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

MATERIALS TRANSACTIONS Vol. 44 (2003), No. 11

  • Kinetics of Normal Grain Growth Depending on the Size Distribution of Small Grains

    pp. 2239-2244

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    DOI:10.2320/matertrans.44.2239

    A theoretical relationship is derived between the kinetics of normal grain growth and the size distribution of small grains. When the distribution of the normalized grain size r is proportional to rm around r=0 in a steady state, the grain growth exponent n is revealed to be m+1. The same relationship is also derived from the analysis of the mean field model for particle growth. The validity of this relationship is confirmed from the consistence with existing grain-growth models and numerical simulations. Experimental consistence is observed for the size distribution on sectional surface. It is found that a log-normal function is not a steady-state size distribution for normal grain growth with n=2, even though it may approximate experimental size distributions. The obtained relationship is also shown to be applicable to particle coarsening by Ostwald-ripening.
  • Computer Simulation of Grain Growth by the Phase Field Model. Effect of Interfacial Energy on Kinetics of Grain Growth

    pp. 2245-2251

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    DOI:10.2320/matertrans.44.2245

    Numerical simulations of grain growth based on the phase field method were performed in order to investigate effect of interfacial energy on kinetics of growth. From the theoretical analysis of the evolution equation with the Ginzburg-Landua type free energy functional, It is known that the gradient energy coefficient ,κ, and a parameter γ in a local free energy function affects interfacial energy of a materials. Numerical simulation results indicate that the average size is proportional to the square root of time and after a short transient time, the grain size distribution and the grain side distribution functions become time-independent. These results are in good agreement with these obtained by the mean field theory and the Monte Carlo simulation. Growth rate becomes larger with the increase of κ and growth rate becomes smaller with the increase of γ . These results are in good agreement with these obtained by conventional theories.
  • Interdiffusion in α Solid Solution of the Al-Cu-Mg-Ag System

    pp. 2252-2257

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    DOI:10.2320/matertrans.44.2252

    Quaternary and ternary interdiffusion experiments of Al-rich α Al-Cu-Mg-Ag alloys have been performed in the temperature range from 793 to 853 K. The concentration profiles indicate that the diffusion distance of Cu is shorter than those of Mg and Ag in the solid solutions. The direct and indirect interdiffusion coefficients are positive in the ternary and quaternary alloys. The ratio of indirect coefficient to direct one suggests that repulsive interactions exist between Cu and Mg atoms in the Al-Cu-Mg-Ag alloys. In addition, the ratio values of converted interdiffusion coefficients in the quaternary alloys suggest that the interactions between Al(solvent) and Cu atoms are attractive in the present alloy.
  • Crystalline Texture in (Nd,Dy)-(Fe,Co)-B Melt Spun Ribbons

    pp. 2258-2261

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    DOI:10.2320/matertrans.44.2258

    In the present paper, the crystalline orientation in melt spun (Nd,Dy)-(Fe,Co)-B ribbons was investigated. The average ribbon thicknesses for Nd15Fe77B8, Nd15Fe75Co2B8 and Nd14DyFe75Co2B8 at V=1 m/s are 300, 380 and 420 μm, respectively. The thickness of (Nd,Dy)-(Fe,Co)-B ribbons decreases with increasing roller speed. There is a certain roller speed V of about 8 m/s, where c-axis preferential orientation of grains on both roller and free surfaces for Nd15Fe77B8 ribbon is pronounced. Compared with Nd15Fe77B8, the addition of Co improves the c-axis orientation of both roller and free surfaces for the Nd15Fe75Co2B8 ribbon at V=1 m/s. With an increase of roller speed (V<20 m/s) for Nd15Fe75Co2B8 ribbons, the c-axis orientation behavior enhances for the free surface but reduces for the roller surface. The addition of Dy in Nd15Fe75Co2B8 at V=5 m/s enhances the c-axis behavior strongly on free surface, but lowers it on roller surface.
  • Preparation and Characterization of Nano-Crystalline Ti-2 at%Fe-10 at%Si Alloy by Mechanical Alloying and Pulsed Current Sintering Process

    pp. 2262-2265

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    DOI:10.2320/matertrans.44.2262

