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ONLINE ISSN: 1347-5320
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MATERIALS TRANSACTIONS Vol. 61 (2020), No. 10

  • Structural and Thermoelectric Characteristics of Sol-Gel based ZnO Thin Films Doped with Elements of Group IB

    Thong Quang Trinh, Tinh Trong Nguyen, Dang Hai Le, Doanh Viet Vu

    pp. 1895-1899

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    DOI:10.2320/matertrans.MT-MN2019014

    Abstract

    This work presents the study results of zinc oxide (ZnO) thin films doped with elements of group IB, namely silver (Ag) and copper (Cu). The films were deposited on the Corning 1737F glass substrate using the solutions derived by sol-gel method. Effect of Ag and Cu doping on structural, surface morphology, and electrical properties were investigated. X-ray diffraction (XRD) analysis showed that both the undoped and doped ZnO thin films are polycrystalline in nature with (002) preferred orientation. The SEM images show the grown films composed with nanoparticles with sizes of about 30 to 50 nm. Doping result was verified by measurements of EDX to check compound elements, Hall effect for basic electric parameters, and UV-VIS. As a result, the EDX measurement revealed the doping ratio of Ag in ZnO as expected. Besides, the concentration of p-type carriers was between 1015 to 1017 cm−3, the resistivity was from 54 to 260 Ω·cm. Particularly, the band gap values were between 3.16 to 3.18 eV for the concentration of dopants between 1 to 3 mol. at% compared to that of 3.20 eV for ZnO films at room temperature. These results confirmed the success of introducing the dopants into ZnO matrix. The transport property of doped films also confirms the material nature throughout the characteristics including temperature dependence of electrical conductivity and Seebeck coefficient. In general, there was a clear change in material nature of ZnO films from n- to p-type with acceptable electrical property for thermoelectric applications.

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  • Dissimilar Metal Joining of Cu and Fe Using Super-Spread Wetting into Surface Fine Crevice Structures

    Jaebong Yeon, Takumi Kageyama, Riku Yamada, Peiyuan Ni, Masashi Nakamoto, Toshihiro Tanaka

    pp. 1900-1906

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    DOI:10.2320/matertrans.MT-M2020120

    Abstract

    Recently, our group employed surface fine crevice structures produced on Cu metal substrates using either laser irradiation or the reduction-sintering of oxide powders to induce region-selective super-spread wetting as a means of joining these substrates. The present work expanded the scope of this method by joining Cu and Fe substrates. Laser irradiation was found to permit the joining of Cu and Fe but generated voids at the join interface. In contrast, the reduction-sintering of mixed oxide powders allowed joining with essentially no voids. Thus, the latter technique is superior to laser irradiation when joining dissimilar metals.

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  • Hydrogen Trapping in Mg2Si and Al7FeCu2 Intermetallic Compounds in Aluminum Alloy: First-Principles Calculations

    Masatake Yamaguchi, Tomohito Tsuru, Ken-ichi Ebihara, Mitsuhiro Itakura, Kenji Matsuda, Kazuyuki Shimizu, Hiroyuki Toda

    pp. 1907-1911

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    DOI:10.2320/matertrans.MT-M2020201

    Abstract

    From first-principles calculations, we estimated the trapping energy of hydrogen atom at the interstitial site of perfect crystals of Mg2Si and Al7FeCu2 intermetallic compounds in the aluminum matrix. We found that Al7FeCu2 trapped hydrogen atoms strongly, whereas Mg2Si did not. The highest trapping energy in Al7FeCu2 is 0.56 eV/atom. We also found that the density of hydrogen trapping can be increased up to about 13 atoms/nm3 while keeping high trapping energy of about 0.40 eV/atom. We inferred that the Al7FeCu2 phase might remove hydrogen from the aluminum matrix, hence, preventing hydrogen embrittlement of aluminum alloy.

