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

MATERIALS TRANSACTIONS Vol. 62 (2021), No. 8

  • Segregation of Carbon in α-Fe Symmetrical Tilt Grain Boundaries Studied by First-Principles Based Interatomic Potential

    pp. 1057-1063

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    DOI:10.2320/matertrans.MT-M2021054

    Carbon segregation is known to have an extreme influence on the cohesive energies and mechanical properties of grain boundaries (GBs) in steel. In this paper, the stability of a series of α-Fe〈110〉 symmetrical tilt grain boundaries (STGBs) with C was systematically investigated based on first-principles calculations. We used the newly-developed Tersoff/ZBL potential of Fe–C interaction constructed from the forces and disolution energies of various defect complexes with C in Fe calculated from first-principles. This potential shows a great effectiveness in considering large complex systems of STGB and determining the segregation sites of various STGB configurations. The stable location of C was analyzed from the view point of free volume formed by the GB systems. We found that the compact GBs were less attractive to C than the open ones. The GBs exhibited a strong attractive interaction with C compared to vacancies, therefore, a higher solubility of C can be expected in GB systems. The present simulation results are consistent with the experimental observations by TEM and APT method, and qualitatively explains the behaviour of C in Fe.
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  • Interfacial Microstructure and Mechanical Properties of Fluxless Arc Brazing of Aluminum Alloy to Steel with the Aid of Milling

    pp. 1064-1070

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    DOI:10.2320/matertrans.MT-M2020307

    Fluxless arc brazing technique using Zn15%Al filler wire with the aid of an additional milling procedure was developed to join aluminum alloy to low-carbon steel. It was found that the addition of a milling procedure during arc brazing could significantly change morphology and distribution of interfacial reaction products of the joint. By adjusting the radial cutting depth (ae), the original continuous layer-wised Fe–Al intermetallic compounds (IMC) was broken into discrete IMC layer. The IMC layer was identified to be θ-FeAl3 with thickness ranging from 2.5 µm to 14.4 µm under optimized ae of 0.10 mm. Upon shear loading, the maximum average interfacial shear strength reaches 182 MPa and the crack propagated through both the steel/braze interface and remaining Zn–Al filler alloy, which is highly favorable in crack deflection and in turn, improving bonding strength. It is thus concluded that milling assisted arc brazing is of great potential to achieve strong bonding between dissimilar Al alloy and steel.
  • Effect of Vanadium Contents on Microstructure and Mechanical Properties of Ti–6Al–xV Components Produced by Wire + Arc Additive Manufacturing

    pp. 1071-1078

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    DOI:10.2320/matertrans.MT-M2021031

    Wire + Arc Additive Manufacturing (WAAM) is an advanced manufacturing technology by inexpensive Gas Tungsten Arc Welding (GTAW) technology. Key microstructural features of the as-built WAAM alloy include large columnar β grains, grain boundary α colonies, and Heat Affected Zone (HAZ) banding, which generally leads to low ductility and anisotropy. In this study, Ti–6Al–xV (x = 0, 2, 4) alloys were prepared by WAAM, the effects of vanadium content on the microstructure, tensile properties and impact toughness were investigated. Irregular-shaped, plate-like features without columnar grains and HAZ banding were obtained in Ti–6Al alloy. Columnar grains were observed in Ti–6Al–2V alloy, and the grain size was further enlarged to more than ten millimeters by 4 mass% vanadium addition. With the increasing of vanadium content, a monotonic increase in yield strength and ultimate tensile strength can be observed, while the fracture strain and impact toughness changed in the opposite trend. Ti–6Al and Ti–6Al–2V alloy exhibited better matching of strength, ductility and impact toughness compared with Ti–6Al–4V alloy.
  • Ex-Situ Electron Microscopy Study of Solid Electrolyte Interphase Formed by Charge-Discharge Reaction of Silicon Negative Electrode in Lithium-Ion Secondary Battery

    pp. 1079-1088

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    DOI:10.2320/matertrans.MT-M2021070

