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

MATERIALS TRANSACTIONS Vol. 58 (2017), No. 10

  • Estimating the Spatial Distribution of Ca Dopants in α-SiAlON by Statistical Analysis of HAADF-STEM Image

    pp. 1341-1345

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    DOI:10.2320/matertrans.MAW201705

    We analyzed the spatial distribution of Ca dopants in α-SiAlON by using a statistical approach that analyzed a high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) image. By evaluating the image intensity of each atomic column, we found that the intensity deviation of columns with Ca atoms was twice that of other atomic columns. The results of HAADF-STEM image simulation suggested that there was a long-range repulsive interaction along the [0001] direction between pairs of Ca atoms in the same Ca column. Moreover, correlation of image intensities in neighboring Ca columns suggested the existence of a short-range repulsive force between first-nearest-neighbor Ca atoms. The origin of such anisotropic interactions between pairs of Ca atoms will be discussed.
  • Effects of Natural Aging on Age-Hardening Behavior of Cu-Be-Co and Cu-Ti Alloys Severely Deformed by High-Pressure Torsion

    pp. 1346-1350

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    DOI:10.2320/matertrans.M2017178

    Solution-treated Cu-1.8 mass%Be-0.2 mass%Co and Cu-3 mass%Ti alloys were subjected to severe plastic deformation using high-pressure torsion (HPT) process to examine the change in densities of grain boundary, dislocation and vacancy during natural aging of the two alloys at 293 K after HPT-straining, and the influence of the natural aging on the age-hardening behavior of the alloys on subsequent artificial aging. Application of HPT processing under an applied pressure of 5 GPa for 10 revolutions at 1 rpm to each alloy produced an ultrafine-grained structure. Aging the HPT-processed alloys at 293 K for the longest period of 2.59 Ms (1 month) did not essentially change the dislocation densities and grain sizes of the alloys; however, the vacancy concentrations of the alloys gradually decreased with increasing natural aging time. The attained peak hardness of the Cu-Be-Co alloy on subsequent artificial aging at 593 K decreased as natural aging time increased, while the natural aging at 293 K for 2.59 Ms had no influence on the age-hardening behavior of the Cu-Ti alloy during artificial aging at 623 K. This result is ascribed to the difference in formation mechanisms of strengthening precipitates between two alloys; G.P. zones are directly formed in Cu-Be-Co alloy while β'-Cu4Ti phase is formed via spinodal decomposition without nucleation events in Cu-Ti alloy. This Paper was Originally Published in Japanese in J. Japan Inst. Copper 55 (2016) 197–201.
  • Microstructures of Ta-Inserted SmCo5/Fe Nanocomposite Thick Film Magnets

    pp. 1351-1355

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    DOI:10.2320/matertrans.M2017035

    Ta-inserted SmCo5/Fe nanocomposite thick film magnets were synthesized by high-speed pulsed laser deposition followed by pulse annealing. The microstructures of the film magnets were characterized by high-resolution scanning electron microscopy and scanning transmission electron microscopy. The as-deposited thick film possessed a multilayered Sm-Co/Ta/α-Fe/Ta structure with amorphous Sm-Co layers and [110]-oriented crystalline α-Fe layers. After pulse annealing, many fine grains of Laves phase TaCo2 were formed, and then the multilayered structure was converted to a granular nanocomposite thick film magnet composed of fine crystalline grains of Sm(Co, Fe)5, α-(Fe, Co), and TaCo2. The volume fractions and grain sizes of hard magnetic Sm(Co, Fe)5, soft magnetic α-(Fe, Co), and TaCo2 were controlled by the thicknesses of the Ta layer, producing a nanocomposite thick film magnet with good exchange coupling.
  • First-Principles Study of Chlorine Adsorption on Clean Al(111)

    pp. 1356-1363

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    DOI:10.2320/matertrans.M2017101

