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

MATERIALS TRANSACTIONS Vol. 43 (2002), No. 6

  • Mechanism of Crack Generation in Carbide Surface Layer of Laser-Clad Iron Alloys

    pp. 1261-1265

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    DOI:10.2320/matertrans.43.1261

    The mechanism of crack generation in the surface chromium carbide layer formed by laser cladding of an iron substrate with a mixed powder of raw materials has been investigated for different laser melting procedures. On the vertical section of the specimens subjected to cross laser scanning, vertical cracks across the surface carbide layer were observed on one side or both sides of the laser melted track, and some interfacial cracks were observed at the interface between the carbide layer and the iron substrate. Stress analysis revealed that vertical cracks may be caused by a tensile stress generated just after the solidification of the laser melted region due to the restriction of the unmelted carbide on both sides, while interfacial cracks may be generated due to a stress resulting from a steep temperature gradient beneath the top surface and the difference in the coefficient of thermal expansion between the chromium carbide and the iron substrate.
  • THE FORTY-SEVENTH HONDA MEMORIAL LECTURE Recent Developments and the Future of Computational Science on Microstructure Formation

    pp. 1266-1272

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    DOI:10.2320/matertrans.43.1266

    The kinetic simulation based on the phase field method has become a very powerful method in fundamental understanding of the dynamics of phase transformation with recent remarkable development of the computer. In the present paper, we briefly explain the theoretical basis of phase field method and then show the simulation results on the dynamics of microstructural changes due to phase transformation. The composition dependence of atomic interchange energy is taken into account to be applicable for the phase diagram of the real alloy systems. The elasticity and the mobility of atoms are assumed to depend on the local order parameters such as the composition, the degree of order, etc. The time-dependent morphological changes are mainly calculated for Fe–Mo, Al–Zn, Fe–Al–Co and GaAsInP alloys. The morphological developments due to the grain boundary motion and dislocation motion are also simulated. The results calculated are quantitatively in good agreement with the experimental facts of the real materials.
  • Quantification of Precipitation Hardening and Evolution of Precipitates

    pp. 1273-1282

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    DOI:10.2320/matertrans.43.1273

    Quantification of precipitation hardening is a challenging subject as it demands combined knowledge of precipitation strengthening mechanism and precipitate growth/coarsening kinetics. Having not seen many attempts on developing new theories in recent years, the authors are aware of the fact that many existing concepts and developed theories are sometimes even neglected or misused. This article therefore aims to describe overview on some aspects which have not been fully addressed and/or misused. Recent developments in this subject include an accurate determination of the equilibrium precipitate fraction and interparticle spacing. The influence from precipitation fraction on precipitate coarsening and hardening kinetics was accounted for quantitatively, which allows the hardening quantification to be carried out in a more accurate manner. In addition, difficulties in quantification of precipitation strengthening effects in commercial systems are discussed. Advanced theories on dislocation precipitate interaction mechanisms are required to improve the accuracy in quantification of precipitation hardening to a high level.
  • Annealing Twinning in Boron-Doped Ni3Al

    pp. 1283-1290

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    DOI:10.2320/matertrans.43.1283

    Dependence of frequency of annealing twinning events on annealing temperature and composition, especially boron content, in polycrystalline Ni3Al alloys has been investigated by means of optical and transmission electron microscopy, and also by evaluation of coherent twin boundary energy via counting the wrong atomic bonds across twin boundaries. Microstructural observation found that after homogenization at temperatures from 1000 to 1360°C both binary and boron-doped Ni3Al alloys without prestrain show annealing twins only on the Al-rich side with respect to stoichiometry below 1200°C. Above 1200°C annealing twinning may also take place on the Ni-rich side of stoichiometry in the binary alloy system but no twins were found in the γ+γ two-phase field. In contrast to plenty of annealing twins in binary alloys, addition of boron decreases greatly the number of twins. 0.2 at% addition significantly decreases the frequency of twinning events and no twins appear above 1200°C. 0.5 at% boron almost cleans out all the twins from the microstructure. Annealing twinning is supposed to be completed by two processes including sweeping of 1⁄3⟨211⟩ superpartial dislocations on successive (111) planes and simultaneous diffusion of boron atoms into the energetically favorable octahedral interstice that are surrounded by six Ni atoms. Boron acts as a strong obstacle to the movement of the superpartial dislocations and greatly increases difficulty in annealing twinning.
  • Formation of Protection Layer during Oxidation of Al-Implanted TiN Coating

    pp. 1291-1297

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    DOI:10.2320/matertrans.43.1291

