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

MATERIALS TRANSACTIONS Vol. 42 (2001), No. 6

  • Electronic Structure and Transport Properties of Pseudogap System Fe2VAl

    pp. 902-910

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    DOI:10.2320/matertrans.42.902

    The Heusler-type Fe2VAl compound exhibits a semiconductor-like behavior with the resistivity reaching 30 \\microΩm at 2 K . Recent band-structure calculations predict that Fe2VAl is a nonmagnetic semimetal with a sharp pseudogap at the Fermi level. The existence of a pseudogap is experimentally confirmed in optical conductivity spectra and is also supported by Hall-effect measurements. A substantial mass enhancement is deduced from electronic specific-heat measurements, reminiscent of a 3d heavy-fermion system. The unusual electron transport is mainly interpreted in terms of the effect of strong spin fluctuations in addition to the possession of a low carrier density. Since a small deviation from stoichiometry causes a large enhancement in the Seebeck coefficient, the pseudogap system Fe2VAl can be regarded as an intriguing candidate for thermoelectric applications.
  • Pseudo-Gap In Quasicrystals: A Key to Understand Their Stability and Properties

    pp. 911-919

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    DOI:10.2320/matertrans.42.911

    The majority of stable quasicrystals known so far is based upon aluminum. It turns out that the valence band of aluminum atoms in such quasicrystals, as well as in the crystals that exhibit a similar icosahedral short range order, shows a pronounced pseudo-gap located at the position of the Fermi energy. In the present paper, we describe how this specific feature of quasicrystals is uncovered and quantified using spectroscopy techniques. We show then that the pseudo-gap, which persists up to the surface, plays an important role in the thermodynamic stability of these non-conventional materials and is involved in transport properties as well as in surface properties of aperiodic intermetallics.
  • Pseudogap in Quasicrystals Studied with Photoemission Spectroscopy

    pp. 920-927

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    DOI:10.2320/matertrans.42.920

    The results of the low-temperature, high energy resolution photoemission studies of a single-grain icosahedral alloy Al70Pd21.5Mn8.5 have been presented. The existence of the theoretically predicted pseudogap has been confirmed. No evidence of the theoretically predicted spikiness of the density of states could be observed. A review of published experimental data on the electronic structure of quasicrystals has also been presented. The photoemission experimental results confirm the existence of a pseudogap in the density of states in icosahedral quasicrystals, but its existence in the decagonal quasicrystals remains still an open question. The predicted DOS spikiness could not be observed in any of the studied quasicrystals.
  • Roles of Quasiperiodicity and Local Environment in the Electronic Transport of the Icosahedral Quasicrystals in Al-Pd-TM (TM=Fe, Ru, Os) Systems

    pp. 928-932

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    DOI:10.2320/matertrans.42.928

    In order to gain an insight into the roles of the local atomic environment and the long-range quasiperiodicity in the electronic transport of the icosahedral quasicrystal, a direct comparison of the electrical resistivity of icosahedral quasicrystals, (1/0, 1/0, 1/0) and (2/1, 2/1, 2/1) crystalline approximants in the Al–Pd–TM (TM=Fe, Ru, Os) ternary systems has been made on the basis of the same alloy system. The trend of the resistivity of 1/0-cubic approximants, which are the lowest order crystalline analogues to the icosahedral phase, already possesses a nonmetallic character, while the 2/1-cubic approximants exhibit quite similar behavior to that of the corresponding quasicrystals. The present result strongly suggests that the effect of the long-range quasiperiodicity beyond the lattice periodicity of 2/1-cubic approximant phase on the electronic transport is of less significance and the electronic transport of approximants and quasicrystals is mainly determined by the local atomic environment of a scale less than 2.0 nm.
  • Direct Evidence of the Hume-Rothery Stabilization Mechanism in Al-Mn-Fe-Si Mackay-Type 1/1-Cubic Approximants

    pp. 933-938

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    DOI:10.2320/matertrans.42.933

    The detailed structure analysis for a series of the Al–Mn–Fe–Si Mackay-type 1/1-cubic approximants has been performed by applying the Rietveld method to the x-ray powder diffraction spectra taken with a synchrotron radiation x-ray source. We found that identical Mackay clusters occupy both the body-center and vertex positions of the cubic lattice but that atom distributions in glue sites connecting the Mackay clusters are unique to each alloy. The number of s, p-electrons per unit cell is calculated by assigning the appropriate number of valence electrons to constituent atoms involved in the refined atomic structure. It is shown that, when the Fe atoms are substituted for Mn atoms, the total number of s, p-electrons per unit cell remains unchanged and is always centered at a value satisfying the so called Hume-Rothery matching rule, where the Fermi sphere well coincides with the relevant Brillouin zone planes. We conclude that the Hume-Rothery matching condition stabilizes the ternary Al–Mn–Fe–Si approximants by compensating an increase in the valence electrons due to the replacement of Mn atoms by Fe atoms by an increase in the number of vacancies in the glue sites.
  • Metal-Insulator Transition in the Amorphous CexSi100-x (4≤ x≤ 83) Heavy Fermion System

    pp. 939-950

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    DOI:10.2320/matertrans.42.939