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  • Isothermal Aging Behaviors of Copper–Titanium–Magnesium Supersaturated Solid-Solution Alloys

    Kaichi Saito, Makio Suzuki, Satoshi Semboshi, Katsuhiko Sato, Yuichiro Hayasaka

    pp. 1912-1921

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    DOI:10.2320/matertrans.MT-M2020149

    Abstract

    Herein, the isothermal aging behavior of copper–titanium–magnesium (Cu–Ti–Mg) supersaturated solid-solution alloys, with different compositions, under test conditions of 450°C for 100 h, has been thoroughly investigated in a comparative study using various electron microscopy and microanalytical techniques. The Vickers hardness and electrical conductivity of the ternary alloys were recorded at slightly elevated (during aging) and reduced levels than their binary counterparts without Mg doping. Hence, it is proposed that the hardness and conductivity values are approximated from the superposition effect of precipitation hardening stimulated by Ti solutes and solution hardening by both Ti and Mg solutes. Furthermore, the tensile tests for these ternary specimens have demonstrated that Mg doping has a substantial effect on the improvement of the tensile strength and fracture elongation properties of binary Cu–Ti alloys. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy imaging combined with atomic-resolution energy-dispersive X-ray spectroscopy mapping analysis confirmed that the same metastable precipitate phase is responsible for peak hardening in ternary and Cu–Ti binary alloys. In addition, a large part of the Mg solutes is homogeneously distributed over the matrix regions, while there is also a smaller part of those present in the precipitates. The potential effects of Mg doping on the microstructures of Cu–Ti alloys were elucidated and the structural environment, which may yield relatively high mechanical properties, was discussed using the aforementioned observations.

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  • Self-Consistent Diffraction Stress Analysis Method for Estimating Stress, Strain-Free Lattice Parameter and Composition of Solid Solutions

    Takashi Harumoto, Ji Shi, Yoshio Nakamura

    pp. 1922-1929

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    DOI:10.2320/matertrans.MT-M2020163

    Abstract

    Self-consistent diffraction stress analysis method is proposed for analyzing solid solutions. Owing to the feedback of the strain-free lattice parameter, it is possible to perform the proposed method even when the exact composition is unknown. Employing an example specimen of (111) fiber-textured palladium cobalt alloy film with different compositions, the validity of the proposed method is confirmed by comparing results with that of the conventional method. This convenient proposal expands the applicability of diffraction stress analysis.

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  • Removal of Boron from Aqueous Solution Using Zero-Valent Magnesium Granules

    Shoji Kasahara, Tomio Takasu, Nobuaki Nagano, Yuki Mikoshi, Hideyuki Itou, Naotaka Sakamoto

    pp. 1930-1939

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    DOI:10.2320/matertrans.M-M2020842

    Abstract

    In order to understand the characteristics of the wastewater treatment method using zero-valent magnesium granules, the reaction between an aqueous solution containing boron and zero-valent magnesium granules was investigated by experiments and a reaction rate model. Particular attention was paid to the effect of adding hydrochloric acid before adding zero-valent magnesium and to the effect of adding sodium hydroxide to adjust the pH to 10.5 after 110 minutes. The following findings were obtained. The relationship between the pH and the dissolved magnesium concentration over time is determined by a reaction formula in which zero-valent magnesium granules react with an aqueous solution to generate Mg2+ ions while generating hydrogen. When magnesium hydroxide is produced, the pH becomes constant over time. Increasing the concentration of hydrochloric acid lowers the pH value reached. This relationship is determined in equilibrium with magnesium hydroxide. The reaction rate of the zero-valent magnesium granules is determined as the first-order reaction of the hydrogen ion activity when the pH was lower than 2.3 or higher than 8.5, and as the zero-order reaction of the hydrogen ion activity at pH from 2.3 to 8.5. The amount of magnesium hydroxide produced without the addition of sodium hydroxide is determined by the above-described reaction rate model of zero-valent magnesium granules. The boron concentration of the solution when the pH is adjusted to 10.5 by adding sodium hydroxide is determined by the Langmuir-type sorption isotherm of boron to the magnesium hydroxide produced. As described above, the behavior of removing boron from an aqueous solution using zero-valent magnesium granules can be well reproduced by the simple reaction rate model used in this study, and it can be said that it is useful for process design.

