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

MATERIALS TRANSACTIONS Vol. 44 (2003), No. 6

  • Is there a Hidden World of New Materials and Effects “between” the Elements of the Periodic Table?

    pp. 1057-1067

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    DOI:10.2320/matertrans.44.1057

    A constrained state of solid matter is known to exist in the cores of crystal defects - for example - in the cores of intercrystalline interfaces. This constrained state of solid matter differs structurally and propertywise from other (unconstrained) solid states such as perfect crystals, glasses etc. in terms of its atomic and electronic structure as well as in its chemical composition. It is the basic idea of nanocrystalline solids to generate a novel type of materials by incorporating such a high density of defect cores into a - formerly - perfect crystal that the total volume of these defect cores becomes comparable to the total volume of the residual lattice regions between the defect cores. The resulting solids are called nanocrystalline solids. Due to the large volume fraction of defect cores, nanocrystalline solids differ from other forms of solids (e.g. single crystals, coarse-grained polycrystals, glasses) in terms of their atomic and electronic structure, their chemical composition and by the fact that the size of the crystalline regions between neighbouring defects is reduced to a few interatomic spacings. As the properties of solids depend on exactly those four parameters (atomic structure, electronic structure, chemical composition and crystal size), the properties of nanocrystalline solids deviate from the ones of crystalline or glassy materials. In this paper attention will be focused on the tuning of the electronic structure of solids by means of their nanostructure. In fact, solids with nanometer-sized microstructures may open the way to generate materials with an excess or a deficit of electrons or holes of up to 0.3 electrons/holes per atom i.e. elements that are electronically “between” the (electrically neutral) elements of the periodic table. Large deviations from charge neutrality may be achieved either by means of an externally applied voltage or by the space charges at interfaces between materials with mobile charge carriers (such as metals or semiconductors) and with different chemical compositions (or combinations of both). As many properties of solid materials depend on their electronic structure, significant deviations from charge neutrality may lead the way into a world of materials with new, yet mostly unexplored properties such as modified electric, ferromagnetic, optical etc. properties. Some existing and conceivable new technological applications of solids deviation from charge neutrality are briefly discussed.
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  • Numerical Analysis of Solidification Effect on Diffusion Measurement in Liquid using the Long Capillary Method

    pp. 1068-1074

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    DOI:10.2320/matertrans.44.1068

    A numerical analysis was carried out to examine the solidification effect on the concentration distribution of a solute in an Ag/Ag–5 at%Au diffusion couple in a liquid diffusion experiment using the long capillary method. Shrinkage-induced fluid flow in the solidifying sample was calculated, and the solute movement due to the fluid flow was evaluated. In the analysis, molten samples in a graphite crucible were cooled in air at three different cooling rates. Flat isoconcentration contours in the horizontal plane of the diffusion couple were formed in the sample with a low cooling rate, whereas concave isoconcentration contours were formed in the samples with high cooling rates. Those features in the simulation agreed with the experimental results. The optimum conditions to avoid the solidification effect in liquid diffusion experiments are discussed.
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  • Crystallization Kinetics in an Amorphous Al-Ni-Mm-Fe Alloy

    pp. 1075-1080

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    DOI:10.2320/matertrans.44.1075

    Differential scanning calorimeter (DSC) has been used to study the crystallization kinetics of Al87.5Ni7Mm5Fe0.5 alloy. Both isochronal and isothermal DSC plots showed that the Al87.5Ni7Mm5Fe0.5 alloy undergoes three stages crystallization process. The first peak corresponds to the precipitation of fcc-Al from the amorphous matrix at around 160°C. Isothermal DSC traces showed that there is a short incubation stage for the formation of nanoscale Al particles. The value of Avrami exponent suggests that the crystallization of Al crystals starts with a nucleation process for a short time and finally controlled by a growth mechanism. The second peak corresponds to the crystallization of Al11(La,Ce)3 from the matrix at around 320°C with a nucleation rate, which decreases with time, and a constant growth rate. The activation energy calculated for the crystallization of Al11(La,Ce)3 phase is 288±5.9 kJ/mol, 324.6±18 kJ/mol, 292±6 kJ/mol by Kissinger, Johnson-Mehl-Avrami and Arrhenius type equations. The difference in the values of kinetic energy is mainly due to different annealing conditions. The third peak at around 340°C corresponds to the precipitation of intermetallic compounds from the remaining matrix.
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  • Microstructural Characterization of a Hot Pressed Si3N4-TiN Composite Studied by TEM

    pp. 1081-1086

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    DOI:10.2320/matertrans.44.1081