    The electronic structure, magnetic properties and electron transport properties have been studied on amorphous CexSi100−x (4≤x≤83) alloys in comparison with non-magnetic amorphous LaxSi100−x (11≤x≤63) and TixSi100−x (6≤x≤41) alloys with a particular emphasis on the formation of the pseudogap at the Fermi level and its effect on the electron transport upon approaching the metal-insulator transition in the heavy fermion system. It is shown that the interaction between conduction electrons and localized moments leads to an anomalous enhancement in the temperature dependence of the measured resistivity below 10 K . We also revealed that the amorphous CexSi100−x alloy system crosses the metal-insulator transition at about 12 at%Ce and that the marginally metallic Ce15Si85 alloy has the resistivity of 1500 \\microΩcm comparable to those in the non-magnetic reference systems but a large electronic specific heat coefficient γ of 22 mJ/mol·K2, which is 50 times as large as the value of 0.4 mJ/mol·K2 for the marginally metallic Ti13Si87 alloy. The set of (ρ300 K, γexp) data fall on an extremely small diffusion constant line in the ρ-γ diagram. This unique behavior is attributed to the existence of Ce-4f electrons at the Fermi level, which give rise to a large value of γ but are localized and immobile even in the metallic regime.
  • Cobalt Oxides and Kondo Semiconductors: A Pseudogap System as a Thermoelectric Material

    pp. 951-955

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    DOI:10.2320/matertrans.42.951

    Layered cobalt oxides and heavy-fermion compounds are quantitatively compared from the viewpoint of thermoelectric materials. These two systems exhibit anomalously large thermopowers with good electric conductivity, which is attributed to a strong electron-electron correlation. In certain materials the pseudogap increases at low temperature, which further enhances the thermopower.
  • Thermoelectric Properties of (Ca, Sr, Bi)2Co2O5 Whiskers

    pp. 956-960

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    DOI:10.2320/matertrans.42.956

    Thermoelectric properties of (Ca, Sr, Bi)2Co2O5 (Co-225) single crystalline whiskers with a layered structure were measured over a wide temperature range 100–973 K . Both Seebeck coefficient and electrical resistivity exhibited fairly complex temperature dependences in this temperature range. The whole temperature range studied is divided into four distinct regions (I) to (IV), depending on observed characteristic temperature dependences of both Seebeck coefficient and electrical resistivity. From more or less linearly temperature dependent Seebeck coefficient in region (I) in combination with unique temperature dependences of both resistivity and Hall coefficient, we conclude the presence of a small pseudogap with a width of a few meV across the Fermi level. Complex magnetic properties are observed: the antiferromagnetic transition at 22 K but the hysteresis in the M-H curve remains up to room temperature. This is taken as evidence for the existence of Co atoms situated in different magnetic environments. The possession of large Seebeck coefficients exceeding 100 \\microV K−1 in this system is attributed to the presence of the pseudogap at the Fermi level.
  • Electronic Structure of Delafossite-Type Metallic Oxide PdCoO2

    pp. 961-964

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    DOI:10.2320/matertrans.42.961

    The electronic structure of metallic oxide PdCoO2 has been investigated by photoemission and inverse photoemission spectroscopies. It is found that the finite density of states at the Fermi level in the spectra is observed at the low photon energy where the ionization cross-section of Pd 4d increases with decreasing photon energy. Resonant photoemission spectra of PdCoO2 at photon energies near the Co 3p to 3d and Pd 4p to 4d absorption thresholds, indicate no density of states at the Fermi level in the partial density of states of Co 3d, and finite density of states at the Fermi level in the partial density of states of Pd 4d, respectively. These results indicate that the main contribution to the density of states at the Fermi level is Pd 4d and that the low resistivity of PdCoO2 is attributable to the itinerancy of the Pd 4d electrons.
  • Damping Capacity of Graphite-Dispersed Composite Steel

    pp. 965-969

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    DOI:10.2320/matertrans.42.965

    Composite steels consisting of an iron-graphite sintered core and steel were prepared by powder metallurgy and their damping capacities were examined. The ferritic grain size in the sintered core decreases with increasing carbon (graphite) content, resulting in a decrease in internal friction due to magnetic domain wall movement. Conversely, internal friction due to the deformation of graphite increases with carbon content. The internal friction of graphite-dispersed composite steel reaches a maximum at a carbon content of approximately 5 mass%. When carbon content exceeds 12 mass%, delamination between the steel part and the sintered core occurs easily and a good composite steel cannot be obtained. Experimental results suggest that a high damping capacity can be provided by appropriate control of the microstructure of the steel part and the ferritic grain size and carbon content of the sintered core.
  • Formation and Functional Properties of Fe-Based Bulk Glassy Alloys

    pp. 970-978

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    DOI:10.2320/matertrans.42.970

    Ferromagnetic bulk glassy alloys were synthesized in a variety of alloy systems by the copper mold casting process for the last five years after 1995. Their typical alloy systems are classified into five groups, i.e., (1) Fe–(Al, Ga)–(P, C, B) and Fe–Ga–(P, C, B), (2) (Nd, Pr)–Fe-(Al, Si), (3) Fe-(Zr, Hf,Nb)-B, (4) Fe–Co–Ln–B, and (5) Fe–(Cr, Mo)–B–C . The Fe-based glassy alloys exhibit a large supercooled liquid region exceeding 50 K before crystallization and the largest value reaches approximately 100 K . The maximum sample thickness of glass formation in the alloy systems belonging to the groups (1) to (5) is about 3 mm, 12 mm, 6 mm, 1 mm and 3 mm, respectively. These bulk glassy alloys exhibit good soft magnetic properties with a maximum saturation magnetization of 1.3 T and low coercive forces below 5 A/m except for hard magnetic properties only for the Nd- or Pr-based alloys. In addition, the application of the consolidation technique using the viscous flow phenomenon to the Fe–(Al, Ga)–(P, C, B) alloys caused the formation of fully dense bulk glassy alloys with rather good soft magnetic properties, e.g., 1.2 T for saturation magnetization, 10 A/m for coercive force, 9000 for maximum permeability and 0.1 W/kg at 50 Hz for core loss. The combination of good magnetic properties, high glass-forming ability and good workability into a bulk form is promising the future development as a new type of magnetic material.
  • Tight-Binding Theory in the Computational Materials Science

    pp. 979-993

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    DOI:10.2320/matertrans.42.979