    A protective nanocrystalline aluminum oxide layer forms during oxidation of Al-implanted TiN thin film coating on stainless steel substrate. TiN itself has poor chemical stability at elevated temperatures in an oxidizing atmosphere. Implantation of Al-ions to a TiN film, prepared by hollow cathode discharge ion plating (HCD-IP), at 4.5×1017 ions·cm−2 has greatly improved thermal oxidation resistance at temperatures up to 973 K and for periods up to 20 h in a pure oxygen atmosphere. Al-implantation significantly reduced the oxidation rate of the TiN. The apparent activation energy for oxidation increased with increasing Al-dose. At the initial stage of oxidation, free metallic aluminum and/or new AlN- or more likely (Ti, Al)N-reacted with oxygen prior to oxidation of TiN. The thin aluminum oxide layer formed on the implanted samples was dense and free from surface flaws. This layer is thought to act as a barrier to oxygen migration protecting the TiN film from further oxidation. The diffusion of Al-atoms was a driving mechanism to activate the protection of TiN at high oxidation temperatures. The alteration of the oxidation kinetics and mechanism of the implanted TiN in an oxygen atmosphere is a consequence of the effective modification of oxide properties through Al incorporation.
  • Compressive Deformation Characteristics of Open-Cell Mg Alloys with Controlled Cell Structure

    pp. 1298-1305

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    DOI:10.2320/matertrans.43.1298

    Mechanical properties of three kinds of open-cell Mg alloys; a cellular Mg alloy with random cell structure (type A), a cellular Mg alloy with controlled cell structure for which an angle between the struts and the load direction is 45° (type B) and a cellular Mg alloy with controlled cell structure for which an angle between the struts and the load direction is 0° (90°) (type C), were investigated by compressive tests. The type C showed the higher collapse stress than the other types. The collapse mechanism and effects of the loading direction on collapse stress for the cellular Mg alloys were investigated from the viewpoint of bending, buckling and yielding of the struts. As a result, it was suggested that collapse for the cellular Mg alloys is associated with yielding of struts.
  • A Mechanism of Nucleation of Crystals

    pp. 1306-1311

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    DOI:10.2320/matertrans.43.1306

    The mechanism of nucleation of crystals is discussed from a view point of atom clusters with the following results: (1) The nucleus size of crystals is larger than those expected up to date, which is at least larger than Φ 3 nm in metals and alloys. (2) Atomistic and electronic structures of atom clusters are very sensitive to their size, so that both the local volume free energy difference and the local surface energy of them widely change until the crystal nucleation is finished. (3) When the size approaches the nucleus size, a hybrid orbital corresponding to the final crystal structure is formed in the center region of each atom cluster. This stage corresponds to formation of the embryo, after which an additional energy is induced by the atomic misfit between the hybridized region and the matrix. (4) The additional energy is very sensitive to the thermal energy, and increases with increasing size of the hybridized region within each atom cluster. As a result, a part of the additional energy is generally relieved during nucleation of crystals. (5) Nucleation of crystals is finished when each atom cluster is fully hybridized, and the nucleus size is determined by a condition under which the total volume free energy difference is always larger than a sum of the total surface energy and the additional energy. The nucleation process is discussed on both isolated and embedded atom clusters.
  • Growth Direction of Cellular and Dendritic Interface in a Constrained Growth Condition

    pp. 1312-1317

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    DOI:10.2320/matertrans.43.1312

