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

MATERIALS TRANSACTIONS Vol. 46 (2005), No. 9

  • An Analysis of Flow Mechanisms in High Temperature Creep and Superplasticity

    pp. 1951-1956

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    DOI:10.2320/matertrans.46.1951

    The flow of crystalline solids at elevated temperatures is dependent upon creep mechanisms associated with the movement of dislocations, the relative displacements of adjacent grains and the stress-directed flow of vacancies. It is convenient to express the steady-state creep rate in terms of the dependences of these various creep mechanisms on the applied stress, the testing temperature and the grain size of the material. In practice, however, there are similarities in some of the predicted dependences for different creep processes and this may lead to experimental difficulties in unambiguously identifying the rate-controlling creep mechanism. The difficulties in identifying the creep mechanism become especially significant at low stresses. This paper reviews the characteristics of the various flow processes occurring in simple metallic systems such as pure metals and metallic alloys and describes procedures that may be adopted to provide an unambiguous identification of the rate-controlling flow mechanism.
  • Stress Relaxation of Poly-Si Film Formed by Excimer Laser Annealing

    pp. 1958-1964

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    DOI:10.2320/matertrans.46.1958

    We investigated the low-temperature processed polycrystalline-silicon (poly-Si) film formed by carrying out low-energy-density and multishot excimer laser annealing (ELA) of amorphous Si (a-Si) film on a glass or quartz substrate. The influence of secondary grain growth which grain size becomes larger than the film thickness on the tensile stress relaxation of the poly-Si film was clarified. Relationship between hydrogens which are supplied to the melt-Si from the bottom catalytic-chemical vapor deposition (Cat-CVD) SiN film and the stress relaxation was also examined. The tensile stress in the poly-Si film relaxes upon the appearance of the secondary grains, and the stress decreases with increasing diameter of the secondary grains. However, the hydrogen supply to melt-Si suppresses the secondary grain growth and the stress relaxation becomes marked as the hydrogen concentration decreases. Furthermore, the relationship between the dangling bond density corresponding to the crystal defect density and grain size was investigated, and the defect site was clarified. Lastly, the recrystallization model by ELA at low energy densities was discussed. The model was constructed based on the experimental data that the secondary grain growth occurs suddenly at the critical energy density and the critical shot number, and it was shown that the present model is consistent with the results related to the hydrogen concentration and the secondary grain growth.
  • Development of Thermally Stable, Solar-Blind Deep-Ultraviolet Diamond Photosensor

    pp. 1965-1968

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    DOI:10.2320/matertrans.46.1965

    A thermally stable, deep-ultraviolet (DUV) photodiode is developed by using tungsten carbide (WC) Schottky and Ti/WC Ohmic contacts for boron-doped homoepitaxial p-type diamond epilayer. Thermal annealing at temperatures lower than 550°C improves the rectifying current-voltage characteristics of the photodiode, resulting in a dramatic enhancement of DUV responsivity at 220 nm by a factor of 3×104. A discrimination ratio between DUV and visible light is measured to be as larger as 105 at a reverse bias as small as 2 V and almost constant after annealing at 550°C for 2.5 h. Development of the thermally-stable WC-based Schottky and Ohmic contacts opens up possibility for stable operation of diamond photosensor at high temperatures.
  • Room-Temperature Operation of Injection-Type 1.5 μm Light-Emitting Diodes with Er,O-Codoped GaAs

    pp. 1969-1974

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    DOI:10.2320/matertrans.46.1969

    Injection-type 1.5 μm light-emitting diodes (LEDs) with Er,O-codoped GaAs (GaAs:Er,O) have been fabricated by organometallic vapor phase epitaxy (OMVPE). Electroluminescence (EL) spectrum from the LEDs under forward bias at room temperature was dominated by the luminescence due to an Er-2O center, an Er atom located at the Ga sublattice with two adjacent O atoms, indicating that injected carriers contribute effectively to the excitation of the Er-2O center. The current density dependence of EL properties revealed an extremely large excitation cross section (approximately 10−15 cm2) of Er ions by current injection. GaInP/GaAs:Er,O/GaInP double-heterostructure (DH) LEDs have also been fabricated. The saturated EL intensity significantly increased with increasing GaAs:Er,O active-layer thickness.
  • Growth of GaN on Nitriding TiN Buffer Layers

    pp. 1975-1978

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    DOI:10.2320/matertrans.46.1975