    Ti-2 at%Fe-10 at%Si amorphous powder was synthesized by mechanical alloying (MA) of elementary Ti, Fe and Si powders using planetary ball milling for 1440 ks. Crystallization temperature of the obtained amorphous powder was ca. 830 K. Ti-2 at%Fe-10 at%Si amorphous powder was consolidated using a pulsed current sintering (PCS) process at a die temperature of 773 K under a pressure of 372 MPa. Then the compact was densified at a die temperature between 743 K and 763 K, which is around the crystallization temperature of the amorphous phase. The obtained Ti-2 at%Fe-10 at%Si alloy consists mainly of nanocrystalline α-Ti phase. The compressive strength of the nanocrystalline compact at room temperature was more than 1.7 GPa, much higher than that of a commercial Ti-6 mass%Al-4 mass%V alloy.
  • Tensile Properties and Press Formability of a Mg-9Li-1Y Alloy Sheet

    pp. 2266-2270

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    DOI:10.2320/matertrans.44.2266

    The formability of an experimentally produced Mg-9%Li-1%Y alloy sheet with a thickness of 0.6 mm is investigated. Uniaxial tension tests and some fundamental press-forming tests, such as stretching, deep-drawing, and bore-expanding tests, are carried out at room temperature. The sheet has sufficiently high ductility under uniaxial tension. However, ductility decreases with an increase in strain rate. Even at room temperature, the flow stress is also sensitive to the strain rate. The stress and the work-hardening rate increase with the strain rate. The strain rate sensitivity affects the formability in press forming. The critical punch stroke in the stretching test decreases with an increase in punch speed. However, the limit drawing ratio increases with the punch speed due to the increase in the work-hardening rate. The Erichsen value is estimated to be 9 mm, and the limit drawing ratio is 2.15. It may be concluded that the sheet has sufficiently high formability.
  • Thermal Stability, Glass-Forming Ability and Mechanical Properties of Mg-Y-Zn-Cu Glassy Alloys

    pp. 2271-2275

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    DOI:10.2320/matertrans.44.2271

    We examined the thermal stability, glass-forming ability (GFA) and mechanical properties of Mg-Y-Zn(-Cu) base alloys produced by melt-spinning and copper mold casting methods. The glass-forming ability of Mg-Y-Zn ternary glassy alloys was not high enough to form a bulk glassy alloy by the copper mold casting method. The Cu addition to form the Mg65Y10Zn25−xCux (x=0 to 25 at%) quaternary alloys resulted in a significant increase in GFA and thermal stability of the amorphous phase. The compressive fracture strength, Young’s modulus and ductility of the Mg65Y10Zn5Cu20 bulk glassy alloy are higher than those of Mg65Y10Cu25 bulk glassy alloy, although the GFA of the Mg65Y10Zn5Cu20 alloy is slightly lower than that of the Mg65Y10Cu25 alloy.
  • Texture Characteristics and Anisotropic Superplasticity of AZ61 Magnesium Alloy

    pp. 2276-2281

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    DOI:10.2320/matertrans.44.2276

    The texture evolution and mechanical anisotropy in an extruded AZ61 Mg alloy are examined. After warm extrusion, the alloy exhibits a refined grain structure and a strong basal texture. The tensile behaviors are examined in extruded specimens with the loading axis oriented at 0°, 45° and 90° to the extrusion direction. The 45° specimens exhibit lower yield stresses and higher ductility. The possible reasons for the mechanical anisotropy based on the Schmid factor calculations are analyzed.
  • Mechanical Properties of in situ Synthesized Titanium Matrix Composites at Elevated Temperature

    pp. 2282-2287

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    DOI:10.2320/matertrans.44.2282