    A SEI (Solid Electrolyte Interphase) is formed on the surface layer of the negative electrode active material of a lithium ion secondary battery (LIB) during the initial charging process, and its morphology and structure significantly affect performance and safety. In this study, by conducting ex situ experiments, SEM, TEM and STEM-EELS observations were performed on Si negative electrodes under charge state within an actual battery and Si negative electrodes directly charged on a TEM thin film, revealed morphology and structure of the SEI. All of the processes from specimen preparation for electron microscopy observation to specimen transport were performed under non-atmospheric exposure conditions.The SEI on the surface of the Si negative electrode grew thicker as the charge depth increased. On the other hands, LixSi amorphous phase due to the lithiation by solid-state reaction was confirmed inside the Si negative electrode. It was found that Li2O was formed on the most surface of the Si negative electrode at the initial stage of charging, and the SEI was mainly composed of Li2O. The SEI of about 1 µm was observed on the Si negative electrode after 40% charge, and the thickness of the SEI decreased to less than 1/5 after discharge. This Paper was Originally Published in Japanese in Japan Inst. Met. Mater. 84 (2020) 382–390.
  • NO + CO Reaction on Polycrystalline Palladium Foils with Specific Surface Crystallographic Orientation: A New Approach to Develop Foil Catalysts Based on Texture Control

    pp. 1089-1096

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    DOI:10.2320/matertrans.MT-M2021050

    Catalytic properties of the metal foils were tuned by the specific surface crystallographic orientation control. The NO + CO reactivity of polycrystalline Pd foils with (101) or (211) orientation, characterized by using electron backscattered diffraction (EBSD), was investigated. The (211)-oriented Pd foils exhibited much greater activity compared with (101) orientated Pd foils, which is in good agreement with the previous structure sensitivity studies on NO + CO reactions over Pd. Hence, the present study introduces the surface crystallographic orientation control of foil catalysts as a new development strategy for the unique catalysts in the material gap.
  • Evaluation of Non-Basal Slip Activity in Rolled Mg–Li Alloys

    pp. 1097-1101

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    DOI:10.2320/matertrans.L-M2021826

    Tensile tests of rolled Mg–6.2 mol%Li and Mg–11.7 mol%Li alloys were carried out at room temperature to clarify effects of lithium addition on the relationship between mechanical properties and activities of slip systems. Ductility increased with increasing lithium content. 0.2% proof stress increased when 6.2 mol%Li was added. However, Mg–11.7 mol%Li showed low 0.2% proof stress, compared to pure magnesium. On the other hand, maximum stress decreased with increasing lithium content. Frequency of non-basal slips increased with increasing lithium content. Also, first order pyramidal 〈c + a〉 slip showed the highest frequency among non-basal slips in Mg–Li alloys. Critical resolved shear stresses for non-basal slips, which were reduced by lithium addition, increased ductility but decreased tensile strength of magnesium. This Paper was Originally Published in Japanese in J. JILM 70 (2020) 117–121.
  • Interfacial Properties of Bonded Dissimilar Materials Fabricated via Spark Plasma Sintering

    pp. 1102-1108

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    DOI:10.2320/matertrans.MT-Z2021001

    This paper describes the characterization of the interfacial properties of bonded dissimilar materials for the fabrication of biocompatible composites and functionally graded materials (FGMs) with high mechanical performance. In the development of biomaterials with conflicting properties such as high strength and toughness for artificial bone, much attention has been paid to composites and FGMs consisting of biocompatible ceramics and metals. Their mechanical properties are influenced by the properties of the interfaces of dissimilar materials, and hence it is important to evaluate the interfacial properties. This study investigated the influence of combinations of materials on the interfacial strength and fracture toughness of bonded dissimilar materials consisting of four types of biocompatible materials: titanium, type 316L stainless steel, partially stabilized zirconia, and alumina. The bonded dissimilar materials were fabricated via spark plasma sintering, which is a powder metallurgy technique utilizing uniaxial load and pulsed direct current in vacuum. The interfacial strength and toughness of the materials were evaluated via compression-bend testing and indentation testing, respectively. The distributions of elements near the interfaces due to atomic diffusion during sintering were evaluated, and the influence of material combinations on the interfacial properties was considered based on the distributions. It was found that the mechanical properties of all interfaces were lower than those of monolithic materials, and the extent of the degradation in mechanical properties was dependent on the material combinations. If atomic diffusion occurred on both sides of the interface, the interfacial fracture toughness and strength tended to be relatively high. This Paper was Originally Published in Japanese in J. Soc. Mater. Sci., Japan 69 (2020) 855–862.
  • Dislocation Cell Structures Formed inside Dislocation Channels of Rapid-Cooled and Tensile-Deformed Aluminum Single Crystals

