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

MATERIALS TRANSACTIONS Vol. 45 (2004), No. 5

  • An Orbital-dependent Correlation Energy Functional in Density-functional Theory for the Study of Strongly-correlated Electronic Systems

    pp. 1402-1410

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    DOI:10.2320/matertrans.45.1402

    An orbital-dependent correlation energy functional Ec to be accompanied by the exact exchange energy functional Ex is proposed for applications of density-functional theory (DFT). The present Ec comprises spin-antiparallel and spin-parallel contributions, Ecσ−σ and Ecσσ. Ecσ−σ is a modification of the spin-antiparallel component of the Hartree energy functional with a factor of gσ−σ (r, r′)−1 and Ecσσ a modification of the spin-parallel component of the same energy functional with gcσσ(r, r′) where gσ−σ(r, r′) (or gcσσ(r, r′)) is the spin-antiparallel (or the correlational part of the spin-parallel) coupling-constant-averaged pair correlation function. The present orbital-dependent gσ−σ(r, r′) and gcσσ(r, r′) fulfill the symmetric property, the Pauli principle and the sum rules. In the limit of uniform density the two correlation functions are reduced to the very accurate analogues of the electron liquid that involve long-, intermediate-, and short-range correlations as well as their exchange counterparts. It is stressed that the correlation energy functional Ec in DFT should by its very nature be defined as a functional only of occupied Kohn-Sham orbitals and occupied Kohn-Sham energies for the purpose of employing the optimized potential method (OPM) to evaluate the correlation potential υc(r). The present scheme for Ec, if applied to finite systems after making a suitable change in the treatment of long-range correlation, can give the correct asymptotic form of υc(r) of order r−4 for large r as well as the van der Waals potential.
  • GW Calculation of a Carbon Oxide Molecule Using an All-Electron Mixed-Basis Approach

    pp. 1411-1413

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    DOI:10.2320/matertrans.45.1411

    An ab-initio calculation for a carbon oxide molecule using the Green's function approach within the GW approximation was performed. We use an all-electron mixed-basis approach, where one wave function is expanded using both plane waves and atomic orbitals. This approach has an advantage to describe the wave function of a carbon and oxide, compared with a pseudopotential approach requiring higher cutoff energy. Obtaied GW quasiparticle energies are in good agreement with avairable experimental value and previous GW calculation.
  • First Principles Calculation of Fe L2,3-edge X-ray Absorption Near Edge Structures of Iron Oxides

    pp. 1414-1418

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    DOI:10.2320/matertrans.45.1414

    X-ray absorption near edge structure (XANES) at L2,3-edge of 3d transition elements is dominated by strong correlation effects among 2p core hole and 3d electrons. In the present study, we have performed systematic configuration interaction (CI) calculations in order to reproduce and interpret Fe-L2,3 XANES of FeO, LaFeO3 and SrFeO3. Relativistic four components wave functions were obtained by solving Dirac equations with density functional theory. CI calculations were made using the relativistic molecular orbitals instead of atomic orbitals, which enables inclusion of the O-2p orbital contributions through covalency. The oscillator strength of the electric dipole transition was then computed. Experimental XANES spectra of three compounds were satisfactorily reproduced by the theoretical spectra obtained for (FeO6)m clusters in octahedral symmetry. Chemical shifts between compounds were quantitatively reproduced as well. Component analysis of CI was systematically made in order to analyze the origin of differences in spectral shapes.
  • First-principles Calculation Method for Electronic Structures of Nanojunctions Suspended between Semi-infinite Electrodes

    pp. 1419-1421

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    DOI:10.2320/matertrans.45.1419

    We develop a method for high-speed and high-accuracy first-principles calculations to derive the ground-state electronic structure by directly minimizing the energy functional. Making efficient use of the advantages of the real-space finite-difference method, we apply arbitrary boundary conditions and employ spatially localized orbitals. These advantages enable us to calculate the ground-state electronic structure of a nanostructure sandwiched between crystalline electrodes. The framework of this method and numerical examples for metallic nanowires are presented.
  • A Second-Variational Prediction Operator for Fast Convergence in Self-Consistent Electronic-Structure Calculations

    pp. 1422-1428

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    DOI:10.2320/matertrans.45.1422

    We propose a new way to reduce the number of iterations required to reach self-consistency in electronic-structure calculations in the framework of the plane-wave pseudopotential method. A prediction operator is derived from the procedure to solve the Kohn-Sham equation approximately on the basis of a second-variational approach, and then combined with a variant of Broyden's algorithm. The self-consistency is reached quite efficiently not only for semiconductor surfaces but also for intermetallic compounds either with large density of states around the Fermi level or near a threshold for the occurrence of the magnetic moment. When the magnetic moment emerges, it converges more smoothly with our prediction operator than otherwise.
  • Growth and Magic Behavior of Metal Encapsulated Silicon Clusters

    pp. 1429-1432

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    DOI:10.2320/matertrans.45.1429

    Metal encapsulated silicon clusters M@Sin (M = Ti and Cr and n = 8−16) have been studied using ab-initio ultrasoft pseudopotential method. Several structures for each cluster have been optimized to obtain the lowest energy isomers. Our results show that cage structures begin to form at the size of n = 12 for Cr@Sin and 13 for Ti@Sin. For Ti@Sin our results are in excellent agreement with the available experimental results. In smaller size, metal doped silicon clusters have basket structures to be of the lowest energy. The bonding nature in these clusters is discussed from the electronic charge distribution.
  • First-Principles Study on Electron Conduction Property of Monatomic Sodium Nanowire

    pp. 1433-1436

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    DOI:10.2320/matertrans.45.1433

    We present first-principles calculations of electron conduction properties of monatomic sodium wires suspended between semi-infinite crystalline electrodes, using the overbridging boundary-matching method. We find that the conductances oscillate depending on the number of atoms in the wire, Natom. Furthermore, the values of conductances are ∼3 G0 (G0 = 2e2/h) for the closed packed structure and ∼1 G0 for single-row wires, which is in agreement with the experimental results of the conductance histogram.
  • Study on Cap Closure Mechanism of Single-Walled Carbon Nanotubes by Molecular Dynamics

    pp. 1437-1441

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    DOI:10.2320/matertrans.45.1437

    The closing mechanism of zigzag single-walled carbon nanotubes (SWCNT) was investigated using the molecular dynamics (MD) simulation at the experimental arcdischarge temperature of 3000 K. The (10,0) SWCNT with a diameter of 0.78 nm showed a dome-shape tip which evolved into a saddle-shaped cap that was caused by double heptagon-octagon pairs. In the case of (18,0) SWCNT with a diameter of 1.404 nm, a zipper-like closing mechanism was observed and the flat cap was obtained.
  • Nanomechanical Behavior of β-SiC Nanowire in Tension: Molecular Dynamics Simulations

    pp. 1442-1449

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    DOI:10.2320/matertrans.45.1442

    The molecular dynamics (MD) simulation employing a Tersoff potential was performed to examine the nanomechanical behavior of the β-SiC nanowire in tension. The elongation was much larger than that of the bulk β-SiC. We observed non-homogeneous deformation, and the fracture behavior was found to depend on size, orientation and temperature of the specimen. The Young's modulus calculated in this study generally decreased with temperatures and increased with the radius, namely, the diameter of the β-SiC nanowire as long as the length scale remained the same. The initial orientation was found to have a more serious effect on the Young's modulus than size and temperature. The [111] Young's modulus is much higher than that of the [001] orientation. The fracture of the β-SiC nanowire in the [001] orientation showed two different modes, which is brittle at 100 K and ductile at 300 and 500 K. The ductile fracture was accompanied by formation of an atomic chain. In the [111] orientation, it was always fractured in the ductile mode and thus an atomic chain was formed before rupture.
  • Stability of Copper Atoms Embedded in Sodium-Chloride Crystals

    pp. 1450-1451

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    DOI:10.2320/matertrans.45.1450

    Because it has been pointed out by Nagasaka et al. that the optical absorption spectrum of copper atoms embedded in alkali-chloride crystals may depend on the position of the copper atoms, we calculate the relation between the optimal position and the total energy of copper atom embedded in NaCl crystal by means of the first-principles pseudopotential plane-wave-expansion method. These results shows that most stable position of embedded Cu+ ion in these alkali halide system are not the substitutional on-center site but off-center site along ‹111› axis.
  • Ab Initio Study of Hydrogen Storage in Hydrogen Hydrate Clathrates

    pp. 1452-1454

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    DOI:10.2320/matertrans.45.1452