    Microstructures and material properties of TiN reinforced-Si3N4 composites, fabricated by hot pressing and using Si3N4, TiN and sponge Ti powders, were investigated. The sponge Ti powders in the Si3N4–TiN-sponge-Ti compacts are fully nitrided into TiN without residual Ti phase, but some residual pores remain. The Si3N4 grain boundaries, Si3N4/TiN interfaces and triple regions are covered with an amorphous phase. However, some reaction compounds estimated to be Si10Ti3W2 phase are observed around fine TiN particles. The values of electrical resistivity, fracture strength and fracture toughness of the composite are 1.2×10−2 Ωcm, 362 MPa and 5.3 MPam1⁄2, respectively. The main reason for the relatively low mechanical properties is the existence of microcracks at the interface of Si3N4/large TiN grains, as well as the microstructure of fine, rod-like Si3N4 grains.
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  • Austenite Grain Size Effects on Isothermal Allotriomorphic Ferrite Formation in 0.37C-1.45Mn-0.11V Microalloyed Steel

    pp. 1087-1095

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    DOI:10.2320/matertrans.44.1087

    Allotriomorphic ferrite is the morphology of ferrite formed at relatively small undercooling below the Ae3 temperature. Because it is the first transformation in austenite decomposition during cooling, allotriomorphic ferrite affects indirectly the subsequent austenite phase transformations, and then its study is of vital importance. The present paper is concerned to the theoretical and experimental study of the isothermal decomposition of austenite in allotriomorphic ferrite in a modern medium carbon vanadium–titanium microalloyed forging steel. This paper deals with the isothermal austenite-to-allotriomorphic ferrite as a whole, considering the specific role of different features such as prior austenite grain size (PAGS) and isothermal temperature, in nucleation and growth processes independently.
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  • Decarburization of 3%Si-1.1%Mn-0.05%C Steel Sheets by Silicon Dioxide and Development of 100< 012> Texture

    pp. 1096-1105

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    DOI:10.2320/matertrans.44.1096

    Decarburization of silicon steel sheets by annealing with oxide separators has been found to cause a high degree of {100}-texture development. Cold-rolled Fe–3%Si–1.1%Mn–0.05%C sheets of 0.35 mm thickness were laminated with separators containing SiO2. They were then annealed under a reduced pressure at about 1300 K in a ferrite and austenite two phase region. It has been observed that carbon concentration notably decreases down to 0.001% during the lamination annealing. Thus an almost complete decarburization of sheet steels was possible, whereas no oxidation of silicon as well as manganese and iron occurred. Associated with decarburization, columnar ferrite grains grew inward from sheet surfaces due to the phase transformation from austenite to ferrite. A {100}⟨012⟩ texture dramatically developed in the columnar grains. Fully decarburized materials consisted of grains of 0.6 mm diameter, more than 90% of which were closely aligned with {100}⟨012⟩ orientation. Another aspect of great interest in the grain structure after decarburization was that there existed convoluted domains of a few mm in width, in which dozens of grains were oriented in a single variant of the texture, (100)[012] or (100)[021]. The decarburization is considered to be caused by the thermo-chemical reaction, 2C+SiO2→Si+2CO. The texture development is most likely to be due to the orientation dependence of surface free energy under an oxidation-free surface condition.
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  • Fine-Grained Doubly Oriented Silicon Steel Sheets and Mechanism of Cube Texture Development

    pp. 1106-1115

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    DOI:10.2320/matertrans.44.1106