    In-situ observation of unidirectional solidification using transparent substance has been performed to investigate the growth direction of solid phase. Cell and/or dendrite, the preferred growth directions of which are not parallel to the heat flow direction, were observed with various solidification conditions. Dimensionless growth direction (π=(angle between heat flow direction and growth direction)/(angle between heat flow direction and preferred growth direction)) changes from zero to unity with increasing growth velocity at a constant temperature gradient. Introducing the normalized growth velocity (VVc, where Vc is the critical growth velocity for breaking down a planar interface), the relation between π and growth condition could be correlated and π could be expressed by a unique line with respect to the normalized growth velocity. Furthermore, the growth directions of cells or dendrites under the condition of unidirectional solidification have been analyzed by the phase-field model. The calculated results agree with the experimental results and the functional relationship between growth velocity and growth direction is qualitatively explained.
  • Surface Morphology and Crystallographic Orientation of Electrodeposited Iron Films

    pp. 1318-1324

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    DOI:10.2320/matertrans.43.1318

    The crystallographic structure and surface morphology of Fe films electrodeposited from three kinds of baths, sulfate, chloride and sulfamate baths, were studied by SEM, XRD. The surface morphology and crystallographic orientation of Fe films deposited on a Ni–P amorphous substrate were varied with electrodeposition bath composition and conditions. The {211} plane appeared to be mainly a preferential orientation of films deposited from all baths examined in this study.
  • Effect of Carbon on Tensile Properties and Wear Behavior of P/M FeAl Alloy

    pp. 1325-1331

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    DOI:10.2320/matertrans.43.1325

    Powder metallurgy (P/M) Fe–40Al and Fe–40Al–3.5C alloys (all compositions are in atomic percent) were produced by hot extrusion of gas-atomized powders to study the effect of carbon addition on microstructure, tensile properties and wear behavior of FeAl. Precipitation of soft graphite phase was observed in the carbon-added alloy. Tensile tests showed that the addition of carbon slightly increased yield strength at temperatures below 800 K, but significantly improved ductility, especially at elevated temperatures. The improvement in ductility was considered to be mainly due to the grain refinement in the carbon-added alloy. The wear behavior was studied by ball-on-disk sliding wear test. The carbon addition greatly improved the wear resistance of the Fe–40Al alloy at room temperature, but the effect was not significant at elevated temperatures. Examination of the worn surface and wear debris revealed that the improved wear resistance of the carbon-added alloy at room temperature was associated with the lubrication effect of graphite.
  • Preparation of Porous Hydroxyapatite/α-Tricalcium Phosphate Composites by a Colloidal Process

    pp. 1332-1335

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    DOI:10.2320/matertrans.43.1332

    Porous hydroxyapatite (HAp)/α-tricalcium phosphate (α-TCP) composites were prepared by a colloidal process. HAp/α-TCP mixed powder was ultrasonically dispersed in an aqueous solution containing an optimum amount of polycarboxylic acid ammonium as polymer dispersant. After the colloidal process, polymethyl methacrylate (PMMA) was added to the slurries, followed by dehydration. A pressure filtration was found to be effective in avoiding the gravity segregation of PMMA. Numerous spherical pores with 100 \\micron in mean diameter were introduced in the composites by mixing PMMA, which was burned out during sintering. The shape, size and volume fraction of pores could be controlled by those of PMMA added to the composites. The bending strength of HAp/α-TCP composites depended strongly on the porosity; the strength decreased exponentially with increasing the porosity. These composites may demonstrate higher solubility and faster bone growth than HAp alone.
  • Fluxless Sn-3.5 mass%Ag Solder Bump Flip Chip Bonding by Ultrasonic Wave

    pp. 1336-1340

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    DOI:10.2320/matertrans.43.1336

    The ultrasonic solder bump flip chip bonding was investigated as a method of fluxless bonding. The 100 \\micron-diameter Sn–3.5 mass%Ag solder bumps were formed at 12 positions on a test Si-die by laser ball bonding process. The test flip chip dies were bonded to a TSM-coated glass substrate on a hotplate at different bonding loads and ultrasonic power condition. The die shear strength was evaluated and fracture surfaces were examined with SEM. The Sn–Ag solder flip chip bonding was possible at lower temperature than the melting point of Sn–3.5Ag solder. The die shear strength increased with increasing bonding temperature, bonding load, and ultrasonic power. However, at excessive bonding load condition over 0.8 N/bump, the die shear strength decreased. The bump height decreased with increasing bonding load.
  • Effects of Mg Addition on δ Precipitation and Mechanical Properties of Al-Li-Zr Alloys Containing Ti and/or V

    pp. 1341-1347

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    DOI:10.2320/matertrans.43.1341