    A density functional theory model is used to investigate the structural, thermodynamic, and electronic properties of chlorine atoms adsorbed on the Al(111) surface within a supercell approach for chlorine coverages of 1/4, 1/3, 1/2, 3/4, and 1 ML. The largest bond length is observed for an atop, hcp, and fcc mixed structure at 3/4 ML coverage. Analysis of the adsorption free energy reveals that the chlorine coverage of 3/4 ML is the most thermodynamically stable over the widest range of chlorine chemical potential and that the coverage of 1 ML is thermodynamically unstable. The electronic charge density distributions, the change in the work function induced by adsorption, and the corresponding electrostatic dipole moment are also calculated. Atop-site adsorption is shown to induce charge transfer and the formation of a dipole structure for low coverage, and the charge transfer decreases with increasing coverage. Surface bonding is investigated using the projected density of states, and aluminum and chlorine 3p-orbitals are shown to be important in Al-Cl bond formation.
  • Application of a Tritium Imaging Plate Technique to Depth Profiling of Hydrogen in Metals and Determination of Hydrogen Diffusion Coefficients

    pp. 1364-1372

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    DOI:10.2320/matertrans.M2017175

    A new methodology for depth profiling of hydrogen in metals is developed applying a tritium imaging plate technique (TIPT) with cross sectional observation. Owing to its high sensitivity and wide dynamic range for tritium detection, depth distribution of hydrogen dissolved in the BCC metals such as tungsten (W) and steels are successfully obtained. The depth distributions enable us to determine reliable lattice diffusion coefficients of hydrogen in W and a ferritic/martensitic steel (F82H) within 20% errors taking into account three dimensional desorption/release from the surfaces of the sample metals. Hydrogen trapped at surface and subsurface are clearly separated from the dissolved one. In BCC metals, since the former could be much larger than the latter, observation of overall hydrogen behavior without knowing detailed depth distributions could lead to wrong estimation of diffusion coefficients and solubility.
  • Deformation and Density Change of Open-Cell Nickel Foam in Compression Test

    pp. 1373-1378

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    DOI:10.2320/matertrans.L-M2017829

    Repeated compression test of open-cell type nickel foam (Celmet) was conducted to investigate deformation behavior with changes in density. Cylindrical billets with two pore sizes were compressed up to 90% in nominal strain. Apparent density, buoyant density and dimensions of billets were measured after each compression step. Compressive stress linearly increases without plateau region until the nominal strain of 0.4 with increase in apparent density and with slight increase in diameter. Apparent density and porosity estimated under assumption of no diameter change show good agreement with the experiments. Above the strain of 0.4, the stress as well as diameter apparently increases with remarkable increase in density.
  • Effect of γ-Phase Stability on Local Deformation Energy of α-γ Duplex Stainless Steel

    pp. 1379-1385

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    DOI:10.2320/matertrans.H-M2017826

    The effect of austenite (γ) phase stability in the local deformation region on the deformation energy of ferrite (α)-γ duplex stainless steel is investigated using tensile testing. The uniform deformation energy values of both the γ-stable and unstable duplex stainless steels are similar, while the local deformation energy of γ-stable duplex stainless steel is larger than that of γ-unstable duplex stainless steel. In γ-unstable duplex stainless steel, the difference in nano-indentation hardness between the α and transformed martensite (α') phases increases remarkably beneath the fractured surface. Low-voltage electron microscopy observations reveal that voids grow rapidly at the grain boundaries between the α and α' phases beneath the fractured surface, resulting in a decrease in the local deformation energy in γ-unstable duplex stainless steel. This Paper was Originally Published in Japanese in J. Jpn. Soc. Heat Treatment 56 (2016) 285–291.
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  • Selective Dissolution of Delta Ferrite Phase in Welded 308 Stainless Steel under Proton Irradiation

    pp. 1386-1391

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    DOI:10.2320/matertrans.MAW201701

