MATERIALS TRANSACTIONS
Online ISSN : 1347-5320
Print ISSN : 1345-9678
ISSN-L : 1345-9678
Volume 45 , Issue 7
Showing 1-50 articles out of 88 articles from the selected issue
  • Masataka Mizuno, Hideki Araki, Yasuharu Shirai
    2004 Volume 45 Issue 7 Pages 1964-1967
    Published: 2004
    Released: June 24, 2005
    JOURNALS FREE ACCESS
    Our recent positron lifetime measurements for metal oxides suggest that positron lifetimes of bulk state in metal oxides are shorter than previously reported values. We have performed theoretical calculations of positron lifetimes for bulk and vacancy states in MgO and ZnO using first-principles electronic structure calculations and discuss the validity of positron lifetime calculations for insulators. By comparing the calculated positron lifetimes to the experimental values, it was found that the semiconductor model well reproduces the experimental positron lifetime. The longer positron lifetime previously reported can be considered to arise from not only the bulk but also from the vacancy induced by impurities. In the case of cation vacancy, the calculated positron lifetime based on semiconductor model is shorter than the experimental value, which suggests that the inward relaxation occurs around the cation vacancy trapping the positron.
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  • Masahito Yoshino, Masahiko Morinaga, Akihiro Shimode, Kousuke Okabayas ...
    2004 Volume 45 Issue 7 Pages 1968-1972
    Published: 2004
    Released: June 24, 2005
    JOURNALS FREE ACCESS
    A universal relation between electron density minima and ionic radii, was discovered from the first principles calculations of electronic structures in ceramics over 60 species, including oxides, borides, carbides, nitride and fluorides. Every ceramic falls on a curve of log(ρminZ−3) vs. 2(Z/n)rmin, where ρmin is the minimum electron density in the line linking the first-nearest-neighbor nuclei, rmin is the distance r at ρmin, Z is the atomic number, and n is the principal quantum number.
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  • Shingo Tanaka, Rui Yang, Masanori Kohyama, Takeo Sasaki, Katsuyuki Mat ...
    2004 Volume 45 Issue 7 Pages 1973-1977
    Published: 2004
    Released: June 24, 2005
    JOURNALS FREE ACCESS
    Atomic structures characterization of Al2O3(0001)/Cu nano-hetero interfaces has been performed by the first-principles pseudopotential method and in cooperation with HRTEM observations. The physical properties of the interfaces depend strongly on the interface stoichiometry. Bonding nature of the O-rich (O-terminated) interface is explained as strong covalent and ionic interactions, whereas that of the stoichiometric (Al-terminated) interface is weak covalent and electrostatic image interactions. The O-terminated interface has quite larger adhesive energy than that of the stoichiometric one. Recent HRTEM observations of the Al2O3(0001)/Cu interface have confirmed the O-terminated interface. However, the observed incoherent interface is not the same as an ideal coherent interface obtained by the first-principles. We explain the relationship between the present coherent interface and the practical incoherent one.
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  • Atsuto Seko, Natsuki Odagaki, Shigeto R. Nishitani, Isao Tanaka, Hiroh ...
    2004 Volume 45 Issue 7 Pages 1978-1981
    Published: 2004
    Released: June 24, 2005
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    We extend our newly proposed calculation method of precipitate nucleation free energy to ternary systems. This method utilized first principles calculations for enthalpy change and interface energy, and the Bragg-Williams approximation for entropy loss from scattered atoms condensing into a cluster. The effect of Ni addition on copper precipitation in the Fe-Cu system was examined by this method. It was revealed that added Ni prefers segregation at the matrix/cluster interface, and reduces the activation energy barrier as well as the interface energy.
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  • A. Togo, I. Tanaka, K. Murase, T. Yamamoto, T. Suga, E. Matsubara
    2004 Volume 45 Issue 7 Pages 1982-1986
    Published: 2004
    Released: June 24, 2005
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    First principles calculations of hydrated polymolybdates complexes have been made with an atomic orbital basis molecular orbital method. Hydration effects are taken into account by the conductor-like screening model (COSMO) using dielectric constant of water. Hydrated heptapolymolybdate, Mo7O246−, shows a low symmetry structure, which agrees well to experimental results, i.e., X-ray diffraction of crystalline salts and X-ray absorption fine structure of the hydrated complex. Contrary to that, the hydrated heteropolymolybdate, NiMo6O2410− prefer to exhibit the high symmetry structure. Inspection of the electronic states found that the Ni ion exhibits trivalent state or d 7 configuration in a formal sense. Jahn-Teller distortion around Ni is therefore evident. Such distortion cannot be found in CrMo6O249− or CoMo6O249−.
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  • Tomoyuki Yamamoto, Fumie Yamashita, Isao Tanaka, Eiichiro Matsubara, A ...
    2004 Volume 45 Issue 7 Pages 1987-1990
    Published: 2004
    Released: June 24, 2005
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    First principles calculations of rutile-type TiO2:S have been performed to investigate the effect of sulfur solutes on the electronic structure. Plane-wave pseudopotentials method has been employed and atomic relaxations were fully taken into account. All possible geometric configurations for sulfur solutes within a 12-atoms supercell have been examined changing sulfur concentration of x = 0, 0.25, 0.5, 0.75 and 1. Theoretical direct band gap is found to decrease as sulfur concentration is increased. The dependence on the sulfur concentration is weaker than that was predicted in literature. Both the optimization of solute configuration and atomic relaxation are found to be essential for quantitative evaluation of the electronic structures in the alloy.
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  • Tomoyuki Yamamoto, Teruyasu Mizoguchi, Kazuyoshi Tatsumi, Isao Tanaka, ...
    2004 Volume 45 Issue 7 Pages 1991-1993
    Published: 2004
    Released: June 24, 2005
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    First principles calculations have been carried out to investigate the core-hole effects on the theoretical fine structures of the X-ray absorption spectra of MgF2 and ZnF2 at F K-edge. Significant differences are found between the calculated spectral fine structures with and without core-holes. Experimental profiles of the near-edge X-ray absorption fine structures are well reproduced by the theoretical ones when the core-hole effect is introduced. The dependence of supercell size on the theoretical fine structures is also examined.
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  • Kouichi Hayashi, Yukio Takahashi, Ichiro Yonenaga, Eiichiro Matsubara
    2004 Volume 45 Issue 7 Pages 1994-1997
    Published: 2004
    Released: June 24, 2005
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    We measured the X-ray fluorescence holograms of Si0.999Ge0.001, Si0.8Ge0.2, Si0.5Ge0.5, Si0.2Ge0.8 and Ge single crystals, and reconstructed the images of second neighbor atoms around Ge. The positional shift of the atomic image across the whole composition range was three times larger than the value predicted from the difference in the lattice constants of pure Si and Ge. We found that imaginary part of the reconstruction strongly affects the positions of the atomic images. Thus, using the negative real parts, the atomic image became sharp and its shift dependent upon Ge composition comes to the reasonable values.