    TiB and TiC reinforced titanium matrix composites were produced by common casting technique utilizing the self-propagation high-temperature synthesis between titanium and B4C. The mechanical properties and fracture mechanism of in situ synthesized titanium matrix composites have been investigated by means of uniaxial tension at elevated temperatures. As temperature increases, the ultimate tensile strength decreases and ductility increases. Compared with the matrix alloy, ultimate tensile strength of the composite was improved obviously because the in situ synthesized reinforcements are very stable at elevated temperatures and can strengthen the matrix alloy effectively. The fracture behavior was dependent on temperature. The composites fail at low strain at room temperature due to the fracture of the reinforcements. As temperature increases, voids are likely to initiate and grow at the interface between the reinforcement and the matrix alloy, and their coalescence eventually leads to the fracture of the composites. The debonding between the reinforcements and the matrix alloy becomes the main reason for the composites failure.
  • Low Temperature Superplasticity and Its Deformation Mechanism in Grain Refinement of Al-Mg Alloy by Multi-Axial Alternative Forging

    pp. 2288-2297

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    DOI:10.2320/matertrans.44.2288

    In practical application, an appearance of low temperature superplasticity (LSTP) is one of necessaries conditions. In this paper, to estimate an appearance and deformation mechanisms of this superplasticity, the role of grain boundary sliding (GBS), intragranular deformation and the change of microstructure during superplastic deformation have been investigated for ultrafine-grained Al-Mg alloy with a grain size of less than 1 μm using Multi-Axial Alternative Forging (MAF) technique. In these materials, it shows that the elongation and strain rate sensitivity (m-value) were 340% and 0.39, respectively, at 473 K under a strain rate of 2.8×10−3 s−1. These results show that superplastic appearance is possible at 473 K. The void formed at 473 K elongated in parallel to the tensile direction, with a length of 15 μm and a width of 5 μm. The intragranular deformation contribution was estimated from the aspect ratio of the grains after deformation and its contribution ratio was about 33.5 %. Therefore, for the appearance of lower temperature superplasticity with large elongation and m-value, the role of intragranular deformation was the most important factor together with GBS under these conditions. As described above, the MAF technique is one of the most effective methods to produce ultrafine-grained material and appearance of lower temperature superplasticity.
  • Wettability Effect on Bubble Formation at Nozzles in Liquid Aluminum

    pp. 2298-2302

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    DOI:10.2320/matertrans.44.2298

    An experimental study was undertaken to determine how wettability affect the volume and formation of gas bubble at nozzles in liquid aluminum. X-ray fluoroscope was used to carry out in-situ observation in the melt. The nozzles were made of steel, silica and alumina to establish different wettability by liquid aluminum. The frequency of bubble formation and the bubbles volume were investigated for low gas flow rate from 0.43 to 12 cm3/s and for various diameters of the nozzle. It was shown that bubble volume increased with wettability worsening both for aqueous and metallic systems. A further insight into the mechanism of the bubble formation was obtained by comparison of the bubble behavior at the tip of the injection devices in liquid aluminum and water.
  • Flow Curve Determination for Metal under Dynamic Recrystallization Using Inverse Analysis

    pp. 2303-2310

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    DOI:10.2320/matertrans.44.2303

    A new method for determining flow curves of hot metal under dynamic recrystallization is proposed in this paper. Thermomechanical finite element analysis of a tool and a workpiece under hot compression is used to evaluate the upsetting force obtained by an experiment, as a function of the distributed flow stress of the workpiece. In order to compensate the effect of inhomogeneous distributions of deformation and temperature on the flow curve, inverse analysis for calculating coefficients in a flow stress curve under dynamic recrystallization is coupled with the thermomechanical finite element analysis. The discrepancy in upsetting force between the experiment and thermomechanical finite element analysis is used as an error estimator in the inverse analysis. A new regression formula of the flow curve, which includes four independent parameters, is introduced. It can express the flow curve of hot metal under dynamic recrystallization as well as dynamic recovery and work hardening. Also, four independent parameters included in the proposed formula have clear physical meanings. The proposed method is applied to the hot compression test of plain carbon steel at elevated temperatures. Flow curves are successfully determined for diversified conditions of strain rate and forming temperature. From the determined parameters of the flow curve by the proposed inverse analysis method, it can be concluded that 1) the critical strain for the onset of dynamic recrystallization is dependent on strain rate as well as temperature, 2) the stain rate sensitivity of m=0.13 is acceptable for plain carbon steel under testing conditions, and 3) the temperature sensitivity A is approximately 3000-3600 [K−1]. Through investigation, it has become clearer that an accurate flow curve can be determined by the proposed method. In addition to a quantitative description of the flow curve as a function of strain, strain rate and temperature, metallurgical parameters such as the critical strain for the onset of dynamic recrystallization and steady-state stress could also be estimated directly by the proposed method.
  • Theoretical Modeling and Experimental Study of Thermal Barrier Coatings