    Recently, for the first time a hydrate clathrate was discovered with hydrogen. Aside from the great technological promise that is inherent in storing hydrogen at high density at modest pressures, there is great scientific interest as this would constitute the first hydrate clathrate with multiple species per cage. The multiple cage occupancy is controversial, and reproducibility of the experiments has been questioned. Therefore, in this study we try to illucidate the stability of the hydrogen hydrate clathrate, and determine the thermodynamically most favored cage occupancy using highly accurate ab initio computer simulations.
  • The Effects of Solute and Vacancy Depletion on the Formation of Precipitation-Free Zone in a Model Binary Alloy Examined by a Monte Carlo Simulation

    pp. 1455-1460

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    DOI:10.2320/matertrans.45.1455

    The effects of vacancy depletion and solute atom depletion on the microstructural evolution of the precipitation-free zone (PFZ) near the grain boundary have been examined by Monte Carlo simulations with Kawasaki dynamics. The solute depletion caused by a precipitation of stable phase at grain boundaries lead to a well-defined PFZ, whose width grew by a t1/2 power law. The average size of precipitates near the PFZ boundary did not change significantly from that inside the grain. On the other hand, the vacancy depletion did not give a well-defined PFZ boundary, and the average size of precipitates changed gradually as a function of distance from the boundary.
  • First-Principles Study of a {122} Σ = 9 Boundary in Cubic SiC: Relative Stability between Zigzag and Straight Models and Comparison with Electron Microscopy Observation

    pp. 1461-1464

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    DOI:10.2320/matertrans.45.1461

    Relative stability between two types of reconstructed models of the {122} Σ = 9 coincidence boundary in cubic SiC has been examined by using the ab initio pseudopotential method based on the density-functional theory. Results are compared with the high-resolution transmission electron microscopy (HRTEM) observation in SiC polycrystalline films. A zigzag model consisting of zigzag arrangement of two sets of five- and seven-membered rings is more stable than a straight model consisting of straight arrangement of five-, six- and seven-membered rings, because of larger bond stretching in the latter model. The former model requires a rigid-body translation parallel to the interface. This may cause large strain at the triple junction with the {111} Σ = 3 boundaries, although this can be avoided by introducing a step at the junction. The HRTEM observation has clearly shown the presence of the zigzag model and the step at the triple junction.
  • Lattice Anomaly of MgB(h-BN) under Anisotropic Compression

    pp. 1465-1468

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    DOI:10.2320/matertrans.45.1465

    We have examined the lattice and electronic properties of a hypothetical compound of magnesium boride (MgB) with a hexagonal boron-nitride (h-BN) type crystal structure using the first-principles molecular dynamics (FPMD) method. We found a lattice anomaly of MgB(h-BN) under uniaxial c-axis compression. Lattice constant a contracts 0.0017 and 0.0051 nm under c-axis compression with Pz = 50 and 100 GPa, respectively. This contraction implies that the Poisson ratio of MgB(h-BN) is negative.
  • Ab initio Modeling of the Stress-Strain Response of SiAlON (Si6-zAlzOzN8-z, z = 0.5 and 1)

    pp. 1469-1472

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    DOI:10.2320/matertrans.45.1469

    A derivative of the Si3N4 ceramic is the quaternary SiAlON solid solution. In this paper the characteristic stress-strain response of the β-and c-SiAlON phases is investigated using an ab initio computational procedure, for the γ11 strain component, where different substitutions of the atomic pairs, Al-O, were performed. From the modeled data the ‘ideal’ strengths and other material constants were estimated for the two polymorphs. Estimates of the elastic constants were found to be in reasonable agreement with existing data.
  • Vibrational Contribution on Nucleation Free Energy of Cu Precipitates in Fe-Cu System

    pp. 1473-1477

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    DOI:10.2320/matertrans.45.1473

    The thermal vibrational effects on the nucleation free energy of the bcc-Cu precipitates in the Fe-Cu system have been explored. Thermal expansion coefficients of bcc-Fe have been correctly predicted by the quasi-harmonic approximation, using the phonon dispersions calculated on the basis of the density functional theory. The Einstein model with a temperature-dependent bulk modulus shows free energy values almost identical to the quasi-harmonic predictions. The vibrational contribution obtained from the Einstein model is reduced to 1.6 × 10−21 J (10 meV), which is almost negligible for the free energy change of cluster nucleation.
  • First-Principles Calculation of L10-Disorder Phase Boundary in Fe-Pd System

    pp. 1478-1484

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    DOI:10.2320/matertrans.45.1478

    L10-disorder phase boundaries for Fe-Pd system are calculated by combining first-principles FLAPW electronic structure calculation with Cluster Variation Method. The total energy calculation at the ground state well predicted the global tendencies of the phase diagram. The calculated transition temperature of L10-disorder based on the free energy formulated by Cluster Variation Method is in good agreement with experimental one. The incorporation of thermal vibration effects further improves the agreement. The shift of the congruent composition from the stoichiometry is, however, not realized by the present calculation.
  • Instability of the B2 Ordered Structure in TiNi Shape Memory Alloys due to Atom Displacements on (110) Planes

    pp. 1485-1488

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    DOI:10.2320/matertrans.45.1485

    Lattice instability due to the formation of lattice displacement waves (LDW) was investigated in the B2 ordered structure of Ti-Ni shape memory alloys using first principle calculations. The modified B2 structure with this LDW of the 1/2(011)[011] phonon mode have structural similarity with the B19′ monoclinic structure, which is martensitic structure known to appear in Ti-Ni alloys at low temperatures. The shift to optimal lattice positions for Ti and Ni atoms is determined by the force acting on each nucleus. Ni and Ti atoms are displaced from their original positions in the B2 structure without any potential barrier. The atomic displacements without any potential barrier can lead to precursor phenomenon in advance of martensitic transformations.
  • Phase Separation of the B2 Structure Accompanied by an Ordering in Co-Al and Ni-Al Binary Systems

    pp. 1489-1498

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    DOI:10.2320/matertrans.45.1489

    The phase separation of the B2 structure in the Co-Al and Ni-Al binary systems has been studied by combining ab initio energetic calculations with the CALPHAD approach. The total energies of the ordered phases based on the bcc lattice were obtained using first-principle band-energy calculations. The cluster expansion method was applied to the results, and the free energies at finite temperatures were calculated for the bcc solid solution. The Co-Al and Ni-Al binary systems were analysed thermodynamically by considering the estimated metastable free energy of the bcc phase. The descriptions of the lattice stability parameters for each pure element were obtained chiefly from the Scientific Group Thermodata Europe (SGTE) datafile. The optimized parameters reasonably reproduced the characteristic features of these binary phase diagrams. The metastable (A2+B2) two-phase field forms in the Co-Al phase diagram, and this equilibrium is closely related to the anomaly in the phase boundaries of the binary system. On the other hand, the phase separation of the A2 and B2 structures are hindered by the presence of the D03 phase in the Ni-Al system. Ground state analysis was performed to clarify the difference in the behaviour of the B2 phase.
  • Effect of the Order-Disorder Transition of the bcc Structure on the Solubility of Be in the Fe-Be Binary System

    pp. 1499-1506

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    DOI:10.2320/matertrans.45.1499

    In the Fe-Be binary system, the solubility of Be in the α-Fe phase deviates significantly from the so-called Arrhenius equation near temperatures of 600°C. The metastable ordering of the bcc structure in this binary system is expected to play a key role in the phase boundary anomaly. Thus, a thermodynamic analysis of the Fe-Be binary system has been performed considering the ordering behaviour of the bcc phase. The total energies of the ordered structures based on the bcc lattice were obtained using ab initio energetic calculations. The cluster expansion method was applied to the results, and the free energies at finite temperatures were calculated for the bcc solid solution. The formation energy of the ζ phase was also calculated using band-energy calculations. The results were analysed together with some experimental data using the sublattice model, and the equilibrium phase diagram was calculated. The results support the formation of a metastable (bcc + B2) two-phase region accompanied by an ordering of the bcc structure. This metastable ordering of the bcc phase was the dominant factor governing the anomalous change in the solubility of Be in the higher temperature range.
  • Thermodynamic Analysis of the Ni-Si-Ti System Using Thermochemical Properties Determined from Ab Initio Calculations

    pp. 1507-1514

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    DOI:10.2320/matertrans.45.1507

    A thermodynamic analysis has been carried out on the Ni-Si-Ti ternary system using the CALPHAD method. A regular solution approximation based on the sublattice model was adopted to describe the Gibbs energy for the individual phases in the binary and ternary systems. The thermodynamic parameters for each phase were evaluated using available experimental data on the phase boundaries and other related thermochemical properties. In addition to the experimental data, the enthalpy of formation for some binary and ternary compound phases as determined by ab initio calculations was incorporated in the present analysis. There was good agreement between the calculated and the experimental phase equilibria in the binary and ternary systems.
  • Numerical Study on LiCaAlF6 Czochralski Crystal Growth

    pp. 1515-1521

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    DOI:10.2320/matertrans.45.1515