    The tight-binding (TB) theory and TB molecular dynamics (TBMD) are now popular and valuable computational schemes available to materials scientists. The simplicity and transparency of the TB schemes enables us to get clear physical insights into the complicated phenomena. In the present review article, the calculational methods of the TB theory and TBMD are outlined and their applications to the important problems in the material sciences will be presented. Recently, linear scaling O(N) (order of N) TB methods have been developed for large scale computer simulations; we analyze the main ideas involved in these O(N) TB methods and their different implementations. The divide-and-conquer techniques for linear-scaling quantum mechanical calculations are reviewed, in conjunction with the catalytic activity of biological molecules. In addition, I also address the genetic and fuzzy algorithms coupled to the TB theory which allows us to find complicated final structures quite efficiently from simple initial structures.
  • Influence of Metallurgical Factors on Secondary Recrystallization of Silicon Steel

    pp. 994-1006

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    DOI:10.2320/matertrans.42.994

    An attempt has been made to describe recent views on grain growth and texture evolution in secondary recrystallization of grain oriented silicon steel. A number of metallurgical factors are known to influence phenomena of secondary recrystallization, such as the occurrence of secondary recrystallization and texture of secondary recrystallized grains. Typically, inhibitors existing in silicon steel, of which the amount and morphology are changed by addition of a small amount of elements, slag reheating conditions etc., affect the size and texture of primary and secondary recrystallized grains. It has also been recognized that a preferred orientation in secondary recrystallzation may be correlated with the texture of primary recrystallized grains, which is formed by cold rolling and subsequent annealing. These phenomena occurring in secondary recrystallization have been systematically studied by changing process conditions, while in the meantime a few techniques for controlling secondary recrystallization have been developed. From the viewpoint of microstructure and texture controls of polycrystalline materials, an overview of these metallurgical factors influencing secondary recrystallization and the correlation between these factors is given.
  • Effect of Alloying Elements on the Cold Deformation Behavior of Cr Phase and the Tensile Strength of Cu-15Cr Based In Situ Composites

    pp. 1007-1014

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    DOI:10.2320/matertrans.42.1007

    Deformation behavior of Cr phase in Cu–15Cr based in situ composites during cold deformation has been examined. Deformation strain partitioning between Cu and Cr phases after cold drawing occurs because the Cr phase tends to elongate less than Cu phase due to its higher flow stress and modulus. The deformation process of Cr phase during cold drawing comprises slight, steady and slow deforming stages, and the Cr phase is primarily deformed in the second stage. At a given drawing strain, the deformation strain of Cr phase increases with increasing hardness ratio of Cu phase to Cr phase, therefore, the finer drawing structure (with smaller interphase spacing and Cr thickness) will be obtained with either increasing hardness of Cu phase or reducing hardness of Cr. Hardening of Cr phase by the addition of alloying elements affects the strength of Cu–15Cr based in situ composites in two opposite ways-increasing the second phase strengthening and reducing the structural refinement strengthening, the latter is more prominent at higher drawing strain. The strength improvement of the as cold drawn Cu–15Cr based in situ composites can be achieved by increasing hardness ratio of Cu to Cr through either softening of the Cr phase or hardening of the Cu phase.
  • Bulk Glassy Alloys in (Fe, Co, Ni)-Si-B System

    pp. 1015-1018

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    DOI:10.2320/matertrans.42.1015

    Multicomponent Fe75−xyCoxNiySi8B17 glassy alloys were found to exhibit a distinct glass transition, followed by a supercooled liquid region before crystallization in a rather wide composition range of 7.5 to 45 at%Co and 7.5 to 60 at%Ni. The largest value of the supercooled liquid region defined by the difference between the glass transition temperature (Tg) and crystallization temperature (Tx), ΔTx (=TxTg) was 54 K for Fe30Co30Ni15Si8B17. Furthermore, the high reduced glass transition temperature (TgTm) above 0.60 was obtained in the range of 15 to 30 at%Co and 37.5 to 52.5 at%Ni in the series of Fe75−xyCoxNiySi8B17. The use of Fe30Co30Ni15Si8B17 with a large ΔTx of 54 K and a high TgTm of 0.65 enabled us to produce bulk glassy rods with diameters up to 1.2 mm. The Tg, Tx, saturated magnetization (4πIs), and coercive force (Hcj) of the as-cast 1.2 mm rod alloy are 780 K, 834 K, 0.90 T, and 3.0 A/m, respectively. In addition, the cast glassy rod exhibits Young’s modulus of 110 GPa, compressive fracture strength of 2800 MPa and fracture elongation of 1.9%. It is noticed that the thermal stability, magnetic properties and mechanical properties are nearly the same as those for the melt-spun glassy ribbon. The first synthesis of Fe–Si–B base bulk glassy alloys with good soft magnetic properties and high mechanical strength is promising for the future development as a new type of engineering material.
  • Compatibility of SiC/SiC Composite Materials with Molten Lithium Metal and Li16-Pb84 Eutectic Alloy

    pp. 1019-1023

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    DOI:10.2320/matertrans.42.1019

