Fig. 3–10, 12, 13 are slightly changed. Reference 11, 12) were removed due to no citation.
  • © 2021 The Japan Institute of Metals and Materials

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