    The breakthrough of GaN epitaxial layer growth with mirror-like surface on the sapphire substrate using a MOCVD (metal organic chemical vapor deposition) technique was made by discovery of an AlN buffer layer prior to GaN deposition about 20 years ago. Since then, extensive efforts have been made to develop a conductive substrate for MOCVD grown GaN layers. In the present study, we explored a possibility of growing continuous, flat GaN layers on a metallic TiN buffer layer, and focused our experiments to investigate the effect of nitrogen contents of the TiN buffer layers on lateral growth of the GaN layers. It was concluded that nitrogen composition in the TiN buffer layers should be higher than that of stoichiometric TiN to grow smooth GaN layers. It was found that nitriding the TiN buffer layers after deposition enriched significantly the nitrogen content compared with increasing the ratio of N2 to N2+Ar (N2⁄(N2+Ar) ratio) during TiN deposition. Choice of the moderate N2⁄(N2+Ar) ratio during TiN deposition and nitriding the TiN buffer layers after TiN deposition were essential to grow continuous, flat GaN layers, since the reduction of the N2⁄(N2+Ar) ratio promoted few opening areas in the GaN layers.
  • Hardness, Yield Strength, and Dislocation Velocity in Elemental and Compound Semiconductors

    pp. 1979-1985

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    DOI:10.2320/matertrans.46.1979

    Current knowledge on macroscopic plasticity indications, i.e., hardness and yield strength, and on microscopic indication, i.e., velocity of individual dislocations, in elemental and IV–IV, III–V, and II–VI compound semiconductors including GaN and ZnSe are reported and discussed on their mutual correlations. The Vickers hardness of the semiconductors can provide conventional information on the material plasticity in a wide temperature range up to their melting points over a wide range of size scales in various material forms. Hardness Hv in diamond- and sphalerite-type semiconductors has a universal relationship on their temperature dependence similar to the yield strength τy with a relation Hv=(70–100) τy in the low temperature region. Yield strength obtained by normal tensile or compression tests are expressed by an experimental equation as a function of the strain rate and temperature. The velocities of various types of dislocations measured directly in several semiconductors are described with an empirical equation as a function of the stress and the temperature. Through the analysis of yield strength data in terms of the collective motion of dislocations during the plastic deformation, the dislocation motion, rate-controlling plastic deformation, are deduced. The activation energy for dislocation motion has a linear relation to the band gap energy, depending on the types of semiconductors, elemental, III–V compounds, and II–VI compounds.
  • Compositional Plane of a New Wide-Gap Solid Solution Semiconductor CaPbSeS and Epitaxial Thin Film Growth of CaSe

    pp. 1986-1990

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    DOI:10.2320/matertrans.46.1986

    We have systematically investigated the compositional plane of solubility range and lattice constants in Ca1−xPbxSe1−ySy system, and as a first step for preparation of the ternary system, we have also investigated the growth and characterization of the CaSe thin films.
    Solubility range and lattice constant of a Ca1−xPbxSe1−ySy system was investigated using powder synthesis under thermal equilibrium condition. A CaSe thin film was grown on a cleaved BaF2(111) substrate by means of a hot-wall epitaxy.
    The solubility limit at 1273 K varies with respect to the Se concentration y, taking a minimum limit of 0.04 at y=0.8 and a maximum of 0.24 at y=0. It is found that the system can be lattice-matched to PbS and InP. The CaSe thin films are grown epitaxially at substrate temperature range between 673 and 873 K. The energy band gap of the film is estimated to be 4.62 eV at RT through measurement of transmittance and reflectance of the film, having full-width at half-maximum (FWHM) of X-ray rocking curve of 0.08° by ω–2θ scan at (222) Bragg reflection.
  • Microstructural Analysis of CdTe Radiation Detectors with Indium Electrodes

    pp. 1991-1995

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    DOI:10.2320/matertrans.46.1991