    Czochralski crystal growth (CZ) is one of the most important single crystal growth techniques. A numerical study on LiCaAlF6 (LiCAF) crystal in CZ is conducted, and three criteria, which concern with growth rate, interface shape and the instability of convection, for estimating an optimal crystal rotation are adopted. Based on a simplified CZ model with Marangoni convection included, the effect of the variable experimental parameters, such as crystal size and melt height etc. on the optimal crystal rotation is investigated by means of the finite volume method.
  • Oscillatory Thermocapillary Convection in Liquid Bridge under Microgravity

    pp. 1522-1527

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    DOI:10.2320/matertrans.45.1522

    Three-dimensional oscillatory thermocapillary convection in both cylindrical and concave non-cylindrical liquid bridge (Pr = 7) under microgravity is investigated numerically by finite volume method. The pattern selection of convection is observed to depend on the shape of free surface. An explanation on the mechanism of convective instability and the nature of pattern selection are presented.
  • Dendritic Morphogenesis by Means of a Fractal

    pp. 1528-1534

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    DOI:10.2320/matertrans.45.1528

    One shows that the dendritic morphogenesis is induced by one-dimensional cnoidal thermal oscillation modes of the solid-liquid interface. Particularly, the one-dimensional l-thermal antisoliton — one-dimensional l-thermal soliton modes imposes laws characterizing dendritic branches growth. One deduces a relation between the dendritic growth speed and the dendritic tip radius that depends on the supercooling degree. Particularly one obtains the Oldsfield's relation. The variation of the fractal dimension with the supercooling degree specifies that the dendritic growth are two-dimensional projections of a higher-dimensional fractal.
  • The Prediction on Initial Discharge Capacity of AB5-Based Alloy with Simulated Annealing

    pp. 1535-1538

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    DOI:10.2320/matertrans.45.1535

    The prediction for initial discharge capacity of AB5-based alloy has tested by employing the simulated annealing method in artificial neural network (ANN). The parameters in cooling schedule were determined and their influences on network prediction performance were discussed. A practical cooling schedule was established and the network with better performances was obtained by associating simulated annealing algorithm with gradient descent algorithm.
  • Fatigue, Fretting Fatigue and Corrosion Characteristics of Biocompatible Beta Type Titanium Alloy Conducted with Various Thermo-Mechanical Treatments

    pp. 1540-1548

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    DOI:10.2320/matertrans.45.1540

    Plain and fretting fatigue properties of β type titanium alloy, Ti-29Nb-13Ta-4.6Zr, underwent various thermo-mechanical treatments were investigated in order to judge its potential for biomedical applications. Ti-29Nb-13Ta-4.6Zr aged directly at 723 K for 259.2 ks after cold rolling shows the greatest fatigue strength in both low cycle fatigue life and high cycle fatigue life regions, and the fatigue limit, which is around 770 MPa, is nearly equal to that of hot-rolled Ti-6Al-4V ELI conducted with aging, which is one of representative α + β type titanium alloys for biomedical applications. Fretting fatigue strength tends in proportion to Young's modulus. Fretting fatigue limits of the forged bar of Ti-29Nb-13Ta-4.6Zr conducted with solution treatment, and aging at 723 K after solution treatment are around two thirds and a half of plain fatigue limits, respectively, and those are around 180 MPa and 285 MPa, respectively. Passive current densities of the plate of Ti-29Nb-13Ta-4.6Zr conducted with a multi-step-thermo-mechanical treatment, where the cold rolling and solution treatment are repeated 4 times, in 0.5%HCl and Ringer's solutions are much smaller than that of Ti-29Nb-13Ta-4.6Zr conducted with general thermo-mechanical treatment, and the values are a little smaller than those of forged Ti-15Mo-5Zr-3Al conducted with annealing and hot rolled Ti-6Al-4V ELI conducted with aging.
  • Dynamic Young's Modulus and Mechanical Properties of Ti−Hf Alloys

    pp. 1549-1554

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    DOI:10.2320/matertrans.45.1549

    Ti−Hf alloys are expected to have lower modulus and higher strength than pure Ti since. Hf has been suggested to have the potential to enhance the strength and to reduce the Young's modulus of Ti alloys at the same time. In the present study, binary Ti−Hf alloys with Hf contents from 5 to 40 mass% were designed and fabricated. The effects of Hf content on the microstructures, dynamic Young's modulus and mechanical properties of Ti−Hf alloys were investigated with the aim of biomedical applications. The microstructures were examined using a scanning electron microscopy and an X-ray diffraction analysis. The experimental results indicate that all the studied Ti−Hf alloys exhibit lamellar HCP martensite (α′) structure under the given solution treatment, and increasing Hf content can gently reduce the Young's modulus but strongly enhance the strength of Ti−Hf alloys.
  • Effect of Composition on Microstructure and Compressive Mechanical Properties in Ti-Cu-Fe-Sn-Nb Alloys

    pp. 1555-1560

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    DOI:10.2320/matertrans.45.1555

    The microstructure and the compressive mechanical properties of the Ti-Cu-Fe-Sn-Nb alloys are significantly affected by the compositions. When the Cu in the alloys is more than 12 mol% and the Fe more than 10 mol%, the microstructure contains the dendritic bcc β-Ti solid solution, the matrix phase and the eutectic structure. With decreasing the contents of Cu and Fe, the dendritic structure increases in volume fraction, while the matrix phase decreases and the eutectic structure vanishes away. Correspondingly, the yield strength and the Young's modulus decrease, but the plastic strain increases. At the higher contents of Cu and Fe, the alloys exhibit the yield strengths of 1442-1515 MPa and the Young's module of 90-101 GPa, but the small plastic strains. At the lower contents of Cu and Fe, the alloys exhibit the yield strengths of 1050-1321 MPa and the Young's module of 47-76 GPa, which are very attractive for the biomedical applications.
  • Microscopic Feature of Fractured Implant in Practical Dental Use

    pp. 1561-1565

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    DOI:10.2320/matertrans.45.1561

    Patients suffer considerable pain from fracture of placed implants. In order to reduce the number of fracturing failures in future and to obtain a better understanding of fracturing, a fractured implant has been investigated by means of radiography, scanning electron microscopy and transmission electron microscopy from a metallurgical viewpoint. The blade type implant of Ti-5V alloy has a virgin microstructure of a low density of dislocation with a dispersion of fine precipitates. Further dislocations and twins are introduced into the implant due to repeated loading in use, resulting in possessing a large amount of lattice defects until fracturing. The dislocations are considered to assist calcium penetration and to bring about intergranular fracturing near the surface of the implant. Striations are extensively observed at a center portion of the implant, indicating that the fracturing is caused by fatigue. Microscopic investigations reveal that the fracturing is initiated by some elements such as calcium in saliva in accordance with high density of lattice defects and developed through the fatigue by repeated loading.
  • The Effect of Cooling Rate from Solution Treatment Temperature on Phase Constitution and Tensile Properties of Ti-4.3Fe-7.1Cr-3.0Al Alloy

    pp. 1566-1570

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    DOI:10.2320/matertrans.45.1566

    Titanium and its alloys are one of very attractive metallic materials for health-care and welfare goods, because these alloys have high specific strength and high biocompatibility. However, high cost of Ti alloys is disadvantage in application to the health-care and welfare goods. To overcome high cost barrier of Ti alloys, Ti-4.3Fe-7.1Cr-3.0Al alloy was developed. This alloy has good tensile properties, i.e. about 1 GPa as tensile strength, about 20% as elongation and about 50% as reduction in area, in solution treated state. It is very important that effect of cooling rate from solution treatment temperature on tensile properties is investigated, because diffusion coefficients of Fe and Cr in beta phase are higher than other beta stabilizers, e.g. V and Mo. When a β stabilizer with higher diffusion coefficient is contained in β phase, isothermal ω precipitation that makes the alloy brittle becomes fast. To suppress isothermal ω precipitation, it is necessary to set the cooling rate higher than an appropriate value. In this study, the effect of cooling rate from solution treatment temperature on phase constitution and tensile properties was investigated by electrical resistivity and Vickers hardness measurement and tensile test in Ti-4.3Fe-7.1Cr-3.0Al alloy. In Ti-4.3Fe-7.1Cr-3.0Al alloy cooled by a cooling rate of 0.46 Ks−1 or more, only β phase was identified by X-ray diffraction, while β and α phases were identified in the alloy cooled by furnace cooling, i.e. 0.024 Ks−1. Resistivity ratio remained almost constant between 0.46 Ks−1 and 34.7 Ks−1. In specimen cooled by 0.024 Ks−1, resistivity ratio significantly decreased and HV drastically increased because of α phase precipitation. Tensile strength remained about 1020 MPa between 0.46 Ks−1 and 34.7 Ks−1. In specimen cooled by 0.024 Ks−1, tensile strength slightly increased. Elongation remained almost constant between 0.85 Ks−1 and 34.7 Ks−1, and then decreased with decrease in cooling rate below 0.46 Ks−1.
  • Tensile Behavior and Cold Workability of Ti-Mo Alloys

    pp. 1571-1576

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    DOI:10.2320/matertrans.45.1571

