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

MATERIALS TRANSACTIONS Vol. 50 (2009), No. 11

  • Evolution Behavior of Carbides in 2.25Cr-1Mo-0.25V Steel

    pp. 2507-2511

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    DOI:10.2320/matertrans.M2009172

    The investigation was carried out on the evolution behavior of carbides precipitated in 2.25Cr-1Mo-0.25V steel during heat treatment. Four types of carbides M3C, M23C6, M7C3 and MC were identified. The mass fraction of each type of carbide was calculated and the dissolution of MC particles was found to occur during tempering. This could be caused by the annihilation of dislocations and the MC carbides precipitated on dislocations became energetically unstable and dissolved into the matrix. In addition, a two-step transformation mechanism was proposed for Cr-Fe-rich carbide. The changes in metallic composition and the average particle size of carbides were also characterized and analysed.
  • Growth and Characterization of Ultrathin Fe Films on Molecule-Adsorbed MgO Surfaces

    pp. 2512-2514

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    DOI:10.2320/matertrans.M2009218

    Effects of the molecule-adsorption on a MgO surface on the growth of ultra-thin Fe films were investigated. Surface observation by reflection high-energy electron diffraction and ultra-high vacuum atomic force microscopy showed that the adsorption of molecules on the MgO surface affected the morphology of Fe films grown on the MgO surface. It was revealed that the surface flatness of Fe films was improved by appropriate molecule-adsorption on the MgO surface.
  • Effect of Cu and P on the Crystallization Behavior of Fe-Rich Hetero-Amorphous FeSiB Alloy

    pp. 2515-2520

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    DOI:10.2320/matertrans.M2009206

    The effect of simultaneously adding P and Cu on the crystallization behavior of hetero-amorphous FeSiB alloy was investigated. Thermal properties and microstructure evolution were studied by means of X-ray diffraction, transmission electron microscopy, differential scanning, and isothermal calorimetric methods. It is found that the addition of P and Cu to the FeSiB alloy changes its crystallization temperature as well as the apparent activation energy for crystallization. The crystallization process is mainly dominated by three-dimensional growth with various nucleation rates for the selected FeSiB(PCu) alloys. The phases formed during crystallization for the different alloys of Fe84Si4B12, Fe83.3Si4B12Cu0.7, and Fe83.3Si4B8P4Cu0.7 are similar. The refinement effect of adding P and Cu to the FeSiB alloys is observed. The mean size of α-Fe phases decreases from 200 nm to about 20 nm.
  • Comparison of the Advantages Conferred by Mobility and Energy of the Grain Boundary in Inducing Abnormal Grain Growth Using Monte Carlo Simulations

    pp. 2521-2525

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    DOI:10.2320/matertrans.M2009209

    Advantages conferred by the mobility and energy of the grain boundary in inducing abnormal grain growth (AGG) were compared by three-dimensional Monte Carlo simulations. The percentage of high-mobility or low-energy grain boundaries between the potential grain for AGG and the other grains was varied. The simulation results showed that the minimum percentage of high-mobility grain boundaries required to induce AGG is 50%, whereas that of low-energy grain boundaries needed to induce AGG is 20%.
  • Evolution of Mechanical Properties and Microstructure in Extruded Mg96Zn2Y2 Alloys by Annealing

    pp. 2526-2531

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    DOI:10.2320/matertrans.M2009214

    Mg96Zn2Y2 alloy with a long period stacking ordered (LPSO) phase was produced by extrusion, and the effects of annealing on its microstructure and mechanical properties were investigated. Annealing at temperatures up to 623 K improved the elongation of the alloy from 5.4 to 9.6% but caused no changes in yield stress and tensile strength, respectively, because the LPSO phase prevented grain growth, the α-Mg phase maintained a fine grain size, kink deformation remained in the LPSO phase, and the Mg3Zn3Y2 phases finely dispersed. When the alloy was annealed at 773 K, the α-Mg and Mg3Zn3Y2 phases became coarse, which reduced the yield stress and tensile strength, respectively, while the elongation improved to 23%.
  • Detection of Hydrogen Evolution during Tensile Deformation and Fracture in Carbon Steel

    pp. 2532-2540

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    DOI:10.2320/matertrans.M2009168