    In order to understand the formation mechanisms of indium contacts which were previously developed for CdTe radiation detectors with p–i–n structure, microstructure of the indium contacts which were deposited at various substrate temperatures on the p-CdTe substrates and subsequently annealed at temperature ranging from 200 to 400°C were analyzed by X-ray diffraction and transmission electron microscopy. The microstructural analysis revealed that reactions between CdTe and In were enhanced by the substrate heating during deposition and growth of In4Te3 or InTe compounds on the CdTe surface was observed. Current leakage and stability of the CdTe radiation detectors were strongly correlated with the microstructure of CdTe/contact interfaces. The indium contacts, which were deposited at substrate temperature of 400 or 350°C and subsequently annealed at 350°C for 1 h in vacuum, displayed rectifying I–V behaviors and satisfied the device requirement. In addition, the excellent thermal stability of these contacts was observed. We found that the formation of the InTe compounds was essential for production of the p–i–n CdTe detector devices which require highly reliable rectifying contacts.
  • Nanoscale Mechanical Properties of Ultrahigh-Purity Aluminum

    pp. 1996-2002

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    DOI:10.2320/matertrans.46.1996

    Nanoindentation data reveal interesting behavior in aluminum with various purities: 99.9999% (6N), 99.99% (4N), and 99% (2N). Nanoindentation is used to investigate the relationship between the purity and the mechanical properties of ultrahigh-purity aluminum at room temperature. The area subjected to nanoindentation would be expected to behave similarly to a perfect, dislocation-free single crystal. Nanoindentation data is also compared with results of conventional tensile and hardness tests. These results highlight the differences between microscopic and macroscopic properties. The tensile strength and the hardness of normal-purity aluminum, 99% (2N), are larger than those of ultrahigh-purity aluminum, 99.9999% (6N), and high-purity aluminum, 99.99% (4N). However, in the nanoindentation test, the penetration depth for ultrahigh-purity aluminum (6N) and high-purity aluminum (4N) is less than that for normal-purity aluminum (2N). Thus, microscopic mechanical properties differ from macroscopic mechanical properties. It is suggested that the surface of high-purity aluminum is harder than that of normal-purity aluminum on the micro-scale. The experimental result shows that the perfect crystals are harder than the imperfect crystals. Furthermore, some recovery of the indentation mark is observed in high-purity aluminum. It comes from the recovery of deformation by indentation due to the mobility of defects in the sample.
  • Phase Field Simulation on Directional Solidification of Succinonitrile (SCN)–Acetone Organic Model Alloy

    pp. 2003-2010

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    DOI:10.2320/matertrans.46.2003

    Phase field simulation on directional solidification of succinonitrile (SCN)–acetone organic model alloy was performed. The development of concentration and phase field profile was calculated in order to clarify the dependences of growth velocity V and temperature gradient G on the morphology of growing crystal.
    Concerning the primary dendrite arm spacing, λ1, the relationship between λ14G2VkΔT0 and V was examined, where k and ΔT0 are partition coefficient and temperature difference between liquid and solid phases, respectively. The calculated values of λ14G2VkΔT0 did not show a universal relationship but the trends of the present calculation can be understood by extrapolation from experimental data. The power law index of V for the λ14G2VkΔT0V plot took similar values between the experiments and the present calculation for some cases. The λ1V relationship did not show dependences on temperature of the low temperature side and nor acetone concentration. For the curvature radius R, the RV relationship can be fitted by a simple function regardless the value of G.
    In addition, the calculated values of R and the power low indexes of V for R were compared to the dendrite growth theory and good accordance was confirmed.
  • High-Magnetic Field X-ray Diffraction Studies on Gd5(Ge2−xFex)Si2 (x=0.05 and 0.2)

    pp. 2011-2014

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    DOI:10.2320/matertrans.46.2011

    We performed the powder X-ray diffraction measurements in magnetic fields up to 5 T for Gd5(Ge1.95Fe0.05)Si2 and Gd5(Ge1.8Fe0.2)Si2. With heating from 8 K, the matrix of Gd5(Ge1.95Fe0.05)Si2 clearly shows a structural transition from an orthorhombic to a monoclinic structure at the Curie temperature (TC=276 K). On the other hand, the matrix of Gd5(Ge1.8Fe0.2)Si2 with the orthorhombic structure in the ferromagnetic state shows two-phases co-existence of the orthorhombic and the monoclinic structures above TC=303 K, indicating that a small amount of the matrix participates in the transformation. For both samples, the monoclinic structure is suppressed but the orthorhombic structure is enhanced just above TC by applying a magnetic field, which closely relates to the magnetization process.
  • Effect of Fabrication Method on Microstructure and Properties of Al2O3–TiC Composites

    pp. 2015-2019

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    DOI:10.2320/matertrans.46.2015