    pp. 2311-2321

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    DOI:10.2320/matertrans.44.2311

    An automated plasma spray apparatus was designed to investigate experimentally the formation of splats (spread and solidified melted particles) by fully controlling key physical parameters (the velocity, temperature and size of the molten droplets impacting a substrate, and the temperature and surface condition of the substrate). The plasma spray apparatus made it possible to obtain a representative set of yttria-stabilized zirconia (YSZ) splats deposited on polished metal substrates (stainless steel or polished CoNiCrAlY sub-layers sprayed on nickel alloy substrates by low pressure plasma spraying) with full control of key physical parameters. The set of splats allowed verification of the theoretical characterization of metal oxide splat formation. Even without introducing empirical coefficients to the theoretical equations, quite good agreement between the calculated and experimental diameters and thicknesses of YSZ-splats was obtained over a wide range of various values of the key physical parameters. The results obtained will be first of all interest for optimizing the deposition of thermal barrier coatings and also for optimizing the deposition of the first monolayer of coatings of metal oxides sprayed onto metal substrates.
  • Glassy Ni-Ta-Ti-Zr(-Co) Alloys with High Thermal Stability and High Corrosion Resistance

    pp. 2322-2325

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    DOI:10.2320/matertrans.44.2322

    Glassy Ni-based alloys with high thermal stability and high corrosion resistance were synthesized in Ni-Ta-Ti-Zr(-Co) alloy system. Glassy Ni60Ta40−xyTixZry alloys exhibit supercooled liquid region exceeding 50 K in a wide composition range and the largest supercooled liquid region of 67 K was obtained at Ni60Ta20Ti15Zr5. Glassy Ni55Ta20Ti10Zr8Co7 alloy exhibits the highest thermal stability among the Ni-based glassy alloys reported up to date, which is evidenced by its high Tg and Tx of 899 K and 977 K, respectively, and a larger supercooled liquid region of 78 K. Bulk glassy Ni55Ta20Ti10Zr8Co7 alloy in a rod form with a diameter of 1 mm can be prepared by copper mold casting. The glassy Ni-Ta-Ti-Zr(-Co) alloys are spontaneously passivated with low passive current density in 1 N H2SO4 and 1 N HCl solutions and their corrosion rate was less than 0.1 μm/year in the above solutions.
  • X-ray Computed Tomography for Micro Porosity in AZ91D Alloy

    pp. 2326-2333

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    DOI:10.2320/matertrans.44.2326

    This research has been clarified the method to precisely analyze characteristics of micro porosity in AZ91D alloy using X-ray computed tomography(XRCT). For analyzing the micro porosity, we used the defect-free sample that had 50 and 100 μm sized artificial holes. The defining of peak in the variation of gray level could be used for analyzing the porosity or compounds. Increasing of image conversion coefficient a, increased the sensitivity of reconstructed image and showed even the difference of matrix. The optimized slice width for measuring the actual dimensions of micro porosity, depends on the size of micro porosity. If the slice width was too thin to analyze only the porosity, even the matrix would be detected as the porosity and the results would include the serious error. The accuracy of data derived from XRCT depends on the slice width related with the amounts of collected data.
  • Solidification-Path and Microstructure of Al-Ti-Cr Alloy Analyzed by Progressive-Type Solidification Equation

    pp. 2334-2338

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    DOI:10.2320/matertrans.44.2334