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  • Yoshitaka Ohkubo, Yukihiro Murakami, Tadashi Saito, Akihiko Yokoyama, ...
    2004 Volume 45 Issue 7 Pages 1998-2002
    Published: 2004
    Released: June 24, 2005
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    The nuclear-electric-quadrupole interactions at 111Cd and 117In nuclei arising from 111mCd and 117Cd, respectively, chemically introduced in pyrochlore ferroelectric Cd2Nb2O7 (TC = 196 K) were studied using the perturbed-angular-correlation technique. At temperatures above TC, there is one type of Cd sites. However, at liquid nitrogen temperature, below TC, there are two types of Cd sites. The ratio of the electric quadrupole frequency of 117In to that of 111Cd is anomalously deviated from a value expected from the purely ionic In and Cd ions in the same lattice environment.
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  • Zhen Zhou, Takanao Komaki, Atsushi Koizumi, Toshitaka Komori, Masahito ...
    2004 Volume 45 Issue 7 Pages 2003-2007
    Published: 2004
    Released: June 24, 2005
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    The photoluminescence (PL) around the wavelength of 1.54 μm from the Er-containing ZnO specimens was measured at room temperature by the indirect excitation of the He-Cd laser (325 nm). The PL intensity varied greatly with the Er concentration and the specimen preparation conditions as well. The specimens with the Er content of about 2.6 at%, sintered at 1623 K, and cooled quickly to room temperature in air showed the highest PL intensity at 1.534 μm. The local structure of the optically active Er centers in ZnO was also discussed, and the appropriate optically active center might be the Er ions existing in the grain boundaries.
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  • Takashi Ohtomo, Saburo Nasu, Masahiro Miyasaka, Shotaro Morimoto, Koji ...
    2004 Volume 45 Issue 7 Pages 2008-2011
    Published: 2004
    Released: June 24, 2005
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    Hf/Fe multilayers are soft magnetic materials and are a good candidate for magnetic heads of storage devices. We have investigated Hf/Fe multilayers using time-differential perturbed angular correlation (TDPAC) spectroscopy of 181Ta (← 181Hf), examining specifically the magnetic nature of the non-magnetic Hf layers sandwiched between two ferromagnetic Fe layers. We prepared multilayer samples with Hf layers of various thicknesses, [Hf(x nm)/Fe]n (x=0.5, 1.0, 2.0, 4.8, 10.0). Three different hyperfine magnetic fields 53(1) T, 43(1) T and 2(1) T were detected in the samples of [Hf(2 nm, 4.8 nm, 10 nm)/Fe]n. The hyperfine magnetic field of 53(1) T corresponds to Hf atoms in a turbulent interface between the Hf and Fe layers. The value of 43(1) T corresponds to Hf atoms inside Hf layers within 0.5 nm from the interface. The value of 2(1) T corresponds to Hf atoms inside the Hf layers within 3.5 nm of the interface.
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  • Shunsuke Fukami, Akichika Ohno, Nobuo Tanaka
    2004 Volume 45 Issue 7 Pages 2012-2017
    Published: 2004
    Released: June 24, 2005
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    Three kinds of FePt-MgO granular films were prepared by a vacuum successive deposition of MgO, Pt, Fe and MgO on a cleaved surface of sodium chloride below 673 K. Their microstructures, electronic structures and magnetic properties were studied by high-resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS) and measurement with a superconducting quantum interference device (SQUID) magnetometer. The TEM observations and selected area electron diffraction patterns revealed that the samples mainly consist of few nm-sized FePt clusters embedded in MgO films with L10-ordered structure and c-axis perpendicular to the film surface. Size effect on the stability of L10 phase in the FePt nano-clusters was directly observed in [MgO/Fe(0.38 nm)/Pt(0.30 nm)/MgO] and the critical size of the transition from L10 to A1 phase was estimated as around 2 nm, that can be considered as smaller than effective size for the transition from ferromagnetism to superparamagnetism. Coercivity of [MgO/Fe(1.0 nm)/Pt(0.8 nm)/MgO] was 1.2 × 105 A/m. The Fe-L2,3 white-line ratios of the present samples measured by EELS were about 4.0, independently on the incident direction of electron beam. The higher white-line ratio may be attributed to their high-spin state by a change of 3d-band structure owing to the hybridization of d-bands between Fe and Pt atoms.
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  • Tomoko Yoshida, Mikio Sakai, Tetsuo Tanabe
    2004 Volume 45 Issue 7 Pages 2018-2022
    Published: 2004
    Released: June 24, 2005
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    In-situ optical reflection measurements have been performed for a Si(100) surface under H+ irradiation to study dynamic change in the electronic structure of the silicon. The relative reflectance at 370 nm decreased almost linearly with displacement per atom (dpa) and was recovered by hydrogen release by 423 K annealing, suggesting that the change in the relative reflectance at 370 nm originates from the increase of hydrogen bonded to Si. With increasing H+ irradiation, the relative reflectance around 700 nm increased due to the formation of Si-H phase near the surface silicon. Furthermore, the broad minima of the relative reflectance shifted from 600 to 500 nm by H+ irradiation, probably correlating with amorphization or appearance of silicon micro/nano-crystal by displacement effect.
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  • Teruyasu Mizoguchi, Tomoyuki Yamamoto, Takeo Suga, Masahiro Kunisu, Is ...
    2004 Volume 45 Issue 7 Pages 2023-2025
    Published: 2004
    Released: June 24, 2005
    JOURNALS FREE ACCESS
    First principles calculations of L3 XANES/ELNES of GaN and InN with both wurtzite and zinc-blende structures have been made using OLCAO (orthogonalized linear combinations of atomic orbitals) method. Supercells with more than 100 atoms were employed. A core-hole was rigorously included in the calculation, and the photo absorption cross section (PACS) between the initial and final states was computed. Quantitative reproduction of experimental spectrum that is available in literature can be found when the PACS was computed. Although spectral shapes of two phases look similar, characteristic differences are predicted to appear at the first peak of the L3 XANES/ELNES. The first peak is notably broader in the zinc-blende phases. The origin of the broadness is analyzed using partial density of unoccupied states (PDOS) and Mulliken charge. We then conclude that the broadness can be related to greater covalency of the zinc-blende phase as compared to the wurtzite phase.
