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

MATERIALS TRANSACTIONS Vol. 46 (2005), No. 5

  • Coarsening in Solidification Processing

    pp. 895-900

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    DOI:10.2320/matertrans.46.895

    Coarsening manifests itself in solidification of metal alloys as 1) growth of larger particles or dendrite arms with simultaneous dissolution of smaller particles or arms (“ripening”), 2) filling in of spaces between particles or dendrite arms (“coalescence”) and 3) breakup of dendrites (“dendrite multiplication”). Ripening of dendrites during solidification results in the well known dependency of dendrite arm spacing on cooling rate or local solidification time. If initial dendritic grain size is sufficiently small, the grains may spheroidize and then grow non-dendritically during isothermal holding in the liquid-solid zone. Dendrite multiplication results from convection combined with rapid cooling during initial stages of dendritic solidification. If the new grains thus formed are sufficiently high in number density, subsequent growth of these grains is non-dendritic. There is engineering interest today in finding the most reliable and economic ways of achieving such a high initial grain density, either by thermo-mechanical means or by grain refinement.
  • Effect of Anisotropy of Surface Energy on the Growth Direction of Solid Phase in Constrained Growth Condition

    pp. 902-908

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    DOI:10.2320/matertrans.46.902

    The growth direction of solid phase, which has a great influence on the grain selection, has been investigated. Transparent organic systems, succinonitrile–H2O alloy and pivalic acid–H2O alloy have been used for unidirectional solidification. Essential parameters of phase diagrams, distribution coefficient of H2O (k) and liquidus slope (m) have been determined experimentally. The relationship is analyzed between dimensionless growth velocity (VVc, where Vc is the critical growth velocity for constitutional supercooling) and dimensionless growth direction (π′). The change in π′ with VVc is common and similar for both alloy systems. π′ increases rapidly at low growth velocity region. Then, π′ increases slowly and approaches unity with increasing VVc. When the anisotropy of surface energy increases, the growth direction of solid phase becomes close to the preferred growth direction even at low growth velocity region, where cellular interface is observed.
  • Influence of Rhenium and Cooling Rate on the Solidification of Ni Base Superalloys, Inconel 718

    pp. 909-915

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    DOI:10.2320/matertrans.46.909

    Influence of 2-6% Re additions has been investigated on the solidification phenomenon of a Ni base superalloy, In718. The solidification of each alloy proceeds in the order of primary γ, eutectic (γ+NbC) and eutectic (γ+Ni2Nb). Higher additions of Re increase the volume fraction of primary γ and enlarge the secondary dendrite arm spacing. The influence of cooling rates at 1, 10 and 100 K/min has also been studied on the solidification process of the alloy containing 3.5% Re. With increasing cooling rate, the crystallization start temperatures of primary γ and eutectic phases decrease and the volume fraction of NbC and Ni2Nb increase. EPMA analysis revealed that the distribution of alloy elements in microstructure also changed depending on cooling rate. The redistribution of alloy elements during solidification was evaluated by Clyne–Kurz and Ohnaka theories.
  • Evolution of Structure Unidirectionally Solidified Sn–Ag3Sn Eutectic Alloy

    pp. 916-921

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    DOI:10.2320/matertrans.46.916

    Sn–Ag3Sn eutectic alloy is known as an attractive candidate to replace Sn–Pb soldering alloy. Since the solidified structure directly relates to the mechanical properties of joints, understanding of solidified structure of this alloy system is very important. Therefore, in order to understand the revolution of solidified structure, unidirectional solidification experiments have been carried out using eutectic Sn–Ag3Sn alloy.
    When the growth velocity is larger than the critical value, Sn-dendrites solidifies as a primary phase. This is attributed to the skewed coupled zone in this alloy system. In this case, the volume fraction of the primary Sn increases with increasing growth velocity and approaches a constant value. Interdendritic eutectic is composed with fibrous Ag3Sn and Sn matrix. On the other hand, when the growth velocity becomes lower, a eutectic structure appears without any primary phase. In this case, a piece of eutectic Ag3Sn becomes large and changes from fibrous to plate-like with decreasing growth velocity. This morphological change may be due to the anisotropy of surface tension of eutectic Ag3Sn.
  • Numerical Analysis of YBCO Crystal Growth in the TFA-MOD Process

    pp. 922-929

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    DOI:10.2320/matertrans.46.922