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  • Kinetic Analysis for Agglomeration-Flotation of Finely Ground Chalcopyrite: Comparison of First Order Kinetic Model and Experimental Results

    Vothy Hornn, Mayumi Ito, Ryosuke Yamazawa, Hiromasa Shimada, Carlito Baltazar Tabelin, Sanghee Jeon, Ilhwan Park, Naoki Hiroyoshi

    pp. 1940-1948

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    DOI:10.2320/matertrans.M-M2020843

    Abstract

    Particle size-flotation rate relationships can be discussed by a first-order kinetic model for flotation, which considers the probability of particle-bubble collision, attachment and detachment; and it was confirmed that recovery rate of finely ground hydrophobic particles in the froths are very low because of the limited particle-bubble collision probabilities. One method to improve the flotation of fine minerals is to agglomerate them before flotation using oil as a bridging liquid, an approached that has been shown to improve the flotation rates dramatically. A mathematical kinetic model for the flotation of agglomerated particles would be useful to design and optimize the agglomeration-flotation process, but no generally applicable model has been established yet. In this paper, flotation experiments of finely ground chalcopyrite were carried out with and without oil-agglomeration as pretreatment and the kinetic data (time-recovery curves) were compared with the conventional first-order kinetic model for flotation. Without agglomeration, time-recovery curves determined by the experiments fitted well with the model calculations, but there were significant deviations between experimental results and model calculations for the agglomerated particles; that is, experimental flotation recoveries were much higher than those calculated by the model. The conventional first-order kinetic model does not consider particle size changes during flotation while the experimental results suggested that the size of agglomerates increased in the flotation cell. This may be one of the reasons why significant deviations between the experimental and modelling results were observed, suggesting that the kinetic model for agglomeration-flotation need to consider the growth of agglomerates during flotation.

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  • Identical-Location Scanning Electron Microscopy Observation of Surface Morphological Changes of Pt–Cu Nanoparticles

    Azusa Ooi, Yuichi Shigihara, Eiji Tada, Atsushi Nishikata

    pp. 1949-1957

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    DOI:10.2320/matertrans.MT-M2020180

    Abstract

    The surface morphological changes of electrodeposited Pt–75 at% Cu (Pt–75Cu) nanoparticles under potential cycling were investigated by identical-location scanning electron microscopy. The electrodeposited nanoparticles consisted of numerous nuclei (∼3 nm in diameter) that agglomerated to form larger secondary particles (30–50 nm). Upon immersion in sulfuric acid, Pt shells formed on the surfaces of the Pt–75Cu nanoparticles due to the selective dissolution of Cu. Although around 40% of the Cu was dissolved from Pt–75Cu, no noticeable surface morphological changes were observed. Thereafter, Pt–75Cu was subjected to potential cycling between 0.05 and 1.0 V, whereupon surface smoothing of the nanoparticles due to the surface diffusion of Pt was observed. Conversely, when Pt–75Cu was potential-cycled between 0.05 and 1.4 V, the particle diameters of the nanoparticles drastically decreased and the nuclei on the surface completely disappeared owing to Pt dissolution and re-deposition. The heat-treated nanoparticles developed numerous pores on their surfaces during immersion and the initial stage of potential cycling. However, their final morphologies were found to be similar to those of the non-heated sample. The formation of pores can be explained by the coarsening of nuclei by heat treatment. This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 84 (2020) 244–252. Abstract and captions of figures are slightly changed.