    Mechanical properties and precipitation structures of Al–Li–Mg–Zr alloys containing Ti and/or V were investigated. In our earlier work, the Al–3 mass%Li–0.2 mass%Zr–0.1 mass%Ti and the Al–3 mass%Li–0.2 mass%Zr–0.1 mass%V quaternary alloys have shown the magnificent mechanical properties due to the grain refinement and the fine precipitates which consist of both the metastable Al3(Zr, Ti or V)-Al3Li composite precipitates and δ(Al3Li) precipitates. In this work, it was observed that, addition of magnesium from zero to 2 mass% in Al–3 mass%Li–0.2 mass%Zr–0.1 mass%Ti alloy or in Al-3 mass%Li–0.2 mass%Zr–0.1 mass%V alloy magnifies the proof and tensile strengths. The Al-3 mass%Li–1.0 mass%Mg–0.2 mass%Zr–0.1 mass%Ti alloy aged at 473 K for 4 h elongates 6–7% with high ultimate tensile strength about 520–540 MPa. But raising the magnesium content to 2 mass%, the elongation decreases to a value of about 4% with a significant increase in proof strength at the same age condition. It is verified that the magnesium content influences significantly the size and number density of δ precipitates.
  • Size Effect of Copper Precipitation Particles on Electrochemical Nanoscopic Galvanic Behavior in Cu-Added Ultra Low Carbon Steel

    pp. 1348-1351

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    DOI:10.2320/matertrans.43.1348

    Electrochemical polarization measurements were applied to Cu-added ultra low carbon steels aged at 773 K for duration ranging from 9×10 to 5×106 s to investigate size effect of copper precipitation particles on passivation of the steel. The anodic polarization curve measurements in 0.5 kmol/m3 H2SO4 solution revealed that the passive current density Ipass showed almost no variation until the aging process reached the overaged stage in which the Vickers hardness of the steel began to decrease. Thereafter, the Ipass increased monotonously with increasing aging time in the overaged stage. This increase in Ipass due to aging was considered to result from the preferential anodic dissolution of grown copper particles. Thus, the corrosion resistance of the Cu-added ultra low carbon steel scarcely deteriorated even if the size of copper particles increased with aging time before the maximum hardness was reached. These results reflected that there was a critical nanometer size of copper particles as a nanoscopic galvanic action beyond which a good protective passive film could not be formed.
  • Formation Mechanism of Low Contact Resistance PdZn-based Ohmic Contacts for p-type InP

    pp. 1352-1359

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    DOI:10.2320/matertrans.43.1352

    Recently, Sb(3 nm)/Zn(20 nm)/Pd(20 nm) ohmic contact materials to p-type InP which provides high reliable, low contact resistance after annealing at temperature around 375°C have been developed in our laboratories (where a slash (/) sign indicates the deposition sequence). The use of these contact materials for photodiodes made it possible to achieve simultaneous preparation of both p and n-type ohmic contacts, which resulted in significant reduction of the fabrication process steps. In the present paper in order to understand the mechanism of ohmic contact formation, the microstructural analysis of the interfaces between the Sb/Zn/Pd ohmic contacts and the Zn-doped p-type InP was carried out by X-ray diffraction, cross-sectional transmission electron microscopy, secondary ion mass spectroscopy, and electrochemical capacitance-voltage measurement. The present experiments proposed a new contact formation mechanism: reduction of the ohmic contact resistances was obtained primarily due to suppression of Zn (doped in the InP) outdiffusing from the InP substrate during annealing by the Pd2InP layers which contained Zn. Thus the direct contact of the Pd2InP layers to the InP substrate was found to be essential to prevent reduction of the net acceptor concentration near the InP surface and also to improve the thermal stability of ohmic contacts. The roles of Pd and Sb added to the contact materials also contributed to the reduction of contact resistance and their roles will be discussed.
  • A Study on Electro-Discharge Machined Surfaces of Ferritic SG Cast Irons

    pp. 1360-1366

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    DOI:10.2320/matertrans.43.1360