    An attempt has been made to obtain strongly cube-textured sheet steels with fine grains employing oxide-separator-induced decarburization. The steels of the chemical compositions around Fe–3%Si–1%Mn–0.05%C were hot-rolled, twice cold-rolled with in-between annealing into 0.35 mm thickness and then subjected to the lamination annealing with separators containing SiO2, which promoted decarburization. During the lamination annealing in a ferrite and austenite two phase region of about 1350 K, the sheet materials were decarburized down to about 0.001% of carbon concentration. At the same time, the cube texture {100}⟨001⟩ remarkably evolved in the columnar ferrite grains which grew inward from sheet surfaces. Cube-oriented nuclei emerged during the primary recrystallization prior to decarburization, and they selectively grew in the columnar grains. After complete decarburization, the sheet materials consisted of ferrite grains of about 0.4 mm diameter, more than 90% of which were well aligned with the cube orientation. The doubly oriented steel sheets thus obtained showed a large magnetic induction of 1.87 T at 800 A/m, a small core loss of 1.2 W/kg at 1.5 T and 60 Hz and a low magnetostriction of 2.5×10−6 at 1.9 T in both the rolling and transverse directions. It is likely that the cube texture component arises from a near-{410}⟨001⟩ component present after intermediate annealing and preferentially grows by surface energy difference. The magnetization processes are also discussed.
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  • Vapor Pressure Measurements for the FeCl2-ZnCl2 System by the Transpiration Method

    pp. 1116-1119

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    DOI:10.2320/matertrans.44.1116

    The vapor pressures of FeCl2 and ZnCl2 in the FeCl2–ZnCl2 system were measured by the transpiration method at 873 K and 917 K. The vapor pressure data were interpreted to indicate the formation of complex ions in the FeCl2–ZnCl2 melts. It is presumed that the complex molecule FeZnCl4 as well as FeCl2 and ZnCl2 exists in the vapors in equilibrium with molten FeCl2–ZnCl2 mixtures. The activities in the FeCl2–ZnCl2 system show a negative deviation from Raoult’s law. These results suggest that the complex ions such as ZnCl42− are formed in molten FeCl2–ZnCl2 mixtures.
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  • Activity Measurements of Copper in Solid Copper-Nickel Alloys using Copper-Beta-Alumina

    pp. 1120-1123

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    DOI:10.2320/matertrans.44.1120

    Copper-β″-alumina was prepared by ion exchange reactions starting with a sodium-β″-alumina. Exchange from sodium ion to copper ion was done by immersing the sample in liquid cuprous chloride. Exchange of Na+ ion in β″-alumina to Cu+ ion was not complete as Na+ ion remained in the β″-alumina. Copper activity in solid copper–nickel alloys was measured by electromotive force (EMF) technique incorporating the partially exchanged (Cu+–Na+)-β″-alumina as a solid electrolyte for temperatures between 870 and 1300 K. The activities of copper and nickel in the solid solution at these temperatures exhibited positive deviations from Raoult’s law. The activity data were compared with other investigators obtained using ZrO2+CaO or ThO2+Y2O3 solid electrolyte.
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  • Behaviour of Iron Ion in the Morphosynthesis of Magnetite Particles

    pp. 1124-1127

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    DOI:10.2320/matertrans.44.1124

    In situ synthesis of magnetite particles has been carried out in a preorganized polyvinyl alcohol (PVA) gel by varying the total number of iron ions in an environment of a fixed number of hydroxyl functional groups of PVA. A systematic change in the morphology and patterning of the magnetite particles on the polymer surface revealed the effect of the total number of cations on the conformation of the supramolecular matrix, responsible for dictating the morphological features of the magnetite particles.
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  • Microstructure and Mechanical Properties of a Rolled Ti-Si-B Alloy

    pp. 1128-1132

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    DOI:10.2320/matertrans.44.1128