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  • Mikhail G. Brik, Takugo Ishii, Alexandra M. Tkachuk, Isao Tanaka
    2004 Volume 45 Issue 7 Pages 2026-2030
    Published: 2004
    Released: June 24, 2005
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    Crystal field analysis of the energy level structure of lithium yttrium fluoride doped with trivalent dysprosium was performed. Assignment of absorption lines in experimental spectrum of absorption was made in a wide spectral region: from IR to UV. Crystal field parameters for the whole series of trivalent rare earth ions in LiYF4 crystal were obtained. The values of the intermediately coupled Lande g-factor were calculated and used to estimate the EPR g-factors g||, g for LiYF4:Dy3+.
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  • Masato Mogi, Tomoyuki Yamamoto, Teruyasu Mizoguchi, Kazuyoshi Tatsumi, ...
    2004 Volume 45 Issue 7 Pages 2031-2034
    Published: 2004
    Released: June 24, 2005
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    High-resolution X-ray absorption spectra of Al, AlN and Al2O3 are measured at the Al K-edge, which have revealed a chemical shift in the threshold energy and significant differences in the spectral fine structures among the three compounds. In order to interpret the chemical shift and the fine structures of these spectra, first-principles calculations using the full-potential linearized augmented plane wave method within the density functional theory are carried out, taking into account the core-hole effect. The resultant theoretical spectra quantitatively reproduce both the chemical shift and the spectral fine structures of the experimental ones. The dependence of the theoretical spectra on the supercell size is also examined.
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  • Sarantuya Myagmarjav, Hideyuki Takahashi, Yoji Sunagawa, Katsutoshi Ya ...
    2004 Volume 45 Issue 7 Pages 2035-2038
    Published: 2004
    Released: June 24, 2005
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    The characterization of Ni-Zn/TiO2 nanocomposite synthesized by the liquid-phase selective-deposition method was studied. Nanoparticles were well dispersed and stabilized by the selective deposition onto TiO2 surface. The particle size was decreased with increasing the amount of Zn added, thus the catalytically active Ni surface area was increased. The selective deposition onto TiO2 surface and addition of Zn to the nanoparticles promoted the catalytic activity of Ni-Zn nanoparticles, e.g. the catalytic activity of Ni-Zn/TiO2 for 1-octene hydrogenation was ca. 10 times higher than that of the unsupported Ni nanoparticles. Ni in the nanocomposite was assigned as metallic, although their surface was oxidized under the atmospheric condition, but Zn and B were deposited as their oxide.
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  • Takeo Suga, Teruyasu Mizoguchi, Masahiro Kunisu, Kazuyoshi Tatsumi, To ...
    2004 Volume 45 Issue 7 Pages 2039-2041
    Published: 2004
    Released: June 24, 2005
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    Silicon nitride thin film was fabricated by pulsed laser deposition using KrF excimer laser and a silicon nitride compact as a target. The deposition was carried out on Al2O3 (0001) at 1173 K in N2 gas pressure of 0.27 Pa. The X-ray diffraction did not provide any structural information of the deposited thin films except that it is composed of amorphous and/or micro-crystalline structure. X-ray absorption near edge structures at Si-K edge revealed that local arrangement of Si is not random. It should be composed of SiN4 unit similar to the case of α-Si3N4 crystal. Metallic Si component cannot be found in XANES.
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  • Kazuya Otsuka, Katsuyuki Matsunaga, Atsutomo Nakamura, Seiichiro Ii, A ...
    2004 Volume 45 Issue 7 Pages 2042-2047
    Published: 2004
    Released: June 24, 2005
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    Ionic conductivities of yttria-stabilized zirconia (YSZ) single crystals deformed at high-temperature were measured by the AC impedance method. A correlation between ionic conductivity and dislocation structures of deformed YSZ single crystals were investigated. Electrical conductivity measurements of the deformed YSZ crystals were performed for two different current directions of [110] and [111]. The [110] direction is parallel to edge dislocations introduced by the primary slip system, while the [111] direction is normal to the edge dislocation lines. Transmission electron microscopy observations showed that the dislocations due to the primary slip system of (001)[110] were mainly generated at the strain of around 1% strain, while the secondary slip systems, such as (111)[101] and (111)[011], were also activated at about 10% strain. It was found that the deformed samples with larger strains exhibited higher electrical conductivities irrespective of the measured current directions. However, the electrical conductivity along [111] was higher than that along [110], suggesting that mobility of oxygen ion is sensitive to the dislocation structures. From the activation energy for oxygen diffusion in the deformed samples, it was found that the oxygen migration enthalpy for deformed samples became smaller than that for undeformed samples, whereas the association enthalpy for the deformed samples became larger. The increase in the association enthalpy might be due to interaction between oxygen vacancies and dislocations. It is thus considered that oxygen vacancies concentrate around dislocations, and are able to move very quickly along the dislocation lines.
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  • Masao Yonemura, Takashi Kamiyama, Yoji Kawamoto, Ryoji Kanno
    2004 Volume 45 Issue 7 Pages 2048-2055
    Published: 2004
    Released: June 24, 2005
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    Low-temperature structures of the lithium manganese spinels were determined using TOF neutron Rietveld analysis. The spinels LiMn2O4-δ with different δ values, (0.016, 0.040, and 0.132) showed the cubic-orthorhombic phase transitions, and the lattice distortion decreased with decreasing δ. Although no anomaly corresponding to the cubic-orthorhombic phase transition was observed in the DSC curve for the spinel with δ ∼ 0.016, the transition was observed by the structure analysis, which is consistent with the broad Cp anomaly at 250 K. The cubic-orthorhombic phase transition is closely correlated to the existence of the vacancy. The charge disproportionation into trivalent and tetravalent state proceeds gradually with decreasing temperature, and the extent of the disproportionation is dependent on the vacancy. Based on the structure analysis, the phase transitions in the spinel are discussed.
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  • Masahito Yoshino, Hiroshi Yukawa, Masahiko Morinaga
    2004 Volume 45 Issue 7 Pages 2056-2061
    Published: 2004
    Released: June 24, 2005
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    The local electronic structures are simulated for a perovskite-type oxide with four polymorphous phases, SrZrO3, using the DV-Xα molecular orbital method. It is found that a series of phase transitions occurs at certain temperatures so as to retain not only the Zr-O bond strength, but also the Sr-O bond strength, by the tilting of ZrO6 octahedra and the attendant accommodation to the Zr-O and the Sr-O interionic distances. The occurrence of such smart phase transitions is observed in other perovskite-type oxides, SrRuO3 and SrHfO3. Another type of phase transition occurs by the cooperative ionic displacements along the [100] direction in BaTiO3. The differences in the local chemical bond are discussed between the two types of phase transitions.