    Three kinds of titanium-molybdenum alloy, Ti-8Mo, Ti-14Mo and Ti-20Mo (mass%), quenched from 1223 K were investigated to clarify the tensile behavior and the cold workability using tensile test and conical cup test. In the quenched state Ti-14Mo showed the superior workability. Ti-20Mo has poor ductility in tensile test, but has relatively good formability in conical cup test. Both Ti-8Mo and Ti-14Mo became brittle through a cold rolling of 50% reduction in thickness; however Ti-20Mo did not change in workability by the cold rolling at all. Hardness remarkably increased with rolling reduction in Ti-8Mo and Ti-14Mo, but was almost constant in Ti-20Mo, especially with Ti-20Mo containing high oxygen. Microstructure of Ti-8Mo and Ti-14Mo rolled until seventy-odd percentage exhibited very fine structure and changed to α′ + β and β structure, respectively. On the other hand Ti-20Mo hardly changed in microstructure by a cold rolling besides the formation of some band-like products. The product was composed of single variant of commensurate ω-phase, whereas the matrix contained four variants of incommensurate ω-phase. It was suggested that the peculiar deformation mechanism of Ti-20Mo was concerned with stress induced transformation of the ω-phase.
  • Effect of Aging on Fatigue-Crack Growth Behavior of a High-Temperature Titanium Alloy

    pp. 1577-1585

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    DOI:10.2320/matertrans.45.1577

    The effects of aging on the room-temperature fatigue-crack growth (FCG) behavior of a newly developed high-temperature near-α titanium alloy (Ti-5.6Al-4.8Sn-2Zr-1.0Mo-0.32Si-0.8Nd) with both bimodal and lamellar microstructures were investigated. The results indicate that aging had a relatively small effect on the FCG behaviors of the concerned alloy. The effect of aging is more pronounced on the lamellar than on the bimodal microstructure. This effect reduces with aging time especially for the lamellar microstructure. TEM and SEM studies were performed to gain insight into the deformation behavior and crack propagation mechanisms of the alloy. Dependence of the FCG rates on aging treatment is rationalized by considering changes in crack path tortuosity, roughness-induced crack closure and deformation uniformity. Slip reversibility is also considered. The work suggests that the amount and size of α2 particles based on Ti3Al is mainly responsible for this dependence in the high ΔK range.
  • Fretting Fatigue Characteristics with Relating Contact Pressure and Surface Roughness of Highly Workable Titanium Alloy, Ti-4.5Al-3V-2Mo-2Fe

    pp. 1586-1593

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    DOI:10.2320/matertrans.45.1586

    The effects of contact pressure and surface roughness on fretting fatigue strength of Ti-4.5Al-3V-2Mo-2Fe conducted with annealing at 1123 K were investigated in this study. Fretting fatigue tests in low and high cycle fatigue life regions of the alloy were carried out at each contact pressure of 10, 15, 30, 45, 75, 105 and 153 MPa using relatively rough surface specimens with roughness number around 120∼140 nm. Furthermore, the fretting fatigue tests were also performed on the relatively smooth surface specimens with roughness number around 13.2∼32.2 nm at a selected contact pressure of 153 MPa. The fretting fatigue crack initiation mechanism is changed by contact pressure. At a contact pressure of 15 MPa, fretting fatigue life tends to be very low in each fatigue life region. The tangential force tends to increase with increasing contact pressure in each fatigue life region. Contrary to this trend, the tangential force coefficient tends to decrease with increasing contact pressure in each fatigue life region. The fretting fatigue life of the alloy with smooth surface is greater about 20% and 10% in low and high cycle fatigue life regions, respectively, than that of the alloy with relatively rough surface.
  • Hydrogen-Assisted Degradation of Titanium Based Alloys

    pp. 1594-1600

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    DOI:10.2320/matertrans.45.1594

    Titanium base alloys are among the most important advanced materials for a wide variety of aerospace, marine, industrial and commercial applications, due to their high strength/weight ratio and good corrosion behavior. Although titanium is generally considered to be reasonably resistant to chemical attack, severe problems can arise when titanium base alloys come in contact with hydrogen containing environments. Titanium base alloys can pick up large amounts of hydrogen when exposed to these environments, especially at elevated temperatures. If the hydrogen remains in the titanium lattice, it may lead to severe degradation of the mechanical and fracture behavior of these alloys upon cooling. As a consequence of the different behavior of hydrogen in α and β phases of titanium (different solubility, different diffusion kinetics, etc), the susceptibility of each of these phases to the various forms of and conditions of hydrogen degradation can vary markedly. This paper presents an overview of hydrogen interactions with titanium alloys, with specific emphasis on the role of microstructure on hydrogen-assisted degradation in these alloys.
  • Formation of Diamond-Like Carbon Based Double-Layer Film on Ti-6Al-4V Substrate by Ionization Deposition

    pp. 1601-1606

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    DOI:10.2320/matertrans.45.1601

    A double-layer film, in which the top layer was a diamond-like carbon (DLC) film and the bottom layer was a compositionally graded film of silicon and carbon compounds with decreasing C/Si atomic ratio to a substrate, was successfully formed on a Ti-6Al-4V substrate by an ionization deposition method. In the film deposition process, a benzene vapor was used for the DLC deposition as a source gas, and hexamethyldisiloxane and benzene vapors were used as source gasses for the compositionally graded film of silicon and carbon compounds. The results of ball-on-disk and scratching tests showed that the double-layer film with graded composition provided a low friction coefficient, high wear resistance and good adhesion with the Ti-6Al-4V substrate compared to a single-layer DLC film. The DLC-based double-layer film developed in this study is much effective in wide applications of the Ti-6Al-4V alloy for which the use to antifriction components has been difficult.
  • One-Pot Process for Anodic Oxide Films of Titanium with High Photocatalytic Activity

    pp. 1607-1612

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    DOI:10.2320/matertrans.45.1607

    Thick anodic oxide film with high photocatalytic activity was prepared successfully in 1.5 kmol/m3 H2SO4−0.3 kmol/m3 H2O2 with 0.03—0.09 kmol/m3 H3PO4 in a one-pot process. Increasing H3PO4 concentration in the range of 0—0.3 kmol/m3 suppressed spark discharge, resulting in a higher anatase-to-rutile ratio and fewer lower-valent titanium oxides in the anodized films. The photocatalytically active films consisted of a mixture of anatase and rutile crystallites with a small amount of Ti3+ ions, and had very rough surface with a large number of pores with a diameter on the order of micrometers. The film prepared in the 1.5 kmol/m3 H2SO4−0.3 kmol/m3 H2O2−0.03 kmol/m3 H3PO4 dispersed with TiO2 nanoparticles was the most active photocatalytically among the films obtained, and exhibited antibacterial activity.
  • Surface Modification of Titanium using Laser Beam

    pp. 1613-1619

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    DOI:10.2320/matertrans.45.1613

    Pure titanium has an excellent biocompatiblity in comparison with stainless steels and Ti-Al-V alloys. We would expect pure titanium to have application for artificial joints and artificial bones if the wear resistance of the pure titanium were to be improved. So the surface modification of the pure titanium was performed using YAG laser beam. The laser power was 1.5 kW and Ar was used as the shielding gas. The shielding gas flow rate was changed from 5 to 40 L/min with a constant laser torch traveling speed of 500 mm/min. First, we investigated effects of the shielding gas flow rate on the Vickers hardness of the laser melted zone. When the shielding gas flow rate decrease, the average hardness increases and the oxygen and nitrogen concentrations of the laser melted zone also increase. We made clear the relationship between the average hardness and the nitrogen equivalent in the laser melted zone as follows. When the square root of the nitrogen equivalent (Neq = N + O/2) was less than 0.1, a plot of the average hardness for the square root of the nitrogen equivalent reveals a linear relationship. However, the average hardness of the laser melted zone increased more than the value indicated by the linear relationship when the square root of the nitrogen equivalent was above 0.1. Next, metallurgical analyses of the laser melted zone were performed using an electron probe micro analyzer (EPMA), an X-ray diffraction method (XRD) and a transmission electron microscope (TEM), and effects of the behavior of oxygen and nitrogen on the hardness of the laser melted zone were studied. A uniform dislocation structures in the laser melted zone is observed over a wide area where there is the linear relationship between the hardness and the square root of the nitrogen equivalent. Lamellar structure, which alternated between two phases of αTi and TiN in the laser melted zone, was formed where the hardness is greater than those indicated by the linear relationship. One phase of TiN contained a large quantity of nitrogen, and the other phase of αTi contained little nitrogen. It is found that the lamellar structure composes of αTi and Ti-nitrides (TiN and TiN0.26). It is also observed that a wide area of αTi possesses a twin structure with a high dislocation density.
  • Ti-Al, Graded Al/AlTi, and Ti-Al-N Coatings Prepared by Supersonic Free-Jet PVD

    pp. 1620-1623

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    DOI:10.2320/matertrans.45.1620