    Numerical analyses of YBa2Cu3O6.5+x (YBCO) polycrystalline film growth in the metal organic deposition (MOD) process using precursor solution containing metal trifluoroacetates (TFA) have been performed. This process is accompanied with both consuming H2O and releasing HF at the growth interface. At first, a one-dimensional numerical growth model of the YBCO was proposed in consideration of the growth kinetics at the interface between the precursor and the YBCO crystalline layers together with the conservation of the gas components, H2O and HF, in the precursor layer. This numerical model was treated as a boundary condition for the convective multi-component diffusion equations in the gas region. Subsequently, the convective multi-component diffusion equations and Navier-Stokes equation in the gas region were solved in a two-dimensional manner by the finite difference method. It was found that this numerical model calculation could make a good estimate for the growth rate distribution in the film and the molar fractions of the components in the gas region. Finally, it was confirmed that the supplied water vapor molar fraction dependence, the positional dependence and the inlet gas velocity dependence of the calculated YBCO growth rate were in good agreement with the experimental results.
  • Analytical Study of the Growth and Formation Processes of Faceted 123 Crystals in Superconductive REBCO Oxide

    pp. 930-934

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    DOI:10.2320/matertrans.46.930

    Growth process and solidification structures of REBCO (RE=Y, Nd etc.) fabricated by conventional unidirectional solidification and infiltration-growth method were studied analytically and numerically to clarify growth and structure-formation mechanism of faceted 123 crystals. The change in freezing front temperature of 123 phase during unidirectional solidification and the critical transition conditions of macrostructures from columnar to equiaxed structures were analyzed by using the growth rate (R)-undercooling (ΔT) and the cooling rate (Vc)-ΔT relations. The formation process of microstructure of faceted 123 crystal was simulated by two-dimensional numerical method. The results on the formation of solidification structures obtained by the above analysis and simulation showed good agreements with the experimental ones.
  • Effect of Twin Growth on Unidirectional Solidification Control of Multicrystal Silicon for Solar Cells

    pp. 935-943

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    DOI:10.2320/matertrans.46.935

    The solidification microstructure and crystal orientation have been investigated for solar cell grade high purity multicrystal silicon through a unidirectional solidification technique. The mechanism of the twin growth on a reentrant corner has been also discussed. A columnar structure is observed at solidification velocities of 1.25–30 μm/s and positive temperature gradient of 20 K/cm in the rod-like silicon specimens in an electric resistance furnace. In the solidification velocity range of 1.25–2.5 μm/s, the grain size enlarges as solidification progresses. Furthermore, large columnar grains contain many twin boundaries. However, the average grain size decreases as the solidification velocity increases and above the critical velocity around 40 μm/s, equiaxed structure appears at the central part of specimens. Therefore, molten silicon must be solidified at the velocity below 2.5 μm/s where twins are always introduced into grains to obtain large columnar crystal grains. The undercooling for directional growth is less than 4 K in the solidification velocity range of 1.25–30 μm/s. A model of two-dimensional nucleation on the reentrant corner was established, and the critical nucleus could be estimated to be 70 to 80% of the radius of the general two-dimensional nucleus. The nucleation undercooling on the surface containing twins also decreased to 70% of the general undercooling. The reduction of the critical radius and undercooling on the reentrant corner could eventually influence on the priority growth direction and the enlargement of the grain size.
  • Functionally Graded Material Fabricated by a Centrifugal Method from ZK60A Magnesium Alloy

    pp. 944-949

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    DOI:10.2320/matertrans.46.944

    Magnesium based functionally graded material (FGM) was fabricated by a centrifugal method from ZK60A (Mg–5.5 mass%Zn–0.6 mass%Zr) alloy. The applied G numbers are 40, 80 and 120, where the G number is the centrifugal force in unit of gravity. The specimen shape was cylindrical with 18 mm in length. Microstructures of the fabricated FGM specimens were observed using SEM. Energy dispersive X-ray analysis was performed to study the chemical compositional gradients within the fabricated FGM specimens. It was found that concentration of Zr in the specimens increases toward the centrifugal force direction, while no or quite small chemical compositional gradient of MgZn2 appears. A hardness change along the centrifugal force direction was also found. The graded structures are caused by the difference in the formation mechanisms of compositional gradient during the centrifugal method between Zr and Zn.
  • Influence of Solidification Microstructure and Distribution of Reinforcement on Fatigue Characteristics of Notched SiC Reinforced AC4B Alloy Composites

    pp. 950-958

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    DOI:10.2320/matertrans.46.950

