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

MATERIALS TRANSACTIONS Vol. 60 (2019), No. 3

  • PREFACE

    pp. 373-373

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    DOI:10.2320/matertrans.MPR2019901

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    1. Equilibrium between Metallic Titanium and Titanium Ions in MgCl2–LiCl Molten Salt MATERIALS TRANSACTIONS Vol.60(2019), No.3
  • Equilibrium between Metallic Titanium and Titanium Ions in MgCl2–LiCl Molten Salt

    pp. 374-378

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    DOI:10.2320/matertrans.MA201801

    The equilibrium between metallic titanium and titanium ions, 3Ti2+ = 2Ti3+ + Ti, in MgCl2–LiCl molten salt system was revaluated by means of best fitting. The measurement was also carried for the MgCl2–LiCl melt with various composition of LiCl at 1023 K. The results illustrate that the values of Kc correspond well with the composition of the melt which defined as polarizing power.
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    1. PREFACE MATERIALS TRANSACTIONS Vol.60(2019), No.3
  • Electrochemical Dy-Alloying Behaviors of Inconel and Hastelloy in Molten LiF–CaF2–DyF3

    pp. 379-385

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    DOI:10.2320/matertrans.MA201805

    The electrochemical Dy-alloying behaviors of Inconel 600 and Hastelloy C-276 were investigated in a molten LiF–CaF2–DyF3 (0.30 mol%) system at 1123 K. Cyclic voltammetry and open-circuit potentiometry suggested the formation of several phases of Dy–Ni alloys. Potentiostatic electrolysis was conducted to prepare alloy samples using Inconel 600 and Hastelloy C-276 plate electrodes at 0.20 V vs. Li+/Li for 60 min. Cross-sectional scanning electron microscopy revealed that Dy–Ni alloys were formed in both samples. X-ray diffraction analysis confirmed the formation of DyNi2 as the predominant phase for both the samples. Phase separation of the alloy layer into the DyNi2 phase and formation of the agglomerated Cr–Fe and Cr–Fe–Mo phases were indicated by EDX mapping analysis.
  • Solubility of CaS in Molten CaCl2

    pp. 386-390

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    DOI:10.2320/matertrans.MA201803

    In order to check the solubility of CaS during sulfide reduction in molten CaCl2, the mixture of CaCl2 and a small amount of CaS was melted in Ar at the range from 1123 K to 1223 K. The melt was sampled by quartz tube and rapidly solidified. The solidified samples showed a lamella structure with CaS particles, which results in a simple eutectic reaction between CaCl2 and CaS. Using ICP-AES analysis, the saturation of CaS was found to be completed within 1.8 ks, and 1.77 ± 0.1 mol%CaS at 1173 K was measured as the solubility limit at the initial composition of 3.0 mol%CaS. A tentative phase diagram of CaCl2–CaS binary system was proposed based on the solubility analysis in these temperatures and eutectic structure.
  • Molten Salt Electrochemical Synthesis, Heat Treatment and Microhardness of Ti–5Ta–2Nb Alloy

    pp. 391-399

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    DOI:10.2320/matertrans.MA201808

    Alloy of composition Ti–5Ta–2Nb (numbers in mass%) was synthesised directly from the corresponding mixed metal oxides via the FFC-Cambridge process. Compacted powder discs of TiO2–Ta2O5–Nb2O5 were cathodically polarised against a graphite anode in a molten CaCl2 electrolyte, and the effect of various process parameters on the synthesis of the alloy was investigated. The samples retained their disc-type shape throughout the oxide-to-metal conversion under appropriate processing conditions. XRD analysis showed that the electrochemically prepared alloy existed as single-phase α-Ti and changed into dual-phase (α+β)-Ti upon heat treatment. SEM/EDX analysis revealed a nodular porous microstructure and confirmed the intended chemical composition. Backscattered SEM analysis after heat treatment provided direct evidence of micron-sized β-phase precipitates at the boundaries of the α-phase particles. Microhardness of the heat-treated alloy compared well with literature data for the same material prepared by conventional metallurgical methods. Overall, the study has demonstrated the feasibility of the single-step electrochemical fabrication of Ti–5Ta–2Nb alloy bodies directly from oxides for structural application.
  • Preparation of Titanium Metal Using Titanium Suboxides in Molten Salt

    pp. 400-404

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    DOI:10.2320/matertrans.MA201807