    In order to obtain the Ti–Si–B alloys with uniform and fine microstructure, the hot workability of Ti–Si–B alloys was investigated by rolling method. The influence of deformation on the microstructure and mechanical properties was studied using optical microscopy, SEM, EPMA and mechanical properties testing. Results show that the alloys exhibit good thermal plasticity from 773 to 1173 K. The deformation induced a significant refinement of microstructure of Ti–Si–B alloys. Both tensile strength and ductility were improved through rolling deformation. The elongation of Ti–0.5Si–0.2B alloy was up to 25.7% and the tensile strength is about 828 MPa when the alloy was rolled with reduction of 50% at 1173 K. Relatively lower rolling temperature can improve the microstructures and mechanical properties of the titanium alloys more effectively.
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  • Control of Crystal Orientation of Hydroxyapatite by Imposition of a High Magnetic Field

    pp. 1133-1137

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    DOI:10.2320/matertrans.44.1133

    A hydroxyapatite is most suitable biomaterial for clinical application, because it is a main component which constitutes bones and teeth of an organism. Since the hydroxyapatite has different biocompatibility and absorptive activity of proteins for its crystal plane, it is necessary to use the appropriate crystal plane for use in vivo. Thus, the crystal orientation of the hydroxyapatite is one of the very crucial subjects in biomaterials processing. In this study, the control of crystal orientation of the hydroxyapatite has been conducted in the colloidal filtration (slip casting) process under a high magnetic field and a new process where the high magnetic field is introduced in the heat substrate method developed by Okido et al. The usefulness of the processes has been confirmed through the orientation index evaluated by X-ray diffraction patterns and scanning electron microscope (SEM) images of hydroxyapatite crystals. The crystal axis of hydroxyapatite aligned by a magnetic field was determined.
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  • Development of Pressure Control Technique of An Arc-Submerged Nanoparticle Synthesis System (ASNSS) for Copper Nanoparticle Fabrication

    pp. 1138-1142

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    DOI:10.2320/matertrans.44.1138

    The synthesis of nano-materials is one of the crucial techniques towards product and process innovation. In this article, low-pressure control methods for an arc-submerged nanoparticle synthesis system (ASNSS) was proposed and developed for copper nanoparticle fabrication. Two technical advances associated with nanoparticle synthesis were achieved. One is the novel pressure control technique developed for nanoparticle fabrication. The other is the verification that the constant low-operating pressure plays an important role in determining the characteristics of the prepared nanoparticles. From the experimental results, pressure control of the ASNSS was identified as crucial to success of metal nanoparticle synthesis. To achieve the desired pressure control, a vacuum chamber was developed as a nanoparticle accumulator and low-pressure reservoir. The chamber was controlled by the proposed flow-valve feedback control system and integrated with the ASNSS. In this study, the pressure control equipment of the ASNSS was effectively developed to prepare desired copper nanocrystalline particles with well-controlled size.
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  • Bulk Glassy Alloys with Low Liquidus Temperature in Pt-Cu-P System

    pp. 1143-1146

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    DOI:10.2320/matertrans.44.1143

    Bulk glassy alloys with low values of liquidus temperature (Tl) and glass transition temperature (Tg) as well as high TgTl were formed at Pt60Cu20P20. Glassy alloys in Pt80−xCuxP20 system were formed over the whole composition range up to 35 at%Cu examined in the present study. The Tg increases significantly with increasing Cu content, shows a maximum value of 522 K at 20%Cu and then decreases slightly with further increase in Cu content. On the other hand, the crystallization temperature (Tx) increases monotonously with increasing Cu content up to 25 at%Cu and then becomes saturated, leading to a monotonous increase in ΔTx(=TxTg) from 25 K at 0%Cu to 83 K at 35 at%Cu. All the ternary glassy alloys crystallize through a single-stage exothermic reaction, accompanied by the precipitation of mixed Pt5P2, Cu3P and unknown phases. The 20%Cu-containing alloy has the lowest Tl of 854 K as well as the smallest temperature interval of 21 K between Tl and melting temperature (Tm) and hence the highest TgTl of 0.61 is obtained for the 20%Cu alloy. The high TgTl as well as the low Tl has enabled us to form bulk glassy alloy rods with diameters up to at least 12 mm by water quenching. The synthesis of the Pt–Cu–P glassy alloys with the low values of Tg and Tl and the large ΔTx is important for future development as precision materials for nano-technology which can be deformed through viscous flow.
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  • Formation, Thermal Stability, Mechanical Properties and Corrosion Resistance of Cu-Zr-Ti-Ni-Nb Bulk Glassy Alloys

    pp. 1147-1152

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    DOI:10.2320/matertrans.44.1147