    The influences of SiC particle distribution, surface notch size and solidification microstructure of the matrix on the fatigue characteristics of SiC reinforced JIS-AC4B alloy composites were investigated. Al–6.79 mass% Si–2.93 mass% Cu–0.17 mass% Mg–0.59 mass% Fe matrix composites with relatively homogeneously dispersed 11 μm SiC particles were fabricated through a combination of pressure infiltration and a melt stirring casting method. The matrix microstructure consisted of a dendritic alpha phase and eutectic Si with a few volume fractions of Fe intermetallic compound among the dendrites. All specimens contained some gas and shrinkage porosities, and all composite specimens contained SiC particle clusters. Vickers hardness of composites clearly increases due to the dispersion of SiC particles and age hardening. The hardening ability increases with an increasing volume fraction of SiC. Rotating bending fatigue tests were carried out on notch-free and notched specimens that had peak aging. In the notch-free matrix alloy specimen, cracks generated from porosities, whereas cracks generated from the SiC particles/the matrix interfaces in the composite specimens. Thus, the fatigue strength decreased with an increase in the SiC volume fraction. In the notch-introduced matrix alloy specimen, where the stress concentration factor is high, the notch becomes the crack generation site and dominated the fatigue strength. The cracks, however, generate near SiC particles instead of the notch bottom in the composite specimen. Moreover, it was found that the fatigue limit stress is unchanged in composite specimen even when the notch is introduced, although the critical stress for crack generation declines. Microstructural observation revealed that the cracks were spread and diverted in and around the cluster of SiC particles, suggesting that crack propagation resistance was improved in the composite specimen.
  • Preparation and Properties of p-Type (Bi2Te3)x(Sb2Te3)1−x Thermoelectric Materials

    pp. 959-962

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    DOI:10.2320/matertrans.46.959

    P-type (Bi2Te3)x(Sb2Te3)1−x thermoelectric materials with various chemical compositions (x=0.16, 0.20 and 0.24) were fabricated through the zone melting (ZM) method and spark plasma sintering (SPS) technique. The dimensionless figure of merit (ZT2σT⁄κ) of the zone-melted crystals increased with increasing the Bi2Te3 content (x). Compared to those of the zone-melted crystals with the same chemical composition, further enhancement by 108–115% in the ZT values was obtained through a combination of ZM and SPS techniques. As a result, the optimal figure of merit reached 1.22 at about 350 K for the composition of 24%Bi2Te3–76%Sb2Te3 with 3 mass% excess Te. The bending strength of the sintered materials was about 65 MPa, which was 5–7 times higher than that of the zone-melted crystals.
  • Hydrogen Dissolution and Structural Changes in Electrodeposited Cr Films

    pp. 963-968

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    DOI:10.2320/matertrans.46.963

    Effects of hydrogen on the structure of electrodeposited Cr films have been investigated by combining X-ray diffraction and thermal desorption spectroscopy, systematically varying the plating conditions. A large amount of hydrogen was dissolved at high current densities in a Cr-rich bath at low temperatures, and caused structural changes from bcc to hcp and fcc hydrides with increasing hydrogen concentrations. Hydrogen dissolved in regular interstitial sites in the hcp and fcc hydrides desorbed at ∼100°C, but its certain fraction remained as bubbles and desorbed at >800°C leaving dimples on the surface.
  • Reactive Diffusion between Ag and Sn at Solid State Temperatures

    pp. 969-973

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    DOI:10.2320/matertrans.46.969

    The reactive diffusion at solid-state temperatures was experimentally studied for the binary Ag–Sn system. Sn/Ag/Sn diffusion couples were prepared by a diffusion bonding technique and then annealed at temperatures between T=433 and 473 K for various periods in an oil bath with silicone oil. After annealing, layers of the ε (Ag3Sn) and ζ compounds were observed to form at the Sn/Ag interface. Furthermore, a thin layer alloyed with Sn was produced into the Ag specimen from the ζ/Ag interface owing to diffusion induced recrystallization (DIR). The thickness is about one order of magnitude greater for the ε layer than for the ζ and DIR layers. The total thickness l of the ε and ζ layers is described as a function of the annealing time t by the equation l=k(tt0)n. Here, t0 is unit time, 1 s. The exponent n is 0.40, 0.38 and 0.36 at T=433, 453 and 473 K, respectively. If the reactive diffusion is controlled by the volume diffusion, n is equal to 0.5. However, n is actually smaller than 0.5 at all the annealing temperatures. This indicates that the grain boundary diffusion contributes to the reactive diffusion and the grain growth occurs at certain rates. As the annealing temperature decreases, the contribution of the grain boundary diffusion should be more remarkable, but the grain growth will become sluggish. This is the reason why the value of n increases with decreasing annealing temperature.
  • Magnetic Domain Structures in Electrical Steel Sheets Studied by Lorentz Microscopy and Electron Holography

    pp. 974-977

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    DOI:10.2320/matertrans.46.974