    Perovskite is an inevitable phase formed in the cathode during electrolytic deoxidation using titanium dioxide as the raw material by FFC process. Here, titanium suboxides (Ti3O5, Ti2O3 and TiO) are used as the raw materials to study whether perovskite can be avoided in the process. The results show that there is not perovskite formed during electrolysis using TiO and perovskite is appeared in the cathode when Ti3O5 and Ti2O3 are used as the raw materials. Perovskite is formed by combining Ca2+ in the molten CaCl2 with the cathode materials after the voltage is applied between the electrodes. Perovskite is mainly produced by electrochemical deoxidation process and amount of it gradually decreases with oxygen content decreasing in the cathode.
  • Production of Fine Titanium Powder from Titanium Sponge by the Shuttle of the Disproportionation Reaction in Molten NaCl–KCl

    pp. 405-410

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    DOI:10.2320/matertrans.MA201811

    Titanium and its alloys are key materials for various fields. Low cost and high-quality titanium powder production methods are crucial for powder metallurgy (PM) and additive manufacturing (AM) of titanium to significantly decrease the manufacturing cost. In this study, the production of titanium powder from raw material titanium sponge was achieved through the shuttle of the disproportionation reaction and the backward reaction of Ti2+ to Ti3+ and Ti metal in molten NaCl–KCl at 750°C. With the addition of a very small amount of TiCl2, over 7∼70 times of titanium powder in mass comparing to the added Ti2+ was obtained. The primary particle size of the powder formed based on the disproportionation reaction was approximately 1 µm, while the secondary agglomerated particle size was in the range of 25∼50 µm. No significant difference of the particle size distribution was found for the experimental runs with different TiCl2 concentrations and holding times. The proposed production method of titanium powder from titanium sponge is expected to significantly decrease the production cost of titanium powder.
  • Solubility of CaS in CaCl2–LiCl Eutectic Melt

    pp. 411-415

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    DOI:10.2320/matertrans.MA201809

    Solubility of CaS in molten CaCl2–65 mol%LiCl eutectic salt was examined by sampling of CaS saturated salt and ICP analysis. The handling in dried environment and an adequate mass of melt were applied for reliable measurements, in addition to suppression of inclusion of CaS particles. The solubility limit was found to be 0.22 ± 0.05, and 0.31 ± 0.05 mol%CaS at 873 K and 973 K, respectively. This saturation value was less than 1.77 ± 0.1 mol%CaS in pure CaCl2 at 1173 K.
  • Phase Transformations and Deoxidation Kinetics during the Electrochemical Reduction of TiO2 in Molten CaCl2

    pp. 416-421

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    DOI:10.2320/matertrans.MA201802

    There are still some different understandings about deoxidization process from TiO2 to Ti by FFC Cambridge process. Herein, an investigation was carried out on the phase transformations and deoxidation kinetics of the electrochemical reduction of TiO2 to Ti in molten CaCl2. The reaction goes through four different stages involving several phase transformations. The first stage is the deoxidation of TiO2 with the formation of CaO and CaTiO3. At this stage, Cl2 is forcibly released from the anode because of the consumption of Ca2+ on the cathode, balanced with Cl at the anode. The second stage is the deoxidation of CaTiO3 and titanium suboxides. These two stages take 45% of the entire deoxidation time from TiO2 to Ti[O]. The third stage is the deoxidation of titanium suboxides. This stage takes about 55% of the total time. The final stage is further deoxidation of Ti–O solid solution by electrochemical reduction or by calcium thermal reduction. There are no obvious kinetic barriers in the formation of CaTiO3, but the rate of deoxidation of titanium suboxides and CaTiO3 is relatively slow. Therefore, the limiting step from TiO2 to Ti by the FFC process in molten CaCl2 is deoxidation of titanium oxides and the interphase of CaTiO3.
  • Phase Composition, Microstructure, Corrosion Resistance and Mechanical Properties of Molten Salt Electrochemically Synthesised Ti–Nb–Sn Biomedical Alloys

    pp. 422-428

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    DOI:10.2320/matertrans.MA201810