    pp. 1109-1117

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    DOI:10.2320/matertrans.MT-M2021021

    Dislocation cell structures inside the cleared dislocation channels in rapid-cooled and tensile-deformed aluminum single crystals were investigated by using transmission electron microscope (TEM). Inside the dislocation channels, dislocation network structures lying on multiple planes were forming the cell structures. One of the planes on which the networks were lying was the primary slip plane, i.e., (1 1 1) plane. Since the Burgers vectors of the dislocations composing the networks were identified as parallel to [1 0 1], [0 1 1] and [1 1 0], the networks were creating the crystallographic rotation of which twisting axis was parallel to the normal direction of the cross slip plane, i.e., (1 1 1) plane. Through the quasi-continuous 3D moving images, these cell structures consisting of the network structures on multiple planes were recognized as “cocoon-like” closure shaped and developed along the primary dislocation channels. Since these structures were consisting of the primary dislocations and the secondary dislocations which were considered to be activated due to the pile-ups of the primary dislocations and distributing independently, origin of the formation of the structures were not “incidental” nor “geometrically necessary” but rather “energetically necessary” driven.
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    1. Numerical Simulation and Fatigue Properties of Laser Spot Weld-Bonding DP590 Dual-Phase Steel Joints MATERIALS TRANSACTIONS Vol.62(2021), No.8
  • Numerical Simulation and Fatigue Properties of Laser Spot Weld-Bonding DP590 Dual-Phase Steel Joints

    pp. 1118-1123

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    DOI:10.2320/matertrans.MT-M2021029

    In this paper, cold-rolled DP590 dual-phase steel sheets with 1.5 mm thickness were weld-bonded by a fiber laser. In order to study the tensile shear strength of laser spot-welding and laser weld-bonding joints, the orthogonal test was designed with three parameters: welding power, welding time and off-focus value. The tensile shear test and the fatigue test were carried out. The vaporization of the adhesive and its disruption to the molten pool during the weld-bonding process were simulated. The results show that the tensile and fatigue strength of the laser weld-bonding joints are higher than the laser spot-welding joints. Simulation results demonstrate that the perforation is caused by the gas produced by the gasified adhesive being discharged from the center of the molten pool under high pressure.
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    1. Nugget shifting in resistance spot welding of multi-stackup sheets QUARTERLY JOURNAL OF THE JAPAN WELDING SOCIETY Vol.29(2011), No.3
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  • Prediction of Work-Hardening Behavior under Various Loading Paths in 5083-O Aluminum Alloy Sheet Using Crystal Plasticity Models

    pp. 1124-1132

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    DOI:10.2320/matertrans.MT-M2021020

    In the present study, crystal plasticity models applicable for reproducing the work-hardening behavior under various loading paths of an A5083-O Al alloy sheet were examined. The loading paths under consideration were tension, reverse loading from compression to tension, simple shear, and biaxial tension. The accumulated-slip-based hardening model with the extended Voce hardening law provided good predictive accuracy for work-hardening behavior under uniaxial loadings. In contrast, it could not reproduce anisotropic hardening under biaxial tension, which was observed in the experimental results. In the case of dislocation-density-based hardening models, the work-hardening behavior under both uniaxial loadings and biaxial tension was reproduced fairly well when an anisotropic property in the interaction matrix was considered. On the basis of these results, a new accumulated-slip-based hardening model was proposed, in which the effects of latent hardening were considered in calculation of accumulated slip. The new model allowed prediction of anisotropic hardening under biaxial tension as in the case of the dislocation-density-based hardening model. This result suggested that the predictive accuracy of anisotropic hardening was more affected by the modelling of the latent hardening and interaction matrix than by the choice of internal variables of hardening, i.e., accumulated slip or dislocation density.
  • Effect of Hydrogen on the Notch Tensile Strength of JIS SNCM439 Steel

    pp. 1133-1140

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    DOI:10.2320/matertrans.MT-M2021046

