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

MATERIALS TRANSACTIONS Vol. 54 (2013), No. 10

  • Mechanical Properties and Phases Derived from TiO2 Nanopowder Inoculation in Low Carbon Steel Matrix

    pp. 1867-1876

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    DOI:10.2320/matertrans.M2013073

    The effect of TiO2 nanoparticle addition on mechanical properties of low carbon steel and the phases originated from this inoculation were investigated. Equilibrium phases were estimated by means of thermodynamic modeling and the results were compared with further microscopy characterization. Both techniques confirmed the dissolution of TiO2 nanoparticles in the molten steel which derived in TiN and Ti4C2S2 nanometric reaction products. The formation of these nanometric phases, were found to result in ferrite grain refinement and consequently in the enhancement of mechanical properties. In addition, the formation of these phases led to C and N depletion from the iron matrix and to a continuous yielding behavior in tensile stress–strain curves.
  • Characterization of Velocity Profile of Highly-Filled GFRP-BMC through Rectangular Duct-Shaped Specimen during Injection Molding from SEM Fiber Orientation Mapping

    pp. 1877-1883

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    DOI:10.2320/matertrans.M2013143

    There is an extensive body of research and texts on numerical simulation to obtain rheological properties of injection molded resins and composites, however to our knowledge there is no or little research on estimating velocity profile from fiber orientation mapping of GFRP-BMCs (glass fiber reinforced polymer bulk molding compounds). This study reports a simple analysis of specific laminar creep flow properties of highly-filled short-fiber GFRP-BMC through a dogbone specimen as a rectangular duct during injection molding from SEM fiber orientation mapping which is found to exhibit a 3-layer [skin-core-skin] structure resembling classical laminar flow through a conduit. Therefore, to characterize the BMC with extremely low Reynolds number ∼2 × 10−4, velocity profile, primary and secondary boundary layers were estimated from Hermans fiber orientation parameter across gauge section thickness.
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    Readers Who Read This Article Also Read

    1. Mechanical Properties and Phases Derived from TiO2 Nanopowder Inoculation in Low Carbon Steel Matrix MATERIALS TRANSACTIONS Vol.54(2013), No.10
    2. PREFACE MATERIALS TRANSACTIONS Vol.55(2014), No.8
    3. Explicit Distinctions between 2D MPF Grain Growth Simulations and EBSD Analyses to Determine Driving Mechanism of Grain Growth MATERIALS TRANSACTIONS Vol.54(2013), No.10
  • Explicit Distinctions between 2D MPF Grain Growth Simulations and EBSD Analyses to Determine Driving Mechanism of Grain Growth

    pp. 1884-1893

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    DOI:10.2320/matertrans.M2013132

    Grain growth, the most fundamental mechanism forming the internal structure in metallic materials, is affected by not only grain boundary (GB) energy but also other driving forces such as internal stress fields of defects. In order to determine the explicit distinctions between the simulated evolutions and the real metallographic observations for discussing the dominant driving force, simulations of polycrystalline grain growth were performed using our proposed multi-phase-field model [T. Hirouchi, T. Tsuru and Y. Shibutani: Compt. Mater. Sci. 53 (2012) 474–482.], which was combined with pure Al and Cu < 110> tilt misorientation and inclination dependences of GB energy based on the molecular dynamics results. The real microstructures in as-annealed Al specimens exhibiting various average grain diameters and in oxygen-free Cu (OFC) specimens after several annealing treatments were characterized using scanning electron microscopy/electron backscatter diffraction (SEM/EBSD). Few low-angle GBs within misorientation angle of 2° were expected to exist in the real microstructures despite the low energies of such GBs in Al, and some Σ3 GBs migrated in the direction opposite to that which decreased their curvature in OFC. These behaviors cannot be explained by only a grain growth mechanism in which total GB energy in the system is reduced as the driving force in the simulations. Models incorporating contribution from the long-range internal stress field due to the defects of GBs and the piled-up dislocations is crucial.
  • Effect of Shot Peening on Surface Characteristics of Ni-Based Single-Crystal Superalloy

    pp. 1894-1897

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    DOI:10.2320/matertrans.M2013153

