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

MATERIALS TRANSACTIONS Vol. 45 (2004), No. 6

  • Crystallization in Partially Amorphous Ni50Ti32Hf18 Melt Spun Ribbon

    pp. 1811-1818

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    DOI:10.2320/matertrans.45.1811

    A partially amorphous Ni50Ti32Hf18 melt spun ribbon has been characterized by means of calorimetry, X-ray diffraction and transmission electron microscopy, showing that the amorphous regions are mostly concentrated in the wheel side as a consequence of a higher cooling rate during the fast solidification (i.e. higher solidification rate). Special emphasis has been given to the crystallization process of the amorphous regions, studying the evolution of the microstructure and the martensitic transformation. Although several crystallization procedures have been carried out by thermal treatments, either slightly under or over the crystallization temperature measured by DSC, TC, the final microstructure and calorimetric behavior of the fully crystalline samples does not depend on the applied temperature. The fully crystalline samples contain regions with small crystallites produced during the thermal treatment (showing that the crystal nucleation energy is rather low for this alloy) together with the large crystals originated during the melt-spinning. At room temperature (RT), both types of crystals are in martensitic state. The new crystallized regions with small grain size show notably lower transformation temperatures and higher hysteresis in comparison to the big crystals already existing before the crystallization treatments. However, an additional mechanism, apart from the crystal size, affects the transformation temperatures, likely related to the presence of more defects (mainly grain boundaries) in the crystallites created by thermal treatments. These defects could act as nucleation sites for the martensite and increase the transformation temperatures with respect to the as cast spherical crystallites of similar sizes.
  • Characterization of Crystalline Inclusions in Cast Bulk Zr-Cu-Ni-Al Glassy Alloy

    pp. 1819-1823

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    DOI:10.2320/matertrans.45.1819

    In order to clarify intrinsic features of bulk glassy alloys, the structural control is the most significant factor. However, it is difficult to obtain a single glassy phase by conventional casting methods. Crystalline inclusions in Zr-based bulk glassy alloy are often seen on the fracture surface. Crystalline inclusions are easily formed in molten alloy state by inhomogeneous nucleation or impurities i.e. oxygen, magnesium dichloride and so on. In Zr55Cu30Ni5Al10 bulk glassy alloy, most of the crystalline inclusions are probably considered as τ3 type intermetallic compound with typical composition of Zr51Cu28Al21. The morphology of the crystalline inclusions is changed by the quantity of absorbed oxygen impurities in bulk glassy alloy.
  • Influence of Primary and Secondary Crystallographic Orientations on Strengths of Nickel-based Superalloy Single Crystals

    pp. 1824-1828

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    DOI:10.2320/matertrans.45.1824

    It has been revealed that the strengths of the notched specimens were affected by the crystallographic orientation not only in the tensile direction (primary orientation) but also in the thickness direction (secondary orientation). In this study, by using single crystals of an experimental superalloy which shows distinct active slip systems, the influence of primary and secondary crystal orientation on creep and fatigue strengths of Ni-based superalloy single crystals was investigated. The influence of crystallographic orientations and plastic anisotropy on the creep and fatigue strengths of single crystals of the Ni-base superalloy was discussed on the assumption that {111} ‹101› and {111} ‹112› slip systems. {111} ‹112› slip is unusual slip. For a tensile stress applied close to the [001] axis, (111)[112] slip system is result of pass of intrinsic-extrinsic superlattice stacking faults pair through the γ′ phase. In the tensile orientation close to [011], (111)[211] slip is occurred by twinning shear through γ and γ′ phases. In the case of creep strength, the results were in agreement with the assumption of the operation of the {111} ‹112› slip in the primary creep and {111} ‹101› slip in the secondary creep. The notched creep behavior was found to be influenced by the additional aging at 850°C for 20 h, which prohibited activity of {111} ‹112› slip systems. The fatigue lifetime and crack growth behavior depended on both plastic anisotropy caused by arrangement of {111} ‹101› slip systems and the stress state.
  • Prediction of the Forming Limit of Porous Metals Using the Finite Element Method

    pp. 1829-1832

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    DOI:10.2320/matertrans.45.1829