    There have been many analytical and numerical studies on microsegregation and prediction of second phase in solidification structures of alloys, and they have both advantages and disadvantages especially on the applicability of variable partition ratio (k) of solute and diffusion coefficient (D) in the solid. In this paper, we propose and apply a new progressive-type solidification equation: CLi=CLi−1·[{1−(1−B·ki−1)fsi}⁄{1−(1−B·ki−1)fsi−1}](ki−1−1)⁄(1−B·ki−1), which has a parameter (B) including variable partition ratio (k) and D. A specific parameter B is given in each progressive solidification model: B=2α (Flemings model), B=2α(1−exp(−1⁄α))−exp(−1⁄2α) (Clyne-Kurz model), or B=2α⁄(1+2α) (Ohnaka model), where α=D·θfL2, L: effective length of volume element (=s·d2⁄2, s: structure factor (=0.5∼1), d2: dendrite arm spacing), θf: local solidification time. The difference in B-values among the above models are small when D-value is relatively small. The solidification-paths of various Al-Ti-Cr alloys, which crystallize L12-type (Al,Cr)3Ti as the primary phase, are analyzed by the progressive-type solidification equation by using the above B values and the data of Al-Ti-Cr phase diagram including k-values (: functions of composition) and experimentally determined diffusion coefficient. The calculated results agree well with the solidification microstructures: the species and the amounts of non-equilibrium eutectic phases and the composition of primary phases.
  • Growth Inhibition of Nano WC particles in WC-Co Alloys during Liquid-Phase Sintering

    pp. 2339-2345

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    DOI:10.2320/matertrans.44.2339

    WC-10Co-xVC alloys were produced using two different sizes of WC powders (200 nm and 4.4 μm) via planetary milling and a liquid-phase sintering technique. When VC was added to the WC-Co alloys, the growth of WC particles was effectively suppressed. However, this process was dependent on the amount of additive used. Since the average WC particle size became less than 1 μm, the system experienced a significant loss in carbon, resulting in the formation of an undesirable η phase. The goal of this study was to determine the role of VC in controlling growth and to verify its effectiveness as a function of average WC size and inhibitor content. It was found from XRD and HREM studies that the (W,M)C phase was formed in WC-10Co-xVC-C systems. The effect of V or VC on the solubility of W in the Co matrix was also examined using HREM/EDS. The findings indicate that the presence of VC tends to reduce the solubility of W in a Co melt, thus inhibiting the dissolution and growth of WC.
  • Thermal Stability and Mechanical Properties of Cu-Hf-Al Base Bulk Glassy Alloys with a Large Supercooled Liquid Region of Over 100 K

    pp. 2346-2349

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    DOI:10.2320/matertrans.44.2346

    The supercooled liquid region defined by the difference between glass transition temperature (Tg) and crystallization temperature (Tx), ΔTx(=TxTg) for Cu50Hf45Al5 glassy alloy increased by the addition of noble metals (Ag, Pd, Pt or Au) and the largest ΔTx reached 110 K for Cu45Hf45Al5Ag5. The large ΔTx of 94 to 101 K were also obtained for the alloys containing the other noble metals of 5 at%. The reduced glass transition temperature (TgTl) of Cu45Hf45Al5M5 (M=Ag, Pd, Pt or Au) glassy alloys are in the range from 0.60 to 0.61. The selection of the quaternary compositions enabled us to form bulk glassy alloys with diameters up to 3 mm. The Vicker’s hardness, Young’s modulus, compressive fracture strength and plastic strain are 603 to 655, 119 to 125 GPa, 2220 to 2330 MPa and 0.1 to 0.6%, respectively, for the Cu45Hf45Al5M5 (M=Ag, Pd, Pt or Au) bulk glassy alloys. A different crystallization behavior is observed for Cu50Hf45Al5 and Cu45Hf45Al5Ag5 glassy alloys. The final crystallization phases are Cu10Hf7 and CuHf2 for the former alloy, and Cu10Hf7, CuHf2 and Cu8Hf3 for the latter alloy.
  • Electrodeposited Co-Fe and Co-Fe-C Alloys for Hydrogen Evolution in a Hot 8 kmol m-3 NaOH Solution

    pp. 2350-2355

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    DOI:10.2320/matertrans.44.2350