    Al2O3–TiC composite powders were prepared by high-energy ball milling (HEBM) and wet-planetary ball milling (WPBM) from different starting materials, and subsequently sintered by Spark Plasma Sintering (SPS). The effect of the fabrication method on the microstructure and properties of Al2O3–TiC composites was investigated. The results showed that Al2O3–TiC composites with fine and homogeneous microstructures were prepared by SPS from high-energy ball milled reactants, which had excellent comprehensive properties: bending strength of 944±21 MPa, Vickers hardness of 21.0±0.3 GPa, fracture toughness of 3.87±0.20 MPa·m1⁄2, and electrical conductivity of 1.2787×105 S·m−1. The relationship between microstructure and properties and the strengthening and toughening mechanisms was discussed. In comparison with the traditional WPBM route, HEBM was a favorable technique used to produce a high homogeneous microstructure and promising mechanical and electrical conduction properties.
  • Mechanical Properties of Ti–6Al–4V Titanium Alloy with Submicrocrystalline Structure Produced by Severe Plastic Deformation

    pp. 2020-2025

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    DOI:10.2320/matertrans.46.2020

    A comparative investigation of mechanical properties of Ti–6Al–4V titanium alloy with microcrystalline and submicrocrystalline structures in the temperature range of 20–600°C has been carried out. The grain sizes under the submicrocrystalline and microcrystalline conditions are 0.4 and 10 μm, respectively. The alloy with the microcrystalline structure has been additionally subjected to a heat-strengthened treatment. The structure refinement of the alloy results in increase in both strength and fatigue limit at room temperature by about 20%. With increasing deformation temperature, the strength of the submicrocrystalline alloy is higher than that of the microcrystalline alloy up to 400°C. However, the creep strength of the submicrocrystalline alloy is slightly lower than that of the heat-strengthened microcrystalline alloy already at 250°C.
  • Nanoindentation-Induced Deformation Behavior in the Vicinity of Single Grain Boundary of Interstitial-Free Steel

    pp. 2026-2029

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    DOI:10.2320/matertrans.46.2026

    Nanoindentation measurements are made on an interstitial-free steel to reveal the effect of single grain boundary on deformation behavior. Three different sites, “on grain boundary”, “near grain boundary” and “grain interior”, were probed to investigate the pop-in behavior on the initial loading curve and nanohardness. The typical pop-in load at the “grain interior” gives a maximum shear stress beneath the indenter as an order of ideal strength. The pop-in load at the “on grain boundary” is significantly smaller than that at the other sites, indicating that the grain boundary acts as an effective dislocation source with a lower applied shear stress. The nanohardness in the “grain interior” is about 20% lower than that at the other sites, suggesting that a single grain boundary has significant resistance to indentation-induced deformation.
  • Influence of Sulfate Ions on the Atomic-Scale Structure of β-FeOOH

    pp. 2030-2035

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    DOI:10.2320/matertrans.46.2030

    Ordinary X-ray diffraction measurements by Mo Kα radiation and anomalous X-ray scattering (AXS) measurements by synchrotron radiation below Fe K absorption edge were used for characterizing the atomic-scale structure of β-FeOOH particles with and without sulfate ions. Fourier-transform infrared spectroscopy (FT-IR) was also used for analyzing a change of the bonding structure in the β-FeOOH particles by addition of sulfate ions. The realistic atomic-scale structures of the β-FeOOH particles with and without sulfate ions were estimated by fitting both the ordinary and environmental interference functions with model calculation using the reverse Monte Carlo (RMC) simulation technique. The results showed that the linkages of FeO6 octahedral structural units in the β-FeOOH particles are distorted by addition of sulfate ions, although the structure of the β-FeOOH particles with and without sulfate ions is fundamentally similar to the ideal β-FeOOH structure. The structural distortion in the β-FeOOH particles with sulfate ions is likely to arise from the incorporation of sulfate ions. The atomic-scale structures visualized for these β-FeOOH particles were discussed coupled with the bonding structure observed in FT-IR.
  • Prediction and Experimental Testing of Spherical Milling Media Wear Rate

    pp. 2036-2040

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    DOI:10.2320/matertrans.46.2036