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  • Hisao Yoshida, Leny Yuliati, Tomoyo Hamajima, Tadashi Hattori
    2004 Volume 45 Issue 7 Pages 2062-2067
    Published: 2004
    Released: June 24, 2005
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    A series of silica-supported cerium oxide samples having various cerium content were prepared by conventional impregnation method, followed by calcination in air. Ce LIII-edge XANES study revealed that highly dispersed cerium oxide species on silica surface were predominantly in trivalent oxidation state and cerium oxide nano-particles on silica contain mainly tetravalent cation like as CeO2 bulk oxide. This elucidated that UV absorption band at 265 nm is assignable to Ce(III) species dispersed on silica.
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  • Takahiro Nakamura, Eiichiro Matsubara, Nobuaki Sato, Atsushi Muramatsu ...
    2004 Volume 45 Issue 7 Pages 2068-2072
    Published: 2004
    Released: June 24, 2005
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    Titanium oxide films were formed using a titanium target by a pulsed laser deposition (PLD) technique under different oxygen pressures from 10−6 to 100 Pa. Their densities and thickness were evaluated from total external X-ray reflection profiles and their atomic structures were determined by grazing incidence X-ray scattering (GIXS), and their surface morphology was observed by atomic force microscopy (AFM). The atomic structures of the films were gradually changed from metal titanium through TiO to Rutile-type titanium dioxide TiO2. The film surfaces became rough above 13.3 Pa oxygen pressure during deposition. Their UV transmission spectroscopy was also observed to predict their photocatalytic activities.
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  • Koji Morita, Keijiro Hiraga, Byung-Nam Kim, Yoshio Sakka
    2004 Volume 45 Issue 7 Pages 2073-2077
    Published: 2004
    Released: June 24, 2005
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    The effect of second phase dispersion on high-strain-rate superplasticity was examined in tetragonal ZrO2 dispersed with 30 vol% MgAl2O4 spinel. The spinel particle enhances the diffusivity of ZrO2 by supplying small amounts of aluminum and magnesium into ZrO2 and suppresses grain growth by grain boundary pinning. After superplastic flow, the spinel particles highly elongate along the tensile direction. In the spinel particles, intragranular dislocations were observed, indicating that the spinel particles may contribute to the relaxation of stress concentrations around grain junctions exerted by grain boundary sliding. Comparison with earlier studies suggests that the dispersion of spinel particles can attain high-strain-rate superplasticity in tetragonal ZrO2 through providing the following positive factors simultaneously; (i) the suppressed grain growth, enhanced accommodation process due to the accelerated (ii) diffusivity and (iii) stress relaxation.
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  • Shuichi Yasuda, Hidehiro Yoshida, Takahisa Yamamoto, Taketo Sakuma
    2004 Volume 45 Issue 7 Pages 2078-2082
    Published: 2004
    Released: June 24, 2005
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    High-temperature creep resistance in cations co-doped polycrystalline Al2O3 was examined by uniaxial compression creep test at 1250°C. The dopant oxides used in this study were 0.1 mol% of YO1.5, ZrO2, SrO, MgO and TiO2. The creep rate in Al2O3 was significantly changed by cations co-doping. For instance, Zr/Y co-doping suppressed the creep rate in Al2O3 by a factor of about 400. A high-resolution transmission electron microscopy (HREM) and nano-probe energy dispersive X-ray spectroscopy (EDS) analysis revealed that Y and Zr cations segregate along grain boundaries. The grain boundary diffusion in Al2O3 was supposed to be retarded by the segregation of Y and Zr cations. A first-principle molecular orbital calculation was made for cations co-doped Al2O3 and cation singly doped Al2O3 model cluster. The creep rate was correlated with the value of net charge in oxygen anion. The net charge of oxygen anion was one of the most important factors to determine the creep resistance in Al2O3.
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  • Young Kyu Cho, Duk Yong Yoon
    2004 Volume 45 Issue 7 Pages 2083-2090
    Published: 2004
    Released: June 24, 2005
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    The shapes and structures of many randomly selected grain boundaries in BaTiO3 with 0.1, 0.3 and 0.5 excess mol% of TiO2 sintered at 1250°C have been examined by TEM. Most of the grain boundaries have flat segments that coincide with low index planes of one of the grain pairs. These low index grain boundaries are likely to be singular. Singular boundary segments appear in a variety of shapes. In the 0.1 mol% Ti specimen, {011} boundary planes appear most frequently, while in the 0.5 mol% Ti specimen, {111} boundary planes are dominant. Some grain boundaries show fine steps with low index terraces indicating that they migrate by the lateral movement of the steps. This step growth mechanism is consistent with the observed slow growth of the matrix grains and abnormal growth of a few grains.
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  • Hui Gu
    2004 Volume 45 Issue 7 Pages 2091-2098
    Published: 2004
    Released: June 24, 2005
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    In many ceramic systems thin amorphous films of about 1 nm thickness often cover grain boundaries. These amorphous films play a key role not only in the formation of microstructures but also in the thermal-mechanical properties of ceramic materials. However, such thin amorphous layers could not be probed directly by an analytical electron beam. With the recent advances in spatially-resolved electron energy-loss spectroscopy technique, chemical and physical parameters of the thin films could be successfully derived using the “spectrum separation” approach. Basic characters and behaviors of variations for the inter-granular films are analyzed in a few silicon nitride (Si3N4) and silicon carbide (SiC) systems. The combined local chemical-structural information reveal new trends on microstructures and properties, and provides further insights in Si-based ceramic materials.
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  • Naoki Takata, Takashi Mizuguchi, Ken-ichi Ikeda, Hideharu Nakashima
    2004 Volume 45 Issue 7 Pages 2099-2105
    Published: 2004
    Released: June 24, 2005
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    In the present study, the energy and atomic structure of the ‹110› symmetric tilt boundaries in palladium were evaluated using the molecular dynamics (MD) simulation and the electronic structures of hydrogen in the bulk and on the grain boundaries of palladium were calculated using the discrete-variational Xα (DV-Xα) method. The MD simulation revealed that the energy of the ‹110› symmetric tilt boundary of palladium depended on the misorientation angle and that there were large energy cusps at the misorientation angles which corresponded to the (111)Σ3 and (113)Σ11 symmetric tilt boundaries. The atomic structure of all ‹110› symmetric tilt boundaries could consist of the combination of the (331)Σ19, (111)Σ3 and (113)Σ11 structural units and (110)Σ1 and (001)Σ1 single crystal units. The DV-Xα calculation showed that the interstitial hydrogen atoms in palladium induced the Pd-H chemical bond which had a different energy level than the Pd-Pd bond. The energy level and component of the Pd-H bonding on the grain boundaries in palladium were similar to those in the bulk palladium.