    Ti–xNb–ySn alloys with different Nb and Sn contents, of x = 24, 35, 42 mass% and y = 4, 7.9 mass%, were synthesised directly from TiO2, Nb2O5 and SnO2 mixtures via the FFC-Cambridge process. Compacted powder discs were employed as the cathode versus graphite as the anode in molten CaCl2 as the electrolyte. XRD analysis of the as-synthesised alloys showed that the two alloys with Nb content of 24 mass% were dual-phase α/β-Ti whereas the other four alloys with Nb contents of 35 and 42 mass% were single-phase β-Ti. SEM analysis showed that the alloys were highly porous, and that particle size decreased with increase in Nb and Sn contents. Alloy samples of each composition were subjected to short-term and long-term immersion tests in Hanks’ simulated body fluid solution. XPS studies then identified a passive oxide film on the surface of the alloy, and a hydroxyapatite layer on top of the oxide. Potentiodynamic polarisation studies revealed excellent corrosion resistance with very small corrosion current densities despite high open porosities. Furthermore, alloy samples were subjected to heat treatment in vacuum. Mechanical testing of these identified a substantial increase of elastic modulus and Vickers hardness. Overall, the experimental programme has brought out that the properties of the Ti–Nb–Sn alloys prepared are influenced markedly by their Nb and Sn contents, and that single-phase β-Ti–35Nb–4Sn holds promise as a candidate material for body implant applications.
  • Characterization of Precipitated Phases and Carbides’ Coarsening in DH36 Shipbuilding Steel during Tempering Process

    pp. 429-436

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    DOI:10.2320/matertrans.M2018309

    The effect of tempering temperature on precipitates in DH36 steel was studied by using OM, SEM and TEM analysis. The results showed that DH36 steel after tempering was composed of ferrite, pearlite and precipitated phases. The grain sizes of ferrite have not increased obviously with increasing of tempering temperature. There were two types of carbides: (Nb,Ti)C and cementite, the sizes of which were more than 80 nm and less than 80 nm, respectively. The precipitation temperature of them were 1370 K and 977 K after calculation. As the tempering temperature increased, the average sizes of precipitated phases increased, while volume fractions and precipitation strengthening decreased. Cementite coarsened much faster than (Ti,Nb)C, which made strength and toughness of steel decrease sharply in 973 K. Meanwhile, moderate amount of precipitates made the toughness reach summit at 873 K. To sum up, the steel after 10800 s at 773 K tempering has the best mechanical properties and finest dispersed precipitates.
  • Ab-Initio Study of (111) to (001) Texture Transformation in Ag Thin Films

    pp. 437-440

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    DOI:10.2320/matertrans.M2018349

    First-principles density functional theory was used to study the (111) to (001) texture transformation in Ag thin films. The texture transformation was experimentally observed and studied during annealing of Ag thin films. Ag(111) showed the lowest surface energy of 0.031 eV/Å2, whereas Ag(001) showed a surface energy of 0.040 eV/Å2. Ag films grown along 〈111〉 and 〈001〉 exhibited epitaxial hardening and softening behavior, respectively. Further, we found that the strain-induced texture transformation of Ag thin films from (111) to (001) can also be realized under a hydrostatic compressive strain of 2.25% or a biaxial compressive strain of 3%. Our results agree greatly with previously reported experimental observations on texture transformation in Ag thin films.
  • Mechanosynthesis and Reversible Hydrogen Storage of Mg2Ni and Mg2Cu Alloys

    pp. 441-449

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    DOI:10.2320/matertrans.M2018293