    Hydrogen evolution behavior during tensile deformation and fracture in hydrogen-charged and uncharged carbon steels was investigated using a hydrogen microprint technique (HMT). A tensile testing machine equipped with a mass spectrometer placed in an ultrahigh vacuum (UHV) chamber was used to detect hydrogen evolution during the tensile test. In the uncharged specimens, the HMT revealed that silver particles, which represented the emission sites of hydrogen, were clearly observed when the applied strain increased. The accumulation of the silver particles along the slip lines was observed at the surface of the highly deformed specimens adjacent to the fracture zone. This indicated that hydrogen atoms primarily dissolved in the specimen were transported by mobile dislocations. In the hydrogen-charged specimen, a large number of silver particles were distributed almost uniformly in the matrix even in the undeformed state; however, the preferential distribution of the silver particles on the slip lines was not clearly identified, probably due to the low fracture strain, which resulted from the hydrogen embrittlement. In all the specimens, HMT showed that silver particles were not visible directly on the Al2O3 inclusions after the deformation. Mass spectrometry analysis in the UHV tensile test revealed that evolution of hydrogen gas increased significantly when the specimens were strained to a level that corresponded to maximum stress.
  • Temperature Dependence of the Solubility of Nitrogen in Liquid Silicon Equilibrated with Silicon Nitride

    pp. 2541-2544

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    DOI:10.2320/matertrans.M2009164

    The solubility of nitrogen in liquid silicon equilibrated with silicon nitride and its dependence on the composition of the atmosphere has been studied in the temperature region 1428–1542°C. High purity silicon was melted in silicon nitride crucibles under Ar and N2 atmospheres. The equilibrium was observed to be established within minutes, after which no evolution of the nitrogen content with time could be observed. The nitrogen transfer between the Si3N4-crucible and the melt was faster than the transfer via the gas-phase to such an extent that the composition of the atmosphere did not influence the solubility limit. The solubility limit as a function of temperature was found to follow: C_N(T,C_B) = 7957.9*exp(-24376/T) At the melting point of silicon, this equation gives the solubility of nitrogen as 42 ppm mass. From this equation, a thermodynamic derivation led to the following expression for the dissolution energy of nitrogen: ΔG^0 = -1.63 * 10^4 + 26T
  • The Effect of Microstructure on Corrosion of Molybdenum-Bearing Titanium Alloys in High Chloride and Acidic Solution at High Temperature

    pp. 2545-2551

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    DOI:10.2320/matertrans.M2009267

    The effect of microstructure on corrosion of heat-treated Ti-15Mo alloys was investigated by Electrochemical impedance spectroscopy (EIS), Field emission scanning electron microscopy (FE-SEM), Transmission electron microscopy (TEM) and Energy dispersive X-ray analysis (EDAX). The sample subjected to solution heat treatment (ST) had a single β phase and samples subjected to aging heat treatment at 600°C had α phase precipitation in β phase. EIS results showed that the corrosion resistance of the aging heat-treated samples was lower than that of the ST sample, but much higher than that of pure Ti in 10% NaCl solution of pH 0.5 at 97°C. Laser micrographs and depth profile of the heat-treated samples indicated that α phase at the grain boundary and in the grain was selectively corroded and caused selective dissolution in NaCl solution. The results of TEM combined with EDAX showed that the Mo content was 18 mass% in the β phase and 0.8 mass% in α phase. Hence, less Mo α phase was selectively corroded in the NaCl solution. Moreover, the sample which had continuous precipitation of α phase had lower corrosion resistance than samples which had separated needle-shape α phases in the base β phase. Thus, it was also found that the form of precipitation of α phase affected the corrosion of these alloys. Finally, it was concluded that it is possible to maintain the high corrosion resistance of heat-treated Ti-Mo alloy by controlling the microstructure of α phase.
  • Wettability between Porous MgAl2O4 Substrates and Molten Iron

    pp. 2552-2556

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    DOI:10.2320/matertrans.M2009204

    The wettability between porous MgAl2O4 substrates and molten iron was investigated by the sessile drop method at 1833 K as a basic study in order to elucidate the interactions between molten iron and a refractory material. For the study, MgAl2O4 substrates with 2, 8, 13, 27, and 39% porosity were prepared. The contact angle increased with the substrate porosity. In substrates with 2%, 8%, and 13% porosity, the contact angles were independent of time. In contrast, for substrates with 27% and 39% porosity, the contact angle decreased rapidly during the first hour and then gradually reached a steady value. The decrease in contact angle with time was attributed to the interfacial free energy. The work of adhesion was 1.27 J·m−2, and it was suggested that the interfacial bond consists of not only a physical bond but also a chemical bond.
  • Characterization of Oxidation of Green Rust (Cl) Containing Copper in Aqueous Solution

    pp. 2557-2562

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    DOI:10.2320/matertrans.M2009279