    This study investigated the characteristics of the surface modification layer of SG cast irons using the electro-discharge machining (EDM). Experimental results indicated that the EDMed surface shows a continuous ridge appearance and the ridge density increases with higher graphite area fraction or nodule count. The modified layer with the high hardness of about Hv 1000 possesses a similar composition to the base material with near-eutectic composition. Microstructural analysis results showed that this rapidly solidified layer comprises mainly cellular γ Fe and intercellular metastable HCP ε phase. Micro-twins can be observed within the γ cells and their formation can be attributed to the residual tensile stress caused by thermal impact.
  • Thermal Stability and Mechanical Properties of Bulk Glassy Cu-Zr-Ti-(Nb, Ta) Alloys

    pp. 1367-1370

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    DOI:10.2320/matertrans.43.1367

    Bulk glassy Cu–Zr–Ti base alloys containing a small amount of Nb or Ta were found to exhibit high strength combined with significant plastic elongation. The glass transition temperature (Tg), crystallization temperature (Tx) and ΔTx(=TxTg) of the Cu-based bulk glassy alloys are in the range of 719 to 731 K, 765 to 769 K and 46 to 44 K, respectively, for the Nb-containing alloy and 716 to 741 K, 752 to 780 K and 44 to 34 K, respectively, for the Ta-containing alloy. With increasing Nb or Ta content to 3 at% for the (Cu0.6Zr0.3Ti0.1)100−xMx alloys, the Young’s modulus, yield strength and compressive fracture strength increase from 114 to 121 GPa, 1785 to 2010 MPa and 2150 to 2250 MPa, respectively, and the plastic elongation shows a maximum of 1.8% at 1%Nb and 3.4% at 1%Ta. The 1%Ta alloy exhibited good combination of high fracture strength of 2250 MPa and large total elongation of 5.4%. The effectiveness of additional Nb or Ta on the increase in elongation and fracture strength is presumably due to the mixed state of atomic pairs with attractive and repulsive bonding natures among the constituent elements with significantly different atomic size mismatches. The success of forming the bulk glassy alloys with high strength and significant elongation is encouraging for future applications as high-strength type materials.
  • Effects of Excess Mg and Si on the Isothermal Ageing Behaviours in the Al-Mg2Si Alloys

    pp. 1371-1380

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    DOI:10.2320/matertrans.43.1371

    The effects of excess Mg and Si contents on the isothermal ageing processes of Al-Mg2Si alloys have been investigated by means of transmission electron microscopy and the following results were obtained. After the formation of β′′ needles inducing large age hardening, cuboid β particles precipitate in both the excess Mg and the quasi-binary alloys, but fine Si particles nucleate in the excess Si alloys. In the quasi-binary alloy, the following reaction is the precipitation of β rods and then β plates form. In the excess Si alloys, various rodlike precipitates form after the precipitation of β′′ needles and Si particles in the sequence: Type-A rods→Type-B rods→β rods. This precipitation sequence can be understood by considering the chemical compositions of them determined by Matsuda et al.
  • Impact Toughness of Weldments in Al-Mg-Si Alloys

    pp. 1381-1389

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    DOI:10.2320/matertrans.43.1381

    6000 series Al–Mg–Si alloys are frequently used for welded structures. The present investigation is conducted to evaluate the level of mechanical properties, especially impact toughness of welded Al–Mg–Si alloys with different amounts of Mn addition and using two types of fillers Al–Si (A4043) and Al–Mg (A5356), respectively. Particular attention is paid to the effect of microstructure on toughness. As one of the most important characteristics of a structural material, toughness is evaluated by the instrumented Charpy impact test method. When Al–Mg filler is used and Mn is not added, recrystallization occurs in the heat affected zone (HAZ), however, recrystallization can be suppressed by a small amount of Mn addition. The structure having large coarsened recrystallized grains is more susceptible to liquation crack that is detrimental to the mechanical properties, especially toughness. The results obtained show that toughness of the welded metal can be improved 20–30% by adding Mn. Using A4043 filler, the level of absorbed energy decreases as the content of Mg increases, and low Mg and without Mn addition results in a good combination of the mechanical properties. However, using the A5356 filler, the level of absorbed energy increases as the content of Mg increases and decreases as the Si content increases. In this case, the highest value of absorbed energy was recorded for alloy 8 (0.7%Mn). When A5356 filler is used, the toughness is about 56% higher than using A4043 filler.
  • Effects of Pulse Current on an Aluminum Powder Oxide Layer During Pulse Current Pressure Sintering

    pp. 1390-1397

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    DOI:10.2320/matertrans.43.1390