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  • Effect of Polyethylene Glycol and Glue on Electrodeposition Behavior of Zn from Electrowinning Solution and Its Crystal Structure

    Keita Uchida, Satoshi Oue, Hiroaki Nakano

    pp. 1958-1966

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    DOI:10.2320/matertrans.MT-M2020182

    Abstract

    To elucidate the effects of polyethylene glycol (PEG) and glue on the deposition of Zn from electrowinning solution and its resulting crystal structure, Zn electrodeposition was performed at a current density of 600 A·m−2 and a charge of 8.64 × 106 C·m−2 in an agitated sulfate solution containing 1.07 and 1.8 mol·dm−3 of ZnSO4 and H2SO4, respectively, at 45°C. With the additions of PEG and glue, the evolution of hydrogen was suppressed at the current density region less than the critical current density for Zn deposition, decreasing the critical current density of Zn. The degree of decrease in the critical current density of Zn was larger with glue than that with PEG. The current efficiency for Zn deposition was higher with PEG and glue than that without at the low current density region because the critical current density of Zn decreased with additives. Since the additives suppressed Zn deposition more than the hydrogen evolution at the high current density region, the current efficiency of Zn decreased by increasing the additive concentration. At the high current density region, little difference was observed in the current efficiency of Zn between PEG and glue. The effect of the molecular weight of PEG on the current efficiency of Zn was rarely observed at the molecular weight above 2000. With the addition of PEG, the deposits became fine platelets with preferred orientation of {1011} and layered pyramidally, while {1120} orientation was obtained, and the platelets grew perpendicularly to the substrate with the addition of glue. The surface roughness of deposited Zn decreased with additives, and it decreased further with PEG compared with that with glue. This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 84 (2020) 58–65.

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  • Yttriothermic Reduction of TiO2 in Molten Salts

    Takara Tanaka, Takanari Ouchi, Toru H. Okabe

    pp. 1967-1973

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    DOI:10.2320/matertrans.MT-M2020123

    Abstract

    A new reduction process for producing titanium (Ti) with an ultra-low oxygen concentration directly from TiO2, employing yttrium (Y) as the reductant, was developed in this study. Several methods for the direct reduction of TiO2 have been proposed to lower the cost of production of Ti. However, none of them have yet been applied industrially. In addition, Y has never been used as a reductant for reducing TiO2 although its reducing ability is the highest among the reductants capable of reducing TiO2 to Ti with low oxygen concentration. In this study, the reduction reactions of TiO2 using Y/Y2O3 equilibrium and Y/YOCl/YCl3 equilibrium, employing various molten salts as solvents, were investigated. TiO2 pellets and metallic Ti pieces were placed in several types of solvents along with sufficient Y and heated at 1300 K for 86 ks. When YCl3 or CaCl2 was used as the solvent, the TiO2 pellets were reduced to metallic Ti. The oxygen concentrations in the Ti pieces after heating in YCl3 and CaCl2 were 90 ± 40 mass ppm O and 350 ± 60 mass ppm O, respectively. However, when NaCl or KCl was used as the solvent, a small amount of metallic Ti and a large amount of complex oxides were obtained. It is considered that the reduction reaction did not proceed sufficiently owing to the low solubility of the oxide ions in molten salts. It was experimentally demonstrated that Ti with an ultra-low oxygen concentration (100 mass ppm O or less) can be directly produced from TiO2 by using Y as the reductant in an appropriate solvent. This method is expected to lead to the development of a new industrial process for the production of Ti with an ultra-low oxygen concentration directly from its ore.

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  • Heat Transfer Modeling between the Mold and the Ingot for Convex Concave Ingot Surface

    Toshio Sakamoto, Akira Matsushita, Yasuhiro Oda, Yuichi Motoyama, Hitoshi Tokunaga, Toshimitsu Okane

    pp. 1974-1980

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    DOI:10.2320/matertrans.MT-M2020162

    Abstract

    In this paper, a heat transfer model between the mold and the ingot with convex and concave surfaces for continuous casting (CC) process of copper and copper alloys is proposed and discussed. Conventionally, during CC process, vibration of the mold leads to ingot with convex and concave surfaces known as oscillation marks. These marks may cause heat resistance between the mold and the ingot. In the model, three areas where heat resistances occurs were considered: (ΔR1) non-contacting area, (ΔR2) concave area derived from decrease of thermal conductivity, and (ΔR3) area where non-effective heat flow exists in solid phase. The heat resistance values were obtained either analytically or by numerical methods for conditions typically observed in the CC process. Quantitative analyses and comparison of heat resistance values indicated that ΔR3 was the most significant factor and that ΔR1 and ΔR2 was negligible. Furthermore, it was found that slight changes in contact condition results in a large change in heat resistance. This Paper was Originally Published in Japanese in J. Japan Inst. Copper 58 (2019) 109–115.