    Electron backscatter diffraction (EBSD) measurements were performed for welded 308 stainless steel corroded under proton irradiation at 473 K. After chemical cleansing and subsequent mechanical polishing to remove oxide layers on the specimen surface for EBSD measurements, selective dissolution of delta ferrite (δ) phase in the dendritic microstructure was detected only for the in-beam corroded specimen. The susceptibility to δ phase dissolution is dependent on the deviation angle of its orientation from the (101) plane on the specimen surface when δ phases are in a grain of austenitic gamma (γ) phase as well as on the γ-γ phase grain boundaries except for the coincidence grain boundaries. Selective dissolution of δ phase would be explained by not only radiolytic products such as radical oxygen acting as electric charge carriers but also the radiation-induced point defects and dislocation structures playing an important role in sustaining anodic reactions at δ phases.
  • Improved Mechanical Properties of Al2O3 Ceramics by Sputtered TiN Coatings

    pp. 1392-1396

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    DOI:10.2320/matertrans.M2017097

    In this study, TiN thin films have been prepared as the coating layers on Al2O3 ceramic substrates to enhance the mechanical properties of Al2O3 ceramics. TiN films with various thicknesses of 0.5 and 1 μm were deposited by DC sputtering at room temperature. The TiN film prepared on the Al2O3 substrate has a columnar structure without large bumps or steps formed on the film's surface. Without the TiN coating, the average friction coefficient of the Al2O3 substrate was 0.51. As 0.5- and 1-μm-thick TiN films were coated on Al2O3 substrates, the average friction coefficients of these two samples reduced to 0.36 and 0.32, respectively, revealing the tribological characteristics of Al2O3 ceramic was enhanced with the TiN coating. Additionally, the 1-μm-thick TiN/Al2O3 sample possesses a slightly higher attrition resistance than that of the 0.5-μm-thick TiN/Al2O3 sample, and this can be also confirmed by performing the elemental mapping method on the wear-treated samples. From the adhesion performances, we observed no critical load of LC2 appeared in these two samples during the scratch test, while the critical load of LC1 values were 11 and 9 N for 0.5-μm-thick TiN/Al2O3 and 1-μm-thick TiN/Al2O3, respectively. Due to the increment of TiN thickness, the internal stress formed in the film would increase. This is the reason why the 1-μm-thick TiN/Al2O3 sample has a lower critical load of LC1. Obviously, the 0.5-μm-thick TiN film coated on the Al2O3 substrate has a better adhesion characteristic. According to these results, the sputtered TiN coating is indeed useful for improving the mechanical properties of Al2O3 ceramics, in particular for the 0.5-μm-thick TiN coating.
  • Fundamental Properties of Ti-6Al-4V Alloy Produced by Selective Laser Melting Method

    pp. 1397-1403

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    DOI:10.2320/matertrans.M2017103

    This study was conducted to comprehensively investigate the fundamental properties of Ti-6Al-4V alloy produced by the selective laser melting (SLM) method. The examined materials were three SLM materials having different axial directions. For comparison, wrought material of Ti-6Al-4V alloy was also examined. The results showed that the densities of the SLM materials were adequately high and more than 99.9% of that of the wrought material. The SLM materials possessed a columnar microstructure extending along the building direction. This microstructure was mainly composed of the acicular α' martensite phase. Due to the generation of the fine α' phase, the static strength of the SLM materials was markedly higher than that of the wrought material although their elongation was lower. There was anisotropy in the static strength and the ductility of the SLM materials. In spite of the higher tensile strength, the fatigue strength of the SLM materials was much lower than that of the wrought material since molding defects may induce stress concentration and accelerate the generation of fatigue cracks.
  • Solid Solution Hardening and Precipitation Hardening of α2-Ti3Al in Ti-Al-Nb Alloys

    pp. 1404-1410

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    DOI:10.2320/matertrans.MAW201707

    Microstructure and mechanical properties of Ti-Al-Nb-based alloys were investigated as they are important in the development of new generation high temperature Ti alloys due to good oxidation resistance. The alloy compositions Ti-15Al-2Nb (to obtain α2 precipitates) and Ti-15Al-2Nb-0.5Si (at%) (to obtain solid-solution hardening by Si) were selected. Heat treatment at 1000℃ after forging and rolling at 900℃ transformed the β phase to α phase and α' martensite during cooling. By heat treatment at 900℃, a single α phase was obtained in both alloys. The α2 phase was formed by aging treatment at 600 and 700℃ of Ti-15Al-2Nb and at 600, 700, and 800℃ of Ti-15Al-2Nb-0.5Si. Compressive strength was investigated for samples heat treated at 900℃ with a single α phase and those aged at 600, 700, 800℃ with α2 precipitates. The solid solution hardening effect of Si was found at all test temperatures. Precipitation hardening effect by α2 precipitates was also observed in both alloys and the effect improved with Si addition. Peaks of precipitation hardening were obtained at 300℃ in both alloys; the precipitation hardening effect decreased with increase of test temperature above 300℃. It is suggested that a shearing mechanism occurs up to 300℃, which changes to a bypass mechanism above 450℃.
  • Acoustic Emission Pattern Recognition Method Utilizing Elastic Wave Simulation