    The pulse current pressure sintering and hot press process were applied for sintering of aluminum powder produced by water and gas atomizing methods. The pulse current pressure sintering process could densify the water atomized powder in a short time compared with the hot pressing because it could remove the H2 gas at lower temperature. The gas prevents sintering of aluminium powder. The tensile strength of the pulse current pressure sintered gas atomized powder which contains low H2 was higher than that of hot pressed specimen. The electric resistivity of sintered specimens obtained by pulse current pressure sintering process was lower than that of hot pressed specimen. The oxide layer destruction was confirmed by this measurement of electric resistivity. The oxide layers were supposed to be fractured by the high temperature region which is attributed to Joule heat by contact resistance.
  • DLC Film Fabricated by a Composite Technique of Unbalanced Magnetron Sputtering and PIII

    pp. 1398-1402

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    DOI:10.2320/matertrans.43.1398

    DLC multilayer films were deposited on an AISI 304 stainless steel substrate by the composite technique of unbalanced magnetron sputtering and plasma immersion ion implantation (PIII). Structure characterization was performed on the films by Raman spectroscopy (RS) and Glancing X-ray Diffraction (GXRD). Composition analysis of the surface layer on the implanted substrates was carried out using auger electron spectroscopy (AES). The mechanical properties of the films were evaluated by nanoindentation. The results showed that the Raman spectra were divided into a “D” disordered peak and a “G” graphite peak with the integrated intensity ratio between them (IDIG) being 1.30. The implanted carbon penetrated the substrate resulting in complete interfacial mixing. The hardness, elastic modulus, fracture toughness and interfacial fracture toughness of the films were about 19.84 GPa, 190.03 GPa, 3.75 MPa·m1⁄2 and 5.68 MPa·m1⁄2 respectively. Compared with that of a DLC coating deposited directly by the PIII technique, the interfacial fracture toughness of the multilayer films increased, which is mainly attributed to the interfacial mixing at the interface.
  • Influence of Substrate Bias Voltage on the Properties of Cu Thin Films by Sputter Type Ion Beam Deposition

    pp. 1403-1408

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    DOI:10.2320/matertrans.43.1403

    Cu thin films have been deposited on Si (100) substrate by using a non-mass-separated ion beam deposition (IBD) system. The effect of the substrate bias voltage on the properties of the deposited films was investigated using X-ray diffraction, resistivity measurement and field emission scanning electron microscopy. In the case of Cu thin films deposited without bias voltage, a columnar structure and small grains were observed distinctly, and the electrical resistivity of the deposited Cu films was very high. By increasing the bias voltage, no clear columnar structure and grain boundary were observed. The resistivity of Cu films decreased remarkably and at a bias voltage of −50 V, reaching a minimum value of 18±1 nΩm, which is close to that of the bulk phase (16.7 nΩm.)
  • Determination of Ti, V, Zr, Nb, Mo and Ta in High-Purity Iron Using Cupferron Co-Precipitation Separation by Axially Viewed ICP-AES with Ultrasonic Nebulization System and a Long Torch

    pp. 1409-1414

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    DOI:10.2320/matertrans.43.1409