    This paper presents a conceptually simple derivation of grinding media mass wear rate model. An experiment of milling one media in ball mill was designed to reveal the relation between media mass wear rate and the contacting points of the media with the other media and/or mill wall and the pressure acting on the point. It was found that mass wear loss of spherical grinding media is proportion to the media contacting points with other media and/or mill wall and the pressure acted on it. A physical model: f=Am−1⁄3t−1+b was established by theory derivation and experiment testing. Two kinds of ceramic ball grinding media, 97 porcelain media and 75 porcelain media, were prepared. The mass wear rate of the two kinds of media was determined by intergrinding with ordinary corundum milling media in ball mill. The experimental results were equated using least square method. The mass wear rate equating results of both kinds of ceramic grinding media was almost the same as the experimental results indicating the accuracy of the model. It is found that the higher the media performance was, the more accuracy the media wear rate as predicted by the model. The possibility of using this model in practical operations was also discussed in this paper.
  • Influence of Additives and Hot-Press Sintering on Mechanical and Lipophilic Properties of Silicon Nitride Ceramics

    pp. 2041-2046

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    DOI:10.2320/matertrans.46.2041

    Fe3O4 and Mo and were added to hot-press sintered Si3N4 ceramics to improve their lipophilic and mechanical properties. The bending strength, relative density, hardness and fracture toughness of Si3N4 ceramics with added Fe3O4 and Mo were improved by hot-press sintering compared with samples prepared with the pressureless process. In particular, the bending strength and relative density were improved by about 200 and 104%, respectively. Both the macro- and micro-lipophilicity of Fe3O4- and Mo-added ceramics were improved when prepared with the hot-press process, compared with those prepared with the pressureless process. This can be attributed to the addition of Fe3O4, and the formation of MoO3 and Si2N2O during hot-press sintering. The lower friction coefficient and high wear resistance of Fe3O4- and Mo-added ceramics have been achieved by applying the hot press process. The high wear resistance is considered to be due to the improvement of hardness and fracture toughness, which decreased the loss of particles during the wear test.
  • Fabrication of High Strength Bonded Abrasive Wheel with Ultrasonic Composite Plating

    pp. 2047-2051

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    DOI:10.2320/matertrans.46.2047

    The life of an abrasive wheel, in which cBN and Ni–P are co-deposited on a steel surface by electroless plating, is dependent on the grit adhesion and projection height. In electroless plating, it is difficult for Ni to deposit on ceramic grains. Therefore, surface shape at the grain periphery is not flat but sunken. In this study, with the aim of achieving higher grain support for the abrasive wheel, the surface shape at the grain periphery is improved with the application of ultrasonic vibration. The grain periphery of the specimen obtained with ultrasonic vibration (amplitude 11 μm) revealed an interface with about a 10 μm embossment at the periphery. In addition, the deposition rate with ultrasonic vibration is 1.6 times faster than that without it. This paper also demonstrated that the ideal interface between the cBN grits and Ni–P matrix is expected to have a heaped-up morphology, based on the results of a horizontal pushing-away test by a micro Vickers hardness tester for measuring the adhesive strength of grits. At the same time, the stress distribution around the grits in applying the lateral force to grit was simulated using the finite element method (FEM).
    The interfacial morphologies were observed between the cBN grits and Ni–P matrix which were prepared with and without ultrasound. It assumed a flat or slightly sunken shape without ultrasound, whereas it exhibited a heaped-up morphology with ultrasound. The heaped-up height and width depended upon the amplitude, and reached their the maximum at an amplitude of 11 μm. With the best interfacial morphology, the adhesive strength becomes 22% greater than that without ultrasound in the horizontal pushing-away tests. If the grit is pushed to 0.03 N, the maximum interface tensile stress without a heaped-up interface is about 160 MPa in stress analysis by FEM, whereas with such an interface (height: 11 μm; width: 7 μm), it is about 210 MPa, for an increase of about 20%.
  • Magnetic Fe–Co and Its Oxide Nanopowders Produced by Chemical Vapor Condensation

    pp. 2052-2056

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    DOI:10.2320/matertrans.46.2052