    To clarify the role of Cu in the oxidation of green rust (GR(Cl)), which contains Fe(II) and Fe(III) ions, Cu-containing GR(Cl) suspensions were synthesized and oxidized under various reaction conditions using nitrogen gas containing oxygen as the oxidant. X-ray diffraction (XRD) measurements were performed for analyzing the solid particles formed in the synthesis and oxidation steps. Upon the addition of Cu ions, a part of GR(Cl) was oxidized to Fe3O4, while the Cu ions were reduced to metallic Cu (Cu(0)) in the GR(Cl) suspensions. The pH and oxidation-reduction potentials (ORPs) of the aqueous solutions were measured during oxidation. The results showed that after oxidation, the pH of the solutions decreased, while the ORP increased. After the introduction of oxygen, GR(Cl) was converted to iron oxides such as γ-FeOOH, α-FeOOH, and Fe3O4. The rate of conversion of GR(Cl) to Fe3O4 was enhanced after the addition of Cu ions. The oxidation processes of the GR(Cl) suspension containing copper revealed a reaction process in the ORP curve, which may be attributed to the reaction of Cu(0)/Cu(II) in the aqueous solution.
  • Influence of Deaerated Condition on the Corrosion Behavior of AZ31 Magnesium Alloy in Dilute NaCl Solutions

    pp. 2563-2569

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    DOI:10.2320/matertrans.M2009191

    The corrosion behavior of AZ31 Mg alloy in the nondeaerated and deaerated dilute NaCl solutions was investigated by electrochemical measurements. The cathodic reaction of AZ31 Mg alloy depends on the hydrogen evolution and the oxygen reduction in nondeaerated media, while that is dominanted by the hydrogen evolution in deaerated media. In the potentiostatic measurements in deaerated media, the current density reached a peak during the initial immersion and then showed a stepwise decrease getting into the passivation state by a homogeneous distribution of the Mg(OH)2 product with increasing polarization time. It revealed that AZ31 alloy has a better corrosion resistance in deaerated media after long immersion. In the 0.01 mol/L solution, passivation zone was observed in deaerated media polarization up to −1.35 V (vs. SCE) while the corrosion and passivation zones were occurred in nondeaerated media. Potentiodynamic measurements in the 0.01 mol/L NaCl solution containing various concentrations NaHCO3 showed that the corrosion behavior of AZ31 alloy depends on the NaHCO3 concentration. It means that the presence of CO2 naturally in nondeaerated media may affect the corrosion process.
  • Effects of Microstructure of Deposited Sn Films and Orientation Index of Cu Foils on Sn Whisker Formation Using Substitutionally-Deposited Sn Films

    pp. 2570-2577

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    DOI:10.2320/matertrans.M2009249

    The effects of the orientation indexes of Cu foils, used as a copper pattern for flexible printed circuits (FPC), and the grain size of substitutionally-deposited crystalline Sn films on Sn whisker formation were investigated. In particular, the relationship between the grain size of substitutionally-deposited Sn films and the structure of intermetallic compound deposits formed at the interface between the substitutionally-deposited Sn films and Cu foils, as a function of aging was examined. Two types of Cu foils were used as substrates in this study. One had granular-shaped grains 0.5∼1.0 μm in size while other had pillar-shaped grains about 5.0 μm in size. We called the former “with the gelatin additive” and the latter “with the Cl ion additive” since we used gelatin and Cl ions for the fabrication of Cu foils. In addition, single crystal Cu (100), (110), (111) samples were also used as substrates. Two types of the Sn deposition bath were used in this study, a hydrofluoroboric acid bath and an organic acid bath. The structures of the deposited Sn films and Cu foils were investigated using TEM and SEM. The number of whiskers formed on the Sn-deposited film increased with aging. The number of whiskers formed on the substitutionally-deposited Sn films using the hydrofluoroboric acid bath was larger than that formed using the organic acid bath. The number of whiskers formed on the deposited Sn films on the Cu single crystal (111) was larger than that formed on the deposited Sn films on the other single crystals. The mechanism of inhibiting whisker formation by heat treatment was also investigated. No whiskers were seen on the substitutionally-deposited Sn films with heat treatment. Analysis of TEM selected-area diffraction patterns obtained from the sample subjected to heat treatment indicated the presence of Cu6Sn5 and Cu3Sn intermetallic compound deposits near the interface between the deposited Sn films and the Cu foils. The results suggest that the microstructures of the substrates strongly affect the Sn whisker formation on substitutionally-deposited Sn films.
  • Effects of Mechanical Vibration on Macrostructure and Mechanical Properties of AC4C Aluminum Alloy Castings

    pp. 2578-2583

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    DOI:10.2320/matertrans.F-M2009822

    The gravity die casting of the AC4C aluminum alloy was conducted when mechanical vibration was incorporated. The specimen with a specification of 25 mm in diameter and 210 mm in length were solidified at various vibration frequencies so as to examine the effect of the vibration on the grain size, the casting defect distribution, and mechanical properties. In comparison with the grains formed in the as-cast state without vibration, the grains in the inner area of a specimen became finer after mechanical vibration. The columnar structure remained in its outer region under all vibration frequency. The average density of specimen increased by imposition the mechanical vibration. The casting defects involved in the specimen reduced and became smaller with the increase of vibration frequency. The scattering in mechanical properties of specimens cast with mechanical vibration decreased because of the decrease in casting defects.
  • Numerical Simulation on Hardness Distribution for a FC250 Gray Cast Iron Brake Disc Casting and Its Experimental Verification

    pp. 2584-2592

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    DOI:10.2320/matertrans.M2009184