    pp. 1411-1417

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    DOI:10.2320/matertrans.M2017104

    Fracture mode analysis via acoustic emission (AE) has attracted research attention as a means of identifying the damage mechanism in machines and engineering structures. However, the estimation of relevant parameters from waveforms detected by AE sensors requires complex analysis because the waveforms are often distorted by the sensor response. Therefore, in this study, we develop a method for the classification of AE signals based on simulated AE waveforms. The simulated AE waveform is a convolution of the simulated elastic wave calculated via the 3D finite-difference time-domain (FDTD) method and the sensor response acquired with the use of the long bar method. The AE signals with different propagation angles obtained with the use of an epoxy-glass fiber composite specimen are classified, and wavelet contour maps and correlation coefficients are used for AE classification. The success of the AE classification is validated by the similarity of waveform features between the experiment and simulation.
  • Effect of Chloride Ions in Electrowinning Solutions on Zinc Deposition Behavior and Crystal Texture

    pp. 1418-1426

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    DOI:10.2320/matertrans.M-M2017827

    To clarify the effect of chloride ions on Zn electrowinning in terms of deposition behavior and crystal texture, the partial polarization curve, current efficiency, and AC impedance for Zn deposition as well as its morphology and crystal orientation were investigated. In solutions containing chloride ions, the partial polarization curve for Zn deposition was evidently depolarized, and the degree of depolarization increased when increasing the concentration of chloride ions from 300 to 6,000 mg/L. The exchange current density i0 for Zn deposition also increased with increasing chloride ion concentration, which indicates that the chloride ions promote the charge-transfer process during Zn deposition. The polarization resistance for Zn deposition measured by AC impedance decreased with increasing chloride ion concentration. Conversely, the capacity of the electric double layer increased with increasing chloride ion concentration from 300 to 1,200 mg/L; however, from 3,000 to 6,000 mg/L, the capacity decreased and approached that obtained from a chloride ion-free solution. On the other hand, the partial polarization curve for hydrogen evolution was almost unchanged by the presence of chloride ions. As a result, the current efficiency of Zn deposition increased by 5–7% and 1.7% in the initial stage of deposition (up until the thickness of Zn reached approximately 2.1 μm) and at 500 A/m2 for 6 hours (482 μm), respectively. With the addition of 300 to 1,200 mg/L of chloride ions, the deposited Zn exhibited a preferred orientation of {0001} plane, and it increased with increasing chloride ion concentration, which is attributed to a decrease in the overpotential for Zn deposition; however, no preferred orientation of a specific plane was shown in the deposits when increasing the concentration of chloride ions above 3,000 mg/L. Some spherical concavities resulting from traces of evolved hydrogen gas were observed on the surface of deposited Zn, and the size of the concavities increased with increasing chloride ion concentration. The size of platelet crystals of the deposited Zn decreased with increasing chloride ion concentration, which indicates that the chloride ions possibly promote the nucleation rate of Zn crystals due to specific adsorption onto the cathode.
  • Equilibrium Modeling of the Extraction of Copper and Ammonia from Alkaline Media with the Extractant LIX84I

    pp. 1427-1433

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    DOI:10.2320/matertrans.M-M2017830