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  • Hidehiro Yoshida, Hitoshi Nagayama, Akihide Kuwabara, Taketo Sakuma
    2004 Volume 45 Issue 7 Pages 2106-2111
    Published: 2004
    Released: June 24, 2005
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    Temperature and strain rate dependence on high temperature elongation to failure in fine-grained ceramics is phenomenologically explained from grain growth behavior during deformation and the superplastic flow behavior. The elongation to failure at temperatures between 1573 and 1773 K was analyzed for 2 mol% TiO2 and 2 mol% GeO2 co-doped tetragonal zirconia polycrystal (TZP), which exhibits excellent high temperature ductility. The improvement in the high temperature ductility in TZP is attributed to dopant cation segregation in the vicinity of the grain boundaries. The phenomenological analysis revealed that co-doping of Ti and Ge cations increases the grain size at the time of failure, as a parameter to describe a limit of an accommodation process for superplastic flow. The parameter of the critical grain size at the time of failure correlates well with the value of overlap population in cation-doped TZP model cluster obtained from a first-principle molecular orbital calculation. The covalent bond at the grain boundaries plays a critical role in the high temperature tensile ductility of TZP.
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  • Masaru Nishi, Tomohito Tanaka, Katsuyuki Matsunaga, Yuichi Ikuhara, Ta ...
    2004 Volume 45 Issue 7 Pages 2112-2116
    Published: 2004
    Released: June 24, 2005
    JOURNALS FREE ACCESS
    Grain boundary structure and current-voltage (I-V) characteristics were investigated for Σ5(210) symmetrical tilt boundary of a Nb-doped SrTiO3 bicrystal. The bicrystal was prepared by a hot-joining technique at high temperature in air. The high resolution transmission electron microscopy (HRTEM) study has revealed that the grain boundaries were perfectly joined at an atomic level without intergranular phases such as amorphous or secondary precipitates. In spite of the high coherency, the boundary tends to be faceted with low index planes of (110) and (410) to reduce grain boundary energy. On the other hand, the I-V property across the boundary was found to exhibit a cooling rate dependency from annealing temperature. In addition, the dependency shows a peak against cooling rates. This fact suggests that the non-linearity in I-V relation is dominantly controlled by a non-equilibrium process of a defect reaction.
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  • Takahiro Ohno, Seiichiro Ii, Naoya Shibata, Katsuyuki Matsunaga, Yuich ...
    2004 Volume 45 Issue 7 Pages 2117-2121
    Published: 2004
    Released: June 24, 2005
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    In this study, the grain boundary structure of [001] symmetric tilt boundary with a tilt angle of 66° was investigated using a rutile-type TiO2 bicrystal. The tilt angle of this boundary has a misfit angle of 1.7° from an exact Σ13 approaching to Σ17 relation. High-resolution transmission electron microscopy study (HRTEM) has revealed that the grain boundary was free from any secondary phases, and the two single crystals contact each other perfectly at an atomic scale. The boundary shows almost straight feature without any step structures whereas a part of the boundary forms facet structures consisting of low index planes such as {310} and {110}. On the other hand, it was found that the contrasts due to strong strain fields existed on the grain boundary plane with a spacing of 7.6 nm by weak beam dark field observation. Comparing with atomic structural analysis using HRTEM, the strain field results from a distorted Σ13 unit structure, which can be predicted from a rigid body model of Σ13 relation. This distorted unit structure has a similar structure of Σ17 relation. Namely, the boundary consists of a periodical array consisting mainly of Σ13 unit structures and partially Σ17-like unit structures. In other words, a misfit angle in this boundary was accommodated by not introducing secondary dislocations, but a transformation of basal unit structure.
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  • Satoru Hanyu, Hitoshi Nishimura, Katsuyuki Matsunaga, Takahisa Yamamot ...
    2004 Volume 45 Issue 7 Pages 2122-2127
    Published: 2004
    Released: June 24, 2005
    JOURNALS FREE ACCESS
    High temperature deformation behavior of [0001] symmetrical tilt grain boundaries of Al2O3 was investigated by using bicrystals. Four kinds of the grain boundaries Σ7/{4510}, Σ21/{4510}, Σ7/{2310} and Σ21/{2310} were selected in the present study, and compressive mechanical tests were performed at 1450°C in air to investigate the sliding behavior of the respective boundaries. It was found that, stresses readily increased with increasing strains for all specimens during compression tests, but the bicrystals showed abrupt sliding along the grain boundary planes to fracture. Among the boundaries studied, Σ7/{4510} exhibited the highest resistance to the grain boundary sliding. The other boundaries showed similar sliding resistance, and yet the stress and strain values at their failure were much smaller than those of Σ7/{4510}. In order to understand the mechanism of the sliding behavior of the respective boundaries, the grain boundary core structures and their atomic densities were examined, based on the structure models obtained in our previous studies. It was found that Σ7/{4510} having the highest sliding resistance showed a larger atomic density at the core, as compared to the others. Therefore, the observed sliding resistance of the boundaries is closely related to the detailed atomic structures at the grain boundary cores.
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  • Sadahiro Tsurekawa, Hiroaki Watanabe, Nobuyuki Tamari, Tadao Watanabe
    2004 Volume 45 Issue 7 Pages 2128-2136
    Published: 2004
    Released: June 24, 2005
    JOURNALS FREE ACCESS
    High temperature oxidation and oxidation-induced embrittlement in β-silicon carbides (SiCs) with different grain boundary microstructures have been studied. SiCs with different grain boundary microstructures were fabricated by hot-pressing with different doping elements like Mg, Al, P. Oxidation experiments were carried out under the oxygen partial pressure ranging from 0.303 Pa to 78.5 Pa at temperatures 1623—1773 K for 7.2—36 ks. Thereafter, the degree of oxidation-induced embrittlement was quantitatively evaluated by three-point bend tests at room temperature in connection with grain boundary microstructure. More severe degradation was observed as a result of oxidation though the passive oxidation took place. It is concluded that the oxidation-induced embrittlement in β-SiC can be improved by decreasing the frequency of random boundaries and the grain size. The potential of grain boundary engineering for a ceramic material has been confirmed.
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  • Takeo Sasaki, Katsuyuki Matsunaga, Hiromichi Ohta, Hideo Hosono, Takah ...