    A series of Mg–Ni or Mg–Cu alloys with Mg content comprised between 55 and 77 at% Mg was prepared by mechanical alloying with the aim of synthesizing Mg2Ni and Mg2Cu phases, respectively. Their morphology and structural properties were characterized by scanning electron microscopy (SEM) and X-ray Diffraction (XRD). High Pressure Differential Scanning Calorimetry (HP-DSC) was used to evaluate their hydrogenation properties. For the Mg–Ni series, 8 hours of milling were enough to synthesize the Mg2Ni alloy. The highest reversible hydrogen capacity (2.8 mass%) was obtained for 70 at% Mg sample without the need of any activation treatment. For Mg–Cu series, 83 mass% of Mg2Cu was obtained after 4 hours of milling. From the view point of alloying composition, Mg-rich samples show better crystallinity of Mg2Cu phase. Under hydrogen, the Mg2Cu powder mixtures decompose and form MgH2 hydride storing 1.56 mass% of hydrogen for 66 at% Mg. For the Mg–Ni series, Cu or Al elements (1 to 10 mass%) were added during milling. HP-DSC runs show that they destabilize the hydride phase due to alloying effects.
  • Evaluation of Work Hardening and Tensile Strength for Press Worked Products Plastic Strained from Multiaxial Direction

    pp. 450-457

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    DOI:10.2320/matertrans.P-M2018854

    Tensile strength and work hardening characteristics in the case that strain was applied from multi-axial directions by press processing and in the case that uniaxial strain was applied by tensile test were compared. Furthermore, in a unified manner with the results of the tensile tests, fatigue tests on plate material subjected to multiaxial strain were conducted, and the strength reliability of the pressed product was evaluated. Tensile strength of mild-steel plate subjected to uniaxial or multiaxial plastic strain was increased. If a tensile test is carried out again after a certain period of time has elapsed by applying strain, not only yield stress but also tensile strength are increased. This trend is due to strain aging inside the material. Degree of work hardening is higher when multiaxial strain is applied compared to when uniaxial strain is applied. For materials work hardened by applying strain, their hardness and the tensile strength are proportional regardless of the load status of strain. Accordingly, tensile strength of a product subjected to different strains can be evaluated uniformly by measuring the product’s hardness. When strain is applied from multiple axes in press working, as well as tensile strength, fatigue strength is improved by work hardening. According to the results the microscopic analysis by X-ray diffraction combined with the full-width-at-half-maximum method, dislocation density is increased when multiaxial strain is applied compared to when single-axis strain is applied, and the degree of work hardening is greater. The full width at half maximum is correlated with tensile strength regardless of the state of strain applied to the material. TEM observation of dislocations revealed that the third-step-pressed product subjected to multiaxial strain has a coaxial, fine-grain dislocation-cell structure. On the other hand, in the case of pre-strained (plastic strain of 20%) material subjected to a uniaxial tensile load, the cell structure was unclear, and even the cells that were produced had coarse grains stretched in the tensile direction.
  • Rice Hull Charcoal for Adsorption of Cesium and Strontium in Aqueous Solution

    pp. 458-463

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    DOI:10.2320/matertrans.Z-M2018855

    The adsorption capacity of rice hull charcoal for cesium and strontium in aqueous solution was examined. Rice husk carbonized at various temperatures was added to aqueous solutions of cesium and strontium and stirred, whereupon, samples carbonized at 400°C adsorbed cesium, and those carbonized at 800°C adsorbed strontium. In view of this result, it was inferred that the mechanisms of adsorption of cesium and strontium on rice hull charcoal are different. FTIR analysis and Boehm titration results indicated that charcoal carbonized at 400°C had acidic functional groups. In addition, the pH of the rice hull charcoal carbonized at high temperature tended to be higher, which is attributed to precipitation of inorganic substances such as potassium on the surface of rice hull charcoal. Pore distribution measurements showed that many mesopores were formed in rice hull charcoal carbonized at 800°C. It was concluded that cesium adsorbs primarily on acidic functional groups on the surface of rice hull charcoal and strontium undergoes ion exchange with potassium, or adsorbs on mesopores. This Paper was Originally Published in Japanese in J. Soc. Mater. Sci., Japan 67 (2018) 898–903.
  • Effect of Heating Rate on Microstructure, Texture and Tensile Properties of AA7085

    pp. 464-470

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    DOI:10.2320/matertrans.M2018337