    The authors previously developed Supersonic Free-Jet PVD (SFJ-PVD) as a new coating method in which a coating film is formed by depositing nanoparticles at very high velocity onto a substrate. This SFJ-PVD provides a rapid deposition rate and produces a mixture of different kinds of nanoparticles formed in different evaporation chambers on the substrate. This paper describes the preparation of graded Al/AlTi, Ti-Al, and Ti-Al-N coating films with SFJ-PVD. Ti-50 at%Al film and graded Al/AlTi film are produced by depositing Al and Ti nanoparticles formed in different evaporation chambers with the controlled evaporation rates of Al and Ti. Ti-Al-N film is produced by depositing nanoparticles formed in the evaporation chamber with a controlled partial pressure of N2 in an atmosphere of He. Mixing Ti and Al nanoparticles by depositing them onto a substrate produces in-situ syntheses of γ-TiAl and α2-Ti3Al intermetallic compounds on the substrate regardless of the substrate temperature in these experimental conditions. A smooth, compact, and defect-free structure is formed both at the interface between the substrate and the coating films and inside the coating films. XRD analysis reveals the crystal structure of the Ti-Al-N film to be TiN, Ti3Al2N2, Ti3AlN, and AlN.
  • Decomposition Processes of β Phase in a Ti-15Zr-4Nb-4Ta Alloy

    pp. 1624-1628

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    DOI:10.2320/matertrans.45.1624

    Decomposition processes of β phase in a Ti-15Zr-4Nb-4Ta alloy during isothermal holding have been examined mainly by transmission electron microscopy. Specimens solution-treated at 1000°C in β phase field were held at temperatures between 400 and 600°C. α phase lath formed in β phase during isothermal holding at 600°C. After holding at 600°C for a short duration, remaining β phase, untransformed β phase into α phase at 600°C, was transformed into α′ martensite during quenching into ice-brine. As increasing holding time at 600°C, remaining β phase was transformed into α″ or athermal ω phases during quenching. After prolonged holding at 600°C, remaining β phase was not transformed during quenching. Enrichment of Nb, Ta and Zr into remaining β phase occurred with increasing holding time at 600°C, resulting in the variation of transformation of remaining β phase during quenching. Microstructures evolved at 400 and 500°C were almost same as those at 600°C.
  • Phase Transformation of α″ Martensite Structure by Aging in Ti-8 mass%Mo Alloy

    pp. 1629-1634

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    DOI:10.2320/matertrans.45.1629

    The orthorhombic α″ martensite was formed in Ti-8 mass%Mo alloy by quenching from 1223 K. The purpose of this study was to investigate phase transformation of the α″ martensite structure by isothermal aging. In differential scanning calorimetry curve of the quenched specimen, an exothermic peak that indicated decomposition from the α″ martensite to α and β phases was observed near 780 K, so that isothermal aging was performed at 723 K and 923 K for 9.0 ks. Optical microscopy, X-ray diffraction and transmission electron microscopy were performed to these specimens. Band-like products that were composed of the single variant of ω phase were observed in the quenched α″ martensite structure. On the other hand, (111)α″ twins were observed in the 723 K-aged α″ martensite structure. The quenched α″ martensite structure indicated low elastic incline and good ductility, whereas the 723 K-aged α″ martensite structure indicated high yield stress and brittleness. It was pointed out that the ω products were formed to relax the volume expansion from the β phase to the α″ martensite, and the (111)α″ twins were formed during the isothermal aging at 723 K with the extinction of the ω products.
  • Effects of Biological Factors on the Repassivation Current of Titanium

    pp. 1635-1639

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    DOI:10.2320/matertrans.45.1635

    The repassivation current of titanium was measured by a newly designed electrochemical cell that can bare a new metal surface momentarily for the determination of the effects of biological factors such as dissolved oxygen, inorganic ions, amino acids, and proteins, on time transient of repassivation current. For this purpose, saline with various concentrations of dissolved oxygen and Hanks' solution with and without amino acids and proteins were employed as electrolytes. Estimated peak current densities and total charges during repassivation were used for the evaluation of reppasivation current. As a result, dissolved oxygen did not influence the repassivation reaction of titanium. Inorganic ions and proteins accelerated the repassivation, while some amino acids delayed it. If these factors are combined, it is important to reveal which factor governs the reaction. Unfortunately, that problem could not be revealed by this study. The above findings may apply to the dissolution amount of metal ion from depassivated titanium in the human body.
  • Self-Division Behavier of TiB Particles in TiB/Ti Composite

    pp. 1640-1645

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    DOI:10.2320/matertrans.45.1640

    The change in dispersion of TiB boride formed through the chemical reaction, Ti+CrB→Cr+TiB, was investigated in the Ti-Cr system powder compact containing CrB boride particles. EPMA analysis revealed that the shape of the TiB particles was granular type at first, but they began to divide into a large number of fine plate-like TiB particles at the sintering temperature (self-division phenomenon). Although this seems to be contrary to the Ostwald ripening mechanism, the phenomenon could be reasonably explained in terms of total interfacial energy in the system: Considering the grain boundary energy within the granular TiB and the low interfacial energy of coherent plate-like TiB/matrix interface, the total interfacial energy was found to be decreased through the self-division. It was concluded that the difference in the total interfacial energy works as driving force for the self-division phenomenon.
  • Shape Recovery Force of TiNi Fiber Reinforced Composite by Electric Heating

    pp. 1646-1652

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    DOI:10.2320/matertrans.45.1646

    The shape-memory effect of shape-memory alloy (SMA) and the character of the super elasticity are common and effective in today's usage. This paper advances these characteristics by discussing the research of an intelligent composite which functions in a wide-ranging ambient conditions TiNi type SMA fiber was used in the experiment and the embedded test piece was used in a polymer matrix. The influence factor of the actuator function by the shape-memory effect was evaluated in real-time by measuring the length change of the test piece during electric resistance heating. Using composite materials with the electric heating method resulted in a quantitative understanding of the primary factor influencing the shape recovery force. It has been understood that the shape recovery force is greatly influenced by the ambient temperature to which the composite material is applied, and this paper describes detailed information for the best composite materials and their condition characteristic mechanical control by the electric heating.
  • Casting Pressure in Two Types of Titanium Casting Equipment

    pp. 1653-1659

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    DOI:10.2320/matertrans.45.1653

    This study theoretically examined the relationship between mold permeability and the casting pressure acting on the molten titanium in two types of pressure casting equipment [two-chamber and one-chamber] for preparing titanium dental castings in order to select the most effective investment material and optimal casting conditions. The casting pressure exerted on the melt can be defined as the pressure difference between the melting chamber and the mold cavity after the molten titanium drops and seals the entrance to the cavity sprue. Differential equations describing the pressure in the mold cavity were derived from the equation of the state of gas as a function of time. Analysis revealed that mold permeability and the operation of each casting unit affect how the casting pressure acts on the melt: a low-permeability mold is appropriate for the one-chamber type but intermediate permeability molds are desirable for the two-chamber type. Using the results of this study and published permeability data on investments, an optimum investment material can be selected for each type of equipment.
  • Fundamental Aspects of Calciothermic Process to Produce Titanium

    pp. 1660-1664

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    DOI:10.2320/matertrans.45.1660