    Fe–Co magnetic nanopowders were prepared by chemical vapor condensation (CVC) process, and some of their characteristics such as size, phase, oxidation behavior, and magnetic properties were investigated. Thermal behaviors of Fe–Co nanopowders were also studied by means of annealing in the air. Iron pentacarbonyl (Fe(CO)5) and dicobalt octacarbonyl (Co2(CO)8) were used as precursors. XRD patterns showed that Fe–Co nanopowders were obtained in pure Ar, but that the Fe and Co oxide nanopowders were produced in the Ar + O2 mixtures. Fe–Co nanopowders obtained in pure Ar consist of a core and a shell with a thickness of 2–4 nm. The saturation magnetization and coercivity decreased with increasing oxygen content in the carrier gases. The CVC nanopowders obtained in different carrier gases showed different Mössbauer spectrums.
  • Infrared Brazing Ti50Ni50 and Ti–6Al–4V Using the BAg-8 Braze Alloy

    pp. 2057-2066

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    DOI:10.2320/matertrans.46.2057

    Microstructure evolution, reaction path and shear strength of infrared brazed Ti50Ni50 shape memory alloy and Ti–6Al–4V joints using BAg-8 braze alloy have been investigated. The braze alloy can readily wet on Ti–6Al–4V, but not on Ti50Ni50. Titanium dissolves from Ti–6Al–4V to enhance the wettability of braze alloy on Ti50Ni50 during brazing. The joint is mainly comprised of hypoeutectic silver and copper for specimens infrared brazed below 850°C. Silver does not react with both substrates, but copper is readily alloyed with titanium and reacts vigorously to form TiCu4, Ti3Cu4, TiCu and Ti2Cu phases in Ti–6Al–4V side and form CuNiTi phase in Ti50Ni50 side. The dramatic microstructure change of joint is observed for specimens infrared brazed at 900°C above 60 s due to the extensive Ti2Ni presence. The average shear strength of the specimens infrared brazed below 850°C is about 200 MPa. Although the presence of interfacial CuNiTi phase is beneficial to the wettability of molten braze alloy on Ti50Ni50 substrate, it is detrimental to the bonding strength of the infrared brazed Ti50Ni50/BAg-8/Ti–6Al–4V joint.
  • Preparation of Directionally Solidified B4C–TiB2–SiC Ternary Eutectic Composites by a Floating Zone Method and Their Properties

    pp. 2067-2072

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    DOI:10.2320/matertrans.46.2067

    Directionally solidified B4C–TiB2–SiC composites were prepared by a floating zone method using B4C, TiB2 and SiC as starting materials. The B4C–TiB2–SiC system was ternary eutectic whose eutectic composition was 51.2B4C–8.1TiB2–40.7SiC (mol%). The ternary eutectic composite showed a lamellar microstructure where B4C(104), TiB2(100) and SiC(111) planes were perpendicular to the growth direction. The Vickers hardness of the B4C–TiB2–SiC ternary eutectic composite was 28 to 32 GPa. The electrical conductivity was 6 to 9×104 Sm−1, and the thermal conductivity was 35 to 45 WK−1m−1 in the temperature range from 298 to 1100 K.
  • Fabrication of Thin Zirconia Rod Using a Traveling-Zone Sintering Method

    pp. 2073-2076

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    DOI:10.2320/matertrans.46.2073