    SiC/SiC composite material is one of the candidates of plasma facing and structural materials for a fusion reactor. In an inertial fusion reactor and a magnetic confinement fusion reactor, interactions of solid and liquid breeder materials as well as coolant materials with various structural materials are considered to be very important. Therefore, corrosion tests of SiC/SiC composite materials with molten lithium and Li16–Pb84 alloy were performed. SiC/SiC specimens of five kinds, different in fiber/matrix interface, manufacturers and manufacturing methods, were used. In the case of lithium, the corrosion temperature was 500 K, and it was 573 K in the case of Li16–Pb84 alloy. Duration for the compatibility tests were about 2.5 Ms (29 days). During the heating of corrosion test, argon gas was made to flow over the liquid metal. The SiC/SiC composite sample, which had a free silicon component within its matrix, was corroded by molten lithium and came to each plane fabrics of fiber. The monolithic specimen that contained a silicon phase was also entirely broken by lithium. While, the attack by Li16–Pb84 alloy was not observed.
  • High Temperature Transport Property of B- and P-Doped GeSi Single Crystals Prepared by a Czochralski Method

    pp. 1024-1027

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    DOI:10.2320/matertrans.42.1024

    B-doped (p-type) and P-doped (n-type) Ge0.07Si0.93 single crystals were prepared by a Czochralski (CZ) method. The anisotropy of Seebeck coefficient (α), electrical conductivity (σ) and thermal conductivity (κ) among the [001], [110] and [111] directions was determined and the values were compared with those of polycrystalline materials. The Seebeck coefficient of B-doped specimen in the [111] direction (α111) was the same as that in the [110] direction (α110), and was greater than that in the [001] direction (α001) measured at all temperature. The α111, α110 and α100 of the P-doped specimens were in agreement above 600 K, but anisotropy of α values was observed below 500 K, (|α001|>|α111|>|α110|). No anisotropy of σ and κ was observed in both B- and P-doped specimens. The thermal conductivity of B- and P-doped single crystal specimens were much greater than that of polycrystalline materials.
  • Diffusion Bonding Associated with Phase Transformation in (γ+β) Micro-duplex Titanium Aluminides

    pp. 1028-1034

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    DOI:10.2320/matertrans.42.1028

    Microstructure and bonding strength of diffusion-bonded γ titanium aluminide alloys have been investigated focusing on phase transformation during diffusion bonding. High resolution scanning electron microscopy (SEM) observation revealed that lamellar grains are evolved near a jointed interface in (γ+β) micro-duplex alloys bonded at high temperatures, while not alloys bonded at low temperatures. This result is consistent with the proposed TTT diagram with the a lamellar nose. The transformation accompanied by the redistribution of Cr is evidenced by calculating Cr composition in each phase. Lamellar structure is also observed at a localized region in the (γ+β) micro-duplex ternary alloy bonded at high temperatures. It is speculated that this region is exposed to a stress high enough to accelerate the transformation, thereby shifting the lamellar nose to shorter time or lower temperature in the TTT diagram. The phase transformation in the (γ+β) micro-duplex ternary alloy is due to the low thermal stability of the β phase, enhancing atomic mass transport by Cr redistribution. Mechanical test revealed the high bonding strength in (γ+β) micro-duplex alloys, in which fracture was characterized by rugged fractography across lamellar boundaries in fracture surface. Thin layer is produced at the jointed interface uniformly in the micro-duplex alloy and inhomogeneously in the other two samples.
  • Effect of Heat Treatment on Tensile Properties of Component with Macro-Interface between Unreinforced Magnesium and Composite

    pp. 1035-1041

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    DOI:10.2320/matertrans.42.1035

    The microstructural and mechanical characteristics of component with the macro-interface between unreinforced magnesium and composite manufactured by squeeze casting were determined. The effect of thermal cycling on the tensile properties of the component with the macro-interface, which is a potential site of cyclic stress formation, was investigated. The macro-interface has diffuse interface characteristics; its thickness is about 7–10 \\micron. A reaction product was formed near the macro-interface. It has been found that the component are weaker than pure magnesium, and the tensile strength of the component decreases with thermal cycling as a function of both the number of thermal cycles and ΔT. The fracture behavior was evaluated by the study of fracture surfaces on both sides of the component fractured at the macro-interface.
  • Evaluation of Diffusion Coefficient of Oxygen in Liquid Silver from AC Impedanceof an Electrochemical Cell based on Solid Electrolyte and Molten Silver Electrode

    pp. 1042-1047

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    DOI:10.2320/matertrans.42.1042

    The diffusion coefficient of oxygen in liquid silver was evaluated by analyzing the relaxation process of the electrode reaction of an AC-polarized solid galvanic cell employing stabilized zirconia as the solid electrolyte and liquid silver as the electrode. The AC impedance of the cell was measured for various amounts of oxygen at the silver electrode in the frequency range from 1 mHz to 1000 mHz at temperatures of 1273–1373 K . The well-defined Warburg impedance, which was expected when the overall electrode reaction rate was controlled by the diffusion of oxygen in liquid silver, was obtained for a certain limited condition. The diffusion coefficients determined from this impedance agreed fairly well with those reported by other authors. The factors causing errors in the evaluation of the diffusion coefficient were examined.
  • Effects of Doping Elements on Oxidation Properties of Low-Activation Vanadium Alloys

    pp. 1048-1051

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    DOI:10.2320/matertrans.42.1048

    V–(4-5)Ti–(4-5)Cr type alloys doped with Si, Al and Y were studied in an effort to develop oxidation-proof vanadium alloys. In order to clarify the effects of the doping elements, the oxidation behavior of V–4Ti–4Cr, V–4Ti–4Cr–0.5Si, V–4Ti–4Cr–0.5Al and V–4T–4Cr–0.5Y alloys was studied. Thermogravimetric analysis (TGA) experiments in air at 400, 500, 620 and 650°C and exposure to air, helium and low oxygen partial pressure atmospheres at 500°C for 250 h were carried out for each alloy. After exposure, measurement of mass gain and tensile tests were performed to study the influence of the exposure atmosphere on each alloy. After exposure to helium atmosphere with oxygen partial pressure PO2=20 Pa gave a smaller mass gain than that by the low oxygen partial pressure atmosphere of PO3=1.3×10−4 Pa. Tensile testing at room temperature showed that the V–4Ti–4Cr–0.5Si alloy has the highest ultimate tensile stress, and V–4Ti–4Cr–0.5Y has the highest total elongation after exposure for all the atmospheres. The results of the TGA experiments in air show that yttrium doping is more effective at higher-temperature exposure. The V–4Ti–4Cr–0.5Y alloy has the highest oxidation-resistance at 620°C in all the alloys. The TGA data fit the parabolic oxidation law, especially in the first stage of oxidation time up to 30 h.
  • Bulk Glassy Fe78-xCoxGa2P12C4B4 Alloys with High Saturation Magnetization and Good Soft Magnetic Properties

    pp. 1052-1055

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    DOI:10.2320/matertrans.42.1052