    Bulk glassy alloys exhibiting high strength and good corrosion resistance were formed in (Cu0.6Zr0.3Ti0.1)100−xyNiyNbx systems. The bulk glassy alloy rods with a diameter of 3 mm are formed in a wide composition range of 0 to 6 at%Ni and 0 to 6 at%Nb by copper mold casting. As the Nb content increases, the glass transition temperature (Tg) increases while the crystallization temperature (Tx) decreases, resulting in a decrease in ΔTx (=TxTg) from 60 K at 0 at%Nb to 36 K at 5 at%Nb. The high fracture strength (σc,f) exceeding 2000 MPa is obtained for the Cu–Zr–Ti–Ni–Nb alloys containing more than 3 at%Nb and their high strength alloys also exhibit distinct plastic elongation of 0.2 to 0.8%. The corrosion resistance of the Cu–Zr–Ti bulk glassy alloy is also significantly improved by the simultaneous addition of Ni and Nb and no loss in sample weight in 1 N HCl and 3%NaCl solutions is detected for the Cu–Zr–Ti–Ni–Nb alloys containing 5 at%Ni and 6 at%Nb or 2 at%Nb, respectively. The usefulness of the simultaneous addition of Ni and Nb on the σc,f and corrosion loss in conjunction with high glass-forming ability seems to affect significant influence on the future development of bulk glassy alloys with special functional characteristics.
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  • Effect of Titanium Carbide Precipitates on the Ductility of 30 mass% Chromium Ferritic Steels

    pp. 1153-1158

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    DOI:10.2320/matertrans.44.1153

    The effect of morphology of Ti carbides on the ductility of 30 mass%Cr ferritic steels containing C and Ti was investigated by tensile test at low temperatures. C and Ti contents were varied from 0.011 to 0.071 mass%, and from 0.094 to 0.60 mass%, respectively. The ratio Ti/C was about nine. Two kinds of heat treatments were adopted to obtain different types of morphology of Ti carbide precipitates, namely numerous fine grain boundary Ti carbide precipitates about 0.05 μm in diameter and coarse globular Ti carbide particles in the 1.5 μm size range. The tensile ductility was evaluated by transition temperatures in reduction of area. The numerous fine grain boundary Ti carbide precipitates lead to an increase in the transition temperature more than uniformly dispersed coarse globular Ti carbide particles. The former brings about the increase in the amount of intergranularly fractured surface in dimple and brittle fracture more than the latter. Cluster-like coarse globular Ti carbides at grain boundaries cause a scatter band of the transition temperature. Microfractographic examinations reveal that initiation of microcracks for brittle fracture is considered to be caused by intergranular fracture cracks which are promoted by decohesion at the interface between the matrix and many grain boundary Ti carbide precipitates and by decohesion at the interface between the matrix and cluster-like coarse globular Ti carbides at grain boundaries.
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  • The Effects of Surface Modification on the Properties of Bonded NdFeB Magnets

    pp. 1159-1162

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    DOI:10.2320/matertrans.44.1159

    Two cladding methods for NdFeB powders, i.e., the modified organic silicon (MOS) cladding and the dichromate passivation reducing (DPR) cladding are exploited respectively for the preparation of polymer-bonded magnet and metal-bonded magnet. The effects of their cladding conditions and coating compositions, together with the effects of modifying additives are discussed. Compared with the conventional polymer-bonded magnet, MOS polymer-bonded sample shows better comprehensive properties including magnetic properties, compression strength and corrosion resistance. Providing a feasible method for Nd–Fe–B type metal-bonded magnet, DPR Sn-bonded NdFeB magnet presents superior compression strength despite a little inferior magnetic property.
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  • Thermal Degradation of Woodceramics under Different Oxygen Concentration

    pp. 1163-1166

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    DOI:10.2320/matertrans.44.1163