    Tailoring of active cobalt alloy cathodes for hydrogen evolution in a hot concentrated sodium hydroxide solution was attempted by electrodeposition. Enhancement of cathodic activity of cobalt for electrolytic hydrogen evolution has been carried out by the formation of Co-Fe and Co-Fe-C alloys containing different content of iron. The carbon addition to Co-Fe alloys was made to enhance the electrolytic hydrogen evolution activity and to prevent open circuit corrosion in 8 kmol m−3 NaOH at 363 K. The best condition for electrodeposition of Co-Fe alloy was pH 4 and the iron sulfate concentration of more than 5 kg m−3. Under this condition the alloy was obtained with nanocrystalline bcc phase as a dominant and with the highest hydrogen evolution activity. The carbon addition slowed down preferential iron dissolution by open circuit corrosion in the hot alkaline solution but was not effective in complete prevention of the open circuit corrosion and in enhancing the hydrogen evolution.
  • Structural Stability and Dehydrogenation of (MgH2+Al, Nb) Powder Mixtures during Mechanical Alloying

    pp. 2356-2362

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    DOI:10.2320/matertrans.44.2356

    High-energy ball milling of MgH2 with appropriate alloying elements is reported to improve the hydrogenation kinetics of MgH2 but the relevant information on dehydrogenation is scarce. Here, a systematic study was carried out to clarify the effects of two key alloying elements, Nb and Al, on the microstructure and the dehydrogenation behaviour of the MgH2. Intensive mechanical alloying was carried out to synthesise (MgH2 + M) powder mixtures (M=Nb, Al). XRD Rietveld analysis revealed the formation of a new bcc phase in the (MgH2+Nb) mixture and a (Al, Mg) solid solution in the (MgH2+Al) mixture. The amount of the newly formed phase in each case increased with milling time, while the level of Nb or Al decreases. SEM analysis of the milled powders showed the existence of nano-particles within 20 hours of milling. Thermogravimetry (TG) results showed that the mechanically alloyed (MgH2+Nb) mixture released about 3.9 mass% H2 and the milled (MgH2 + Al) about 5.4 mass% H2 at 300°C within 10 minutes, compared with only 1.5 mass% of the milled MgH2 powder and 1.0 mass% of the as-received MgH2 under the same conditions.
  • Ultrasonic Evaluation of Temper-Embrittlement for Martensitic Stainless Steel

    pp. 2363-2368

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    DOI:10.2320/matertrans.44.2363

    This research studied the possibility of non-destructive detection of temper-embrittlement in tempered CA-15 martensitic stainless steel (MSS). It was found that secondary hardening phenomenon existed in tempering temperature range of 573-673 K for the MSS in this study. Ultrasonic responses, both in terms of acoustic velocity and attenuation, exhibited changes with respective to the microstructure variation. Microstructural constituent of chromium carbide (Cr23C6 type) precipitates was found to have resulted in the temper-embrittlement of the MSS and also was responsible for the changes of the ultrasonic behavior of the material.
  • Thermal Diffusivities and Conductivities of Molten Germanium and Silicon

    pp. 2369-2374

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    DOI:10.2320/matertrans.44.2369

    Thermal diffusivity values of molten germanium and silicon were measured by a laser flash method. Simple but useful sample cell systems were developed to keep the molten germanium and silicon shape uniform for a given thickness. In the present experimental condition, it is necessary to consider the effect of not only the radiative heat loss but also the conductive heat loss at the interface between the molten sample and the cell material under the present experimental conditions. However, the computer simulation results suggest that the conductive heat loss is found to be negligibly small. The thermal diffusivity values of molten germanium and silicon are given in the following equations (unit: m2/s). 2α_Ge & =1.40×10^-8(T-1218)+2.29×10^-5 & & 1218≤T≤1398 (unit: K)
    α_Si & =4.48×10^-9(T-1685)+2.23×10^-5 & & 1685≤T≤1705 (unit: K)
  • Sn-Ag Based Solders Bonded to Ni-P/Au Plating: Effects of Interfacial Structure on Joint Strength

    pp. 2375-2383

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    DOI:10.2320/matertrans.44.2375