    The microstructure, texture and tensile properties of a deformed AA7085 alloy during solution treatment were investigated in the temperature range of 350 to 500°C with heating rates of 1 and 600°C/min. The results show that the alloy sheet solution treated with rapid heating rate exhibits a finer grain structure and narrower grain size distribution compared to the alloy sheet solution treated with slow heating rate, indicating that driving force is more important than sub-boundaries coalescence and mobility during rapid heating process. Heating rate affects the qualitative recrystallization textures, whereas it has no significant changes on the quantitative texture of the individual components. After two-stage ageing, the samples with low heating have slightly higher tensile properties than those with high heating rate.
  • Preparation for WC–Ni3Al Hard Metal by Vacuum Sintering

    pp. 471-475

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    DOI:10.2320/matertrans.Y-M2018856

    WC–Ni3Al hard metal of the composition for WC–25 vol% (Ni–24 at%Al–0.5 at%B) was prepared by vacuum sintering. The powder mixtures of WC, Ni, NiAl (Ni–50 at%Al) and B were treated by wet milling with a tumbling ball mill machine for 24 h or 48 h. The average particle size of the powder mixtures after wet-milling for 24 h and 48 h were 1.69 µm and 1.23 µm, respectively. These powder mixtures were press-moulded into the test-pieces and sintered in vacuum of (7∼9) × 10−3 Pa at various temperature for 1 h. The densities, microstructures and mechanical properties of the obtained sintered compacts were investigated. When 48 h wet-milled powder was used and the green compact was sintered at 1425°C for 1 h, the compact had relative density more than 99%, transverse rapture strength of 2.2 GPa and Vickers hardness of 1330 HV. On the other hand, using 24 h wet-milled powder, the sintered compact had lower mechanical properties than the compact of 48 h wet-milled powder because of low density and inhomogeneous microstructure. This Paper was Originally Published in Japanese in J. Jpn. Soc. Powder Powder Metallurgy 64 (2017) 615–620.
  • Directional Magnetic Modification of Iron Rhodium Compound by Ion Irradiation and Annealing

    pp. 476-478

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    DOI:10.2320/matertrans.M2018262

    Depth-directional magnetic modification of FeRh samples by energetic ion beam irradiation and post-annealing and its quantitative analysis are discussed. Iron-rhodium foil samples were irradiated with a 2-MeV He ion beam at various ion fluences. Post-thermal annealing at 373 K–673 K was performed for the irradiated samples. The depth-directional magnetic profiles were quantitatively estimated through the TRIM ion transport simulation, as well as previously known relations between irradiation-induced ferromagnetic moment and elastic deposition energy by ion beam irradiation. The magnetic profiles were considered to be successfully estimated because the values of the total magnetic moment measured are very consistent with the results calculated by TRIM. The effect of thermal annealing and possibility of three-dimensional magnetic modification will be also discussed.
  • Role of Surface Iron Oxides in Coercivity Deterioration of Sm2Fe17N3 Magnet Associated with Low Temperature Sintering

    pp. 479-483

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    DOI:10.2320/matertrans.M2018358

    Sm2Fe17N3 powders were analyzed by X-ray photoelectron spectroscopy to elucidate the role of surface Fe oxides in the significant deterioration of coercivity observed when Sm2Fe17N3 is sintered at relatively low temperatures. It was demonstrated that the Fe oxides on the powder surface completely disappear after heat treatment at 500°C in Ar. This suggests that the chemical reaction which causes the coercivity deterioration involves consumption of the Fe oxides. A reaction model that postulates a redox reaction between the Fe oxides and Sm2Fe17N3 to produce soft-magnetic α-Fe consistently explains both the coercivity drop and the consumption of the Fe oxides. The conventional model of simple Sm2Fe17N3 decomposition, on the other hand, cannot rationally explain the disappearance of the Fe oxides. It is therefore reasonable to consider the redox reaction to be the primary mechanism of coercivity deterioration, in which the Fe oxides play a role as one of the major reactants.

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