    An analytical method was established for the determination of trace amounts of refractory metal elements (Ti, V, Zr, Nb, Mo and Ta) in high-purity iron samples by axially viewed inductively coupled plasma atomic emission spectroscopy (ICP-AES). We investigated the analytical method of a trace amount of refractory metal elements in high-purity iron matrix using cupferron coprecipitation separation procedure and by axially viewed ICP-AES with ultrasonic nebulization system and a long torch. The established analytical procedure was as follows. A 1.0 g of high-purity iron sample was decomposed with 150 cm3 of hydrochloric acid (1+4) by heating on a hot plate at 453 K. After cooling the sample solution to room temperature, we added 10 cm3 of ascorbic acid solution and cupferron solution to it, and then separated analyte elements. After filtration, the precipitate and residue were decomposed with 10 cm3 of nitric acid and 10 cm3 of perchloric acid, and evaporated to dryness. After leaving cool, salts were dissolved with nitric acid. The sample solution was determined by axially viewed ICP-AES with ultrasonic nebulization system and a long torch. For the controlled matrix concentration, the good precisions and accuracy were obtained by using matrix-matched standard solutions for calibration curves to the ultrasonic nebulization system method. The limit of determination is considerably low in the ultrasonic nebulization system method, which thus demonstrating the effectiveness of the ultrasonic nebulization system method. The recoveries of added six elements were 100% for Ti, Nb and Mo, and 101% for V and Zr, 102% for Ta; the limits of detection were 0.03 ng cm−3 for Ti and Zr to 1.00 ng cm−3 for Ta.
  • Effect of W Alloying and NbC Dispersion on High Temperature Strength at 1773 K and Room Temperature Fracture Toughness in Nb5Si3/Nb In-situ Composites

    pp. 1415-1418

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    DOI:10.2320/matertrans.43.1415

    The effects of W alloying and NbC dispersion on high temperature strength at 1773 K and room temperature fracture toughness are investigated using Nbss/Nb5Si3 and Nbss/Nb5Si3/NbC in-situ composites with the hypoeutectic composition, where Nbss denotes Nb solid solution. With increasing W content, 0.2% offset yield stress for both composites increases at 1773 K. The increase in the yield strength is attributable to the solid solution strengthening in Nbss by adding W, low diffusivity of W in Nbss and microstructural change caused by W addition. With increasing W content, fracture toughness decreases in the Nbss/Nb5Si3, but no marked change is observed in the Nbss/Nb5Si3/NbC. A crack propagates straightly in Nb–16Si–5Mo–15W, while crack deflection and branching take place in the Nbss/Nb5Si3/NbC. Fractography reveals that Nbss/Nb5Si3 fractures completely in a transgranular mode, while Nbss/Nb5Si3/NbC fractured in a mixed mode of transgranular fracture and interface decohesion between NbC and Nbss or Nb5Si3 and Nbss. It is suggested that the presence of NbC dispersoids in Nbss/Nb5Si3 is effective to increase high temperature strength without a significant decrease of fracture toughness.
  • Comments on Correlation Entropy Theory by Baranyai and Evans

    pp. 1419-1421

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    DOI:10.2320/matertrans.43.1419

    The correlation entropy theory by Baranyai and Evans produces non-negligible entropy contributions arising from the four-body and even higher order terms of correlations. If we express this as S(x), the estimated value of S(x) is 0.6 (±0.1) NkB near the melting point when the structure factor of a liquid metal is of hard-sphere form (I. Yokoyama and S. Tsuchiya: Mater. Trans. 43 (2002) 67–72.). The purpose of the present paper is to propose a way for avoiding the large positive value of S(x) and to show the recalculated results of the total entropy, diffusion coefficient, viscosity coefficient and surface tension of the liquid metals studied in the previous paper using the proposed pair and triplet correlation entropies definitions.
  • Mechanically Induced Solid-State Reaction for Synthesizing New Multicomponent Ta55Zr10Ni10Al10Cu15 Glassy Alloy Powders with Extremely Wide Supercooled Liquid Region

    pp. 1422-1425

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    DOI:10.2320/matertrans.43.1422

    New multicomponent Ta-based glassy alloy powder was synthesized by mechanical alloying a mixture of elemental Ta55Zr10Ni10Al10Cu15 powders at room temperature, using a low-energy ball mill. The glassy powders of the final-product (1080 ks) in which its glass transition temperature (Tg) lies at a high temperature (834 K), crystallize through a single sharp exothermic peak at 1004 K (Tx). The total enthalpy change of crystallization (ΔHx) is −10.32 kJ/mol. The supercooled liquid region before crystallization (ΔTx) of the synthesized glassy powder shows the widest value (170 K) of any reported metallic glassy system.

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