    This study investigates interfacial reaction and joint strength of Sn-Ag and Sn-Ag-Cu solder balls bonded to electroless Ni-P plating with various thicknesses of Au coating (50, 250 and 500 nm). For the Sn-Ag solder joints, a P-rich layer formed after reflow-soldering at the joint interface, regardless of thickness of the Au coating. However, for the Sn-Ag-Cu solder joints, a P-rich layer formed at the joint interface only in those samples with Au coating of 250 and 500 nm. Fractures were found to occur at the interface, and the joint strength degraded at Au thickness of >250 nm for both the Sn-Ag and Sn-Ag-Cu solder joints. An Au thickness of 50 nm resulted in the best joint strength.
  • Composition/Phase Structure and Properties of Titanium-Niobium Alloys

    pp. 2384-2390

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    DOI:10.2320/matertrans.44.2384

    This study attempts to develop Ti-Nb alloys with elastic moduli that approach that of human bone. The experimental results reveal that the microstructure of a Ti-Nb alloy that contains 14 mass% Nb consists of α and β phases, with α phase being the dominant one. The proportion of the α phase decreases gradually as the Nb content increases, and the microstructure becomes completely the β phase when the Nb content exceeds 34 mass%. Moreover, the ω phase can be detected using XRD and TEM in alloys with a Nb content from 30 to 34 mass%. Over the Nb range studied (14 to 40 mass%), the elastic modulus decreases from 14 to 26 mass% Nb, and then increases to a maximum at 34 mass% Nb, before falling again as Nb content is increased further. The elastic modulus of the Ti-Nb alloys is closely related to the microstructure (or Nb content) of the alloys. The fall in the elastic modulus with the increasing Nb content from 14 to 26 mass% is associated with a gradual decrease in the proportion of the α phase in the microstructure, while the precipitation of the ω phase accounts for the increase in the elastic modulus over the intermediate range of Nb (30 to 34 mass%). The tensile strength of Ti-Nb alloys increases slightly from 14 to 26 mass% Nb, and then increases markedly with a Nb content of up to 34 mass%, before falling drastically as Nb content is increased further. A similar pattern was obtained for 0.2% proof stress, while the elongation vs. %Nb curve was just the reverse of the T.S. vs. %Nb curve, as expected. A Ti-Nb alloy with a relatively high Nb content (above 36 mass%) is preferred to other compositions for use in medical implants with a reduced stress shielding effect.
  • Enhanced Shot Peening Effect for Steels by Using Fe-based Glassy Alloy Shots

    pp. 2391-2395

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    DOI:10.2320/matertrans.44.2391

    The use of Fe-Co-Ni-Mo-B-Si glassy alloy balls as peening shots was found to cause a significantly enhanced shot peening effect for steel sheets, i.e., the increase in the thickness of the shot region, higher hardness and higher compressive residual stress in the shot region, and the generation of much distinct crater-like pattern on the shot surface in comparison with those for conventional cast steel shots and high speed steel shots. It was also found that the endurance life time of the glassy alloy shots is 8 to 10 times longer than those for the conventional crystalline alloy shots. The enhanced effect was interpreted to originate from unique mechanical properties of the Fe-based glassy alloy shots such as lower Young’s modulus, larger elastic elongation limit and higher tensile strength which cannot be obtained for the conventional crystalline steel shots. The finding of the effectiveness of the Fe-based glassy alloy as peening shots is promising for future applications.
  • Thermal Decomposition Behavior of Expandable Pattern Including Blended Metal or Alloy Powder in Evaporative Pattern Casting Process of Cast Iron

    pp. 2396-2403

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    DOI:10.2320/matertrans.44.2396