    Here we examined the equilibrium distribution ratios of copper(II) and ammonia between an ammonium salt solution (nitrate, sulfate, or chloride) and the extractant LIX84I (active component, 2-hydroxy-5-nonylacetophenone oxime) dissolved in a non-polar diluent Shellsol D70 at pH 2–10 and 298 K. The effects of pH, ammonium salt concentration in the aqueous phase, counter anion type, and phase ratio on the co-extraction of copper and ammonia were investigated. The values of the extraction constant of ammonia and the distribution constant of copper-oxime complex were independent of the type and concentration of ammonium salt, and the logarithm of the extraction constant of copper linearly increased with the ionic strength of the aqueous phase. Consequently, we constructed an equilibrium model that enabled quantitative calculation of the distribution ratios of copper and ammonia at a given equilibrium pH. Full loading of copper was found to minimize the accumulation of ammonia in the organic phase, which may reduce the need for ammonia scrubbing in practical operations.
  • Measurement and Thermodynamic Analysis of Carbon Solubility in Si–Cr Alloys at SiC Saturation

    pp. 1434-1438

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    DOI:10.2320/matertrans.M2016465

    Si–Cr alloy is one of the predominant solvents for rapid solution growth of 4H–SiC crystals. The solubilities of carbon in Si–40 mol%Cr alloy at SiC saturation at 1773–2273 K and in Si–Cr alloys of various chromium contents at 2073 K were measured by equilibrating the Si–Cr alloy with a 4H–SiC single crystal. Carbon solubility in Si–40 mol%Cr alloy increased with temperature from 0.22 mol% at 1773 K to 3.59 mol% at 2273 K. At 2073 K, carbon solubility at SiC saturation increased with the chromium content in the liquid from 0.18 mol% in Si–20 mol%Cr to 16.4 mol% in Si–80 mol%Cr. A thermodynamic analysis of the Si–Cr–C alloy was also conducted. Although the sub-regular solution model is often adopted to estimate phase relations in solution systems, this predicted a carbon solubility in Si-40 mol%Cr at SiC saturation more than two times higher than the measured value. In contrast, a quasi-chemical model that considered the competition between substitutional Si and Cr atoms bonding to interstitial carbon atoms reproduced the activity coefficient of carbon in Si–Cr alloys of 60–100 mol%Si composition, in which the carbon solubility at SiC saturation was less than 1.5 mol%, fairly well. This quasi-chemical model enabled the precise phase relation to be evaluated when designing the solution growth of SiC using a Si–Cr solvent.
  • Aluminum Co-Doping Method to Increase the Boron and Phosphorous Concentration Limits Allowed in Silicon Feedstock for Solar Cells

    pp. 1439-1443

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    DOI:10.2320/matertrans.M2017149

    In the present work, an aluminum co-doping method was employed to improve the electrical properties of multi-crystalline silicon containing phosphorous and boron. The method is based on the compensation theory and on the different segregation coefficients of aluminum (kAl = 2.8 × 10−3), boron (kB = 0.8), and phosphorous (kP = 0.35) in silicon during solidification. The carrier type and carrier concentration throughout the height of the silicon ingot with co-doping of aluminum were measured. The results show that no polarity inversion occurs and a relatively low carrier concentration, i.e., low net doping concentration can be achieved with aluminum co-doping. Moreover, the resistivity and minority carrier lifetime throughout the height of the silicon ingots with and without co-doping of aluminum were measured. The results show that a relatively high and uniform resistivity and minority carrier lifetime can be achieved by adding a controlled amount of aluminum. In addition, the effect of the grain boundary on the electrical properties was studied.
  • Formation of Fe-Cr-Mo Alloy Metallic Glass Coating Using High-Velocity Oxy-Fuel Apparatus with Hydrogen Gas and Its Microstructural Transition at Elevated Temperatures

    pp. 1444-1450

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    DOI:10.2320/matertrans.T-M2017828