    Calciothermic reduction incorporated with electrolytic reclamation of calcium in a single reactor is a new method for extracting titanium from its oxide. Titanium dioxide is typically reduced in molten calcium chloride by calcium existing in this medium as soluble state, not by elemental calcium, to give also soluble calcium oxide and isolated granular sponge titanium to be recovered. This soluble calcium is reproduced simultaneously by electrolyzing the dissolved calcium oxide in the vicinity of the reduction zone of the reactor. In this paper, an outline of this advanced reactor is demonstrated especially from physico-chemical points of view with regard to the following items: (1) Thermochemistry of the CaCl2-CaO-Ca melt, (2) The electrolytic dissolution of calcium into molten calcium chloride. (3) Measure of impurities control, (4) Calciothermic reduction of titanium dioxide in molten calcium chloride.
  • Reduction of TiO2 in Molten CaCl2 by Ca Deposited during CaO Electrolysis

    pp. 1665-1671

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    DOI:10.2320/matertrans.45.1665

    Fundamental experiments are conducted to confirm the calcium reactivity with titanium oxide in the molten calcium chloride. The TiO2 samples placed in the molten salt could be reduced without any electron supply to TiO2. In the close vicinity of cathode, TiO2 could be successfully reduced to α-Ti with 1600 mass ppm oxygen. However, the strong stirring of the melt disturbed the calcium distribution near the cathode and the reduction was incomplete. These findings supported the proposed mechanism that Ca deposited on the cathode and it dissolved immediately into the molten salt. The parasite reactions consumed the dissolved Ca quickly, and they suppressed the effective reduction and subsequent deoxidation.
  • Effect of Ni Addition on Fatigue Properties of Bulk Glassy Zr50Cu40Al10 Alloys

    pp. 1672-1678

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    DOI:10.2320/matertrans.45.1672

    We examined Wöhler curves of Zr50Cu40Al10 and Zr50Cu30Ni10Al10 bulk glassy alloys (BGAs) using a rotating-beam fatigue test to evaluate the effect of addition Ni on fatigue strength. As a result, we found that the fatigue limit was increased from 250 MPa to 500 MPa by adding 10 at% Ni instead of Cu to a Zr50Cu40Al10 BGA. Zr50Cu40Al10 BGAs exhibit wider fatigue-fractured region with striation-like marks than that of Zr50Cu30Ni10Al10 BGAs. Additive Ni element limits the fatigue-fractured region to half of the whole fractured surface. We also measured dimensions of fatigue-fracture area of Zr50Cu40Al10 and Zr50Cu30Ni10Al10 BGAs to estimate the value of fatigue-fracture toughness. Especially, the fatigue fracture toughness decreases suddenly around 103 ∼ 104 cycles, similar to the Wöhler curves. The significant decrease of fatigue-fracture toughness originates from embrittlement/hardening in front of the fatigue crack tip during fatigue test.
  • Stress-Temperature Relationship in Compression Mode in Cu-Al-Ni Shape Memory Alloys

    pp. 1679-1683

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    DOI:10.2320/matertrans.45.1679

    Compression tests were performed at different temperatures in an 82.5%Cu-13.5%Al-4.0%Ni (mass %) single crystal, a composition giving the β′ (18R) thermally induced martensitic phase. In all cases, the non-twinned γ′ (2H) martensite was obtained after compression, so, the β′ → γ′ or β → γ′ martensitic transformations were induced, depending of the test temperature. The critical stress vs. temperature, σcT, relationship was established for both types of transformations, obtaining a negative slope dσc/dT (considering compressive stresses as negative) for the austenite-martensite transformation and a small positive slope for the martensite-martensite. The experimental dσc/dT values were compared with those calculated from the Clausius-Clapeyron type equation and reasonable good agreement between them was obtained. For these calculations, the entropy changes ΔSβ′−γ′ and ΔSβ−γ′ were directly obtained from the calorimetric runs performed after each mechanical test.
  • Photoluminescent Properties of Europium-Activated Zn2SnO4 Phosphors

    pp. 1684-1686

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    DOI:10.2320/matertrans.45.1684

    Zn2SnO4, which has an inverse spinel structure, was adopted as the host material of a new green emitting phosphor. The photoluminescent (PL) properties of a series of europium-activated zinc stannate synthesized by vibrating milled solid state reaction have been investigated. One broaden emission band from the Zn2SnO4: Eu2+ phosphor calcined at 1000°C to 1200°C for 3 h in air atmosphere is clearly observed at 525 nm under 374 nm UV ray excitation. The emission band from Zn2SnO4: Eu3+ can also be observed under 464 nm-ray excitation. The reduction of Eu3+ → Eu2+ was firstly discovered in stannate phosphor of Zn2SnO4: Eu synthesized in air condition.
  • Influence of Purity on the Formation of Cube Texture in Aluminum Foils for Electrolytic Capacitors

    pp. 1687-1692

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    DOI:10.2320/matertrans.45.1687

    In the present study, the crystal orientation and residual strains in hot rolled sheets, cold rolled foils, partially annealed foils, and additionally rolled foils of aluminum of 99.9% (3NAl) and 99.99% (4NAl) purity used for the fabrication of electrolytic capacitors, were evaluated by the SEM/EBSP method. The additionally rolled foils were annealed at 573 K and the behavior of the growth of cube-oriented grains and the grain boundary character were analyzed. In the hot rolled sheets and the partially annealed foils, cube-oriented grains in 4NAl were larger in number and size than those of 3NAl. From the result, it was clarified that purity of the aluminum affected the growth of cubeoriented grains during the thermo-mechanical treatment. In the additionally rolled foils annealed at 573 K, the growth of the cube-oriented grains in 4NAl was faster than that in 3NAl. In both 3NAl and 4NAl, the residual strains and the grain boundary character were similar. Accordingly, it is concluded that impurity in 3NAl could have segregated at the grain boundaries around cube-oriented grains and supressed the growth of cube-oriented grains.
  • Oxidation Resistance of TiAl Improved by Ion Implantation of Beta-Former Elements

    pp. 1693-1699

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    DOI:10.2320/matertrans.45.1693

    TiAl coupon specimens were implanted with Fe, Mo, Ta or W ions and then cyclically oxidised with temperature varying between room temperature and 1200 K in a flow of purified oxygen under atmospheric pressure. The surface modification by the ion implantation was characterised by glancing angle X-ray diffractometry (GAXRD), Auger electron spectroscopy (AES) and transmission electron microscopy (TEM). The oxidised specimens were examined by AES, GAXRD, X-ray diffractometry, scanning electron microscopy and electron probe microanalysis. The oxidation resistance of TiAl is significantly improved by the implantation of Mo, Ta or W ions with a dose of 1021 ions·m&minus2; at acceleration voltages ranging from 50 to 340 kV. The acceleration voltage has a small influence. The oxide scales consist predominantly of α-Al2O3 and are very adherent to the substrates even after 20 cycles (400 h). On the other hand, the implantation of Fe has a little effect. The significant effect brought about by the implantation is attributable to the formation of a thin β-Ti phase layer in the modified area. Therefore, a possible explanation for the improved oxidation resistance is the formation of β-Ti phase, which is a solid solution where diffusion of Al seems much faster than in γ-TiAl which has an ordered structure. The enhanced Al diffusion results in the formation of a thin but continuous Al2O3-rich layer in the scale during the initial oxidation stages. The enrichment of Al relative to Ti by the implantation was thought playing some role. The so-called doping effect of Mo, Ta and W is also contributing to the early formation of Al2O3-rich layers by retarding TiO2 growth.
  • Mechanical Properties Related to Microstructural Variation of 6061 Al Alloy Joints by Friction Stir Welding

    pp. 1700-1705

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    DOI:10.2320/matertrans.45.1700

    The microstructural change related with the mechanical properties of a friction stir welded 6061 Al alloy has been investigated under various welding conditions. Frictional heat and plastic flow during friction stir welding produced fine and equiaxed grains in the stir zone, macroscopically upset and elongated grains in the thermo-mechanically affected zone caused by dynamic recovery and recrystallization. The heat-affected zone, characterized by coarse precipitates, was formed beside the weld zone. Hardness distribution near the weld zone was strongly related to the behavior of precipitates and dislocation density. Especially, hardness of the SZ at a higher tool rotation speed was higher than that of a lower tool rotation speed due to higher density of spherical shaped re-precipitates. The joint strength was approximately 200 MPa which was lower than that of the base metal, 270 MPa, because softening region was formed around the weld zone.
  • Tear Toughness of Permanent Mold Cast and DC Cast A356 Aluminum Alloys

    pp. 1706-1713

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    DOI:10.2320/matertrans.45.1706