    To predict the forming limit diagram of porous metals, we use the elasto-plastic finite element method in conjunction with a critical relative density criterion to simulate the deformation behavior in the upsetting of porous metallic specimens. We predict the strain instability of porous cylindrical specimens by calculating the strain paths during deformation. Strain instability is the point where axial strain starts to increase in compression. By using strain instability as a ductile fracture criterion of porous metals, we calculate a forming limit that agrees well with the experimental forming limit. The calculated relative density in crack initiation is higher than the critical relative density.
  • Improvement of Fatigue Properties by Means of Continuous Cyclic Bending and Annealing in an Al-Mg-Mn Alloy Sheet

    pp. 1833-1838

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    DOI:10.2320/matertrans.45.1833

    The fatigue properties in the Al-Mg-Mn alloy sheet subjected by the continuous cyclic bending (CCB) and annealing have been investigated. By the CCB and subsequent annealing, the gradient microstructure with coarse-grained surface layers and fine-grained central layer is produced in the sheet. The fatigue life of the CCBent and annealed sheet is 2—4 times longer than that of the as-received one. The existence of the coarse-grained surface layers makes the crack propagation rate lower by a factor of 1/2 to 1/3. The fatigue crack growth in the sheet consisting of such different microstructure layers is characterized by the fractography. Further, factors improving the fatigue properties are discussed.
  • Crack Control in Titanium Nickel Fiber Reinforced Polycarbonate Composites

    pp. 1839-1846

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    DOI:10.2320/matertrans.45.1839

    Titanium nickel fiber reinforced composites have great potential as intelligent materials. In this paper, titanium nickel fiber reinforced polycarbonate composites are developed with different prestrains of the embedded titanium nickel fiber. The effect of reducing the stress concentration, the enhancement of mechanical properties and the resistance to deformation of the titanium nickel fiber reinforced polycarbonate composites were investigated. The stress intensity factor, KI, was determined using photoelasticity and digital image processing to examine the crack closure effect in the titanium nickel fiber reinforced polycarbonate composites. The result shows that the crack closure effect is dramatically improved. The shape memory effect and the thermal expansion behavior of the matrix, caused by temperature increases, improve the resistance to fracture by decreasing the stress intensity factor, KI. The effect of crack closure is attributed to the compressive stress field in the matrix due to the shrinkage of the titanium nickel fibers above the austenitic finishing temperature (Af).
  • Thermodynamic Properties of the SiO2-GeO2 and Pt-rich Pt-Ge Systems at 1623 and 1723K

    pp. 1847-1851

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    DOI:10.2320/matertrans.45.1847

    Thermodynamic properties of the SiO2-GeO2 melt were investigated from the measurement of the GeO2 activity in the SiO2-GeO2 melt by equilibrating it with solid platinum under controlled oxygen partial pressure. The excess Gibbs energy for the SiO2-GeO2 melt was evaluated as the following equation in the quasi-regular solution formalism.
    ΔGM,ex = (−47600 + 18.2T)XGeO2XSiO2.
    Also, by equilibrating solid platinum with the GeO2 melt under controlled oxygen partial pressure and investigating the phase equilibrium between solid platinum and the liquid Pt-Ge alloy, thermodynamic properties of Pt-rich Pt-Ge system above the eutectic temperature of Pt3Ge was clarified.
  • Effect of A Degassing Treatment on the Quality of Al-7Si and A356 Melts

    pp. 1852-1858

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    DOI:10.2320/matertrans.45.1852

    A degassing treatment is necessary to assure the quality of aluminum alloy castings. Its effect on the mechanical properties of the A356 alloys has been well studied and documented. This study assessed the effect of different degassing diffusers, a lance and a porous bar, on the areas of the foggy marks and the pore counts as revealed in the chilled Al-7Si and the A356 alloy sample sections. The degassing bubbles rose in the melt floating with induced convection loops during degassing. The porous diffuser generated a strong and wide convection loop in the melt. There are two bubble explosion sites when a porous diffuser was used: one in the area close to the wall of the crucible and another near the diffuser. Using a lance diffuser generated a coarse-bubble cloud near the diffuser during degassing. For Al-7Si alloy, different diffusers produced a close ratio of foggy marked area and a close pore count but the quality of A356 alloy melt was affected by diffusers and/or Sr addition. During rotating bending tests, the fatigue life of the specimens was affected by the pore count and the relative porosity, but was irrelevant to the foggy marks.
  • Solvent Extraction Equilibria of FeCl3 with TBP

    pp. 1859-1863

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    DOI:10.2320/matertrans.45.1859