    To improve high-velocity oxy-fuel (HVOF) sprays and reduce CO2 emission, an iron-based metallic glass coating produced using HVOF apparatus with hydrogen gas without a fusing process was investigated. Crystallization phenomena of the metallic glass were also evaluated at elevated temperatures. The Fe-Cr-Mo-based alloy was sprayed on a mild steel substrate using a specific gun with hydrogen gas, and metallographic observation revealed that the alloy was successfully coated on the substrate even when using the hydrogen gas. In addition, the corrosion resistance was investigated by performing a combined cyclic corrosion test. Significant corrosion was prevented until 1578 h by a sealing treatment even without a fusing process. The metallic glass coating was heat-treated at 500 to 800℃, and then X-ray diffraction analysis was performed. In the X-ray diffraction profile, the intensity of the observed broad peak from the metallic glass decreased with increasing temperature and holding time, while sharp peaks from the crystal phase appeared. The crystallization process was successfully predicted from the Johnson–Mehl–Avrami equation regarding nucleation and growth of crystal grains from the glass phase. Although the Vickers hardness of the as-sprayed specimen was 778 HV, it was improved to 1029 HV at approximately 80% crystallinity; thus, the nanoscale crystals enhanced the hardness of the metallic glass. This Paper was Originally Published in Japanese in J. Japan Thermal Spray Society 53 (2016) 48–54.
  • Development of New T-Bar Production Technology by Tandem Universal Rolling

    pp. 1451-1457

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    DOI:10.2320/matertrans.P-M2017830

    Application of T-bars in shipbuilding has increased recently. Slim T-bars having a web height of at least twice the flange width are generally required in ships. Because the dimensions of existing hot-rolled T-bars are unsuitable, T-bars fabricated by welding two plates are currently used in shipbuilding. To manufacture slim T-bars by hot rolling, we devised a new T-bar rolling process using two universal mills and an edging mill. The two universal mills have different roll shapes. The horizontal roll width of the first mill is wider than the web height of the T-bar, whereas that of the second mill is narrower than the web height in order to reduce the web height with a vertical roll. Multi-pass rolling experiments with pure lead were performed using the new process, and a T-bar having an excellent cross section was obtained. Finite element analyses of universal rolling were also carried out to investigate the rolling deformation behavior in this process in detail. Following these investigations, a T-bar hot rolling test was carried out at an actual structural steel mill. As a result, steel T-bars with an excellent cross section were successfully produced, and the capability of the new T-bar rolling technology was clearly demonstrated. This Paper was Originally Published in Japanese in J. JSTP. 58 (2017) 53–59.
  • Synthesis of Lead-Free Solder Particles Using High-Speed Centrifugal Atomization

    pp. 1458-1462

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    DOI:10.2320/matertrans.M2017129

    We investigated the relationships between the centrifugal force and the disc size and rotation speed during the production of fine particles. In this study, an empirical formula was developed with the goal of obtaining an average particle size of D50. Moreover, particle production was performed at a disc rotation speed of at least 100,000 rpm, which was greater than that reported in previous studies. Our results showed that with Sn-13 mass%Sb (called Sn-13Sb), the average particle size produced at a disc rotation speed of 115,000 rpm was 12.8 μm, with 96.5% of the particles smaller than 25 μm.
  • Crystallization Behavior of Thermally Rejuvenated Zr50Cu40Al10 Metallic Glass

    pp. 1463-1468

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    DOI:10.2320/matertrans.MAW201703

    The recovery annealing technique was used to clarify the crystallization behavior of a rejuvenated Zr50Cu40Al10 metallic glass. Specific heat and thermal expansion measurements showed that the glassy states of fast cooled and recovery annealed samples are almost identical from the energetic and volumetric points of view. Dynamic mechanical analysis also suggested that their internal microstructures resemble each other. These results indicate that the glassy state and the structure can be controlled according to the final cooling conditions from the supercooled liquid region. However, a clear difference of the incubation time for crystallization was observed between the fast cooled and recovery annealed samples, implying that thermal accumulation seems to mostly correlate to the crystallization rather than to atomic diffusion through the newly created excess free volume in the rejuvenated sample. In addition, a transmission electron microscopy image of the sample, performed on double sets of the recovery annealing process, shows a precipitation of the crystalline phase with approximately 50 nm in diameter in the amorphous matrix. These results suggest that the recovery annealing independently leads to two different thermal paths, one towards structural rejuvenation and the other towards crystallization. The high-resolution transmission electron microscopy result of the recovery annealed sample indicates that these two regions co-exist in the amorphous matrix.
  • Effects of Crack Size Distribution and Specimen Length on the Correlation between n-Value and Critical Current in Heterogeneously Cracked Superconducting Tape

    pp. 1469-1478

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    DOI:10.2320/matertrans.MAW201702