    2004 Volume 45 Issue 7 Pages 2137-2143
    Published: 2004
    Released: June 24, 2005
    JOURNALS FREE ACCESS
    Thin Ni films were deposited on the (111) surface of YSZ at 1073 K by a pulsed laser deposition technique. The interfacial atomic structure of Ni/YSZ was investigated by high-resolution transmission electron microscopy (HRTEM). It was found that Ni was epitaxially oriented to the YSZ surface, and the following orientation relationship (OR) was observed: (111)Ni || (111)YSZ, [110]Ni || [110]YSZ. Geometrical coherency of the Ni/YSZ system was also evaluated by the coincidence of reciprocal lattice points (CRLP) method. It was found that the most coherent OR predicted by CRLP method was (705)Ni || (111)YSZ, [010]Ni || [110]YSZ, which was not consistent with the experimentally observed OR. To understand the detailed atomic structure, HRTEM image simulations were performed. However, simulated images based on both O-terminated and Zr-terminated interface models were quite similar to the experimental image, and thus it was hard to determine which model is comparable with the actual interface only by the HRTEM image simulations. In order to clarify the termination layer at the interface, electronic structures of the Ni/YSZ interface were investigated by electron energy-loss spectroscopy. It was found that significant differences were observed in O-K edge spectra between the interface and the YSZ crystal interior, and the spectrum from the interface showed similar features to the reference spectrum of bulk NiO. This indicates that the Ni-O interaction occurs at the interface to terminate the oxygen {111} plane of YSZ at the Ni/YSZ interface. In addition, the density of Ni-O bonds across the interface in the experimental OR was larger than that in the most coherent OR predicted by CRLP method, which also suggests that the on-top Ni-O bonds stabilize the Ni/YSZ(111) interface.
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  • Akihide Kuwabara, Syu Yokota, Yuichi Ikuhara, Taketo Sakuma
    2004 Volume 45 Issue 7 Pages 2144-2149
    Published: 2004
    Released: June 24, 2005
    JOURNALS FREE ACCESS
    Fine-grained 3Y-TZP has been known to show high superplasticity. Addition of a small amount of metal oxide influences the superplastic behavior in 3Y-TZP. In this study, 3Y-TZP doped with 1 mol% GeO2, TiO2 or BaO were fabricated, and respective grain boundary energy has been systematically measured by a thermal grooving technique with atomic force microscopy. It has been found that addition of Ge4+ or Ti4+ ions decreases the grain boundary energy to stabilize the grain boundaries in TZP whereas doping of Ba2+ ion increases the grain boundary energy to destabilize the grain boundaries. A change in the grain boundary energy should be due to segregation of dopant at grain boundaries. It has been also found that the elongation to failure of cation-doped 3Y-TZP is directly proportional to the stability of grain boundary. Grain boundary energy is thus one of the principal factors to determine the tensile ductility of TZP. In order to reveal the effect of dopant on the grain boundary energy, lattice static calculations and first principles molecular orbital calculations have been performed for supercells and model clusters including the present dopant, respectively. A series of results shows that substitution of Ge4+ or Ti4+ ion for Zr4+ ion increases the covalency of TZP, but the covalency of TZP is reduced by addition of Ba2+ ions. The grain boundary energy is found to have a relationship with covalency nearby grain boundaries in TZP.
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  • Matthew Robert Barnett, Xiang Ma, Alexis Oudin
    2004 Volume 45 Issue 7 Pages 2151-2156
    Published: 2004
    Released: June 24, 2005
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    Of considerable importance to the generation of ultrafine microstructures is the development of high misorientations. The present work examines the effect of the crystallographic rotation field in simple shear upon the evolution of misorientation during plastic working. A series of Taylor simulations are presented and it is shown that the rotation field is such that small differences in orientation in the region of the main torsion texture components are considerably increased with the application of shear strain. This did not occur in simulations of rolling. The torsion simulations compare favourably with the nature of the misorientations evident in hot worked 1050 Al and Ti-IF steel. It is concluded that shear deformation, by its nature, facilitates the generation of higher misorientations.
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  • Pavel Cizek, Fang Bai, W.Mark Rainforth, John H. Beynon
    2004 Volume 45 Issue 7 Pages 2157-2164
    Published: 2004
    Released: June 24, 2005
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    Shear bands formed during both cold and hot plastic deformation have been linked with several proposed mechanisms for the formation of ultrafine grains. The aim of the present work was to undertake a detailed investigation of the microstructural and crystallographic characteristics of the shear bands formed during hot deformation of a 22Cr-19Ni-3Mo (mass%) austenitic stainless steel and a Fe-30 mass%Ni based austenitic model alloy. These alloys were subjected to deformation in torsion and plane strain compression (PSC), respectively, at temperatures of 900°C and 950°C and strain rates of 0.7 s−1 and 10 s−1, respectively. Transmission electron microscopy and electron backscatter diffraction in conjunction with scanning electron microscopy were employed in the investigation. It has been observed that shear bands already started to form at moderate strains in a matrix of pre-existing microbands and were composed of fine, slightly elongated subgrains (fragments). These bands propagated along a similar macroscopic path and the subgrains, present within their substructure, were rotated relative to the surrounding matrix about axes approximately parallel to the sample radial and transverse directions for deformation in torsion and PSC, respectively. The subgrain boundaries were largely observed to be non-crystallographic, suggesting that the subgrains generally formed via multiple slip processes. Shear bands appeared to form through a co-operative nucleation of originally isolated subgrains that gradually interconnected with the others to form long, thin bands that subsequently thickened via the formation of new subgrains. The observed small dimensions of the subgrains present within shear bands and their large misorientations clearly indicate that these subgrains can serve as potent nucleation sites for the formation of ultrafine grain structures during both subsequent recrystallisation, as observed during the present PSC experiments, and phase transformation.
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  • Jeong-Ho Lee, Il-Heon Son, Yong-Taek Im
    2004 Volume 45 Issue 7 Pages 2165-2171
    Published: 2004
    Released: June 24, 2005
    JOURNALS FREE ACCESS
    In this study, the effect of material properties on deformation pattern and strain distribution of the commercially-available pure titanium (CP-Ti) specimen during an equal channel angular extrusion (ECAE) was investigated. Finite element analyses were carried out for two-dimensional plane strain condition at elevated temperatures. Material properties were assumed by hardening, softening, and no hardening behaviors based on the measured experiment data. The effects of friction conditions, die geometries, and processing routes were also examined under the same processing condition. Based on the value of average strain and standard deviation of the strain distribution, the magnitude and uniformity of deformation was determined depending on process conditions. In addition, damage parameters based on plastic work and Cockroft and Latham criteria were calculated to check the likeliness of surface cracking. Finally, non-isothermal analyses were carried out to explore the effect of processing temperature in the ECAE process.
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  • Hyoung Seop Kim, Pham Quang, Min Hong Seo, Sun Ig Hong, Kyeong Ho Baik ...