    Solvent extraction equilibria of FeCl3 with TPB from chloride solutions were analyzed by considering chemical equilibria, extraction reaction, mass and charge balance equations. The activity coefficients of solutes in aqueous phase were calculated by Bromley equation while ideal behavior was assumed for species in organic phase. Extraction reaction of FeCl3 with TBP in the experimental range of this study was determined from the dependence of the distribution coefficient of iron on the concentration of chloride ions. By applying ionic equilibria to the results of solvent extraction experiments, equilbrium constant of the extraction reaction was estimated to be 4.5 × 102. The equilibrium concentrations of solutes in both phases after extraction could be calculated with the ionic equilibria developed in this study. The predicted distribution coefficients of iron agreed well with those measured.
  • Effect of Simultaneous Laser Irradiation on a Cr3C2-Ni-Cr Coating Produced by High-Velocity Oxy-Fuel Spraying Process

    pp. 1864-1868

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    DOI:10.2320/matertrans.45.1864

    The thermal spraying technique is used in many industries, for example, the automobile industry and the steel-making industry. However, the process suffers from several problems, because cohesion between particles in the coating may be incomplete. It is very difficult to anneal the coating by heat treatment, because of deformation of the substrate and the hardness of the coatings is less than that of sintered coatings. An yttrium aluminum garnet (YAG) laser was used during high-velocity oxy-fuel (HVOF) spraying to improve the properties of the applied coating. The spray material was Cr3C2-Ni-Cr, and the substrate was stainless steel (SUS304). The YAG laser was combined with HVOF spraying by irradiating the spraying point on the substrate. The coatings obtained were compared with a coating sprayed by HVOF spraying without the YAG laser. The hardness of coatings sprayed by HVOF with YAG laser treatment was higher, and the weight loss in a blast-erosion test was smaller, than for comparable coatings applied by HVOF spraying alone. The particles deposited in the coatings obtained by HVOF spraying combined with a YAG laser were shown to be very fine. Laser irradiation to the HVOF flame results in strong adhesion between particles, and the deposition of fine carbide particles in the coating.
  • Effect of Substrate Surface Change on Flattening Behaviour of Thermal Sprayed Particles

    pp. 1869-1873

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    DOI:10.2320/matertrans.45.1869

    We have pointed out in our research that in the thermal spraying of practical powder materials, the splat shape change transitionally to a disk shape from a splash shape on a critical substrate temperature range. Substrate temperature increasing may accompany the change in something on the substrate surface, because the changing effect is maintained till the substrate is cooled down to the room temperature. Thus, a certain non-reversible change in substrate surface due to the heating might be the possible domination for the transition phenomenon in the flattening of thermal sprayed particles. In this study, AISI304 steel substrate surface once heated to 673 K was analyzed precisely by atomic force microscope (AFM), X-ray photoelectron spectroscopy (XPS) and auger electron spectroscopy. The results obtained revealed that the change of the substrate surface occurred not in the chemical composition but mainly in the surface roughness, especially surface morphology in nano-meter scale. Substrate heating may bring about the change in the physical way on the substrate surface and this change induces the transition phenomenon.
  • Evaluations of Ceramic Spraying Processes by Numerical Simulation

    pp. 1874-1879

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    DOI:10.2320/matertrans.45.1874

    A plasma spraying process from ceramic particle injection to coating formation was evaluated computationally by integrating particleladen plasma flow, splat formation and coating formation models. Since the plasma flow is unsteady, radial distributions of particle impact temperature and the particle impinging point are obtained. The particle impact temperature is effectively increased by applying an RF electromagnetic field. Furthermore, the coating thickness distribution is strongly influenced by particle size and the RF electromagnetic field.
  • Effect of Plasma Cleaning on Fluxless Plasma Soldering of Pb-free Solder Balls on Si-wafer

    pp. 1880-1885

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    DOI:10.2320/matertrans.45.1880