    Magnetic domain structures of doubly oriented and non-oriented electrical steel sheets, the former of which has two perpendicular magnetic easy axes in the sheet plane, have been observed by Lorentz microscopy and electron holography. In the demagnetized state, the doubly oriented electrical steel sheet shows straight lines of domain walls with homogeneous distribution of lines of magnetic flux. On the other hand, non-oriented electrical steel shows irregular distribution of domain walls with curved lines of magnetic flux. Through the in situ observation of magnetization process of these electrical steel sheets, it has been found that domain walls in doubly oriented electrical steel sheets move continuously, while domain walls in non-oriented electrical steel sheets move discontinuously. It has been also clarified that the domain walls in non-oriented electrical steel sheets are pinned at the precipitates which are considered to consist of Al2O3, SiO2, MnO and TiO2. These differences in the movement of domain walls are briefly discussed with their magnetic properties.
  • Method for the Determination of Non-Chemical Free Energy Contributions as a Function of the Transformed Fraction at Different Stress Levels in Shape Memory Alloys

    pp. 978-982

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    DOI:10.2320/matertrans.46.978

    General equations allowing the determination of the elastic and dissipative energy contributions, as the function of the transformed fraction, ξ, to the martensitic transformation in shape memory alloys are derived. It is shown that the derivatives of these energies by ξ can be calculated from the measured hysteresis loops. It is illustrated, by the example of our previous experimental results obtained on CuAlNi shape memory alloys at different stress levels, σ, that our evaluation method gives consistent results with those obtained form the DSC measurements. Furthermore, the ξ- and σ-dependence of the above derivatives as well as their integral values for (0,1) and (1,0) ξ intervals, i.e. the dissipated energy and the elastic energy stored and released during the forward and reverse transformations, are determined as the function of stress.
  • Statistical Description of Mechanical Stabilization of Cu–Al–Ni Martensite

    pp. 983-989

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    DOI:10.2320/matertrans.46.983

    A statistical description of the superelastic behavior of martensitic alloys has been applied to the stress–strain curves obtained for a Cu69.6Al26.8Ni3.6 at% single crystal under compression which shows a mechanical stabilization effect of the stress-induced γ′ martensite. In order to analyse very different stress–strain behaviours, the same model has been applied to a Cu68.3Zn15.4Al16.3 (at%) single crystal showing the β–β′ transformation under tension with almost “ideal” superelasticity.
    The model, which reproduces accurately the experimental σ–ε curves, allows to compute the volume fractions of martensite at every stage of the loading–unloading cycle by introducing a probability distribution function or, if the elastic stiffness coefficients are known, by performing some calculations directly from the experimental stress–strain loops.
    The mechanical stabilization of γ′ martensite in the Cu–Al–Ni alloy is manifested through the large differences in the probability distributions between the forward and reverse transformations. The dependence of the mechanical stabilization degree on the amount of previous deformation is also quite well reproduced by the model.
  • Remediation of Contaminated Soil by Fly Ash Containing Dioxins from Incineration by using Flotation

    pp. 990-995

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    DOI:10.2320/matertrans.46.990