    2004 Volume 45 Issue 7 Pages 2172-2176
    Published: 2004
    Released: June 24, 2005
    JOURNALS FREE ACCESS
    Equal channel angular pressing (ECAP) is a viable forming procedure to extrude material by use of specially designed channel dies without a substantial change in geometry and to make an ultrafine grained material by imposing severe plastic deformation. Because the evolution of microstructures and the mechanical properties of the deformed material are directly related to the amount of plastic deformation, the understanding of the phenomenon associated with strain development is very important in the ECAP process. The plastic deformation behaviour during pressing is governed mainly by die geometry (channel sizes, a channel angle and corner angles), material properties (strength and hardening behaviour) and process variables (temperature, lubrication and deformation speed). There is a need for modelling techniques which may permit a wider study of the effects observed for better process control and the understanding of process related phenomena. In this study, we describe a range of our continuum modelling results of the ECAP process in order to illustrate the modelling applicability. Firstly, the finite element results of ECAP modelling for various geometric factors are described. Secondly, the inhomogeneous deformation due to the hardening property of the material is explained. Lastly, modelling the temperature field coupled with stress as a typical process variable in ECAP is presented.
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  • Seong-Hee Lee, Hiroshi Utsunomiya, Tetsuo Sakai
    2004 Volume 45 Issue 7 Pages 2177-2181
    Published: 2004
    Released: June 24, 2005
    JOURNALS FREE ACCESS
    An ultra low carbon interstitial free (IF) steel was severely deformed by the six-layer stack accumulative roll-bonding (ARB) process for improvement of the mechanical properties. As-received material with 1 mm in thickness showed a recrystallization structure with average grain diameter of 27 μm. The ARB was conducted at ambient temperature after deforming the as-received material to 0.5 mm thick by cold rolling. The ARB was performed for six-layer stacked, i.e. 3 mm thick sheet, up to 3 cycles (an equivalent strain of ∼7.1). In each ARB cycle, the stacked sheets were, first, deformed to 1.5 mm thick by the first pass, and then reduced to 0.5 mm thick, equals to the starting thickness, by multipass rolling without lubrication. The specimen after 3 cycles of ARB was annealed for 1.8 ks at various temperatures ranging from 673 K to 1073 K. The tensile strength of the ARB processed materials increased largely with the number of ARB cycles, after 3 cycles it reached a maximum of 1.12 GPa, which is about 4 times larger than that of the initial material. The elongation dropped largely after the cold rolling prior to the ARB, however it remains almost constant during the subsequent ARB process. Transmission Electron Microscopy revealed that the ARB processed materials exhibited a dislocation cell and/or subgrain structure with relatively high dislocation density. The selected area diffraction (SAD) patterns suggested that the orientation difference between neighboring cells was very small. The annealing up to 873 K resulted in gradual decrease in the strength due to the static recovery. The annealing above 873 K resulted in recrystallization and normal grain growth, and thereby a significant drop in the strength and recovery in ductility.
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  • Giuliano Angella, Paola Bassani, Ausonio Tuissi, Maurizio Vedani
    2004 Volume 45 Issue 7 Pages 2182-2186
    Published: 2004
    Released: June 24, 2005
    JOURNALS FREE ACCESS
    Intermetallics evolution in a commercial 6082 aluminium alloy severely deformed by Equal Channel Angular Pressing was investigated. Chemical electron probe microanalyses allowed to state that in the severely deformed alloy, Si-rich phases were progressively dissolved whereas the amount and composition of the Fe-Mn-Si containing intermetallics remained substantially unchanged. The moderate hardening effect measured on isothermal aging at 130°C of the ECAP processed samples was accounted for by the reprecipitation of the phases dissolved during severe plastic deformation. Tensile tests and fractographic analyses on the broken specimens showed that the 6082 alloy featured a relatively high ductility and a substantially unaltered fracture mode even after several ECAP passes.
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  • Alexei Vinogradov, Yasushi Suzuki, Takahiko Ishida, Kazuo Kitagawa, Vl ...
    2004 Volume 45 Issue 7 Pages 2187-2191
    Published: 2004
    Released: June 24, 2005
    JOURNALS FREE ACCESS
    The properties of multi-functional CuCr and CuCrZr alloys with ultra fine grains (UFG) produced by equal-channel angular pressing (ECAP) have been investigated in dependence of concentration of alloying elements. A special attention is paid to optimization of fatigue performance together with strength, thermal and electric properties after ECAP and subsequent aging. Substantial improvement of fatigue life is evidenced in ECAP-fabricated alloys when compared with conventional tempers.
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  • Rimma Lapovok, Peter F. Thomson, Ryan Cottam, Y. Estrin
    2004 Volume 45 Issue 7 Pages 2192-2199
    Published: 2004
    Released: June 24, 2005
    JOURNALS FREE ACCESS
    Tensile ductility of magnesium alloy ZK60 pre-strained by warm equal channel angular extrusion (ECAE) was investigated at 300°C. It was shown that a significant ductility (262%) is achievable with this technique for a fairly high strain rate of 3·10−3 s−1. The effect of pre-straining by warm ECAE on twinning under room temperature deformation was also investigated. It was shown that a bi-modal grain structure, whose characteristics are determined by the ECAE route, number of passes and temperature, has a strong effect on the propensity for twinning and mechanical properties, including ductility.
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  • T. Suzuki, A. Vinogradov, S. Hashimoto
    2004 Volume 45 Issue 7 Pages 2200-2208
    Published: 2004
    Released: June 24, 2005
    JOURNALS FREE ACCESS
    The significant enhancement of strength is reported for pure K24 gold and its K18 alloys subjected to grain refinement by equal channel angular pressing (ECAP) in comparison with their annealed or ordinary cold-worked counterparts. The effect of strain path on the tensile behavior is discussed from the stand point of mechanical properties and damage evolution during straining. It is shown that although different processing can give rise to substantially different structures, the mechanical properties depend on the pre-strain history only slightly. The ECAP appears to be particularly effective for strengthening of pure gold and Au-Ag solid solutions where the strength can increase by a factor of 3 or 4 while in the precipitation hardenable alloys the effect of grain refinement is masked by other strengthening mechanisms and the strength increases by a factor of 2. The strain localization and features of plastic deformation in ECAP materials are discussed in detail upon in-situ and post-experimental surface observations in differently fabricated samples.
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  • Yoshikazu Todaka, Minoru Umemoto, Shuji Tanaka, Koichi Tsuchiya
    2004 Volume 45 Issue 7 Pages 2209-2213
    Published: 2004
    Released: June 24, 2005
    JOURNALS FREE ACCESS
    Microstructure of martensite steel near drill hole surface was investigated. The microstructure of drill hole surface can be divided into 3 layers with depth. From top surface, nanocrystalline layer, equiaxed submicron grain layer and deformed martensite structure layer. Nano-grained layers have extremely high hardness, similar to those observed in the specimens treated by shot peening, ball milling, ball drop and particle impact deformation. The measured amount of shear strain produced by drilling was found to be an exponential function of depth from the hole surface. The amount of true strain at the hole surface region was estimated to exceed 7 which is considered to be the necessary amount of strain to produce nanocrystalline structure.