    Cast products of A356 with different microstructural features were prepared; permanent-mold cast (PM) and direct-chill cast (DC) products. For each casting, a sharp notched plate specimen was subjected to static tensile loading until a crack initiated at the notch root and propagated across the width of the specimen. Both maximum load and energy to fracture (the integrated area under the load-displacement curve) increased with decreasing dendrite arm spacing (DAS). The curve of DC was smooth and the energy to fracture was quite large. The load-displacement curve was divided into two segments by a vertical line through the maximum load. The area under the first segment is a measure of the energy necessary to initiate the crack. The second segment represents the energy necessary for crack propagation. Unit energy was computed by dividing the measured energy by the net area of the specimen. Refinement of DAS and grain size increased unit energies for crack initiation (UEi) and propagation (UEp). Comparison among PM products revealed that DAS refinement was effective for increasing UEi. Among the present castings, the DC product with the finest DAS exhibited a significant increase in UEp. Observation of the crack propagation path revealed that the fracture surface was normal to the loading direction for PM. In contrast, for DC, a slanted crack path was dominant through the specimen ligament. The features of the crack propagation path are considered to have affected quantitative balance between UEi and UEp. The increased UEp in DC is considered to be due to the introduced slanted crack. Tear tests were confirmed to provide useful information concerning the effect of solidification structure on toughness, which can serve as a guide for further toughening of aluminum alloy castings.
  • Tear Toughness Evaluation of a Permanent Mold Cast A356 Aluminum Alloy Using a Small-size Specimen

    pp. 1714-1721

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    DOI:10.2320/matertrans.45.1714

    Tear toughness evaluation of a permanent mold cast A356 aluminum alloy was carried out by using two kinds of specimen with different size. One was equivalent to the specimen size designated in ASTM B871. The other one was about 30% as large as that of standard one in volume. Unit energies for tear fracture were obtained from load-displacement curves, and their specimen size, thickness and microstructure dependency were examined. Unit crack initiation energy (UEi) increased with increase in specimen thickness. Meanwhile, unit crack propagation energy (UEp) monotonically decreased in accordance with increase in specimen thickness. In order to make sure if the UEp values reflected the characteristics of local microstructure difference, the small-size tear specimens were collected from various parts in a single cast product. Larger UEp was obtained in the specimen with finer dendrite arm spacing (DAS). These findings suggest that the tear test using a small-size specimen is useful for toughness evaluation of cast aluminum alloys.
  • Rock Fragmentation Control in Blasting

    pp. 1722-1730

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    DOI:10.2320/matertrans.45.1722

    Rock fragmentation, which is the fragment size distribution of blasted rock material, is used in the mining industry as an index to estimate the effect of bench blasting. It is well known that the rock fragmentation in bench blasting is affected by blast condition such as specific charge, spacing and burden, involving rock heterogeneity, dynamic fracture phenomena, etc. Rock fragmentation in bench blasting was examined using a numerical approach, based on dynamic fracture process analysis and image analysis. Five models were used to consider the effect of specific charge and geometry in bench blasting. To investigate the influence of blast condition on fragmentation in bench blast simulations, the fragmentation obtained using the numerical approach was compared and analyzed. To discuses controlled fragmentation related to the blast pattern, widely spaced blast patterns were simulated. The fracture process and fragmentation were compared with that in general bench blasting. Controlled fragmentation with respect to delay timing was also discussed. Ultimately it was discussed that the optimal fragmentation in the field with respect to delay time depends strongly on the gas flow through the fractures caused by the stress wave.
  • Creep Characteristic of NiAl-9Mo Eutectic Alloy

    pp. 1731-1737

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    DOI:10.2320/matertrans.45.1731

    The creep behavior of NiAl-9Mo eutectic alloy has been studied in an effort to understand the characteristic that the alloy exhibits of the low stress exponent of 4.75 and the high apparent activation energy of 410 kJ/mol during the steady-state creep of under the test conditions. The material exhibits threshold behavior with a threshold stress of 17.9 MPa. TEM observation reveals that the creep deformation is mainly governed by dislocation climb in NiAl matrix phase. Large recoverable strain is observed after unloading under the transient creep and increases with the increase of stress or temperature. It exhibits an activation energy 232 kJ/mol, suggesting the processes is controlled by the flow behavior of the matrix phase. The apparent creep activation energy under constant stress is a combination of the creep flow activation energy of the matrix and the activation energy for the damage process at the phase interface.
  • High-loading Velocity Tensile Properties and Fracture Behavior of SiCp/AC4CH Composite

    pp. 1738-1742

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    DOI:10.2320/matertrans.45.1738

    This study conducted a high-loading velocity tensile test for a SiCp/AC4CH composite and an AC4CH alloy (Al-6.7%Si-0.3%Mg alloy). Microstructures of both materials before and after tensile testing were carefully examined with an optical microscope and SEM. Experimental results demonstrate that the ultimate tensile strength (UTS) of the SiCp/AC4CH composite increased with increasing loading velocity up to 10 m·s-1. Compared to the AC4CH alloy, the fracture elongation of the SiCp/AC4CH composite is more sensitive to the strain rate. The AC4CH alloy yield strength (YS) shows more sensitivity than that of UTS with increasing strain rate, especially in the range of loading velocity higher than 1 m·s−1. The composite failed by coalescence of microcracking/microvoiding. The ratio of broken SiC particles increases with increasing loading velocity.
  • The Damping Capacity of AZ91 Magnesium Matrix Composites Reinforced with the Coated Carbon Fiber Fabric

    pp. 1743-1747

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    DOI:10.2320/matertrans.45.1743

    A novel technology to obtain high damping capacity in magnesium matrix composites was developed by designing a special interface layer. The interface layer was fabricated by coating a pyrocarbon on the surface of carbon fibers. Experimental results reveal that the carbon coating on carbon fibers may improve the overall damping of composite from room temperature to about 170°C. The relevant damping mechanisms are ascribed to dislocation damping, interface damping and the intrinsic damping of the constituents. At temperature above 170°C, the damping of the composite without the coating exceeds that with the coating owing to the contribution of interface damping and grain boundary damping.
  • Effect of Temperature on Equilibria in Synthetic Sulfuric Acid Leaching Solution of Zinc Calcine

    pp. 1748-1753

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    DOI:10.2320/matertrans.45.1748

    In order to simulate the leaching of zinc calcine by sulfuric acid solutions, we prepared synthetic solutions with the composition ZnSO4-Fe2(SO4)3-Na2SO4-H2SO4-NaOH-H2O and measured the solution pH at various temperatures. The effect of temperature on the ionic equilibria of these solutions was analyzed by considering chemical equilibria, mass balance and charge balance equations. The activity coefficients of solutes and the activity of water were calculated by using the Pitzer equation. The mole fractions of iron species were greatly affected by the solution temperature and the concentration of Fe2(SO4)3. In the experimental ranges of ionic strength of solution up to 9 m, the pH values calculated in this study agreed well with those measured.
  • Measurement of the Density of Binary Ni-X (X=Co, W, Ta, Al) Alloys

    pp. 1754-1763

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    DOI:10.2320/matertrans.45.1754

    The modified sessile drop method (MSDM) and modified pycnometric method (MPM) have been employed to measure the densities of liquid Ni-Co, Ni-W, Ni-Ta and Ni-Al alloys precisely. In the case of Ni-Al alloys measurements were also made in the “mushy” state where both solid and liquid coexist. There was a good agreement between the values measured by the two methods. The density of liquid Ni-base binary alloys from the melting points to 1833 K and that of Ni-Al alloys in the mushy state decrease linearly with increasing temperature. The temperature coefficient of the density of Ni-base binary alloys measured in this work was regressed as a quadratic function of the concentration of alloy element. The measured density of Ni-base binary alloys at any required temperature can also be expressed as a quadratic function of the concentration of alloy element. The recommended equations for the density of Ni-base binary alloys were obtained as a function of composition and temperature by regressing the density values measured by both MSDM and MPM. The calculated values from the recommended density equations show good agreement with those obtained with both methods.
  • Prediction of the Occurrence of Swirling Liquid Jet

    pp. 1764-1768

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    DOI:10.2320/matertrans.45.1764

    A liquid bath contained in a cylindrical vessel is strongly agitated by a swirl motion of a liquid jet. This phenomenon is beneficial for the enhancement of the reaction efficiency of metals refining processes as well as wastewater treatment. The condition for the occurrence of the swirl motion was determined based on water model experiments as a function of the aspect ratio of the bath and a modified Rossby number. The physical properties of liquid hardly affected the occurrence of the swirl motion under the present experimental conditions. Empirical equations were proposed for predicting the occurrence of the swirl motion.
  • Thermal Expansion Behavior of SiCP/Aluminum Alloy Composites Fabricated by a Low-Pressure Infiltration Process

    pp. 1769-1773

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    DOI:10.2320/matertrans.45.1769