    The purposes of this study are to develop a technique of numerically simulating the hardness of a FC250 gray cast iron brake disc casting and verified by experimental measurements. As the numerical model is proven reliable, numerical experimentation is then conducted to homogenize the hardness distribution of a brake disc to obtain better casting quality. The Oldfield’s model was adopted to simulate the nucleation and grain growth during solidification of the casting. A calibration brake disc casting was first made. By comparing the hardness of the calibration brake disc casting with the simulated results using different nucleation and growth coefficients (Ae,Be) in Oldfield’s model, the most appropriate set of values for Ae and Be was obtained. Then, this set of values was applied to the hardness simulation of a test brake disc casting and confirmed by experimental measurements. Through this approach, a set of nucleation and growth coefficients was obtained for the brake disc casting. Subsequently, numerical simulations were conducted for the brake disc casting with different shake-out times to evaluate its distribution of hardness and an optimized shake-out time was then proposed based on the simulated results. The predictions of hardness were validated by comparison with experimental measurements and actual track testing.
  • Tungsten Oxide Nanopowders and Nanorods Prepared by a Modified Plasma Arc Gas Condensation Technique

    pp. 2593-2597

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    DOI:10.2320/matertrans.M2009068

    Tungsten oxide nanopowders and nanorods were synthesized by using a modified plasma arc gas condensation technique that involved the introduction of a mixed gas with controllable partial O2 pressure into a gas condensation system. Experimental results showed that yellow and blue tungsten oxide nanopowders, prepared under an Ar to O2 ratios of 1:1 and 100:1, exhibited a major WO3 phase and W19O55 phase, respectively. The nanopowders grew into W5O14-phase nanorods along the [001] direction when H2 was used as the reduction gas.
  • Friction Stir Lap Welding of Magnesium Alloy and Zinc-Coated Steel

    pp. 2598-2603

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    DOI:10.2320/matertrans.M2009022

    AZ31 Mg alloy and zinc-coated steel were lap welded using friction stir welding technology. The microstructures and mechanical properties of the joints were examined. The lap shear tensile test results showed that the welding speed had a significant effect on the failure loads of the joints at the rotation speed of 1500 rpm. The maximum failure loads of 3.4 kN, 65% of that of the zinc-coated steel base material, could be obtained when the welding speed was 150 mm/min. Microstructure analysis showed that the intervention of zinc coat promoted the formation of Mg-Zn low-melting-point eutectic structure at the interface. The joining mechanism and the role of Zn coat on friction stir lap welding of Mg alloy and zinc-coated steel were put forward.
  • Effects of Mechanical Vibration on Cooling Rate and DAS of AC4C Aluminum Alloy Gravity Die Castings

    pp. 2604-2608

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    DOI:10.2320/matertrans.M2009247

    Gravity die casting of AC4C aluminum alloy with mechanical vibration (0–120 Hz) was conducted. Columnar rod specimens (φ25mm×L210mm) were cast to investigate the effect of mechanical vibration on the cooling rate and the dendrite arm spacing of AC4C aluminum alloy castings. The cooling rate increased by imposition of the mechanical vibration, and increased with the increase of the vibration frequency. When the mechanical vibration imposed, the mold temperature increased quickly and reached higher temperature compared with no vibration condition. The dendrite arm spacing in outer region of the specimen decreased by mechanical vibration, and decreased with the increase of the vibration frequency. In the case without mechanical vibration, the specimen showed smooth surface. But the surface of specimen became rough by the imposition of the mechanical vibration.
  • Synthesis of Porous Al3Ti/Al Composite and Effect of Precursor Processing Condition on Cell Morphology

    pp. 2609-2614

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    DOI:10.2320/matertrans.L-M2009823

    Porous Al3Ti/Al composite was fabricated by a combustion foaming process, which makes use of exothermic reactions between titanium, aluminum and boron carbide powders. Boron carbide powder was used as an exothermic agent to increase the heat of reaction. Fundamental combustion foaming behavior and effects of processing parameters (Al/Ti blending ratio, addition of exothermic agent and powder compacting pressure) on the foaming behavior were investigated. Combustion foaming took place over temperature ranges between melting points of aluminum and Al3Ti. Exothermic agent addition turned out to be effective to increase the porosity of porous Al3Ti/Al composite. Relative density of the precursor was an important factor, which needed to be higher than a threshold level (0.67) to prevent extremely inhomogeneous pore morphology. By conducting hot extrusion to the precursor, both pore size and porosity were increased. As a result of compressive test, high plateau stress and absorbed energy were achieved by increasing Al/Ti mole blending ratio from 4.0 to 7.0.
  • Fatigue Property of Nickel-Free High-Nitrogen Austenitic Stainless Steels in Simulated Body Fluid

    pp. 2615-2622

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    DOI:10.2320/matertrans.M2009216