    The effect of Co on the magnetic properties and glass-forming ability for the Fe78−xCoxGa2P12C4B4 glassy alloys was investigated. The addition of 4 to 8 at%Co was found to be effective for the extension of the supercooled liquid region (ΔTx) defined by the difference between the glass transition temperature (Tg) and crystallization temperature (Tx). The ΔTx value is 26 K for the Fe78Ga2P12C4B4 alloy and increases to 37 K for the 4 at%Co alloy. Besides, the addition of a small amount of Co was also found to be effective for increasing the saturation magnetization (Is) and decreasing the coercive force (Hc). These glassy ribbon alloys exhibit good soft magnetic properties of 1.36 to 1.43 T for saturation magnetization and 1.5 to 1.8 A/m for coercive force. The largest saturation magnetization (Is) is 1.43 T for Fe74Co4Ga2P12C4B4 alloy. Based on its large ΔTx, we tried to prepare bulk glassy Fe74Co4Ga2P12C4B4 rods in a cylindrical form with different diameters. The glassy single phase was obtained in the diameter of 0.5 and 1.0 mm. These bulk glassy alloys also exhibit high saturation magnetization of about 1.42 T.
  • Effect of Adsorbed Oxygen on Initial Growth of Fe on Cu(111)

    pp. 1056-1059

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    DOI:10.2320/matertrans.42.1056

    Epitaxial growth mode and morphology of Fe on Cu(111), affected by the pre-adsorbed oxygen in the form of a “29” structure of the Cu(111) surface, were investigated by scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED). According to RHEED observation, the γ-Fe structure was kept to more than 3 monoatomic layers (ML) and fcc and bcc structures coexisted at 5 ML . In contrast, the RHEED pattern of 1 ML Fe on a clean surface already shows the coexistence of fcc and bcc structures. By STM observation, at about 0.4 ML of Fe on the oxygen absorbed Cu(111) surface, small islands appeared on the surface with a high density instead of decorating the step edges with large islands about 6 ML high for the iron growth on a clean Cu(111) surface. As the surface diffusion was suppressed due to oxygen adsorption, the growth mode was significantly changed. However, no strong surfactant effect was observed as in Fe growth on oxygen-adsorbed Cu(001) surface where the γ-Fe structure was kept to 45 ML with a surface O (\\sqrt2×\\sqrt2)R45° reconstruction and all the oxygen floated to the surface.
  • Effects of Composition and Annealing on Shape Memory Behavior of Ti-Rich Ti-Ni Thin Films Formed by Sputtering

    pp. 1060-1067

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    DOI:10.2320/matertrans.42.1060

    Ti-rich Ti–Ni thin films of Ti–45.2, 46.1, 47.0, 47.9, 48.5 at%Ni were prepared by sputtering. The sputter-deposited thin films were annealed at 773, 823 and 873 K for 1 h. Transmission electron microscopy revealed that Ti–45.2 at%Ni thin films contain randomly oriented Ti2Ni particles, while the other films contain Ti2Ni precipitates with the same orientation as that of the TiNi matrix. In addition to these Ti2Ni precipitates, GP zones were also observed in Ti–47.9 and 48.5 at%Ni thin films annealed at 773 K for 1 h. The shape memory behavior of these Ti–Ni thin films was investigated with a thermomechanical tester. The following results were obtained with respect to the martensitic transformation. (1) The transformation temperature decreases with increasing Ti content and decreasing annealing temperature. However, thin films containing GP zones show low transformation temperatures despite of the low Ti contents. (2) The critical stress for plastic deformation associated with the transformation increases with increasing Ti content and decreasing annealing temperature. GP zones seem to increase the critical stresses of Ti–47.9 and 48.5 at%Ni thin films annealed at 773 K for 1 h. (3) The recoverable strain increases with decreasing Ti content and decreasing annealing temperature. Especially, the thin films containing GP zones show large recoverable strains. On the other hand, the R-phase transformation showed the following features. (1) The transformation temperature is insensitive to film composition and heat treatment, being almost 335 K, though thin films containing GP zones show relatively low R-phase transformation temperatures. (2) No residual strain is detected for any specimen after a thermal cycle. These characteristics suggest that the shape memory effect due to the R-phase transformation is suitable for practical use.
  • Crystallization and Glass Forming Ability of Supercooled Pd-Cu-Ni-P Melt

    pp. 1068-1073

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    DOI:10.2320/matertrans.42.1068