    Metal or alloy powder capable of exothermic reaction with cast iron melt was blended into expandable polystyrene patterns. The behavior of thermal decomposition of the expandable patterns in evaporative pattern casting of cast iron was investigated. In evaporative pattern casting in which patterns including blended powder of metallic Si, Fe-Si alloy, Fe-Si-Mg alloy, or Fe-Si-Ca alloy were used, the temperature of the melt was higher than that in casting with original non-blended patterns. When the blending ratio was increased, the volume and pressure of decomposition gas during casting process were increased and, on the other hand, the filling rate of the melt was reduced. The use of patterns blended with powder caused no defect and abnormal structure in castings. These results imply that blending of metal or alloy powder into a pattern accelerates thermal decomposition of the pattern.
  • Thermal Decomposition Behavior of Expandable Pattern Including Blended Metal or Metal Oxide Powder in Evaporative Pattern Casting of Al-Si System Alloy

    pp. 2404-2409

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    DOI:10.2320/matertrans.44.2404

    We investigated thermal decomposition behavior of expanded patterns including thermal decomposition accelerators such as Ni, NiO, CuO and Cu2O (hereinafter referred to as “powder-blended expanded patterns”) in an aluminum alloy evaporative pattern casting process; the following results were obtained. When these metal or metal oxide powders were blended in EPS (expandable polystyrene) patterns, an exothermic reaction occurred between metal or metal oxide powder and aluminum alloy melt. This reaction prevented, temperature drop at the top of the melt flow. In addition, these kinds of powder accelerated pattern thermal decomposition. As a result, the pattern thermal decomposition gas volume increased and aluminum alloy melt filling decreased.
  • A New Soft Magnetic Bulk Metallic Glass with Dual Glass Phases

    pp. 2410-2413

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    DOI:10.2320/matertrans.44.2410

    Bulk metallic glass composites with dual amorphous phases and soft magnetic properties were produced successfully by using hot-pressed technique. Zr-based amorphous powder was mixed homogeneously with Fe-based amorphous powder, and then hot-pressed in their liquid supercooled region. The compacts still display dual amorphous phase structure, and continuous viscosity connection among different amorphous phases. The magnetization curve shows that the structures still exhibit soft magnetic properties originating from the addition of the soft magnetic phase. The successful processing of new bulk metallic glasses (BMGs) with dual amorphous phases gives us a new design concept for new materials, especially for new glass function materials making use of unique softening characteristics of BMG alloys in their liquid supercooled regions.
  • Reductive Deposition of Ni-Zn Nanoparticles Selectively on TiO2 Fine Particles in the Liquid Phase

    pp. 2414-2416

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    DOI:10.2320/matertrans.44.2414

    A novel metal nanoparticle synthesis method has been developed, in which metallic Ni nanoparticles with an amorphous-like structure were selectively deposited on TiO2 fine particles through the reduction from the liquid phase. The addition of Zn proved to decrease the nanoparticles size, leading to the increase in the total area of catalytically active Ni surface. In addition, nanoparticles were highly stabilized by the deposition on TiO2, so that the catalytic activity of Zn-added TiO2-supported Ni nanoparticles (Ni-Zn/TiO2) in the 1-octene hydrogenation was ca. 10 times higher than that of unsupported Ni nanoparticles.
  • Structural-Color-Control by AlN Coating on Rough-Surface Al Film

    pp. 2417-2419

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    DOI:10.2320/matertrans.44.2417

    The surface roughness of the rf-sputtered Al film has been controlled by using a method of two steps deposition at two different substrate temperatures (473 K and 300 K). One of the Al films with a proper surface roughness shows low specular reflection and high diffuse reflection in visible region, and looks like white colour without gloss. When a transparent (50-400 nm thick) AlN film is deposited on the roughened Al film, its color is changed into wheat, light-skyblue, khaki, and light-plum depending on the AlN film-thickness. It is revealed that the major part of the whole reflection from the specimen is diffuse reflection due to the surface roughness, and the variety of color without gloss is caused by the interference effect by the deposition of AlN film.

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