    Two kinds of high nitrogen-containing Ni-free austenitic stainless steels for medical applications were used for evaluation of fatigue behavior. One was an Fe-23%Cr-1%N stainless steel heat-treated in a N2 gas atmosphere (NA alloy) and the other was an Fe 24%Cr-2%Mo-1%N stainless steel fabricated with an electro-slag remelting method in a pressurized N2 gas atmosphere (P-ESR alloy). Fatigue tests were carried out both in air and in the phosphate-buffered saline solution, PBS(-). Cyclic stress with a sinusoidal waveform was applied to the specimen in a tension-to-tension mode with a stress ratio of 0.1 at a frequency of 20 Hz in air and 2 Hz in PBS(-). During testing, the saline solution was kept at 310 K and at the pH value of 7.5. A nitrogen-4% oxygen gas mixture was bubbled into the PBS(-).
    The results obtained are as follows. There was no difference between S-N (stress-number of cycles to failure) curves in air and in PBS(-) for each stainless steel. The fatigue strength at 107 cycles for the NA alloy was 245 MPa. No difference was found in the fatigue strength between the NA specimens heat-treated for 259.2 ks and 129.6 ks. The fatigue strength at 107 cycles for the P-ESR alloy was 320 MPa. The fatigue life at various stress amplitudes depended on preparation of the test specimens for the P-ESR alloy. The fatigue life was decreased by the plastic deformation layer and/or cracks on the specimen surface caused by machining and increased by elimination of the deformation layer and/or cracks. The fracture surface exhibited a cleavage-like appearance at the fatigue crack initiation site for both alloys.
  • Measurement of Thermal Conductivity of Magnesia Brick with Straight Brick Specimens by Hot Wire Method

    pp. 2623-2630

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    DOI:10.2320/matertrans.M2009098

    As a result of a revision of the JIS system, the application of the hot wire method for determining thermal conductivity has been expanded from heat-insulating bricks to refractories with higher heat conductivity. In this regard, focusing on magnesia bricks, which are used in the safety lining of converters and a wide variety of other applications, the authors studied the possibility of applying the non-stationary hot wire method in the determination of thermal conductivity up to the high-temperature range by using straight brick specimens. To perform measurements on magnesia bricks, whose thermal conductivity is higher than that of heat-insulating bricks, a set of measurement equipment including a high-capacity electric furnace was constructed, and as a result, it was possible to confirm the linear relationship between the increase in temperature θ of the hot wire and the logarithm of time logt by using straight brick specimens. It is possible to determine the thermal conductivity accurately by identifying the critical point where the θ−logt relationship ceases to be linear and calculating the thermal conductivity of the target material on the basis of the linear relationship.
  • Effect of Heat Treatment on TEM Microstructures of Zirconium Carbide Coating Layer in Fuel Particle for Advanced High Temperature Gas Cooled Reactor

    pp. 2631-2636

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    DOI:10.2320/matertrans.M2009185

    Japan Atomic Energy Agency (JAEA) has started to study and develop zirconium carbide (ZrC) coated fuel particles for advanced high temperature gas cooled reactors. The ZrC coating layer has been fabricated using the bromide process at JAEA. The coated particles with IPyC layers reported in a previous study were annealed at around 2073 K for 1 h, under which compact sintering will be done in a practical process, in order to study effects of the heat treatment (annealing) on their microstructure evolution. Then the microstructures of the ZrC layers in the cases (batches) of C/Zr = 1.11 and 1.35 were characterized by means of TEM and STEM. Microstructural evolution such as changes in the shape and size of voids or free carbons region caused by the heat treatment were found in the cases of both batches. After the heat treatment, the voids or free carbons region showed a clod like feature with diameters of 50 to 100 nm. The grain growth of ZrC was also observed in both cases: In the ZrC layer with C/Zr = 1.11, the fibrous carbons grew as if to stand from the PyC to ZrC layers on some places in the IPyC/ZrC boundary.
  • Martensitic Transformation of Cold-Rolled and Annealed Ti50Ni40Cu10 Shape Memory Alloy

    pp. 2637-2642

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    DOI:10.2320/matertrans.M2009254