    Thermal cycling test was carried out for 20 vol% and 40 vol% SiC particle/Al-4 mass%Cu alloy composites to evaluate the bonding strength of the SiC particle/matrix interface in PRMMC fabricated by a low-pressure infiltration process (LPI process). The SiC particles were distributed homogenously in the specimens and a reaction layer of less than 1 μm thickness was observed at the SiC particle/matrix interface before thermal cycling test. This reaction layer was identified as Al4C3 formed by the reaction between the alloy melt and SiC particle during infiltration. The θ phase, which might form a coherent interface with Al4C3, crystallized around the SiC particles through the Al4C3 layer. The coefficient of thermal expansion was hardly changed during thermal cycling for both the 20 vol% and 40 vol% SiC particle/Al-Cu composites. No significant change in the microstructure and no detachment at the SiC particle/matrix interface were observed after thermal cycling test. The interfacial structure consisting of SiC, Al4C3, θ phase and α-Al in order was considered to exhibit a strong bonding of SiC particles to the matrix. The Vickers microhardness measured on the SiC particles in the specimens having a strongly bound interface show hardness values with a small scatter, while those for the specimens having a weakly bound interface exhibit a large scatter in the hardness values. It is suggested that the bonding strength of the reinforcement particle/matrix interface could be evaluated qualitatively from the Vickers microhardness test.
  • Mechanical Alloying of Fe25Al75−xTix Mixed Powders

    pp. 1774-1777

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    DOI:10.2320/matertrans.45.1774

    Elemental aluminum, titanium and iron powders with compositions of Fe25Al75−xTix (x =0, 3.75, 7.5, 11.25 and 15) were mechanical alloyed in a planetary ball mill. The transformations and thermal stabilities of milled powders were characterized by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Supersaturated solid solutions were found in low Ti concentration (x = 0−7.5). The addition of Ti shortened the milling time for the formation of solid solution. The amorphous phases were obtained with high Ti concentration (x = 11.25 and 15). The activation enthalpies of crystallization for amorphous phases increased with Ti. The crystallization temperatures of amorphous alloys were calculated, which are in good agreement with the experimental values.
  • Evaluation of Degradability of CaTiO3 Thin Films in Simulated Body Fluids

    pp. 1778-1781

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    DOI:10.2320/matertrans.45.1778

    In vitro dissolution behaviors of CaTiO3 thin films deposited with ion beam assisted deposition (IBAD) and sputter-deposition techniques were investigated using Rutherford backscattering spectrometry (RBS) and Auger electron spectroscopy (AES). In vitro test was carried out by immersion in a simulated body fluid at 37°C for 5, 24, and 888 h. The dissolution of titanium ions from CaTiO3 film was almost independent of deposition techniques while that of calcium ions was dependent on those. In the specimens deposited by sputter-deposition, the dissolution rate of the calcium ions increased proportionally with time, and in the case of IBAD, it increased proportionally with the logarithm of time. Furthermore, in IBAD, the dissolution rate of the calcium ions was also intimately related with the total kinetic energy of the titanium ions when implanted. The results reveal that IBAD CaTiO3 coating film makes it possible to control dissolution rate of calcium ions from the film.
  • Bone-like Layer Growth and Adhesion of Osteoblast-like Cells on Calcium-deficient Hydroxyapatite Synthesized at Different pH

    pp. 1782-1787

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    DOI:10.2320/matertrans.45.1782

    Calcium-deficient hydroxyapatite (DAp) was synthesized by hydrolysis of α-TCP at different pH. Ca/P ratio of DAp increased with increasing pH during the hydrolysis. In addition, DAp which was synthesized at lower pH, showed higher solubility in the simulated body fluid (SBF) and had a more negatively charged surface than that at higher pH. Bone-like crystals were formed on DAp which have been soaked in SBF at 309.5 K in vitro. DAp demonstrated faster formation of a bone-like layer than stoichiometric hydroxyapatite (HAp). Moreover, the formation of the crystals on DAp synthesized at lower pH was much faster than that at higher pH. The adhesion of MC3T3-E1 osteoblast-like cells was also influenced by pH during the synthesis of DAp; the number of adhering cells increased with a decrease in the pH. Therefore, high solubility and negative charge of DAp, especially that synthesized at lower pH resulted in fast formation of bone-like crystals and good cell adhesion.
  • Mechanism of Vertical Microcracking in CaO·SiO2-CaO·ZrO2 Sprayed Thermal Barrier Top Coating

    pp. 1788-1793

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    DOI:10.2320/matertrans.45.1788

    The durability of thermal barrier coatings (TBCs) at gas turbine has been improved substantially by instituting more strain tolerant zirconia-ceramic top coatings with vertical cracks artificially, such as ZrO2-Y2O3 plasma-sprayed coating with purposely segmentation. Under the necessity of TBCs with thermal shock resistance and durability at high temperature, we have developed the top coating of 2CaO·SiO2-10∼30 mass%CaO·ZrO2 (C2S-10∼30%CZ) with many vertical microcracks that is formed with the spraying process only, namely one vertical microcrack does not pierce through the sprayed layer but remains within a single flattened ceramic particle of the top coating. In order to discuss the mechanism of the vertical microcracking, the flattening-solidification behavior of the C2S-10∼30%CZ sprayed splat was investigated. Microcrack was observed within the splat of C2S-10∼30%CZ particle after solidification on the polished SUS304 stainless steel.
    It is assumed that spraying particle liquefied in plasma flame collides and strongly adheres on the substrate or the accumulated particles and change the phase from liquid to brittle solid due to rapid cooling, at the same time, the tensile stress is generated on the external surface of the particle and fine vertical microcrack occurs in the particle which could not withstand this stress.
    From strong correlation between the vertical microcracking and amorphous like behavior of the C2S-10∼30%CZ coating, the liquid phase of C2S-10∼30%CZ particle sublimated some CaO in plasma flame is considered to take an important role in vertical microcracking formation due to shrinkage during the cooling and solidifying process.
  • Study of Micro-Droplet Behavior for a Piezoelectric Inkjet Printing Device Using a Single Pulse Voltage Pattern

    pp. 1794-1801

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    DOI:10.2320/matertrans.45.1794

    The aim of this study is to investigate the formation and ejection behavior of droplets created by a squeeze mode piezoelectric inkjet printing device using a single pulse voltage pattern. The test liquids are de-ionized (DI) water and ethylene glycol. The experimental results and acoustic wave theory are used to discuss the effects of operating frequency, positive voltage keeping time and pulse voltage magnitude on the volume and velocity of the droplets. For this study, a squeeze mode piezoelectric printhead is employed. By coordinating an LED flash with droplet ejection, a CCD camera could be used to capture images of the droplets at different points in the formation and ejection process. These images were then used to estimate the volume and velocity of the droplets. The experimental results are consistent with the propagation theory of acoustic waves. The maximum allowable pulse frequency in DI water and ethylene glycol are 1500 Hz and 14000 Hz respectively. If the positive voltage keeping time equals the time required for the acoustic wave to propagate through the printhead, optimal ejection behavior is achieved. As the pulse voltage increases, both the velocity and volume of the droplet become larger.
  • Formation of New Ti-based Metallic Glassy Alloys

    pp. 1802-1806

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    DOI:10.2320/matertrans.45.1802

    New Ti-based bulk glassy alloys were synthesized in a multi-component alloy system of Ti-Cu-Ni-Al-Zr-Si-B. In a wide composition range of Ti > 50 at%, single-phase glass rods with diameters up to 2.5 mm were prepared using a conventional copper-mold casting method. The new Ti-based glassy alloys show compressive strength above 1900 MPa. The plastic (serrated) strain under compression is lager than 0.01. The new Ti-based glass phase exhibits relatively high thermal stability with a large supercooled liquid region (ΔTx) above 50 K. These glassy alloys, however, exhibit complex crystallization behavior characterized by multiple exothermic-reaction peaks on their differential scanning calorimeter (DSC) curves. The glass-forming ability of this complex system is discussed on the basis of knowledge available for bulk glass formation.
  • Bending Properties of Nanocrystalline Ni-18 at% W Alloy Produced by Electrodeposition

    pp. 1807-1810

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    DOI:10.2320/matertrans.45.1807

    A nanocrystalline Ni-18 at% W alloy with the grain size of 5.3 nm was produced by an electrodeposition method and its bending properties were investigated in the temperature range between room temperature and 673 K. At room temperature and 373 K, the nanocrystalline Ni-W alloy could be bent to an angle of 180 degrees without fracturing. At 423 K and above, the alloy fractured before reaching 180 degree during bending. Significant springback behavior was observed after 180 degree bending at room temperature under the condition that a compressive load of 9 N was applied during bending. As the compressive load was increased from 30 N to 135 N, the magnitude of springback decreased. Thus, the nanocrystalline Ni-W alloy exhibited excellent bendability at room temperature.

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