    The notch tensile tests of JIS SNCM439 steel were conducted using various notch root radii at room temperature in a 20 MPa high-pressure hydrogen gas atmosphere to investigate the hydrogen embrittlement of the steel specimen at the notch root. The maximum load in hydrogen gas decreased with the increasing stress concentration factor (Kt) as the notch root radius decreased, and the value was reduced compared to that in air at any Kt, beyond which the load abruptly decreased and the specimen broke. Moreover, the local strain value of the notch root at the maximum load point increased with an increase in Kt in air, but that value remained almost constant in hydrogen gas regardless of Kt. Furthermore, the fracture strain observed during the notch tensile test in high-pressure hydrogen gas exceeded the local deformation during the smooth tensile test. In other words, cracks initiation in hydrogen gas were significantly affected by strain value, cracks were generated when voids began to form in the material, and lead to fracture. The specimens in hydrogen gas had quasi-cleavage fracture surfaces with no cracks directly below the notch root. Cracks developed in these fracture surfaces near the center of the specimen, and the propagation of these cracks resulted in the formation of ultimate fracture surfaces with dimples. The quasi-cleavage fracture surface directly under the notch root was considered as the area that yielded before cracking, and the fracture surface accompanied by cracks was considered as the area where the cracks had grown under the effect of high-pressure hydrogen gas. The fracture surface with cracks depended on the load at the time of crack growth, and the percentage of intergranular surfaces increased as the load decreased.
  • Reduction of Volatile Impurities in Zinc Chloride Melt with Metallic Iron and Its Effect on Vacuum Separation Behavior

    pp. 1141-1150

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    DOI:10.2320/matertrans.M-M2021830

    Metallic Zn production currently generates toxic gases and residues that cannot be recycled, and consumes a large amount of energy. To develop more efficient process using chlorination reaction of Zn resources, purification methods for crude ZnCl2 melt have been developed with limited success. In this study, we have investigated a new type of pyrometallurgical purification that combines reduction reactions of FeCl3 and CuCl2 in ZnCl2 melt using metallic Fe with vacuum distillation. Metallic Fe reduced FeCl3 and CuCl2 into FeCl2 and CuCl/Cu, respectively, which show lower vapor pressures than ZnCl2. Vacuum distillation of the crude ZnCl2 melt after the Fe reduction successfully produced a high-purity ZnCl2 deposit with less contamination by Fe and Cu. This study reveals that reduction treatment with metallic Fe converts FeCl3 and CuCl2 into less volatile substances and suppresses contamination by such substances into ZnCl2 recovery.
  • Microstructure and Joint Strength of Magnetic Pulse Welded Aluminum/Aluminum-Coated Steel Joint

    pp. 1151-1159

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    DOI:10.2320/matertrans.L-M2021829

    Welding of aluminum and aluminum-coated steel plates was performed using magnetic pulse welding. A1050 pure aluminum and Al–Si-coated steel plates were used in this study. The aluminum and the aluminum-coated steel plates were used for a flyer plate and a parent plate, respectively. The welding interface was observed with an optical microscope, scanning electron microscope and scanning transmission electron microscope. Tensile-shear tests were conducted for evaluation of the joint strength. After the welding, thickness of the aluminum coating slightly decreased at the welded area and Si particles containing in the aluminum coating were refined. The welding interface exhibited characteristic wavy morphology. A banded structure consisting of fine aluminum grains with diameters of approximately 500 nm was formed along the wavy interface. When the welding was performed at longer plate gap condition, void formation was observed in the banded structure and fracture occurred at the welding interface by tensile-shear test. The void formation is considered to lead to a decrease in joint strength.
  • Improving the Pitting Corrosion Resistance of AA1050 Aluminum by Removing Intermetallic Particles during Conversion Treatments

    pp. 1160-1167

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    DOI:10.2320/matertrans.MT-M2021071