    This study investigates the cold-rolling effect on the formation of Ti(Ni,Cu)2 precipitates and on the change of transformation temperatures for hot-rolled (HR)/annealed (HRA) and HR/cold-rolled/annealed (CRA) Ti50Ni40Cu10 specimens. Particle-like Ti(Ni,Cu)2 precipitates formed in CRA are more numerous and smaller than plate-like Ti(Ni,Cu)2 precipitates formed in HRA and thus the hardness and storage modulus of the former are higher. The broader transformation peaks exhibited in CRA compared to HRA can be explained by the assistance of the tensile stress field around the plate-like Ti(Ni,Cu)2 precipitates. The different transformation peak temperatures of B2→B19 and B19→B19′ in between HRA and CRA result from the different Cu content in the matrix. A relaxation peak appears in CRA with the activation energy of 64.4 kJ/mol. The cold-rolling process may be an important factor in enhancing the appearance of the relaxation peak.
  • Selective Sorption of Co2+ over Ni2+ Using Biogenic Manganese Oxides

    pp. 2643-2648

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    DOI:10.2320/matertrans.M-M2009821

    Preferential sorption of Co2+ ions over Ni2+ ions was achieved using biogenic Mn oxides produced by the Mn-oxidizing fungus Paraconiothyrium sp. WL-2 strain with a maximum selectivity coefficient (αCo) of 18. The selective sorption was based on different sorption mechanism for Co2+ and Ni2+ and unique properties of biogenic Mn oxides. The octahedral Co2+ ions occupy vacancies of central metal sites and edge sites in the octahedral Mn oxide unit structures of biogenic Mn oxides, where they are immobilized by oxidation to CoOOH by Mn(III). In contrast, Ni2+ ions are sorbed primarily on layer edges at circumneutral pHs without oxidation. Selective sorption of Co2+ over Ni2+ on the biogenic Mn oxides results from more vacant sites, higher Mn(III) contents, and larger specific surface areas compared to synthetic Mn oxides.
  • Hybrid Interconnection Process Using Solderable ICAs (Isotropic Conductive Adhesives) with Low-Melting-Point Alloy Fillers

    pp. 2649-2655

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    DOI:10.2320/matertrans.M2009109

    In order to overcome several crucial limitations to the traditional electrically conductive adhesive bonding used in the electronics field, a novel low-melting-point alloy (LMPA)-filled ICA (Isotropic Conductive Adhesive) was synthesized. Also, a hybrid interconnection process using it was developed, which can be achieved by melting-coalescence-wetting behavior of LMPA fillers in ICA. In order to ensure excellent coalescence and wetting characteristics of the LMPA filler particles, the fluxing capability of the resin should have been sufficient to remove the oxide layer on the surfaces of the filler particles and the electrode materials. The effect of reductant on the hybrid interconnection formation was examined.
    ICAs with and without reductant were formulated with a 40% volume fraction of filler. The QFP chip had a size of 14×14×2.7 mm and a 1 mm lead pitch. The test board had a 18 μm thick Cu daisy-chained pattern. Thermal characteristic of the ICA was observed by differential scanning calorimetry (DSC), and the temperature-dependant viscosity characteristic of the polymer matrix was observed by torsional parallel rheometer. Based on the results, the reflow profile of the interconnection process was determined. The dipping interconnection method was applied in the QFP bonding process. After the QFP bonding process, the electrical characteristic of ICA was measured with a multimeter. The wetting, bondability, coalescence characteristics of the LMPA filler particles and the morphology of the conduction path were observed by microfocus X-ray inspection systems and optical microscopy. As a result, the LMPA-filled ICAs with reductant had stable contact resistance, forming the metallurgical interconnection. A good electrical conduction path was achieved with the coalescence and wetting characteristics of the molten solder particles in the ICA.
  • The Effect of Water Vapor on High Temperature Oxidation of Fe-Cr Alloys at 1073 K

    pp. 2656-2663

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    DOI:10.2320/matertrans.M2009212

    The effect of water vapor on high temperature oxidation was studied based on Wagner’s theory of binary alloy oxidation. The oxidation of Fe-Cr alloys was carried out at 1073 K in dry and humid conditions. The oxidation was conducted in a closed apparatus at 1073 K and the oxygen partial pressure of 1.1×10−14 Pa, which was fixed by a Fe/FeO buffer. To prepare the humid condition, Ar-5% H2 gas mixture of 3×104 Pa was filled in the apparatus, which provided the water vapor pressure of 3.3×102 Pa. The transition of internal and external oxidation was observed in Fe-8Cr in the dry condition and in Fe-12Cr in the humid condition.
    Interdiffusion experiment of Fe/Fe-16Cr diffusion couples in dry and humid environments showed that the diffusion coefficient of Cr was not influenced by dissolved hydrogen.
    The oxygen permeability in α-Fe was determined by means of internal oxidation of Fe-5Cr alloy at 1073 K and the oxygen partial pressure of 1.1×10−14 Pa in a dry and two humid conditions with water vapor of 1.1×102 Pa and 3.3×102 Pa. The oxygen permeability in humid condition increases by a factor of 1.4. Dissolved hydrogen increases the oxygen permeability, thus increases the minimum concentration of Cr to form external scales in humid conditions. The presence of dissolved hydrogen changes the oxide shape from discrete spherical particle to spike-like precipitates, which enhances the oxygen transport along the metal/oxide precipitates interface.
  • Machinability of Short Alumina Fiber Reinforced Al-Si-Cu-Ni-Mg Alloy Composite

    pp. 2664-2668

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    DOI:10.2320/matertrans.M2009263