    An Ar-10 vol%H2 plasma was applied as a cleaning medium on UBMs (Under Bump Metallizations) to improve the bondability of solder balls prior to plasma reflow. Each UBM area comprised four layers deposited on a Si-wafer. Sequentially, the deposit comprised 0.4 μm of Al, 4μm each of Ni and Cu, and 20 nm of Au from bottom to top of the metallization. Two compositions of lead-free solder balls (Sn-3.5 mass%Ag and Sn-3.5 mass%Ag-0.7 mass%Cu) and a lead containing one (Sn-37 mass%Pb) as a reference were selected for the experiment. The solder balls, of 500 μm diameter, were placed on the UBM's and fluxlessly soldered under Ar-10%H2 plasma (with or without prior plasma cleaning). Additionally, air reflow with flux was also performed for comparison. Experimental results showed that the spreading ratios of the solders by plasma reflow after plasma cleaning were 20—40% higher than those plasma reflowed without cleaning. The shear strengths of the solder balls processed by the plasma-cleaned plasma reflow showed around 58—65 MPa, which is 60—80% and 15—35% higher than that of plasma reflow without plasma cleaning, and that of fluxed air reflow, respectively. From this study, plasma cleaning of UBM's using Ar-10%H2 gas was shown to be quite effective to improve the bond strength of solder balls.
  • Forging Condition for Removing Porosities in the Hybrid Casting and Forging Process of 7075 Aluminum Alloy Casting

    pp. 1886-1890

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    DOI:10.2320/matertrans.45.1886

    In this study, the ranges of compression ratio for the successful implementation of hybrid casting and forging process for 7075 aluminum alloy are investigated. Hybrid casting and forging process combines the advantages of casting and forging processes. In this process, a near net shape preform is first cast and the casting is subsequently forged. The casting defects such as shrinkage porosity can be eliminated by forging and the number of forging steps can also be reduced. Therefore, the production cost can be reduced. In this study, castings with stepwise cross section are die cast with 7075 aluminum alloy. The stepwise cross section has three different thicknesses, which are 15 mm, 10 mm and 5 mm. The aim of this study is to obtain how high compression ratio is required to eliminate the porosities inside the preform castings with various thicknesses, where porosity contents are different, and what are the maximum compression ratios that can be tolerated before these preform castings fail. These data are crucial to the design of the preform casting for the hybrid casting and forging process. The experimental results show that for 7075 aluminum alloy, it requires a compression ratio of 30% to eliminate all the porosities in the casting section of 5 mm in thickness and a maximum compression ratio of 44% can be tolerated. For the casting section of 10 mm in thickness, it requires 41% of compression ratio to eliminate all the porosities and a maximum compression ratio of 64% can be tolerated. For the casting section of 15 mm in thickness, it requires 52% of compression ratio to eliminate all the porosities and a maximum compression ratio of 64% can be tolerated.
  • Properties of High Density Magnetic Composite (HDMC) by Warm Compaction Using Die Wall Lubrication

    pp. 1891-1894

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    DOI:10.2320/matertrans.45.1891

    Powder magnetic cores are candidates for a new type of motor core. However, both their magnetic flux and their strength are less than those of laminated steel. The lesser properties are caused by a small green density. In order to increase the green density, the cores need to reduce the admixed lubricant and use high-pressure compaction. We developed a new warm compaction using die wall lubrication with lithium stearate. By using this method, a very high-density compact could be fabricated without scoring. In this instance a starting powder was Somaloy500 without resin, the powder magnetic core compacted at 1176 MPa with heat treatment at 673 K had properties as follows: d = 7.77 Mg/m3, μm = 1000, B2k = 1.41 T, B5k = 1.61 T, B10k = 1.80 T, bHc = 300 A/m, ρ = 4 μΩm, TRS = 210 MPa, σmax = 150 MPa, and in case of Bmax = 1.0 T and f = 400 Hz, Pc = 900 kW/m3, Ph = 350 kW/m3, Pe = 550 kW/m3.
  • Effect of Magnetic Field on Deformation Properties in Ferromagnetic Ni2MnGa Shape Memory Alloy

    pp. 1895-1902

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    DOI:10.2320/matertrans.45.1895