    Flotation was investigated to clean up the dioxin (Polychlorinated Dibenzo-p-Dioxins/Furans, PCDD/Fs and co-Planar Polychlorinated Biphenyls, co-PCBs)-contaminated soil, originated from fly ash contaminated by dioxins dispersed into soil. The primary purpose was to reduce the concentration of dioxins in soil by examining the conditions of flotation to remove selectively unburned carbons including a high dioxin concentration of fly ash generated from incineration processes. Three kind of materials were used, such as fly ash from an ESP (electrostatic precipitation) of an incinerator on vapor gas treatment (FA), artificial contaminated soil (mixture of soil and fly ash containing dioxins, ACS) and real dioxin-contaminated soil which excavated from a site of Japanese domestic incinerator area (CS). As a result, in the case of fly ash and artificial soil, fly ash containing dioxins would be enriched and then separated as float products by flotation under the conditions as follows, additional amount of kerosene and Dow 250, pulp density and pH were 40 kg/t, 7 kg/t, 5–30% and 2.8, respectively. In addition, it could be possible to recover approximately 80% of soil from dioxin-contaminated soil. The concentration of dioxins in the soil analyzed by Ah-Immunoassay after flotation was decreasing from 15 to 0.68 ng-DEQ/g and was satisfied with Japanese environmental regulation i.e. less than 10 ng-DEQ/g (analyzed by Ah-Immunoassay) or 1 ng-TEQ/g (analyzed by GC-MS methods).
  • Effect of Chemical Composition on the Optical Properties and Fracture Toughness of Transparent Magnesium Aluminate Spinel Ceramics

    pp. 996-1003

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    DOI:10.2320/matertrans.46.996

    Polycrystalline transparent magnesium aluminate “spinel” ceramics were fabricated by hot-pressing and hot isostatic pressing (HIPing) using commercially available MgO and Al2O3 powders. Al2O3 content of spinel was systematically changed that can be expressed as MgO·nAl2O3 with n=1.0, 1.5 and 2.0. UV/visible and near-IR wavelength region light reflection and transmission behaviors of the spinel ceramics were quantitatively correlated to their microstructure to account for the optical quality of the fabricated materials. The stoichiometric spinel ceramic with n=1.0 revealed a relatively poor optical transparency due to pronounced light scattering at the microcracked grain boundaries with a specular light transmission of ∼20–40% in the visible wavelength range. On the other hand, Al2O3 rich compositions revealed a specular transmission of ∼40–60% in the same wavelength range with a high degree of transparency. Additionally, effect of chemical composition on the fracture toughness of spinel ceramics was investigated applying indentation and chevron notched specimen fracture toughness measurement techniques. The spinel ceramic with n=2.0 revealed the highest fracture toughness with a mean value of ∼2.02 MPa·m1⁄2. Based on their optical and mechanical properties, potential of Al2O3 rich non-stoichiometric polycrystalline spinel ceramics for engineering applications requiring high optical transparency and improved fracture toughness was addressed.
  • Atmospheric Rust Formation Process on Fe–Cr and Fe–Ni Alloys under Wet/Dry Cycles Observed by Synchrotron Radiation X-ray Diffraction

    pp. 1004-1009

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    DOI:10.2320/matertrans.46.1004

    In situ observation of initial process of rust formation on Fe–5 mass%Cr and Fe–9 mass%Ni alloys using SPring-8 synchrotron radiation was carried out under wet/dry cycling with 3.5% Na2SO4 and NaCl solution films. It was shown that the rust constituents detected during the quite initial wet/dry cycles were Fe(OH)2 and Fe(OH)3 independent of alloying elements. Most characteristic X-ray diffraction peaks obtained under the Na2SO4 solution film are indexed as those from goethite, α-FeOOH, both in the Fe–Cr and Fe–Ni alloys. In the case of the Fe–Cr alloy, coexistence of Cr3+ ion in the Na2SO4 film weakened the α-FeOOH diffraction peaks probably due to formation of ultrafine goethite crystals. Akaganéite, β-FeOOH, appears only under the NaCl solution film. Especially, the Fe–Ni alloy forms a considerable amount of akaganéite, which can be interpreted in terms of formation of the monoclinic akaganéite.
  • Formation and Properties of Ni-Based Amorphous Metallic Coating Produced by HVAF Thermal Spraying

    pp. 1010-1015

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    DOI:10.2320/matertrans.46.1010

    A highly dense Ni59Zr20Ti16Si2Sn3 partial amorphous metallic coating, about 500 μm thick, was fabricated by means of HVAF (High Velocity Air Fuel) thermal spraying. Amorphous alloy powders produced by using gas atomization were used for HVAF thermal spraying to produce an amorphous metallic coating. Microstructural investigation showed that crystallization and oxidation occurred during thermal spraying. The volume fraction of amorphous phase in the coating is 44% as determined by using differential scanning calorimetry. Studies of the wear and corrosion properties of the resulting coating were also performed. The corrosion characteristics of the coating are sensitive to aqueous solutions selected. It exhibits good corrosion resistance in 0.05 kmol/m3 H2SO4 + 0.05 kmol/m3 Na2SO4 aqueous solution due to low passive current density and wide passive region.
  • A Modified Plasma Arc Gas Condensation Technique to Synthesize Nanocrystalline Tungsten Oxide Powders

    pp. 1016-1020

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    DOI:10.2320/matertrans.46.1016