    A Monte Carlo simulation study was carried out to reveal the effects of crack size distribution and specimen length on the correlation between n-value and critical current in heterogeneously cracked superconducting tapes. First, it was shown that, with increasing distribution width of crack size, the distribution widths of critical current and n-values increase, and, the average critical current-value and average n-value of specimens decrease. Also, it was shown that the decrease in average n-value with increase in distribution width of crack size is more intense than the decrease in average critical current-value and this feature is more pronounced in longer specimens. Then it was revealed that plural n-values can exist for one critical current value since the n-value of specimen was dependent on the positional relation among the voltage-current curves of the sections, of which specimen is constituted. This phenomenon could be described by the difference in the resistance value in the current shunting circuit by application of a single equivalent crack-current shunting model in which the cracks within a specimen are replaced by a single equivalent crack. Based on this result, an approach, in which the resistance value in the current shunting circuit is used to describe the upper-lower bounds and the center of n-value in the correlation diagram between n-value and critical current, was presented. It was shown that the correlation diagrams at various distribution widths of crack size and specimen lengths, obtained by simulation and experiments, are described well by this approach.
  • Training Effect on Microstructure and Shape Recovery in Ti-Pd-Zr Alloys

    pp. 1479-1486

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    DOI:10.2320/matertrans.MAW201706

    The training effect of microstructure and shape recovery on Ti-50Pd-xZr (x = 7 and 10) at% and Ti-50Pd-xZr-(5-x)V (x = 1, 2.5, and 4) high-temperature shape memory alloys were investigated. Zr was selected as an alloying element as it is known to improve the shape recovery of TiPd. As a further alloying element, V was selected because it is effective in strengthening TiPd. The dependence of Zr content and V addition on the martensitic transformation (MT) temperature, shape recovery, and training effect were investigated. For example, Mf, decreased with increasing Zr from 480℃ in Ti-50Pd to 302℃ in Ti-50Pd-10Zr. In Ti-50Pd-xZr-(5-x)V, when the total amount of Zr and V was 5 at%, the MT temperatures did not change drastically. The MT temperatures ranged between 350 and 550℃.Shape recovery was investigated using the thermal cyclic test under a constant applied stress in the range of 15 to 200 MPa. Perfect recovery was obtained at low stresses, while irrecoverable strain was observed at high stresses. For Ti-50Pd-2.5Zr-2.5V and Ti-50Pd-1Zr-4V, creep deformation was observed above 150 MPa. To obtain perfect recovery, training (repeated thermal cyclic tests under a constant applied stress) was performed. Perfect recovery was obtained for the alloys by training, except for Ti-50Pd-4Zr-1V. Ti-50Pd-10Zr achieved perfect recovery up to 200 MPa, while Ti-50Pd-1Zr-4V achieved perfect recovery up to 150 MPa. Other alloys achieved perfect recovery at lower stresses of 65 or 50 MPa. The microstructure changed from a random martensite variant to a specific orientation during training, to accommodate the large strain during deformation. It was found that a strong texture led to perfect shape recovery.
  • Strength and Ductility at High-speed Tensile Deformation of Low-carbon Steel with Ultrafine Grains

    pp. 1487-1492

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    DOI:10.2320/matertrans.M2017191

    Tensile properties at various deformation speeds up to ~1500 mm/sec of low-carbon steel with elongated ultrafine grains were examined, focusing on both strength and ductility. Fine-grained samples were prepared by producing severe plastic deformation by caliber rolling. The ultrafine-grained samples showed an increase of elongation while maintaining high strength at a higher deformation speed. At a deformation speed of ~1500 mm/s, the absorbed energy exhibited by the fine-grained samples was as large as that of the conventional-grained sample, regardless of the difference in strength.
  • Fabrication of YBa2Cu3O7 Superconducting Film on {100}<001> Textured Cu Tape via Conductive Buffer Layers

    pp. 1493-1499

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    DOI:10.2320/matertrans.M2017052