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  • Inlet Condition for Mold Filling Simulation in Gravity Casting of Aluminum Alloy

    Shingo Taniguchi, Yasuhiro Maeda

    pp. 1981-1986

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    DOI:10.2320/matertrans.F-M2020840

    Abstract

    The casting CAE is useful tool for casting design in order to obtain the sound castings without defects. It is very important to know the influence of casting conditions on the mold filling. In the present study, the inlet condition for the mold filling simulation in gravity casting of aluminum alloy is investigated to simulate the real phenomena changing with the difference in the casting shape. The direct observation experiment is carried out with six types of sprue designs using an aluminum alloy and sand mold. There are three types of sprues: the elbow, bend and stair-step shapes. The molten metal of Al–7%Si alloy is poured using the stopper ladle. The velocity change in accordance with the different casting condition is analyzed using images observed by the video camera. The velocity of flow front in the early stage of the mold filling is changed intensely. Then, the velocity after the middle stage of the mold filling is decreased calmly. It is difficult to simulate the present phenomena by the mold filling simulation using the velocity inlet boundary. In order to reproduce the difference of each filling behavior, new inlet condition, which is named to “OverFlow pouring Basin” (OFB), is proposed in this study. The mold filling behavior and filled-up time simulated using the inlet condition of OFB are more or less agreed with six real phenomena according with three sprue shapes and two sprue heights.

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  • Effect of Heating Conditions before Quenching on Residual Stress in High-Speed Steel Type Cast Iron Rolls by Centrifugal Cast

    Kazunori Kamimiyada, Yuji Konno, Ayaka Yanagitsuru

    pp. 1987-1993

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    DOI:10.2320/matertrans.F-M2020841

    Abstract

    High-speed steel type cast iron rolls (HSS-rolls) used in the hot rolling process of steel are composite rolls manufactured by the CPC method or the centrifugal casting method (CCM). Comparing the two, the centrifugal cast HSS-roll is advantageous in terms of production cost, and since the shaft material is ductile cast iron, the thermal crown during rolling is small, and the plate threading property is excellent. For this reason, Centrifugal cast HSS-rolls are often used as standard type HSS-rolls, and some of them are also used in the later stands of the hot strip mills. On the other hand, forged steel is used for the shaft material of CPC-HSS-rolls, and the strength of the shaft material is superior to that of ductile cast iron, so there is very little risk of fracture accident (thermal breakage) from the center. The thermal breakage of the roll is defect that occurs when the tensile residual stress of the shaft and the thermal stress generated during rolling exceed the material strength of the shaft. Therefore, in order to prevent thermal breakage, it is considered extremely effective to suppress the tensile residual stress of the shaft portion. Therefore, in this study, an attempt was made to significantly reduce the tensile residual stress of the shaft by improving the heat treatment conditions. Furthermore, the influence of heat treatment conditions on the compressive residual stress of the outer layer was also investigated. As a result, it was confirmed that by applying the surface rapid heating method to heating conditions before quenching, the tensile residual stress inside the roll can be reduced by about 30% compared to the conventional uniform heating method. Effective results were obtained by deciding the manufacturing conditions for significantly improving safety against thermal breakage in centrifugal cast HSS-rolls. This Paper was Originally Published in Japanese in J. JFS 92 (2020) 144–150. Figures 10 and 11 were slightly changed.