    Short alumina fiber-reinforced aluminum alloy composites were fabricated by squeeze casting, and the effects of the fiber reinforcement on the machinability of the alloy under various cutting conditions were investigated. Al-Si-Cu-Ni-Mg alloy (JIS-AC8A alloy) was used as the matrix metal. The mean values of the cutting force of the AC8A alloy were reduced by the fiber reinforcement. The lower the hardness of the fiber in the composite, the lower the cutting force of the composite. The range of the variation in the cutting force during the cutting of the composite reinforced with lower-hardness alumina fibers was almost the same as that of the AC8A alloy. The roughness of the machined surface decreased by the fiber reinforcement under every cutting condition, and the roughness of the composites was almost the same as the theoretical roughness when the cutting speed and feed rate were high. This result indicates that the fibers in the composite suppress the formation of the built-up edge. The machined surfaces and chip forms indicated that the fibers in the composite facilitated the shear deformation of the chips because the fibers were easily sheared by the cutting. These results lead to the conclusion that the machinability of the composites is superior to that of the AC8A alloy.
  • Kinetics of the Oxidative Roasting of Low Grade Mongolian Molybdenite Concentrate

    pp. 2669-2674

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    DOI:10.2320/matertrans.M2009197

    Molybdenite concentrate is the major mineral for the molybdenum industry. The industrial processing of molybdenite concentrate is first to convert to technical grade molybdenum trioxide by its oxidative roasting, followed by its purification by distillation or its ammonia leaching. In the present research, detailed experimental results for the oxidative roasting of low grade Mongolian molybdenite concentrate are presented. The experiments were carried out in the temperature range of 778 to 838 K under air atmosphere by using a thermogravimetric analysis technique. The particle size of the molybdenite concentrate was varied between 53 and 103 μm. As an example of the oxidative roasting of low grade Mongolian molybdenite concentrate, more than 95% of 53 μm particle size molybdenite was converted to molybdenum trioxide in 40 min at 823 K. The Jander equation was found to be useful in describing the rates of the oxidative roasting, which had an activation energy of 215.0 to 259.3 kJ/mol (51.4 to 62.0 kcal/mol) for various sizes, such as 53 μm, 67 μm and 103 μm.
  • Effects of Sintering Temperature on Thermoelectric Device of La-Doped Strontium Titanate in the Combination of Combustion Synthesis and Spark Plasma Sintering

    pp. 2675-2679

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    DOI:10.2320/matertrans.M2009225

    Thermoelectric properties of La-doped SrTiO3 were investigated in order to determine the optimum sintering temperature for its fabrication by using a combination of combustion synthesis and spark plasma sintering. Combustion-synthesized samples (Sr1−xLaxTiO3, x=0.08) were subjected to spark plasma sintering at temperatures from 1513 to 1663 K. The average grain size of sintered Sr0.92La0.08TiO3 enlarged as sintering temperature rose up. The maximum average grain size was 23.5 μm for a sintering temperature of 1663 K. The thermoelectric properties of sintered Sr0.92La0.08TiO3 were measured from room temperature to 1073 K. The optimum sintering temperature in the experimental sintering temperature range was 1633 K. Among the samples, the Sr0.92La0.08TiO3 sample sintered at 1633 K showed the maximum power factor of 1.51×10−3 Wm−1K−1 at 375 K. Further, we investigated the effects of pressing direction during sintering on the thermoelectric properties of combustion-synthesized Sr0.92La0.08TiO3. The combustion-synthesized samples were sintered well along the pressing direction during sintering; therefore, the electric conductivity measured along the pressing direction during sintering was more than twice that measured along the direction perpendicular to the pressing direction during sintering. Thus, we concluded that pressing direction during sintering affected the electric property of Sr0.92La0.08TiO3.
  • Nanoparticle Deposition of Al2O3 Powders on Various Substrates

    pp. 2680-2684

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    DOI:10.2320/matertrans.M2009272