    Long-length coated conductors (CCs) have recently become commercially available, serving as a promising candidate for use in electric power applications. However, the material and manufacturing costs are high, which discourages their use in a wide range of commercially feasible products.REBa2Cu3O7 (REBCO; RE: Y or rare-earth elements) superconducting films with high critical current density (Jc) have been grown on cube-textured metal tapes to develop CCs for high temperature, high magnetic field applications. In the standard approach, a biaxially oriented YBCO layer is deposited on a Y2O3/Y2O3-stabilized ZrO2/CeO2 buffered Ni-W tape. CCs become highly resistive when they are quenched; therefore, for reliable and safe application, it is necessary to attach low-resistivity metal layers, such as Cu and/or Ag, to the CCs to stabilize and protect them from damage due to quenching. Presently, insulative oxides are used for the buffer layers; thus, thick Ag and Cu layers are required to be deposited as stabilizer layers on the YBCO layer. However, the high material and process costs for obtaining the Ag and Cu layers are one of the major obstacles to achieving low-cost CCs. The use of conductive buffer layers instead of insulative reduces the cost of CCs. In this paper, we propose a new configuration for CCs: YBCO deposited on a conductive Sr(Ti0.95Nb0.05)O3 buffered Ni-electroplated {100}<001> textured Cu and SUS316 lamination tape. Sr(Ti0.95Nb0.05)O3 was epitaxially grown on the Ni-electroplated {100}<001> textured Cu tape, and its resistivity was as low as 2.5 mΩ-cm at 77 K. An excellent Jc of 2.6 × 106 A/cm2 was achieved at 77 K under a magnetic self-field for the YBCO/Sr(Ti0.95Nb0.05)O3/Ni/Cu/SUS316 tape. We believe that Sr(Ti0.95Nb0.05)O3 is a promising candidate for the conductive buffer layer material. This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 80 (2016) 428–433.
  • Leaching of Copper from Cuprous Oxide in Aerated Sulfuric Acid

    pp. 1500-1504

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    DOI:10.2320/matertrans.M2017147

    The leaching behavior of copper from Cu2O in H2SO4 solution was investigated to establish the leaching process for cathode powders produced by the recycling of waste printed circuit boards. When air was not introduced in sulfuric acid solution, the dissolution of copper from Cu2O was inhibited by the formation of elemental copper (Cu0). The dissociated cuprous ions (Cu+) transformed into elemental copper (Cu0) or cupric ions (Cu2+) owing to the instability of Cu+ in H2SO4. Cu+ can be reduced to elemental copper (Cu0) by accepting an electron generated from the oxidation of another Cu+ to Cu2+, which is known as a “disproportionation reaction.” The introduction of air enhanced the leaching efficiency of copper due to the role of oxygen in the air as oxidant by accepting the electron generated from the oxidation of Cu+ to Cu2+. In the leaching test using Cu2O reagent, the leaching efficiencies of copper increased with increasing air flow rate, temperature and agitation speed, but decreased with increasing pulp density. Copper leaching efficiency increased to up to 99% within 60 min in the aerated sulfuric acid solution at 30℃, 400 rpm, and pulp density of 2%.
  • Brittle Fracture Stress of Ultrafine-Grained Low-Carbon Steel

    pp. 1505-1508

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    DOI:10.2320/matertrans.M2017135

    The variation of brittle fracture stress on grain size in a wide range of 138 μm to 1 μm was studied in low-carbon steel. The steel bars with an ultrafine elongated grain structure and equiaxed grain structure were fabricated by caliber rolling. The brittle fracture stress was quantitatively estimated through a three-point bending test at 77 K with a notch sample and finite element analysis. A reduction in grain size leads to higher fracture stress, compared with yield strength, and steel with a grain size of 1 μm was estimated to have a very high fracture stress of 6.8 GPa. It has been shown that significant improvement of brittle fracture stress is one of the advantages of grain refinement.
  • Editor's Announcement

    pp. 1509-1509

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    DOI:10.2320/matertrans.EAM2017702

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