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  • Laser Ultrasonic Technique to Non-Destructively Detect Cracks on a Ni-Based Self-Fluxing Alloy Fabricated Using Directed Energy Deposition (DED)

    Harumichi Sato, Hisato Ogiso, Yorihiro Yamashita, Yoshinori Funada

    pp. 1994-2001

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    DOI:10.2320/matertrans.MT-M2020086

    Abstract

    Miniaturization of bearing rollers used in autos and robots will require a manufacturing system that combines a deposition method that can fabricate thin jigs without defects and a non-destructive inspection method that can detect cracks on such jigs. Here, we are developing a system that uses directed energy deposition (DED), which is a 3D-printing (additive manufacturing, AM) process, to fabricate thin jigs, and then uses laser ultrasonics (LU) to inspect the jigs. Here, deposited layers having a 0.4 × 0.6 mm2 cross-section were fabricated using DED, and then non-destructively inspected using LU. However, using LU on such a small area has three problems: the effect of overlapping of the excitation and detection laser beams, difficulty in separating the multiple types of waves due to the simultaneous generation, and complexity of the acoustic field. Therefore, first, the acoustic field was examined using the finite element method (FEM), and then LU was used to inspect a small area of the deposited layer using complex discrete wavelet transform. Results show successfully detection of spontaneously occurring cracks, thus confirming the effectiveness of LU for non-destructive inspection of a thin jig.

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  • Surface Modification of Molybdenum by Iron-Powder Pack Treatment

    Yasuhiro Morizono, Takateru Yamamuro, Sadahiro Tsurekawa

    pp. 2002-2007

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    DOI:10.2320/matertrans.MT-M2020177

    Abstract

    Molybdenum sheets were embedded in mixtures of iron, graphite and alumina powders and heated at 1073–1373 K for 1.8–14.4 ks in a nitrogen flow. This process is a new surface modification technique called “Iron-powder pack (IPP) treatment”. The amount of alumina added as an anti-sintering agent was fixed in the powder mixtures, and the volume ratio of iron, graphite and alumina powders was varied from 0:10:2 to 6:4:2. An XRD pattern of the surface of the molybdenum sheet heat-treated at 1273 K for 3.6 ks using a 0:10:2 mixture had some small peaks for α-Mo2C. However, it could be identified by optical microscopy and scanning electron microscopy. On the other hand, the use of mixtures containing iron powder led to the formation of an α-Mo2C layer. When IPP treatment using a 4:6:2 mixture was carried out at 1273 K for 3.6 ks, the α-Mo2C layer with a thickness of approximately 14 µm formed on the molybdenum surface. The layer began to be observed at a heating temperature of 1073 K, and grew toward the inside of the molybdenum via the diffusion of carbon from the powder mixture. The sheet covered with the thick α-Mo2C layer showed a surface hardness of approximately HV = 1500.

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  • Effects of Eccentric Mold Electromagnetic Stirring on Continuous Casting Large Steel Round Blooms

    Liang Niu, Junxue Zhao, Shengtao Qiu

    pp. 2008-2016

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    DOI:10.2320/matertrans.MT-M2019376

    Abstract

    In this paper, a coupled 3D mathematical model is established to study the electromagnetic, flow and temperature fields of round blooms with different degrees of eccentric mold electromagnetic stirring (M-EMS). The results show that under the action of severely eccentric M-EMS, the magnetic flux density and time-averaged electromagnetic force near the external arc side of a Φ380 mm round bloom are greater than those near the inner arc side; the inertial impingement jet from the nozzle is deflected toward the external arc sides, and the temperature of molten steel on the external arc side is higher. As the degree of M-EMS eccentricity decreases, the differences in the electromagnetic, flow and temperature fields between the inner and external arc sides of the round blooms gradually decrease. The molten steel temperature on the inner arc side increases significantly after moving the nozzle position to the inner arc side of the Φ380 mm round bloom, so this method is not suitable to eliminate the effects of M-EMS eccentricity on Φ380 mm round blooms.