    Measurements of the open circuit potential of AA1050 were performed in 10 mM CrO3, and the Al-matrix around the intermetallic particles was found to dissolve locally. Pre-immersion in 1 M NaOH promoted the dissolution of the Al-matrix around intermetallic particles, resulting in the improvement of pitting corrosion resistance in NaCl solutions. The pitting corrosion resistance provided by molybdate treatment is effectively improved by pre-immersion in NaOH. The maximum pit depth of the specimen treated in MoO3–HNO3 solution after pre-immersion in 1 M NaOH was small compared to the chromate-treated specimen. It was determined that immersion in an acidic solution containing an inhibitor after pre-immersion in NaOH is an effective method to improve the pitting corrosion resistance of aluminum alloys.
  • Development of Strip Lateral Movement Simulator for Temper Rolling

    pp. 1168-1176

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    DOI:10.2320/matertrans.P-M2021832

    The temper rolling process of hot-rolled strips is the final rolling step to improve the flatness of the strips and shape slippage of the coiled strips. Poor flatness of a hot-rolled strip causes lateral movement of the strip during the temper rolling process. Manual leveling operations to control this movement result in a much lower line speed and productivity. Moreover, the quality of manual leveling is not consistent, as it depends on the operator’s experience. Lack of understanding of the lateral movement phenomenon has sustained manual operation and discouraged the development of an automatic control system. To solve this problem, the authors propose both a new lateral movement model and a theoretical method. A lateral movement model for temper rolling and a large-deflection strip model are important components in the new model. In the method, lateral movement stability is equivalent to an eigenvalue problem with lateral movement static equations. Their usefulness is confirmed by comparing the results of experimental rolling in the laboratory with those of numerical calculations. The simulation results obtained using the proposed models confirm that actual problems can be solved more exactly than with the conventional linear model. Thus, simulation using the proposed models can support the investigation of lateral movement problems and the development of an automatic control system. This Paper was Originally Published in Japanese in J. Jpn. Soc. Technol. Plast. 61 (2020) 107–114. The captions of figures were slightly modified.
  • Amorphization at the Welded Boundary between 5052 Aluminum Alloy and Zirconium by Friction Stir Diffusion Bonding

    pp. 1177-1183

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    DOI:10.2320/matertrans.L-M2021827

    Microstructure at the boundary between 5052Al and zirconium foils subjected to friction stir diffusion bonding (FSDB) was examined by transmission electron microscopy (TEM). 5052Al and zirconium foils were welded by travelling of a rotating tool with microindention only into the 5052Al foil. The welding strength of the specimens was higher than the fracture strength of the 5052Al foil. An amorphous layer with a thickness of 2 to 100 nm was found at the welded boundary by edge-on TEM observation. An amorphous phase at a welded boundary has been also reported for other dissimilar metal welding, e.g. aluminum and iron alloys, which exhibits high welding strength. Microstructural evolution at the welded boundary is discussed for dissimilar welding of 5052Al and zirconium. This Paper was Originally Published in Japanese in J. JILM 70 (2020) 523–529.
  • Experimental and Numerical Analyses of Wavy Interface Formation and Local Melting Phenomena at the Magnetic Pulse Welded Al/Fe Joint Interface

    pp. 1184-1193

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    DOI:10.2320/matertrans.L-M2021828

    A 1050-O aluminum plate and a low carbon steel (SPCC) plate were seam welded by magnetic pulse welding (MPW). Experimental and numerical analysis methods were combined to investigate the wavy interface formation and local melting phenomena at the Al/Fe joint interface. The metallographic observation results showed that Al/Fe joint interface presents a trigger-like wave shape with two types of intermediate layers intermittently formed at the interface. Collision angle between two plates and impact velocity of Al sheet driven by electromagnetic force was numerically solved by coupled mechanical and electromagnetic fields analysis. The metal jet emission behavior, wavy interface formation process and the temperature change during MPW collision process was analyzed by SPH method. The numerical results showed that the temperature near the interface exceeds the melting point of both Al and Fe at extremely high pressure, which leads to the formation of a local melting zone (LMZ) where Al and Fe were mixed in melted state. The numerically reproduced wavy interface morphology showed a good consistency with the experimentally observed results.
  • Influence of Free Nitrogen on Tendency of Chill Formation in Spheroidal Graphite Iron Castings

    pp. 1194-1202

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    DOI:10.2320/matertrans.F-M2021831