    Al2O3 powders of 100 nm diameter were deposited on Al, Cu, and Si substrates using a micro-nozzle in nano-particle deposition system (NPDS). This procedure allowed the production of fine-scale depositional patterns or templates not possible using conventional semiconductor processing techniques. For a given set of depositional conditions, the Al2O3 powder layers developed different thicknesses on different substrates. Following deposition, the powders were sintered to provide ceramic layers. In the first instance, the depositional behavior of Al2O3 was determined by Stokes number, where a number greater than 1 meant that more powder was deposited. However, the extent of deposition was also influenced by substrate type, where a Si substrate yielded the thickest powder layer. This phenomenon was related to both substrate hardness and melting temperature. Most powder particles fragmented on impact (with pieces deposited on the substrate) when substrate hardness was high. In addition, when the melting temperature of a substrate was low, more powder accumulated as a result of the kinetic energy of a colliding powder particle being transformed into heat. The Al substrate, with its relatively low melting point, developed a thicker powder deposit than those formed on Cu. Therefore, hardness and melting temperature of substrates are the key parameters influencing the depositional behavior of powders.
  • Enhanced Tensile Strength and Plasticity of Zr-Cu-Al Bulk Glassy Alloys at Cryogenic Temperatures

    pp. 2685-2690

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    DOI:10.2320/matertrans.M2009186

    No data are available about mechanical behavior of bulk glassy alloys (BGAs) in tension at cryogenic temperatures. In this study, we investigated the effect of temperature on the mechanical behavior of ternary eutectic and hypoeutectic Zr-Cu-Al BGAs fabricated by an arc tilt casting method. Tensile tests were performed for the BGA plates with gauge dimensions of 5 mm in length, 1.2 mm in width and 0.5 mm in thickness at temperatures of 295, 223, 173 and 77 K, at an initial strain rate of 5×10−4 s−1. Measurements of elastic parameters were also made at temperatures from 97 to 342 K by an ultrasonic pulse method. It is found that the tensile strength and elongation for both BGAs increase with decreasing testing temperature, which is reported for the first time under a tensile condition. At cryogenic temperatures, the tensile elongation of the hypoeutectic Zr59Cu31Al10 BGA tends to be higher than that of the eutectic Zr50Cu40Al10 BGA, although the difference is small. Multiple shear bands are observed on the side surface deformed at lower temperatures. The Young’s and shear moduli, and Debye temperature monotonically increase with decreasing temperature. This indicates that the BGA becomes rigid and the effective atomic distance decreases at cryogenic temperatures, leading to the increase of the tensile strength at cryogenic temperatures.
  • Dynamic Coercivity of Advanced Ferritic Steel during Long-Term Isothermal Ageing

    pp. 2691-2694

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    DOI:10.2320/matertrans.M2009170

    The object of this study is to characterize the microstructural evolution in advanced ferritic steel for power plant during long-term isothermal ageing by measuring the reversible permeability. Ageing was observed to coarsen the tempered carbide (Cr23C6), generate the Laves (Fe2W) phase, and reduce the mechanical strength. The dynamic coercivity decreased monotonously during isothermal ageing. The decrease in coercivity physically depends on the domain wall movement, related with the domain wall pinning by nonmagnetic particles and the dislocations. The experimental results show that the dynamic coercivity is very sensitive to the damage accumulations due to isothermal ageing of advanced ferritic steel.
  • Investigation of the Dynamic Reactive Wetting of Sn-Ag-Cu Solder Alloys on Ni(P)/Au Coated Cu Substrates

    pp. 2695-2698

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    DOI:10.2320/matertrans.M2009242

    In the electronic components and device packaging process, the Cu conductor surface is generally coated with Au and Ni. The Au coating is applied to prevent the oxidation of the Cu surface and enhance the solderability, while the Ni coating is applied as a diffusion barrier between the solder alloy and the Cu substrate, in order to restrict the formation and growth of intermetallic compounds. The dynamic reactive wetting characteristics of Sn-Ag-Cu alloys are related to the properties of the coating materials. In the present study, the dynamic reactive wetting behavior of Sn-Ag-Cu alloys on Cu substrates coated with Ni and Au was firstly investigated on millisecond scale. On the bare Cu surface, the metal droplet rebounded several times due to the poor wettability and started to spread from 1 s and became equilibrated by 20 s, whereas on the Ni/Au coated surface, the metal droplet settled down without rebounding because of the good wettability, which caused it to react instantaneously, and became equilibrated by 1 s.
  • Removal of NOx Using the Reductive Properties of TiOx (0<x<2)

    pp. 2699-2701

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    DOI:10.2320/matertrans.M2009282

    Removal of NOx is one of the most urgent environmental issues. NO decomposition has been carried out utilizing the reductive properties of TiOx (0<x<2) (i.e., low valence materials of titanium oxides), as represented by Ti-coated Al2O3 balls prepared by fine particle bombardment, or TiOx powders (for example, so-to-speak Ti-black powders used for liquid crystal displays as the black matrix). In the absence of oxygen, NO is found to mostly decompose into NH3 by means of Ti-coated Al2O3 balls or TiOx powders at approximately 150°C, accompanied by traces of N2O. A similar result is also obtained even in the presence of about 20% oxygen. The formation of NH3 is further effective in reducing NO to give water and N2 as shown by the urea-SCR (selective catalytic reduction) system used currently in practice in diesel engines.

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