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  • Relationship between Microstructure and Fatigue Properties of Forged Ti–5Al–2Sn–2Zr–4Mo–4Cr for Aircraft Applications

    Saki Tanaka, Toshikazu Akahori, Mitsuo Niinomi, Masaaki Nakai

    pp. 2017-2024

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    DOI:10.2320/matertrans.MT-M2020184

    Abstract

    Titanium alloys have applications in air frames for commercial aircraft, and jet engine components such as fans and compressor disks, which function at low temperatures (up to 673 K). Near β-type Ti–5Al–2Sn–2Zr–4C–4Mo (Ti-17) exhibits greater strength, crack propagation resistance, and creep resistance at intermediate temperatures compared to the (α + β)-type Ti–6Al–4V. It is important to estimate the fatigue life of engine components made of Ti-17. This requires problem quantitative relationship between the fatigue properties and microstructural factors of Ti-17. Therefore, the fatigue properties including tensile properties and microstructures of Ti-17 samples fabricated by hot-forging at various temperatures, followed by high- and low-temperature solution treatment (ST), and same aging treatment were investigated to define a quantitative relationship between the fatigue properties and the microstructures.The microstructures of all forged Ti-17 samples exhibit elongated prior β-grains composed of two microstructural feature regions: acicular α and fine equiaxed α-phase regions. The volume fraction of the acicular α region decreases with increasing ST temperature. The Vickers hardness, 0.2% proof stress and tensile strength increases with increasing ST temperature. However, the elongation and reduction of area exhibit a reverse trend. The Ti-17 samples forged at 1173 K followed by solution treatment at 1073 K and aging treatment exhibits the highest fatigue limit of around 975 MPa. The fatigue strength of the forged Ti-17 samples is strongly related to the microstructural factor such as the volume fraction of the equiaxed α-phase region, which is one of the crack initiation sites in the forged Ti-17 samples subjected to low temperature ST and aging, and the strength difference between the acicular α-phase and the fine (α + β)-phase, which leads to the crack initiation in the forged Ti-17 sample subjected to high temperature ST and aging. This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 84 (2020) 200–207.

  • The Critical Point of Average Grain Size in Phonon Thermal Conductivity of Fine-Grained Undoped Lead Telluride

    Mongkol Bumrungpon, Issei Morioka, Ryusuke Yasufuku, Toshiharu Hirai, Kenichi Hanasaku, Kenji Hirota, Katsuhiro Takagi, Kazuhiro Hasezaki

    pp. 2025-2029

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    DOI:10.2320/matertrans.MT-M2020069

    Abstract

    Undoped PbTe was melted at 1123 K, ball milled (BM) at rotation speeds from 90 to 180 rpm and hot pressed (HP) at 147 MPa and 650 K. Milling at 120 rpm produced the minimum phonon thermal conductivity of 1.29 W m−1 K−1 and average grain size of 0.80 µm. Phonon thermal conductivity was constant from coarse grain size to fine grain size of 1 µm and decreased suddenly at 0.80 µm. This tendency of phonon thermal conductivity corresponded to theoretical calculations with grain boundary scattering. However, the observed critical point of 1 µm was much larger than the calculated value of 0.03 µm. There was a significant inverse relationship between phonon thermal conductivity and FWHM of X-ray diffraction peaks. The low phonon thermal conductivity was associated with not only grain boundary scattering but high internal strain.

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  • Selective Extraction of Chromium from EAF Stainless Steel Slag by Pressurized Oxidation in a NaOH Solution

    Qing-Zhong Zhao, Ya-Nan Zeng, Jun-Guo Li, Ya-Jun Wang

    pp. 2030-2039

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    DOI:10.2320/matertrans.MT-M2020050

    Abstract

    Chromium was selectively extracted from electric arc furnace (EAF) slag by pressurized oxidation in a NaOH solution. The effect of the temperature (T), NaOH concentration, oxygen pressure and reaction time on the extraction ratio of the chromium was investigated. It was found that the chromium extraction rate was significantly impacted by the temperature. At the optimum conditions of T = 170°C, NaOH concentration of 40 wt.%, oxygen pressure of 1.6 MPa and reaction time of 4 h, a maximum chromium extraction rate of 60.04% was obtained. It was observed by scanning electron microscopy (SEM) with energy dispersive spectrometry (EDS) that the residual particles were enclosed by a “secondary precipitation layer” mainly composed of Fe and O, which prevented further extraction of the chromium from the core residues. The leaching process of chromium was controlled by the diffusion of the solid phase product layer, and the activation energy of the extraction reaction was 56.67 kJ/mol.

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