    A traveling-zone sintering method prototype machine was designed and built to fabricate high-density homogeneous long rods from powder materials. A thin zirconia rod was fabricated using this prototype machine with the aim of verifying its performance. Electric power was successfully transmitted to the cylindrical mold by inserting a graphite sheet between the electrodes and the mold, even during the movement of the heating zone, allowing the sintering temperature to be controlled precisely to the desired value at any position. The distribution of density in the produced zirconia rod was examined. The relative density near both ends indicated over 99%, while that around the central section remained at 95%. The problem seemed to be caused by the insufficient load transferring to the central section due to the friction between the material and the mold. A modified zone sintering process, in which the applied load was compensated for each sintering section taking into account the frictional loss, was able to improve the density in the central section and successfully produced a zirconia rod with dimensions of φ8×51 mm. The above result suggests that our traveling-zone sintering machine has good potential to produce homogeneously densified thin zirconia rods with an aspect ratio of more than 6.
  • Quantitative Parameters and Definition of Stages of Anodic-Cathodic Microplasma Processes on Aluminum Alloys

    pp. 2077-2082

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    DOI:10.2320/matertrans.46.2077

    Positive (anode) and negative (cathode) voltage-time charts have been registered for half-wave anodic, AC polarization, and AC-C polarization and for negative polarization (C-packets), respectively. During the process, the visual characteristic changes of the microdischarges were also analyzed, namely: the color and dimension of the microdischarges and the character of the movement on the surface. The quantitative parameters for the microplasma process stages were also defined. For different aluminum alloys, it was found that spark voltage and microarc voltage are not dependent on the alloy composition and the current density. It was likewise shown that a negative current has no influence on the process parameters at the initial stages of the AC microplasma processes. The study’s hypothesis on the role of a negative current in initiating phase transition was therefore confirmed.
  • Fatigue Properties of Stainless Steel Wire Ropes for Electrodes in Functional Electrical Stimulation Systems

    pp. 2083-2088

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    DOI:10.2320/matertrans.46.2083

    The fatigue properties of stainless steel wire ropes with 19 strands developed for a functional electrical stimulation (FES) electrode in simulated body fluids were investigated. The wire ropes were made of a new austenite stainless steel (Fe–22Cr–10Ni–6Mn–2Mo–0.4N alloy) and Type 316L stainless steel. Fatigue properties of the wire ropes were evaluated in Hanks’ solution and 1 mass% lactic acid solution at 310 K using a dual axel-driven rotating-bending fatigue machine. After fatigue testing, the wire ropes were examined by scanning electron microscopy and the metal ion concentrations in the solutions were quantitatively determined. Corrosion products in the solutions were analyzed by transmission electron microscopy. Under low maximum cyclic bending stress level, the Fe–22Cr–10Ni–6Mn–2Mo–0.4N alloy wire rope exhibited a higher cyclic life than the Type 316L stainless steel wire rope. Metal ion concentrations in the lactic acid solution increased with increasing of number of fatigue cycles, and no corrosion products were detected. In Hanks’ solution, ferric oxyhydroxides (FeOOH) were formed as corrosion products during fatigue testing and the concentrations of metal ions were lower than those in the lactic acid solution for cyclic tests over 104 cycles.
  • Oxidation Resistance of Boiler Steels with Al2O3–Y2O3 Nano- and Micro-Composite Coatings Produced by Sol–Gel Process

    pp. 2089-2092

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    DOI:10.2320/matertrans.46.2089

    Al2O3–Y2O3 nano- and micro-composite coatings have been deposited on boiler steel substrates using sol–gel composite coating technology. The processes include dipping samples in a sol–gel solution dispersed with fine ceramic powders, which is prepared by high-energy ball milling. Scanning electron microscopy (SEM) and XRD analyses show that the one-layer coating (i.e. dipping for one time) has a thickness of more than 2 μm. The coating is mainly composed of α-Al2O3 and γ-Al2O3, and is relatively dense without cracking after drying and sintering treatments. The oxidation tests performed in air at 600°C show that the coatings possess much improved resistance to high temperature oxidation and scale spallation. It is believed that the nano-structured composite particles and reactive elements are integrated into the coatings, which played an important role in improving their oxidation resistance.
  • Guidelines for Developing Amide-Based Hydrogen Storage Materials

    pp. 2093-2097

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    DOI:10.2320/matertrans.46.2093