    Woodceramics has been studied as an Ecomaterial, and is expected to have applications in a wide variety of fields. In this report, thermo-gravimetry and differential thermal analysis of Woodceramics with changing concentration of oxygen was conducted. Experiments in chemical reaction kinetics were also performed in order to clarify the thermo-physical and thermo-chemical properties of Woodceramics in the presence of oxygen. The experimental results show that both mass loss curves and differential heat curves are dependent on the concentration of oxygen. In addition, the reaction rate law of the thermal degradation of Woodceramics with change in concentration of oxygen is reported to verify the suitability of the method.
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  • Thermal Stability and Mechanical Properties of Glassy and Amorphous Ni-Nb-Zr Alloys Produced by Rapid Solidification

    pp. 1167-1171

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    DOI:10.2320/matertrans.44.1167

    Rapidly solidified Ni–Nb–Zr alloys in a ribbon form with a thickness of about 20 μm were prepared by a single roller melt-spinning technique. Amorphous alloys were formed in the composition range of 20 to 75 at%Ni, 0 to 60 at%Nb and 0 to 80 at%Zr. Furthermore, glassy alloys were also synthesized in the range of 50 to 70 at%Ni, 5 to 35 at%Nb and 5 to 45 at%Zr. The maximum temperature interval of the supercooled liquid region was 51 K for Ni60Nb20Zr20 alloy. The tensile fracture strength and Vickers hardness of the glassy Ni60Nb20Zr20 alloy are 2160 MPa and 700, respectively. Wide ribbons with a width of 50 mm of amorphous Ni42Nb28Zr30 and glassy Ni60Nb20Zr20 alloys were produced at a circumferential velocity of the copper roller of 20 m/s.
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  • Effects of Particle-Dispersion on the Strength of an Alumina Fiber-Reinforced Aluminum Alloy Matrix Composite

    pp. 1172-1180

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    DOI:10.2320/matertrans.44.1172

    Aluminum alloy matrix hybrid composites, in which alumina particles were dispersed among continuous alumina fibers, were fabricated by squeeze casting, and the influence of temperature and the effects of the particle-dispersion on the strength of the composites were then investigated. The particle-dispersion among the fibers minimized preform contraction and fiber-to-fiber contact due to the melt infiltration during the squeeze casting. The tensile strength, 0.2% proof stress and elastic modulus of the composites in the longitudinal direction increased with increasing the fiber volume fraction, retaining nearly the same values up to 623 K. These values of the hybrid composite were larger than those of the particle-free composite at every temperature. This is because the fibers were distributed uniformly owing to the particles that prevented the fiber-to-fiber contact, leading to the reduction of stress concentration at the points of direct fiber contact, and stress transmission between the fiber and the matrix becomes easy. At every temperature, the transverse tensile strength and proof stress of the particle-free composite was lower than that of the unreinforced alloy, because the fracture was initiated at the fiber-to-fiber contact point and the cracks propagated mainly along the fiber-matrix interface. In contrast, the strength of the hybrid composite was close to that of the unreinforced alloy even at high-temperature, because the cracks propagated mainly throughout the matrix owing to the uniform distribution of the fibers and the strong fiber-matrix bond.
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  • Nano-Characterization of Ceramic Top-Coat/Metallic Bond-Coat Interface for Thermal Barrier Coating Systems by Plasma Spraying

    pp. 1181-1189

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    DOI:10.2320/matertrans.44.1181