    The thermal stability, GFA and crystallization behavior of a highly purified Pd40Cu30Ni10P20 supercooled liquid was examined. Under continuous cooling, the critical cooling rate for glass formation for the highly purified alloy is the same as that (0.100 K/s) for the fluxed ordinary alloy, though the magnitude of supercooling is enhanced by about 80 K upon by the purification treatment. The enhancement is presumed to result from the elimination or decrease of the quenched-in nuclei. The Time-Temperature-Transformation (TTT) diagram was constructed experimentally under isothermal annealing of the supercooled melt. The nose point in the TTT diagram is located at 683 K and 80 s. By utilizing the high thermal stability of the supercooled liquid, in-situ TEM observation was successfully carried out. In the isothermal annealing at 683 K, crystalline particle with a diameter of about 15 nm abruptly precipitated from the molten particle with a diameter of 40 nm and no significant grain growth was observed during further annealing. The incubation time was measured to be 3180 s. This value is much longer than that of the sample obtained by the HV/HT-DSC measurement. The difference between the two incubation times indicates that the nucleation is the event dominated by statistical probability.
  • Microstructures and Mechanical Properties of Equal Channel Angular Pressed 5083 Al Alloy

    pp. 1074-1080

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    DOI:10.2320/matertrans.42.1074

    An ultra-fine grained (UFG) structure was introduced in a commercial 5083 Al alloy with an initial grain size of ∼ 200 \\micron using the equal channel angular pressing (ECAP) technique. ECAP was successfully conducted at 473 K on the same sample up to a total of 8 pressings through the die such that the sample was rotated 180° around its longitudinal axis between pressings. The microstructure was reasonably homogeneous after a single pressing and consisted of parallel bands of elongated substructure having an average width of 0.2 \\micron and an average length of 0.8 \\micron. An equiaxed ultra-fine grained structure of ∼ 0.3 \\micron was obtained in the present alloy after 4 pressings. The ultra-fine grains were thermally stable at 523 K . The yield stress of as-received 5083 Al alloy was 129 MPa, whereas it increased to 249 MPa after a single pressing and finally became 290 MPa after 8 pressings, which was superior to that of a conventional 5083-H321 Al alloy. In addition, in this study, the feasibility of low temperature superplasticity of a UFG 5083 Al alloy was examined. It was found that the 5083 Al alloy with a grain size of ∼ 0.3 \\micron exhibited a superplastic-like behavior with elongation to failure in excess of 200% below 523 K.
  • The Structure of Type-II Al-Ni-Co Decagonal Quasicrystal Studied by Atomic-Scale Electron Microscopic Observations

    pp. 1081-1084

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    DOI:10.2320/matertrans.42.1081

    The structure of an Al–Ni–Co decagonal quasicrystal, showing electron diffraction patterns called “type-II”, in an Al71.5Ni12.5Co16 alloy, has been examined by high-angle annular detector dark-field scanning transmission electron microscopy and high-resolution transmission electron microscopy. Its structure has a columnar cluster of atoms with a decagonal section of about 2 nm in diameter as a structural unit, and forms a heterogeneously aperiodic tiling of pentagonal and rhombic tiles in a two-dimensional arrangement of the columnar clusters. Translational arrangements of pentagons and rhombuses in the tiling can be observed locally. Also, the characteristic of a NaCl-type ordered arrangement of two kinds of columnar clusters of atoms, which have different orientations of pentagonal symmetry, is observed in the tiling.
  • Environmental Effects on Fatigue Crack Growth in Austempered Ductile Irons

    pp. 1085-1094

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    DOI:10.2320/matertrans.42.1085

    Corrosion fatigue crack growth (CFCG) behavior has been investigated for two different grades of austempered ductile irons (ADIs) which were heat treated respectively at two different austempering temperatures, 573 and 633 K . Fatigue crack growth (FCG) tests using compact tension (CT) specimens were conducted in humid air, lubrication oil and three aqueous environments: distilled water (pH=7.6), sodium chloride solution (pH=7.3) and sulfuric acid solution (pH=3). The results showed that fatigue crack growth rates (FCGRs) at 20 Hz in humid air and aqueous environments were comparable and faster than those in lubrication oil. The FCGRs in aqueous environments were found to increase with a reduction in loading frequency or with an increase in solution temperature. The crack growth enhancement by different aqueous media appeared to be equivalent, although the ion species and pH in the bulk solutions were different. The degree of sensitivity to environmental effects for CFCG response in ADI was not essentially influenced by the austempering treatment as the relative difference in FCGR between the two grades of ADIs was not considerably changed in any of the studied.
  • Multicomponent Chemical Short Range Order Undercooling and the Formation of Bulk Metallic Glasses

    pp. 1095-1102

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    DOI:10.2320/matertrans.42.1095

    Multicomponent chemical short range order (MCSRO) undercooling principle was proposed as a criterion to evaluate the glass forming ability (GFA) of alloys. The thermodynamic model of MCSRO was established in order to calculate the MCSRO undercooling. Comprehensive numerical calculations using MCSRO software were conducted to obtain the composition dependence of the MCSRO undercooling in Zr–Ni–Cu, Zr–Si–Cu, and Pd–Si–Cu ternary systems. By the MCSRO undercooling criterion, the composition ranges with great GFA in these ternary systems were predicated. It is shown that the prediction by MCSRO undercooling principle is in general consistent with the well-known empirical rules proposed by Inoue. According to the MCSRO undercooling principle, the composition with great GFA in the range of Zr–Ni–Cu system is Zr=62.5–75, Cu=5–20 and Ni=12.5–25, (Ni/Cu=1–5), which is in agreement with the recent experimental results of the quaternary Zr–Ni–Cu–Ti alloy. The calculation also illustrates that Pd-based alloys which easily form a metallic glass exhibit an extraordinary deep MCSRO undercooling. By calculating TTT curves in Zr–Ni–Cu system, it is shown that the average critical cooling rates are estimated to be as low as ∼ 100 K/s for the alloy with deep MCSRO undercooling. As an example of an effective bulk metallic glass (BMG) design method, a new kind of Zr–Si–Cu BMG is explored based on the MCSRO undercooling principle.
  • Structural Change from Amorphous to Nano Icosahedral Quasicrystalline Phase with Quenching Rate in Zr-Pt Binary Alloy

    pp. 1103-1108

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    DOI:10.2320/matertrans.42.1103