    Ferromagnetic shape memory alloy Ni-25 at%Mn-23 at%Ga prepared by melting in an electric furnace was investigated to study compression properties at various temperatures with and without magnetic field. The magnetic field is applied parallel to the compression axis at a temperature below martensite finish temperature and at a temperature above austenite finish temperature. Triggering stress necessary for rearrangement of martensite variants is reduced by 0.76∼1.68 MPa (i.e., 2.5∼5.5%) when a magnetic field of 0.4∼0.6 T is applied to martensite state. The stress reduction is larger at a higher magnetic field. On the other hand, triggering stress for stress-induced martensite transformation is essentially unchanged under the same magnetic field applied to austenite state. This is due in part to very small changes in Gibbs free energy under magnetic field because of large temperature differences between Ms and Tc. and because of essentially no difference in magnetization between martensite and austenite phases.
  • Electron Beam Processed Silica Glass with Multi-Property

    pp. 1903-1906

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    DOI:10.2320/matertrans.45.1903

    This paper mainly describes that surface density changes in dangling bond and charging and impurity atom adsorption caused the multiproperties of silica glass irradiated by electron beam (EB). The irradiated silica glass with high wettability, misting free, sterilization, and high strength has been successfully obtained. Dangling bond formation and charging generally attract the poling water molecules. Thus, EB irradiation decreased the water contact angle of silica glass. Scattering of light reflection of fine sessile drops of water usually causes the misting. When the water wettability is caused by EB irradiation, the water thin film is formed. Thus, the electron beam irradiation prevents the misting. On the other hand, the strengthening, generated by EB irradiation, was mainly induced by the stress relaxation induced by dangling bond in network structure of silica glass irradiated.
  • Reduction of Iron Oxides by Nano-Sized Graphite Particles Observed in Pre-Oxidized Iron Carbide at Temperatures around 873 K

    pp. 1907-1910

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    DOI:10.2320/matertrans.45.1907

    Iron oxides containing nano-sized graphite particles were prepared from porous iron carbide and reaction characteristics of the oxides/graphite composite mixture were investigated by thermogravimetry in combination with microstructure analysis using X-ray diffractometry and optical microscopy. A drastic mass reduction was observed in a temperature range between 833 K and 1023 K, and the loss of mass reached 2.8%. The mass reduction was found to correspond to a growth of Fe and FeO at the expense of Fe3O4 and Fe2O3 in the composite mixture, suggesting that the oxides were reduced by graphite. The oxides-graphite reaction is presumably indirect reduction caused by CO and H2 that are generated through the water gas reaction, C + H2O → CO + H2.
  • Materialographic Investigation on the Mechanism of Hydrogen Production through the Reaction between Iron Carbide and Steam at a Temperature of 673 K

    pp. 1911-1914

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    DOI:10.2320/matertrans.45.1911

    Materialographic investigation was applied to the microstructure evolution of iron carbide brought by the reaction with steam at 673 K in order to clarify the mechanism of hydrogen production through the iron carbide-steam reaction. Optical and scanning electron microscopy in combination with X-ray diffractometry revealed that a layer of iron oxide, mainly magnetite, forms on the external and internal surfaces of porous particles of iron carbide as a result of the direct oxidation of iron carbide. The oxide phase contains nano-sized graphite particles that are highly crystallized and dispersed uniformly in the oxide layer. The oxidation of iron carbide by steam was confirmed to be responsible for the hydrogen production at 673 K.
  • Separation of Copper and Zinc Ions by Hollow Fiber Supported Liquid Membrane Containing LIX84 and PC-88A

    pp. 1915-1919

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    DOI:10.2320/matertrans.45.1915

    The separation of Cu2+ and Zn2+ in mixed solutions of zinc and copper sulfate has been studied using hollow fiber supported liquid membrane (HFSLM). The extractants for Cu2+ and Zn2+ were LIX84(anti-2-hydroxy-5-nonyl acetophenoneoxime) and PC-88A(2-ethyl-hexylphosphonic acid mono-2 ethylhexyl ester) which were diluted in kerosene, respectively. The effects of operational variables on the permeation rate and the separation factor of Cu2+ and Zn2+ were investigated. At metal concentrations in the feed solution between 1 to 200 mol·m−3 under pH 2.0, the permeation rate of Cu2+ by 10 mass% LIX84 HFSLM was 1 × 10−6∼5 × 10−6 mol·m−2·s−1 while that of Zn2+ with PC-88A was 10−6∼10−5 mol·m−2·s−1. By total recycle operation, copper sulfate solution with 99.6% purity and 99.9% pure zinc sulfate solution could be enriched from the dilute solution in which the concentrations of both Cu2+ and Zn2+ were 1 g/L, respectively.
  • Nanostructure of Protective Rust Layer on Weathering Steel Examined Using Synchrotron Radiation X-rays

    pp. 1920-1924

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    DOI:10.2320/matertrans.45.1920