    In this study, the most influential factor for causing chill formation in spheroidal graphite iron castings was surveyed. A 30 ton electric arc furnace with magnesia linings using in commercial operation was used for melting. Chill depth of wedge samples was tested through the process from melting to pouring, and the tendency was compared with the changes of temperature, chemical compositions, and gas elements at the same stages. As the results, it was cleared that chill depth was mostly influenced with the change of free nitrogen (NF). Chill depth was deeper when NF was higher value. This indicates that chill structure will be able to be avoided by denitrification. It was considered that NF might replace carbon (C) atom in ledeburite cementite (Fe3C) crystal structure and might promote chill formation. The denitrification would be able to promote full graphitization and might prevent chilling. On the other hand, it looked an important to minimize nitrification after denitrification.
  • Swelling-Pressure and Hydraulic Conductivity of Compacted Clays Focusing on the Clay-Mineral Type

    pp. 1203-1209

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    DOI:10.2320/matertrans.Z-M2021833

    It is very important to notice clay minerals, for resource development, stability evaluation of underground utilization, and others. However, few quantitative data on the effects of clay-mineral type on the swelling characteristics and permeability of clays and clay minerals have been reported to date. This study was conducted to investigate the effects of clay-mineral type on the swelling characteristics and permeability of compacted clays using one-dimensional swelling-pressure and constant-pressure permeability tests. Comparative tests revealed that the difference of clay-mineral type in the clays influences the swelling-pressure and hydraulic conductivity. The swelling-pressure and hydraulic conductivity were closely associated with the specific surface area of clays. Furthermore, hydraulic conductivity was almost consistent that measured with the Kozeny-Carman equation. This result suggests that hydraulic conductivity can be estimated based on a specific surface area and void ratio of compacted clays. This Paper was Originally Published in Japanese in J. Soc. Mater. Sci., Japan 70 (2021) 272–278. The caption of Fig. 2, and Figs. 2 and 4 are slightly changed.
  • Effect of Si Concentration of a Brazing Precursor on the Bonding Strength of Aluminum Foam Bonded via Foaming Bonding

    pp. 1210-1215

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    DOI:10.2320/matertrans.MT-M2020393

    Pure Al foams were bonded via foaming bonding using Al–10.5Si brazing alloy. However, the pores around the bonded area were coarsened because Si diffused into the Al foam from the brazing foam, where it reduced the melting temperature, leading to melting of the cell walls of the Al foam. The bonding strength of the bonded foam varied greatly. An Al–Si brazing precursor with a low Si concentration was prepared and used for foaming bonding of two Al foam specimens. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy were used to investigate the bonding state. Four-point bending tests were conducted to evaluate the bonding strength. The brazing foam was metallurgically bonded with the Al foam when brazing precursors with 9.4–10.5 mass% Si were used. The bonding interfaces were partially observed by SEM. The bonding strength of the Al foam bonded using 10.5 mass% Si brazing foam was lower than the minimum strength of the Al foam. The bonding strength of the specimen bonded using the 9.4 mass% Si brazing foam was lower than the minimum strength of pure Al foam and the bonding strength of the other bonded specimens. An Al–Si brazing precursor with an appropriate Si concentration prevents pore coarsening around the bonding interface. The bonding strength became low when the Si concentration in the Al–Si brazing precursor was excessively low.
  • Magnetic Properties and Magnetic Domain Observation of Nd–Fe–B Sintered Magnets Treated by Grain Boundary Diffusion Process with Dy–Al Co-Sorption

    pp. 1216-1224

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    DOI:10.2320/matertrans.MT-M2020389