    In the present study, nanocrystalline tungsten oxide powders were prepared by a modified plasma arc gas condensation technique where a gas nozzle was introduced to provide blowing gas. With the aid of the blowing gases, nanocrystalline tungsten oxide powders can be prepared under various chamber pressures ranged from 4.9 to 101.3 kPa. The mean grain size and powder production rate of the as-prepared nanocrystalline WO3 powders increased with increasing chamber pressure. For an increasing chamber pressure from 4.9 to 101.3 kPa, asignificant increase in powder production rate from 0.374 to 13.658 g/h can be noticed, while the mean grain size only enlarged acceptably from 5.9 to 15.7 nm. Meanwhile, by controlling the partial oxygen pressure of the mixed gas, nanocrystalline blue tungsten oxide powders can be prepared successfully. The blowing mixed gas from the nozzle not only suppressed the nucleation and growth for powders from the gas phase, but can be used to prepare stoichiometric or nonstoichiometric tungsten oxide powders.
  • Variation of the Thermal Stress in the Al18B4O33W/Al Composite during Low Temperature Cycling

    pp. 1021-1025

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    DOI:10.2320/matertrans.46.1021

    With the method for measurement of triaxial stress in material by the X-ray diffraction, the variation of the thermal stress of matrix in a 25 vol%Al18B4O33W/6061Al composite during low temperature cycling was investigated. It was found that the matrix of composite undergoes a process of tensile mismatch plastic strain when cooling, while it undergoes a process of unloading for mismatch stress when warming. After low temperature cycling, the thermal residual stress of composite was decreased obviously. If the secondary low temperature cycling was carried out, the thermal residual stress can be decreased again when the temperature of secondary cycling is lower than that of the first cycling.
  • Microstructures and Hydrogen Permeability of Nb-Ti-Ni Alloys with High Resistance to Hydrogen Embrittlement

    pp. 1026-1031

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    DOI:10.2320/matertrans.46.1026

    Microstructures and hydrogen permeability (Φ) of as-cast Nb-Ti-Ni alloys have been investigated by scanning electron microscopy (SEM) and by the gas permeation technique, respectively. The Φ value increases with increasing temperature and the amount of the Nb content for every alloy. In these alloys, the Nb39Ti31Ni30 alloy, consisting of the primary bcc-(Nb, Ti) solid solution and the eutectic {(Nb, Ti)+TiNi} phase, shows the highest Φ, which is equivalent to that of Pd. On the other hand, the Nb10Ti50Ni40 alloy, consisting of the primary B2-TiNi compound and the eutectic {(Nb, Ti)+TiNi} phase, shows the lowest Φ value among the alloys for which Φ is measurable. Eutectic microstructures suppress the hydrogen embrittlement, while the primary (Nb, Ti) phase contributes to the hydrogen permeation in these alloys. The present work demonstrates that duplex alloys containing eutectic microstructures are promising for hydrogen permeation membranes with high resistance to the hydrogen embrittlement.
  • Hot Corrosion, Oxidation and Their Effects on the Tensile Strength of SiC Fiber in Alkaline Melts

    pp. 1032-1035

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    DOI:10.2320/matertrans.46.1032

    An experimental study on the behavior of hot corrosion and oxidation of SiC fiber (Hi-NicalonTM) coated with the film of varied alkaline melts was investigated at 1000°C. The hot corrosion and oxidation of SiC fiber were characterized by specific mass loss. The tensile properties on the fibers before and after hot corrosion were evaluated by room temperature single filament test technique. The specific mass loss and strength retention after hot corrosion indicated that the corrosion effect of Hi-NicalonTM fiber in sodium carbonate was more severe than in sodium sulfate. The corrosion effects of Na2SO4 could be enhanced by addition of small amount of Na2CO3/NaNO3. Scanning electron microscopy (SEM) and X-ray diffractometer (XRD) revealed that the degradation of SiC fiber in corrosive and oxidative environment was mainly related to the interaction between the alkaline melts and silica scale. Furthermore, the excess carbon in Hi-NicalonTM fiber must enhance the hot corrosion and oxidation leading to the degradation in microstructure and mechanical properties.
  • The Effect of Target Purities on Grain Growth in Sputtered Copper Thin Films

    pp. 1036-1041

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    DOI:10.2320/matertrans.46.1036