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  • Hossein Beladi, Georgina L. Kelly, Peter D. Hodgson
    2004 Volume 45 Issue 7 Pages 2214-2218
    Published: 2004
    Released: June 24, 2005
    JOURNALS FREE ACCESS
    In the present study, wedge-shaped samples were used to determine the effect of nominal equivalent strain (between 0 and 1.2) and carbon content (0.06—sh0.35%C) on ferrite grain refinement through dynamic strain-induced transformation (DSIT) in plain carbon steels using single-pass rolling. The microstructural evolution of the transformation of austenite to ferrite has been evaluated through the thickness of the strip. The results showed a number of important microstructural features as a function of strain which could be classified into three regions; no DSIT region, DSIT region and the ultrafine ferrite (UFF) grain region. Also, the extent of these regions was strongly influenced by the carbon content. The UFF microstructure consisted of ultrafine, equiaxed ferrite grains (<2 μm) with very fine cementite particles. In the centre of the rolled strip, there was a conventional ferrite-pearlite microstructure, although ferrite grain refinement and the volume fraction of ferrite increased with an increase in the nominal equivalent strain.
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  • Joo-Hee Kang, Hu-Chul Lee
    2004 Volume 45 Issue 7 Pages 2219-2223
    Published: 2004
    Released: June 24, 2005
    JOURNALS FREE ACCESS
    The orientation distribution of ferrite grains formed by dynamic and static transformation was examined using the electron backscattered diffraction technique. The orientation of ferrite grains, transformed both dynamically and statically from deformed austenite, showed a large deviation from the Kurdjumov-Sachs (K-S) orientation relationship. The deformed austenite grain structure modeled using Ni-30Fe alloy showed inhomogeneous and localized deformations within the austenite grains. A large orientation variation was observed across this localized deformation zone, and the large deviation of the ferrite orientation from the K-S relationship was attributed to the nucleation of ferrite from this localized deformation region.
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  • Akio Ohmori, Shiro Torizuka, Kotobu Nagai, Naoshi Koseki, Yasuo Kogo
    2004 Volume 45 Issue 7 Pages 2224-2231
    Published: 2004
    Released: June 24, 2005
    JOURNALS FREE ACCESS
    Ultrafine grained structure formed dynamically through a severe warm deformation in the temperature range from 773 K to 923 K has been investigated in a 0.16%C-0.4%Si-1.4%Mn steel. The effects of the deformation conditions such as deformation temperature and strain rate on microstructural evolution were examined using a single-pass compression technique with a pair of anvils. A large plastic strain up to 4 was imposed on the specimen interior at a strain rate of 1 or 0.01 s−1. Ultrafine ferrite grains surrounded by high angle boundaries, whose nominal grain size ranged from 0.26 to 1.1 μm, evolved when the equivalent plastic strain exceeded the critical value about 0.5 to 1, and increased with an increase in strain without any large-scale migration of high angle boundaries. The effects of deformation conditions on microstructural evolution of ultrafine grained structures can be summarized into the Zener-Hollomon(Z-H) parameter dependences. The average size and the volume fraction of newly evolved ultrafine grains depend on the Z-H parameter. Decreasing Z-H parameter enhances the formation of equiaxed ultrafine grains. These indicate that the mechanism forming ultrafine grained structures through the warm severe deformation in the present study is similar to “continuous recrystallization” or “in-situ recrystallization” and that some activation process during or after the deformation plays an important role in the microstructural evolution.
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  • Oleg Sitdikov, Taku Sakai, Alexandre Goloborodko, Hiromi Miura, Rustam ...
    2004 Volume 45 Issue 7 Pages 2232-2238
    Published: 2004
    Released: June 24, 2005
    JOURNALS FREE ACCESS
    Effect of pass strain (Δε) on grain refinement was studied in multidirectional forging (MDF) of a coarse-grained 7475 Al alloy at 490°C under a strain rate of 3 × 10−4 s−1. Samples of rectangular shape were deformed up to accumulated strains of around 6 with subsequent changes in loading direction 90° from pass to pass. The pass strains in each compression (Δε) were 0.4 and 0.7. The cumulative flow curves integrated by each compression exhibit significant work softening just after yielding, followed by apparent steady state plastic flow at high strains. Structural changes were characterized by grain fragmentation due to frequent development of deformation and/or microshear bands followed by full evolution of new fine grains in the original grains. Increasing Δε accelerates significantly the kinetics of grain refinement, leading to more clear reduction of flow stresses at moderate to high strains. MDF of Δε = 0.7 results finally in formation of a finer grained structure with an average size of around 7.5 μm at strains of above 3.5, while, the processing with Δε = 0.4 develops a slightly coarser grain structure at higher strain of about 6. The effect of MDF on new grain evolution and the mechanisms of grain refinement are discussed in details.
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  • Setuo Takaki, Masaaki Fujioka, Shuji Aihara, Yasunobu Nagataki, Takako ...
    2004 Volume 45 Issue 7 Pages 2239-2244
    Published: 2004
    Released: June 24, 2005
    JOURNALS FREE ACCESS
    Progress of Ferrous Nano-Metal Project is introduced in the present paper. In the project, maximum use of copper clusters and precipitates is pursued for achieving better strength-ductility balance than that of conventional high-tensile strength steels. Fundamental aspect of clustering and precipitation of Cu in Fe-Cu alloys was studied using Optical Tomographic Atom-Probe (OTAP). It was found that Cu precipitation during aging was enhanced by plastic deformation. The observed Cu precipitation behavior was well related to the age-hardening behavior, that is, aging started at lower temperature and maximum hardness was higher for plastically deformed and aged ferrite. Aging behavior and associated tensile properties were further examined for Fe-C-Mn-Cu martensitic steel. Higher value of tensile strength times elongation was achieved in Fe-C-Mn-Cu steel than Fe-C-Mn steel. Finally, effect of Cu precipitation on grain-refinement of ferrite was studied for Fe-C-Mn-Cu steel. Ferrite grain smaller than 1 μm was obtained in both processes of strain-assisted ferrite transformation from heavily deformed austenite and dynamic recrystallization of heavily deformed ferrite. Ferrite grain size was found to decrease by addition of Cu at the both processes. It was suggested that a simple additivity rule does not hold in terms of the strengthening by grain-refinement and that by precipitation, especially at grain-size range less than 1 μm.
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