    Effective grain boundary diffusion (GBD) process with Dy–Al co-sorption is applied to enhance the coercivity of Nd–Fe–B sintered magnets. The coercivity of the magnet (HcJ = 1789 kA·m−1) subjected to the present GBD treatment was observed to be superior to that of the untreated magnet (HcJ = 1003 kA·m−1) and the conventional GBD magnet (HcJ = 1661 kA·m−1) treated with DyAl alloy. In the present GBD magnet, the Dy–Al co-sorption process facilitated Dy diffusion into the center region of the magnet (thickness: 3.5 mm), resulting in high coercivity. Further, magnetic domain observations were made using magnetic force microscopy (MFM) to observe the thermal demagnetization behavior of the present GBD magnet. The present GBD magnet suppressed the continuous domain reversal of adjacent grains; thus, the partially persistent single-domain structure remained, even at 453 K.
  • Reduction of Spatter Generation Using Atmospheric Gas in Laser Powder Bed Fusion of Ti–6Al–4V

    pp. 1225-1230

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    DOI:10.2320/matertrans.MT-M2021059

    Laser powder bed fusion (LPBF), a typical additive manufacturing (AM) process, is a promising approach that enables high-accuracy manufacturing of arbitrary structures; therefore, it has been utilized in the aerospace and medical fields. However, several unexplained phenomena significantly affect the quality of fabricated components. In particular, it has been reported that the generation of spatters adversely affects the stability of fabrication process and degrades the performance of the fabricated components. To realize high-quality components, it is essential to suppress the generation of spatters. Thus far, the suppression of spatter generation has been attempted based on the process parameters; however, this has not been adequately discussed in terms of the fabrication atmosphere. Therefore, in this study, we focused on the fabrication atmosphere and investigated spatter generation using gas with different physical properties rather than conventionally used argon. It was observed that the spatter generation during the fabrication of the Ti–6Al–4V alloy could be significantly suppressed by changing the atmospheric gas, even under constant LPBF process parameters. We proved that the fabrication atmosphere is an important factor to be considered, apart from the process parameters, in AM technology.
  • Producing CrFeCoNiSi-Based High Entropy Alloy by Spark Plasma Sintering

    pp. 1231-1238

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    DOI:10.2320/matertrans.MT-M2020387

    A high-entropy alloy (HEA) is a multi-component alloy obtained by blending at least five metal elements at compositions of 5–35%. Contrary to expectations, HEAs have a simple microstructure and exhibit unique properties such as excellent high-temperature strength, high tensile strength, and extremely slow diffusion rate. They are mainly produced by casting method, and heterogeneous microstructures have been found in the as-cast structure. Powder metallurgy has many advantages over ingot-metallurgical methods such as casting, including excellent material efficiency and ease of conversion to complex shapes. In this study, we produced a CrFeCoNiSi HEA and evaluated its properties. The Si content was changed within the range of 5–15 at% to investigate the characteristics of the corresponding HEAs. A Vickers hardness test and a corrosion test were performed. The Vickers hardness test revealed that the hardness of the samples of (CrFeCoNi)95Si5 and (CrFeCoNi)90Si10 was approximately 550 HV, whereas that of (CrFeCoNi)85Si15 was approximately 700 HV. (CrFeCoNi)95Si5 and (CrFeCoNi)90Si10 showed significant improvement in corrosion resistance. In constant, deterioration was observed for the (CrFeCoNi)85Si15 sample due to the presence of excess Si, which formed a stable silicide with metal elements.
  • Effects of Thermal Aging on the Mechanical Properties of FeCrAl-ODS Alloy Claddings

    pp. 1239-1246

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    DOI:10.2320/matertrans.MT-M2021057