    Grain growth rates in sputtered Cu thin films were found to be influenced by impurity levels of the sputtering targets. The Cu thin films with thickness of 100 nm or 1 μm were deposited on the rigid substrates by a sputter-deposition technique using the Cu target with purity of 99.99% (4N) or 99.9999% (6N), then subsequently annealed at room temperatures. The microstructures of the Cu films were analyzed by scanning-ion microscopy and the sheet resistivities was measured by a four-point probe method. Significant grain growth and reduction of the electrical resistivities was observed during room-temperature storage in these sputtered Cu films. For the Cu films with a thickness of 1 μm, the grain growth rates of the Cu films were not influenced by the impurity levels of the targets. However, for the films with a thickness of 100 nm, the rate of the grain growth in the 6N-Cu films was found to be slower than that in the 4N-Cu films. This was contradictory to the grain growth mechanism of bulk Cu. The grain growth rates of the Cu films at room temperature, which were strongly influenced by the existence of a small amount of impurities in the Cu films, were well explained by the difference of the strain relaxation mechanisms in the films.
  • Contact Resistance of the Chip-on-Glass Bonded 48Sn–52In Solder Joint

    pp. 1042-1046

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    DOI:10.2320/matertrans.46.1042

    An average contact resistance of a COG joint bonded with 48Sn–52In solder was characterized using daisy chain structure. The average contact resistances of the 48Sn–52In solder joint of 29 μm diameter and 14 μm height were 132 mΩ/bump, 28.5 mΩ/bump, and 8.6 mΩ/bump on Ti(0.1 μm)/Cu(1.5 μm), Ti(0.1 μm)/Cu(1.5 μm)/Au(0.1 μm), and Ti(0.1 μm)/Cu(3 μm)/Au(0.1 μm) UBMs, respectively. Such difference in the average contact resistance of the 48Sn–52In solder joint on each UBM could be attributed to partial oxidation of the Cu UBM layer during solder evaporation and a difference of the thickness of the Cu UBM layer remaining underneath the intermetallic compounds after soldering.
  • Effect of Titanium on the Tensile Ductility of 30 mass% Chromium Ferritic Steels

    pp. 1047-1051

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    DOI:10.2320/matertrans.46.1047

    The effect of titanium on the ductility of 30% chromium ferritic steels was investigated by tensile testing at low temperatures. Carbon content is 0.023±0.002 mass%. Titanium content is varied from 0.051 to 0.26 mass%. The ratio of titanium content to carbon content, Ti/C, ranges from 2.1 to 12.4. Two kinds of heat treatment with or without solution treatment were adopted to obtain different types of morphology of carbide precipitates. Aging after solution treatment gives rise to grain boundary precipitation of coarse Cr carbides and numerous fine Ti carbides, depending on Ti/C ratio, while annealing without solution treatment produces coarse globular carbide particles within the grains. The Effect of Ti/C ratio on the ductile-brittle transition temperature (DBTT) determined by the tensile tests is not as remarkable as that on the DBTT determined by Charpy impact tests. In the case of specimens containing globular carbide particles within the grains, there is no noticeable influence of varying Ti/C ratio on fracture modes, while in the case of specimens containing grain boundary carbides, the effect of Ti/C ratio on fracture modes is remarkable and the amount of intergranular fracture first decreases with increasing Ti/C ratio up to 8 and then increases with the ratio at testing temperatures above 200 K.
  • Hydrogen Production from Waste Aluminum at Different Temperatures, with LCA

    pp. 1052-1057

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    DOI:10.2320/matertrans.46.1052

    The recycling of waste metallic aluminum with high chemical exergy, which consumes a large quantity of electricity in the refining process, is insufficient. In particular, the so-called dross generated during the remelting process a part of recycling requires expensive treatment, particularly when the metallic concentration is less than 20%, before it can be landfilled. The purpose of this study is to produce hydrogen from waste aluminum sources, such as dross, using an aqueous solution of sodium hydroxide in a beaker and an autoclave. During the study, the effects of temperature of the aqueous solution on the rate of hydrogen generation are to be chiefly examined. The result obtained from an XRD analysis showed that the white product that precipitated during the experiments contained aluminum hydroxide, the rate of hydrogen generation significantly increased with the concentration of sodium hydroxide and temperature of the aqueous solution, and the activation energy was 68.4 kJ mol−1. In the autoclave experiments, hydrogen is released quickly, along with an increase in the inner pressure to a minimum of 1.0 MPa and an increase in the temperature above 473 K. The results suggested a possibility of a new cost effective process of hydrogen production from waste aluminum along with the by prodution of sodium hydroxide. The life cycle assessment (LCA) of the proposed process for producing not only 1 kg of hydrogen but also 26 kg aluminum hydroxide from waste aluminum was carried out to assess the energy requirement and amount of carbon dioxide emissions. Results suggest that the energy requirement of our process is only 2% and the amount of carbon dioxide emissions is 4%, in comparison to a conventional method.
  • Dynamical Desorption Process of Oxygen on Platinum by Using a Gas Controllable H2 | H+Electrolyte | Pt Cell