    Characterization at the ceramic top-coat/metallic bond-coat interfacial region was conducted for several kinds of the plasma sprayed thermal barrier coating (TBC) systems by means of a transmission electron microscope (TEM), an electron probe micro-analysis (EPMA) and so forth, in order to find out the optimum compositional and structural conditions of the coating components together with the optimum coating processing condition for designing the advanced TBC systems. Specimens with different coating features were prepared systematically by using different coating parameters such as the top-coat spraying conditions and reheat-treatment conditions. Especially, the reheat-treatment was applied to the TBC specimen with different temperature either in air or in inert argon (Ar) gas atmosphere. It was found that in the case of reheat-treatment in air the thermally grown oxide (TGO) was developed at the interface as multiple oxide layers; one is Al2O3 layer developed discontinuously at directly above the bond-coat and another is the mixed oxides layer consisting of the Al, Cr, Co, Ni oxide particles on the Al2O3 layer. Such a TGO layer was heterogeneous and imperfect layer with containing many kinds of defects. On the contrary, the TGO layer formed by the reheat-treatment in Ar was composed dominantly of the continuous and fairly purified Al2O3 layer with large grain size and homogeneous layer thickness. The growth mechanism and influencing factors for TGO were discussed in some detail on the basis of the nano-characterization and quantitative evaluation of TGO.
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  • Marking Oxide Films on the Section of Al-XSi Alloys by Ultrasonic-vibration Treatment

    pp. 1190-1197

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    DOI:10.2320/matertrans.44.1190

    Oxide films entrapped in Al–XSi alloys with an X from 0% to 13% are different, and can be identified by the presented ultrasonic-vibration treatment. After polishing and ultrasonic-vibration treatment, the surfaces of samples or castings then show differently shaped foggy marks, including lumps, flakes, strips or spots. Oxide films fracture and particles become detached from the film during ultrasonic-vibration treatment. The polished surface thus becomes partly eroded after treatment, and these eroded areas are visibly as differently shaped foggy marks. This paper presents a sequential summary of the formation of these eroded areas, foggy marks, in samples of pure aluminum and Al–XSi alloys during ultrasonic-vibration treatment.
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  • Capacity for Deformation and the Evaluation of Flow Stress of Hot Extruded Mg-8Al-xRE Alloys at Elevated Temperatures

    pp. 1198-1203

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    DOI:10.2320/matertrans.44.1198

    This paper investigated the optimal deformation temperature Tc, and the optimal Zener-Hollomon parameter Zc, of both as-extruded Mg–8Al and Mg–8Al–2RE alloys, to examine the capacity of these alloys for deformation. Additionally, an equation that involves the Zener-Hollomon parameter Z was derived to evaluate the flow stress of these magnesium alloys at various temperatures and strain rates and was validated using a statistical method. These alloys were prepared by melting and casting in a vacuum induction furnace, and extruded at 633 K with a reduction ratio of 90:1. Tensile tests were performed at 473 to 723 K and at strain rates ranging from 8.3×10−4 to 8.3×10−1 s−1. The implications of the experimental data were discussed in detail.
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  • Nondestructive Evaluation of Thermally Degraded 2.25Cr-1Mo Steel by Electrical Resistivity Measurement

    pp. 1204-1208

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    DOI:10.2320/matertrans.44.1204

    An attempt was made to evaluate nondestructively the degradation of thermally aged 2.25Cr–1Mo steel by electrical resistivity measurement. Artificial aging was performed to simulate the microstructural changes in 2.25Cr–1Mo steel arising from long time exposure at 540°C. Microstructural parameter (amount of dissolved Mo in the matrix), mechanical property (ductile-brittle transition temperature) and electrical resistivity were measured to investigate the relationship among these parameters. Both the amount of dissolved Mo and the electrical resistivity decreased rapidly in the initial 1000 hours of aging and then changed little thereafter. On the other hand, the ductile-brittle transition temperature (DBTT) increased rapidly in the initial stage of aging and then saturated afterward. Electrical resistivity was found to have linear correlation with the amount of dissolved Mo and DBTT, respectively. Electrical resistivity was suggested as a potential nondestructive evaluation parameter for assessing DBTT of the thermally degraded 2.25Cr–1Mo steel.
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  • Effect of Microstructural Refinement on Ductility Deterioration of High Silicon Ferritic Spheroidal Graphite Cast Iron Caused by Cyclic Heating

    pp. 1209-1218

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    DOI:10.2320/matertrans.44.1209