    Structural change with quenching rate in the Zr80Pt20 alloy was studied by controlling the roll speed in the melt-spinning technique. The as-quenched ribbon had an amorphous structure at a roll speed of 50 ms−1. An icosahedral quasicrystalline phase was directly formed at a roll speed in the range of 40 to 30 ms−1. The critical roll speed for the formation of the icosahedral phase was 25 ms−1, where the volume fraction of the icosahedral phase was very small. Several crystalline phases precipitated in the melt-spun ribbon at roll speeds below 20 ms−1. The DSC curve of the sample prepared at 50 ms−1 clearly exhibited two exothermic peaks. The first exothermic peak at 717 K, corresponded to the transformation from the amorphous to the icosahedral phase and the second peak was due to the decomposition reaction of the icosahedral phase. The size of the icosahedral phase in the as-quenched and annealed samples lay in the diameter range of 5 to 20 nm and the particles were distributed homogeneously. Thus, it was clarified that the nano icosahedral phase can be formed in the as-quenched state from the melt and in the annealed state from the amorphous phase in the Zr80Pt20 binary alloy. The icosahedral phase transformed to Zr, ZrPt, Zr9Pt11 and Zr5Pt3 phases at a temperature of approximately 900 K . The formation of the nano scale icosahedral phase indicated the possibility that icosahedral short-range order exists in the melted state of the Zr–Pt binary alloy. The strong chemical affinity between Zr and Pt probably contributed to the stabilization of the icosahedral short-range order and restraining the long-range rearrangement of constitutional elements to form a stable crystalline phase, which is an important factor in the formation of an icosahedral phase.
  • Corrosion Behavior of a Zr-Base Bulk Glassy Alloy and Its Crystallized Counterparts

    pp. 1109-1111

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    DOI:10.2320/matertrans.42.1109

    A Zr-base bulk glassy alloy with a nominal composition of Zr52.5Cu17.9Ni14.6Al10Ti5 (at. percents) was evaluated by conducting corrosion experiments in 0.5 kmol·m−3 H2SO4 and 0.5 kmol·m−3 NaCl electrolyte respectively using electrochemical measurements. Potentiodynamic polarization experiments were carried out with a scan rate of 20 mV/min. Passive current density and pitting corrosion potential were evaluated from potentiodynamic polarization curves. The results indicate that the full glass has a higher resistance to general corrosion and good resistance to pitting corrosion than the counterparts of the partially and completely crystallized glassy alloy. The resistance to general corrosion decreases with increase of the volume fraction of nanocrystalline phases. This is mainly due to the inhomogeneity of microstructure, the structural defects such as grain boundaries, as well as the formation of crystalline phases that have lower corrosion resistance in the partially and completely crystallized samples.
  • Electronic States Calculation of Cobalt by Using the DV-Xα Cluster Method

    pp. 1112-1118

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    DOI:10.2320/matertrans.42.1112

    Using the DV-Xα cluster method, two alloying parameters were calculated in fcc Co. One was the bond order (Bo) which is a measure of the covalent bond strength between atoms and the other was the d-orbital energy level (Md) of alloying transition elements. These two alloying parameters were found to change monotonically with the atomic number of alloying 3d transition metals, M . The changes in the Bo with alloying elements could be understood in terms of the difference electron density maps. Also, the Co–M binary phase diagrams could be classified well using Bo-Md map. In addition, local spin moments at the alloying 3d transition metals were obtained. These results will provide us some guide for the design of cobalt-based alloys.
  • Fabrication of Al2O3/ZrO2 Micro/Nano-Composite Prepared by High Energy Ball Milling

    pp. 1119-1123

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    DOI:10.2320/matertrans.42.1119

    An Al2O3-5 vol%ZrO2 micro/nano-composite was successfully fabricated by sintering the high-energy ball milled mixture of commercial Al2O3 powder and zirconium alkoxide. The composite could be sintered to a nearly full density at 1450°C in the ambient atmosphere. The microstructure consisted of the fine dispersions of ZrO2 particles, both intra-granular type of ∼ 50 nm and inter-granular type of ∼ 0.2 \\micron, and Al2O3 matrix grains of ∼ 0.6 \\micron. The role of high energy ball milling in the present nano-composite formation was primarily the refinement of the Al2O3 grains and the introduction of strains in them.
  • Oxygen Permeability of Y2SiO5

    pp. 1124-1130

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    DOI:10.2320/matertrans.42.1124

    The oxygen permeation through a wafer of Y2SiO5 has been measured in the temperature range from 1973 to 2033 K and the oxygen permeability constant of Y2SiO5 has been determined, because Y2SiO5 is favorable for the outer layer of our proposed oxidation protection double layered coating on C/C composites. We presented a data reduction, which can separate lattice diffusion through the wafer from other contributions. The experimental data are in agreement with previous data reported in the literature for Ca stabilized zirconia and alumina in a maximum error of 10−12 kg/(m·s). The oxygen permeability constant of Y2SiO5 at 1973 K is 10−10 kg/(m·s). A 100 \\micron-thick Y2SiO5 outer layer would extend a life time of a 100 \\micron-thick SiC inner layer up to 70 hours, assuming that the SiC layer is consumed by oxygen permeated through the Y2SiO5 layer. The mechanism of the oxygen transport is discussed in accordance with the activation energy of the oxygen permeation and relationship between the oxygen permeability constants and oxygen partial pressure. Experimental results indicate that vacancy diffusion is dominant below 1913 K and interstitial diffusion is dominant above 1913 K . It is estimated that there is a mechanism to switch vacancy diffusion to interstitial diffusion with increasing temperature, because interstitial diffusion should not be active at higher temperature.
  • High-Resolution Electron Microscopy of a Subgrain Boundary in Strontium Titanate Single Crystal

    pp. 1131-1134

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    DOI:10.2320/matertrans.42.1131