    pp. 1058-1063

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    DOI:10.2320/matertrans.46.1058

    The dynamical desorption process of oxygen on platinum was investigated by an electrochemical method. By using a gas controllable H2 | H+electrolyte | Pt cell, the value of electromotive force (EMF) has been measured as a function of lapse of time after an oxygen adsorbing treatment on the Pt electrode. The time dependence of EMF value has been classified into three stages. In the base stage, the value of EMF was discussed on the oxygen concentration on the Pt electrode. The 1st stage was interpreted by using the electrochemical theory on a diffusion model. The 2nd stage has been analyzed by using a random desorption model. The behavior of EMF was in good agreement with these models. From above analyses, the activation energy of oxygen diffusion process and the adsorption energy of oxygen on the Pt electrode were evaluated. The inflection point as a boundary point of the 1st to 2nd stage was related to the dynamical desorption process of oxygen.
  • A Comparative Study of Two Shear Deforming Processes in Texture Evolution

    pp. 1064-1069

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    DOI:10.2320/matertrans.46.1064

    The effects of deformation history on the texture evolution in an aluminum alloy during two different shear deforming processes, i.e., equal channel angular pressing (ECAP) and dissimilar channel angular pressing (DCAP) were comparatively investigated. Simulation based on the Taylor model showed that the texture evolution during DCAP was very similar to that of ECAP up to the region, a so-called ‘zero dilatation line’. Beyond this line, the texture continued to evolve by rotating about the transverse direction (TD) by 10–15° during ECAP, whereas the texture hardly evolved during DCAP. These analytical observations were verified experimentally.
  • Beta TiNbSn Alloys with Low Young’s Modulus and High Strength

    pp. 1070-1078

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    DOI:10.2320/matertrans.46.1070

    Young’s modulus and tensile strength were investigated in relation to phase transformation and microstructural changes occurring during cold rolling and subsequent heat treatment using β (Ti–35 mass%Nb)–4 mass%Sn and (Ti–35 mass%Nb)–7.9 mass%Sn alloys. Stress-induced α″ martensite is generated on cold rolling of (Ti–35Nb)–4Sn whose martensitic transformation start temperature is around room temperature. Young’s modulus in the rolling direction is lowered by the generation of stress induced α″ phase with preferred texture, while it is recovered by the reverse martensitic transformation to β at 523 K. The reverse transformation yields fine β grains which are elongated approximately along the rolling direction and have an average grain size in width of less than 1 μm. This fine microstructure leads to high strength over 800 MPa with keeping low static Young’s modulus of 43 GPa. In contrast, mechanical properties of (Ti–35Nb)–7.9Sn in which matensite is not stress-induced are not so significantly improved by cold rolling and heat treatment.
  • Electron Diffraction Study on Fe–Zn Γ intermetallic Phase of a Galvannealed IF Steel Sheet

    pp. 1079-1082

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    DOI:10.2320/matertrans.46.1079

    In order to investigate the structure of Fe–Zn Γ phase in a commercial galvannealed IF steel sheet, the electron diffraction and composition of Γ phase in a commercial galvannealed IF steel sheet were studied by transmission electron microscope (TEM) and X-ray energy dispersive spectroscopy (EDX). The cross-sectional TEM specimen was prepared using focused ion beam (FIB) technique which was also introduced briefly. The results show that there exist superlattice reflections in the selected area diffraction patterns (SADPs) of Γ phase which indicate that the Γ phase is a kind of ordered intermetallic compound. The results of computer simulation of electron diffraction patterns based on the single crystals X-ray diffraction of previous researchers give a advice to the atomic coordinates of Γ phase in a commercial galvannealed IF steel sheets.

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