    The X-ray absorption fine structure (XAFS) spectrum of pure goethite around the Fe K absorption edge and that of the protective rust layer formed on weathering steel exposed for 17 years in an atmospheric environment around the Cr K edge, have been examined using synchrotron radiation X-rays. It was found that the rust layer on the weathering steel mainly consisted of Cr-goethite. By examining the fine structure at the Cr K edge and the Fe K edge, we concluded that Cr3+ in the rust layer is coordinated with O2− and is positioned in the double chains of vacant sites in the network of FeO3(OH)3 octahedra in the goethite crystal. This Cr3+ site indicates that the protective effect of the rust layer is due to the dense aggregation of fine crystals of Cr-goethite with cation selectivity.
  • Characterization of Gallium-induced Intergranular Fracture Surface and the Auger Electron Spectroscopic Analysis for Mg Grain Boundary Segregation in AA6061 T4 Al-Mg-Si Alloy

    pp. 1925-1932

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    DOI:10.2320/matertrans.45.1925

    This investigation examines the grain boundary segregation of Mg and Si in AA6061-T4 alloy, using Auger electron spectroscopy technology. Liquid metal embrittlement by gallium was conducted to fracture the AA6061-T4 alloy intergranularly to obtain compositional information directly from the grain boundary facets. The amount of liquid gallium applied is controlled to break the alloy intergranularly at room temperature, importantly without Ga-bearing particles or a film covering the fracture surface. The method of generating a fully intergranular fracture surface for AA6061 is elucidated. The AES analysis reveals that Mg in 6061 T4 alloy is segregated at grain boundaries, but Si does not. The segregation of Mg depends on the rolling direction. The mean peak-to-peak ratio IMg/IAl of the specimen whose longitudinal axis is perpendicular to the rolling direction is about three times that of the specimen whose axis parallel thereto. The grain boundary segregation is not result oxidation-induced; surface segregation also makes no contribution.
  • Tribological Behavior of Friction Materials Affected by Boehmite Impregnation

    pp. 1933-1938

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    DOI:10.2320/matertrans.45.1933

    Focus of this study has been placed on the effect of boehmite impregnation methods at densification on tribological and mechanical behavior of friction materials. Experimental results indicate that open porosity decreases with the numbers of impregnation cycle. Both bulk density and flexural strength increase with the numbers of impregnation cycle. The friction coefficient and the mass loss of the impregnated specimen is shown more stable and lower than that of the green specimen. Morphological observations show that the number of open pores for these materials decreases but they exhibit a denser and smoother morphology as the number of impregnation cycle increases. Furthermore, as the number of densification cycle increase, the friction behavior of impregnated specimens becomes more sensitive to the presentation of γ-Al2O3 particles. Therefore, the friction coefficient of specimens becomes higher but more unstable; in the mean time, the mass loss becomes larger.
  • Effect of the Intensity and Frequency of Electromagnetic Vibrations on Refinement of Primary Silicon in Al-17%Si Alloy

    pp. 1939-1943

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    DOI:10.2320/matertrans.45.1939

    Electromagnetic vibrations, which are the interaction between a static magnetic field and an alternating electric field, were applied to the structural refinement of Al-17%Si hyper-eutectic alloy. The relationship between the intensity and frequency of the electromagnetic vibrations and the refinement of primary silicon particles is quantitatively investigated. In the frequency range from 10 Hz to 30 kHz, the vibration frequency near 1 kHz was most effective for the refinement of silicon particles, and silicon particles of sizes up to approximately 5 μm were refined at that frequency. The level of refinement increased with increasing the intensity of electromagnetic vibrations. A primary silicon particle was sufficiently refined with a magnetic field intensity of only approximately 1.6 Tesla (T).
  • Effect of the Electromagnetic Vibration Intensity on Microstructural Refinement of Al-7%Si Alloy

    pp. 1944-1948

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    DOI:10.2320/matertrans.45.1944