    The FeCrAl-ODS alloy claddings were manufactured and Vickers hardness, ring tensile tests and transmission electron microscopy (TEM) observations of these claddings were performed to investigate the effects of thermal aging at 450°C for 5,000 and 15,000 h. The age-hardening of all FeCrAl-ODS alloy cladding was found. In addition, the significant increase in tensile strength was accompanied by much larger loss of ductility. It was suggested that this age-hardening behavior was attributed to the (Ti, Al)-enriched phase (β′ phase) and the α′ phase precipitates (content of Al is <7 mass%). In comparison with FeCrAl-ODS alloys with almost same chemical compositions, there was significant age-hardening in both alloys. However, the extrusion bar with no-recrystallized structures was keeping good ductility. It was suggested that this different behavior of reduction ductility was attributed to the effects of grain boundaries, dislocation densities and specimen preparation direction.
  • Change in Electrical Resistivity in Nickel Processed by Accumulative Roll Bonding

    pp. 1247-1252

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    DOI:10.2320/matertrans.MT-M2021063

    Pure nickel was processed by accumulative roll bonding (ARB), and the change in electrical resistivity measured at 77 K was about 2.2 nΩm after 8 ARB cycles. The change in electrical resistivity was estimated based on the microstructural parameters, such as, dislocation density of about 3 × 1014 m−2 and density of grain boundaries of about 8 Mm−1 after 8 ARB cycles. Those values were evaluated using X-ray diffraction and electron backscattering diffraction in a field emission-scanning electron microscope, respectively. The change in the electrical resistivity was associated with the above-mentioned microstructural parameters.
  • Recycling of Steelmaking Slag in Seawater as an Iron Supplier: Effects of Slag Composition, Carbonation and Usage of Gluconic Acid

    pp. 1253-1262

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    DOI:10.2320/matertrans.MT-M2020346

    Disappearing of benthic seaweeds in coastal areas has gradually become a serious environmental problem worldwide. One possible reason is iron deficiency in seawater. As a major byproduct generated in steelmaking industry, steelmaking slag is rich in Fe and has the potential to restore seaweeds by supplying soluble Fe in seawater. The authors aim to develop a sustainable approach for ecosystem restoration in coastal areas by utilizing steelmaking slag. This work investigates the releasing behavior of elements from synthesized steelmaking slags in seawater and clarifies the effects of slag composition, carbonation and usage of gluconic acid. Slag with larger CaO/SiO2 ratio (= 3.0) has difficulty in releasing Fe into seawater directly, due to drastic increase of pH. Combination of slag carbonation and gluconic acid usage is necessary and effective in boosting release of Fe. Slag with smaller CaO/SiO2 ratio (= 1.0) would not cause drastic increase of pH in seawater and thus releasing of Fe is easier. These results have proved the potential value and feasibility of using steelmaking slag as an underwater iron fertilizer.
  • Evaluation of Hydrogenated Amorphous Silicon Oxide Photo-Absorber from Quantum Efficiency in Thin-Film Solar Cell by Fourier Transform Photocurrent Spectroscopy

    pp. 1263-1269

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    DOI:10.2320/matertrans.MT-M2020379

    We demonstrated evaluation of sub-gap state and gap-state defect in hydrogenated amorphous silicon oxide (a-SiOX:H) photo-absorber within solar cell structure from internal quantum efficiency (IQE) measured by Fourier transform photocurrent spectroscopy (FTPS). In IQE spectra for a-SiOX:H thin-film solar cells, exponential tail and IQE corresponding to gap-state defect was observed. We also investigated light-induced degradation in a-SiOX:H photo-absorber within solar cell structure by FTPS. IQE related to gap-state defect increased and conversion efficiency decreased by light irradiation, which corresponds to light-induced degradation. Urbach energy obtained from IQE spectra increased by light irradiation.
  • Recent Development of Joining and Conductive Materials for Electronic Components

    pp. 1270-1276

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    DOI:10.2320/matertrans.MT-M2021060

    This study introduces research trends in the electronics materials, such as joining materials including high-temperature lead-free solder, sintered materials using metal particles and transient liquid phase (TLP) materials, and conductive materials such as aluminum and copper alloys. These studies include the content of the special issue published in Materials Transactions (Vol. 57, No. 6 and Vol. 60, No. 6) entitled Frontier Researches Related to Interconnection, Packaging and Microjoining Materials and Microprocessing for Such Materials. It also introduces interesting research contents, leading the development of next-generation electronics.
  • Erratum

    pp. 1277-1277

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    DOI:10.2320/matertrans.ERM202102

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