    This investigation applies cyclic heating and cooling to elucidate the effect of microstructural refinement on the tensile elongation deterioration of ferritic spheroidal graphite cast iron. In order to eliminate the oxidation factor, the cyclic heating/cooling test was performed in a 1.33∼0.133 Pa ambient vacuum atmosphere with cyclic heating at a maximum temperature of 1023 K. Severe embrittlement accompanied by intergranular fracture occurred after the ferritic spheroidal graphite cast iron was subjected to a certain number of heating and cooling cycles. A fair amount of inevitable inclusion particles were found to agglomerate in the eutectic cell boundary region, and so the cyclic heating induced embrittlement can be recognized to be strongly dependent on the solidification cooling rate of the materials. Based on experimental evidence, the cracking evolution can be divided into three steps: (1) crack initiation from the vicinity of the eutectic cell boundary at the surface, (2) crack linking and major crack formation, and (3) major crack inward extension. Cyclic heating cracks are mainly initiated at the eutectic cell boundary where a fair amount of MgO inclusions dispersed, and consequently propagated along the annealed eutectic cell boundary. While investigating the plastic deformation behaviors around the above mentioned MgO inclusions pertaining to the crack initiation and crack propagation, typical etch pit evidence was observed in the vicinity of the cell boundary area.
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  • Ageing in Parent Phase and Martensite Stabilization in a Ni50Ti30Hf20 Alloy

    pp. 1219-1224

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    DOI:10.2320/matertrans.44.1219

    The effect of ageing in parent phase and martensite stabilization on melt spun Ni50Ti30Hf20 ribbons were investigated by Differential Scanning Calorimetry and X Ray Diffraction. Thermal treatments in parent phase generally shift the martensitic transformation range as far as an equilibrium state is reached. The equilibrium state, here found for a thermal treatment at 450°C (723 K), was adopted as start state to investigate ageing in martensite (commonly indicated as martensite stabilization). Martensite stabilization involves diffusion processes with an activation energy in the order of 1.2 eV: results add a piece of evidence to the general principle of Symmetry-Conforming Short-Range Order advanced by Ren and Otsuka.
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  • Effect of Graphite Content on the Tribological Behavior of a Cu-Fe-C Based Friction Material Sliding against FC30 Cast Iron

    pp. 1225-1230

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    DOI:10.2320/matertrans.44.1225

    Focus of the study has been placed on the effect of graphite content on tribological behavior of a Cu–Fe–C based friction material sliding against FC30 cast iron. Experimental results indicated that, after sintering in air, the densities of Cu–Fe–C specimens decreased, oxygen contents increased, and essentially all original metallic constituents turned into oxides. As graphite content increased, the amounts of Fe2O3, Cu2O and Cu–Sn decreased, while CuO and FeSn2 contents increased. The friction coefficient profiles of Cu–10%Steel wool–10%Al–10%Al2O3–10%Sn (vol%) material (C0) and Cu–10%Steel wool–10%C–10%Al–10%Al2O3–10%Sn (vol%) material (C10) are almost identical. After sliding for a few minutes, their friction coefficients start to decay. The friction coefficient of Cu–10%Steel wool–20%C–10%Al–10%Al2O3–10%Sn (vol%) material (C20) maintained to be high and stable throughout tests. The mass loss of C20 was larger than that of C0 or C10. The mass loss of FC30 sliding against C20 was smaller than that of FC30 sliding against C0 or C10. While both particulate and film types of debris were observed on each Cu–Fe–C specimen, more particulate debris was observed on C0 and C10 than on C20 surface. Chemical analysis of worn surfaces indicated that Fe was dominant on C0 and C10 surfaces, while large amounts of Fe and Cu were detected on the worn surfaces of both C20 and FC30.
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