    An effective method for developing amide-based high-performance hydrogen storage materials is to prepare appropriate combinations of amides and hydrides. We have proposed that a mixture of an amide with a low decomposition temperature and a hydride showing rapid reaction to ammonia would be an appropriate combination. According to this proposal, the mixture of Mg(NH2)2 (Mg amide) and LiH (Li hydride) was investigated. The dehydriding temperature of the mixture of Mg(NH2)2 and 4·LiH is lower than that of the mixture of LiNH2 (Li amide) and 2·LiH. A method for preventing ammonia release is increasing the LiH ratio in the mixtures, which results in a reduction in the amount of desorbed hydrogen. The homogeneous dispersion between Mg(NH2)2 and LiH might be also an important factor for preventing ammonia release.
  • Commercial Purity Titanium Processed by Rotary-Die Equal Channel Angular Pressing Method

    pp. 2098-2101

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    DOI:10.2320/matertrans.46.2098

    Commercial purity titanium cylindrical samples with a diameter of 11.5 mm and a length of 24 mm were processed under the condition of 773 K, 2.4 mm/s punch, 1–4 equal channel angular pressing passes by a new severe plastic deformation process called the rotary-die equal channel angular pressing (RD-ECAP) process. Using this process, it was possible to conduct an ECAP process of 2 passes or over without sample removal, and the temperature of the commercial purity titanium could be controlled. Following the RD-ECAP process, the commercial purity titanium contained no cracks, and fine-grained microstructures were observed in the samples.
  • Kinetics of the Chlorination Reaction of Tantalum Pentoxide with Carbon Tetrachloride Gas

    pp. 2102-2106

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    DOI:10.2320/matertrans.46.2102

    The chlorination reaction of tantalum pentoxide is one of the important intermediate steps to produce tantalum metal from its oxide. The results of experiments on the kinetics of the reaction between tantalum pentoxide and carbon tetrachloride gas are presented in this paper. The experiments were carried out using a thermogravimetric analysis technique at temperature ranges between 723 and 798 K and carbon tetrachloride partial pressures between 36.5 and 81.1 kPa. Spherical shrinking core model was found to fit well the chlorination reaction rate over the entire temperature range. The chlorination reaction was controlled by surface chemical reaction, and an activation energy of 46.3 kJ/mol (11.1 kcal/mol) was obtained. The reaction order with respect to carbon tetrachloride partial pressure in a gaseous mixture with nitrogen was 2.15.
  • In Situ Observations of Solidification and Melting of Aluminum Alloy Using Ultrasonic Waveguide Sensor

    pp. 2107-2113

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    DOI:10.2320/matertrans.46.2107

    The in situ observations of solidification and melting of an aluminum-silicon alloy (Al–12.6%Si) using an ultrasonic waveguide sensor are presented. The ultrasonic sensor consists of a conventional piezoelectric ultrasonic transducer, a cooling system and a titanium (Ti) rod as an ultrasonic waveguide. The sustainability of the Ti rod in the molten Al alloy is investigated by immersion tests for 1, 4, 8 and 16 h at 800°C. The formation of a layer consisting of globular TiAl3, disperse AlSi2Ti and α-Al phases has been observed at the interface between the Ti and the Al alloy. Ultrasonic pulse-echo measurements of the Al alloy during solidification and melting have been performed using the ultrasonic sensor in temperature range from 200 to 800°C. The longitudinal wave velocity of the Al alloy shows a rapid and significant change from about 3900 to 5600 m/s around the eutectic point. An attempt to measure a solid/liquid interface of the Al alloy has been made at frequency of 2.25 MHz. The reflected echo from the interface undergoing directional solidification has been observed. The position and growth rate of the interface have also been determined from the reflected echo.
  • High Speed Deposition of Y2O3 Films by Laser-Assisted Chemical Vapor Deposition

    pp. 2114-2116

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    DOI:10.2320/matertrans.46.2114

    Thick yttria (Y2O3) films were synthesized at high speeds by laser-assisted chemical vapor deposition (LCVD) using an Y(dpm)3 (dpm = dipivaloylmethanate) precursor. The effects of deposition conditions on the deposition rate and their microstructure were investigated. While the deposition rate was less than a few microns per hour at low laser powers (PL) less than 100 W, significantly high deposition rates of more than 200 μm/h (56 nm/s) were obtained at PL more than 160 W. The highest deposition rate in this study was 300 μm/h (83 nm/s) being 100 to 1000 times greater than those of conventional CVD processes. Deposited films were dense and isotropic with no preferred orientation showing cauliflower-like microstructure.

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