    The static magnetic field and the alternating electric field were simultaneously imposed on the melt of Al-7%Si hypo-eutectic alloy, and primary α-dendrite particles were refined by electromagnetic vibrations imposed during solidification. The refinement mechanism of primary α-dendrite particles was quantitatively investigated in terms of vibration intensity. When the vibration intensity was increased, coarse equiaxed dendrite particles decreased, and primary α-dendrites approached a globular shape that size was about 25 μm in diameter. By contrast, dendrite arm spacing (DAS) remained intrinsically unchanged even if vibration intensity was changed with electric current intensity. Refinement of primary α-dendrite particles was likely to be caused by collapse of dendrite arms due to the micro-explosion and stirring of the melt.
  • Pasty Ranges and Latent Heat Release Modes for Sn-9Zn-xAg Lead-free Solder Alloys

    pp. 1949-1957

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    DOI:10.2320/matertrans.45.1949

    The pasty ranges and latent heat release modes of Sn-9Zn-xAg alloys, where x varies between 0.5 and 3.5, are examined in this study. The effects of alloy composition and cooling rate on the pasty range and latent heat release modes are also investigated. A Computer Aided-Cooling Curve Analysis (CA-CCA) technique is used to determine the pasty ranges and latent heat release modes for the alloys. To comply with the requirements of CA-CCA, the heat transfer conditions of the experimental setup must closely resemble of a lumped system, in which the alloy is cooled in a uniform fashion. To confirm this, two thermocouples are inserted into the crucible where the alloy is melted and subsequently resolidified. The crucible is used with and without a covering of insulating material to obtain different cooling rates. The temperature readings show that the heat transfer conditions in the experimental setup indeed meet the requirements of a lumped system. The solid fraction versus temperature relationship (fs-T) for the various alloy compositions and cooling rates are obtained from the temperature data through CA-CCA. The experimental results show that as the silver content of the Sn-9Zn-xAg alloy increases, the liquidus temperature rises and the pasty range broadens. The pasty range is approximately 2°C for 0.5Ag, 9°C for 1.5Ag, 14°C for 2.5Ag, and 18°C for 3.5Ag. As long as the silver content is below 0.5 mass%, silver has little effect on the microstructure, which is basically the eutectic Sn-9Zn and the fs-T relationship is nearly a vertical line. However, as the silver content exceeds 1.5 mass%, the formation of Ag-Zn intermetallic compounds becomes obvious. This causes the alloy composition to deviate from eutectic and to lean towards the tin side of the Sn-Zn phase diagram. This in turn causes the proportion of the primary tin phase to increase and that of the zinc-tin eutectic phase to decrease. This is reflected on the plot of the fs-T relationship by two distinct vertical regions. One corresponds to the primary tin phase and the other to the eutectic phase. As the silver content further increases, the effects of intermetallic compound formation become even more obvious. As an alternative to CA-CCA, Huang's model can be used to obtain a quantitative fs-T function. As the silver content increases, the primary solid fraction for Huang's model increases. As the cooling rate increases, the primary solid fraction and the nonlinearity factors ne and np decrease.
  • New Cu-Zr-Al-Nb Bulk Glassy Alloys with High Corrosion Resistance

    pp. 1958-1961

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    DOI:10.2320/matertrans.45.1958

    New Cu-based bulk glassy alloys with good mechanical properties were formed in Cu-Zr-Al-(Nb) system by copper mold casting. They exhibit a large supercooled liquid region (ΔTx) of 50 K-74 K and high glass-forming ability with maximum diameters of 2.0 mm-3.0 mm. The addition of Nb to the glassy alloys is effective for improving the corrosion resistance in all the solutions. The corrosion rate of the 5 at% Nb alloy decreases by one order of magnitude compared to those of the 0 at% Nb alloys in 3 mass% NaCl solution. In 0.5 mol·dm−3 H2SO4 solution, the Cu-Zr-Al-(Nb) alloys are passivated in wide passive region and low passive current density of the order of 10−2 A·m−2, indicating high corrosion resistance in this solution.

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