    The microstructure of subgrain boundaries in as-annealed non-doped SrTiO3 single crystals was studied. When the crystal quality is good, it is difficult to observe lattice defects using only transmission electron microscopy (TEM). In such cases, pre-observation by X-ray topography is useful. Hence, the author used both TEM and X-ray topographic techniques. X-ray topographs showed the defect distribution throughout the crystals. [011]-Type crystals generally have subgrain textures. High-resolution transmission electron microscopy revealed that the subgrain boundaries were small-angle tilt boundaries formed by partial dislocations of 1⁄2⟨110⟩ Burgers vectors and there were no segregation of impurities.
  • Refinement of γ Precipitate with Melt Undercooling in Nickel-based Superalloy

    pp. 1135-1138

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    DOI:10.2320/matertrans.42.1135

    The effect of melt undercooling on the γ precipitation is investigated in nickel-based superalloy by employing the method of molten salt denucleating combined with thermal cycles. The application of SEM and TEM technique allows to further confirm and interpret γ formation mechanism here. In connection with the classical nucleation theory, it is found that not only the size of precipitated γ but its distribution in the as-solidified structure is also drastically influenced by the melt undercooling.
  • The Effect of Calcium Addition to Magnesium on the Microstructure and Compositional Changes of Oxide Film Formed at High Temperature

    pp. 1139-1141

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    DOI:10.2320/matertrans.42.1139

    Microstructural and compositional changes of oxide films were investigated in pure magnesium and Mg–1.51 mass%Ca alloy oxidized at high temperature. The oxide films of pure magnesium formed at high temperature above 500°C were porous, whereas those of Ca-containing alloy were compact, thin and dense. It was found that the oxide films of Ca-containing alloy were basically in the amorphous state containing a high volume fraction of CaO, while those of liquid oxidation were identified as a mixture of amorphous and microcrystalline structure. The retardation and suppression of solid and liquid oxidation at high temperature in Ca-containing magnesium alloy was caused by the formation of compact amorphous oxide films.
  • Soft Magnetic Properties of (Fe, Co)-RE-B Amorphous Alloys with a Large Supercooled Liquid Region

    pp. 1142-1145

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    DOI:10.2320/matertrans.42.1142

    Fe-based amorphous alloys with high B concentrations in (Fe, Co)-RE-B system were found to exhibit a large supercooled liquid region (ΔTx) in a composition range 22.5–30 at%B, 0–30 at%Co and 2.5–6 at%RE . The ΔTx values exceeding 50 K were obtained for a series of Fe62Co9.5RE3.5B25 (RE=Pr, Nd, Sm, Gd, Tb, Dy, Er) amorphous alloys. The Fe62Co9.5RE3.5B25 amorphous alloys exhibit good soft magnetic properties of 1.35 to 1.43 T for saturation magnetization, 1.0 to 3.8 A/m for coercive force, and 8300 to 12500 for permeability at 1 kHz in the annealed state for 600 s at 773 K . The synthesis of the (Fe, Co)-RE-B amorphous alloys exhibiting a large supercooled liquid region and good soft magnetic properties with a high saturation magnetization is promising for future development as a new type of soft magnetic material.
  • Structure of a Pentagonal Quasicrystal in Al72.5Co17.5Ni10 Studied by High-Angle Annular Detector Dark-Field Scanning Transmission Electron Microscopy

    pp. 1146-1148

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    DOI:10.2320/matertrans.42.1146

    The structure of a pentagonal quasicrystal, which has been called “basic Co-rich state of Al–Co–Ni decagonal quasicrystals”, in an Al72.5Co17.5Ni10 alloy annealed at 900°C for 40 hr has been studied by high-angle annular detector dark-field scanning transmission electron (HAADF-STEM). An observed HADDF-STEM image clearly shows that there exist columnar clusters of atoms with a decagonal section of about 2 nm in diameter as a structural unit, and that the atomic clusters with the same orientation of pentagonal symmetry are arranged with a pentagonal quasiperiodic lattice. Thus, the basic Co-rich state can be concluded to be a pentagonal quasicrystal with a pentagonal quasiperiodic lattice.
  • Thermal and Mechanical Properties of Cu-Based Cu-Zr-Ti Bulk Glassy Alloys

    pp. 1149-1151

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    DOI:10.2320/matertrans.42.1149

    New Cu-based bulk glassy alloys of Cu–Zr–Ti system were formed by copper mold casting. The maximum diameter is 4 mm for the Cu60Zr30Ti10 alloy. The addition of Ti to Cu60Zr40 resulted in an increase in the glass-forming ability (GFA). As the Ti content increases, the glass transition temperature (Tg), crystallization temperature (Tx) and temperature interval of supercooled liquid region ΔTx(=TxTg) decrease, while the liquidus temperature (Tl) has a minimum of 1127 K around 20%Ti, leading to a maximum TgTl of 0.63 in the vicinity of 20 at%Ti. The high GFA of the Cu-based alloys was obtained at the compositions with high TgTl. The bulk glassy Cu60Zr30Ti10 alloy exhibits Vickers hardness of 660, Young’s modulus of 114 GPa, fracture strength of 2150 MPa and compressive plastic elongation of 1.7%. The finding of the Cu-based bulk glassy alloy with high TgTl above 0.60, high fracture strength above 2000 MPa and distinct plastic elongation is encouraging for future development as a new type of bulk glassy alloys which can be used for structural materials.

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