    The effect of shot peening on surface characteristics of DD3 Ni-based single-crystal superalloy including microstructures, texture and residual stress were investigated, utilizing XRD analysis. Results showed that the polycrystals was introduced into the surface of single-crystal specimen by shot peeing and the initial texture (200) was erased after 60 s-processing. The variation in microstructure was mainly influenced by the processing time and finer domains as well higher microstrain were obtained after 60 s-processing. The value of residual stress depended upon processing time and measurement direction. In the early period of processing (∼10 s), residual stress was anisotropy, being significant smaller in <110> direction. With the processing time increased, the anisotropic residual stress gradually changed to isotropic residual stress, due to domain-orientation randomization, domain refinement and increase in plastic strain. Also increasing processing time could significantly enhance the magnitude of residual stress and the microhardness. In addition, the influence of processing time on work hardening and residual stress was discussed, based on the deformation mechanism.
  • Rapid Age-Hardening Behavior of Al–Mg–Cu (–Ag) Alloys and Incubation Stage in the Low-Temperature Aging

    pp. 1898-1904

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    DOI:10.2320/matertrans.MAW201315

    The characteristic age-hardening response of Al–3.0Mg–1.0Cu (mass%) alloys with and without Ag addition has been investigated by the hardness measurement, differential scanning calorimetry (DSC) and electrical resistivity measurement. The alloy compositions locating in the (α+S+T) phase field of the Al–Mg–Cu phase diagram are known to be effective to harden in two stages separated by a distinct and often prolonged hardness plateau. The first stage of hardening occurs very rapidly (e.g., within 60 s at 443 K) and contributes to increase hardness as much as 50% of the total age-hardening. After the first stage of hardening, a clear plateau stage is observed in the hardness curve. In the low temperature aging, the characteristic incubation stage at which no clear hardness increase and electrical resistivity increase appears for a long time before the first stage of hardening. The origin of the both incubation and plateau stages is discussed based on the nanocluster formation and vacancies. The influence of the Ag addition on the precipitation behavior is also discussed in terms of the atom-atom interaction.
  • Computer Simulation of Precipitation Process in Si/Ge Amorphous Multi-Layer Films: Effects of Cu Addition

    pp. 1905-1909

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    DOI:10.2320/matertrans.M2013191

    We have simulated the precipitation process in an amorphous Si/Ge multi-layer film, with and without Cu addition, by a molecular dynamics method. Four specimens were prepared for this study: Si/Ge layers, Si/(Ge + Cu) layers, (Si + Cu)/(Ge + Cu) layers and Si/Cu/Ge/Cu layers. After the multi-layered films became amorphous, we tracked the movement of individual atoms at 1000 K, the annealing temperature. When Cu was present in the Ge layer or both the Si and Ge layers, the precipitation of nano-clusters was less than that in Cu-free Si/Ge layers. We think that the Cu atoms block the precipitation and make the Si and Ge become more stable in the amorphous state. If Cu atoms are note present in a layer, however, like the Si layer in Si/(Ge + Cu) and Si/Cu/Ge/Cu specimens, the precipitation of nano-clusters in the Cu-free layer is enhanced. Therefore we conclude that precipitation of nano-clusters in Si/Ge layers can be controlled by how Cu atoms are added to the amorphous Si/Ge system, and that this will improve the thermoelectric performance.
  • Mechanism of Pipe End Deformation after Cutting of Square Steel Pipe Formed by Roll Forming

    pp. 1910-1915

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    DOI:10.2320/matertrans.P-M2013822

    Square steel pipes are reshaped from welded round pipes by roll-forming. Pipe end deformation after cutting of the square steel pipe was investigated by experimentation and by three-dimensional finite element simulation. Each direction of the residual stresses composed of longitudinal stress and longitudinal-peripheral shear stress becomes reverse with respect to the outer and inner surfaces. A large opening deformation arises at the back end of the product by the release of those residual stresses. The side part receives bending deformation in the surrounding area of the contact zone with a roll. The residual stresses comprising longitudinal stress and longitudinal-peripheral shear stress increase as a result of the reverse bending deformation downstream of the roll center position. The absolute value of residual stress decreases with decreased initial wall thickness.
  • Estimation of Fracture Toughness of SiC Fiber and Statistical Analysis of Change in Fracture Strength Distribution with Notch Size

    pp. 1916-1924

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    DOI:10.2320/matertrans.MAW201302

    Distribution of fracture toughness value of amorphous SiC fiber (Tyranno ZMI, Ube Industries, Ltd.) and statistical feature of distribution of fracture strength at various notch sizes were studied analytically. The fracture toughness values, estimated for the fiber specimens with mode I type straight-fronted edge notch introduced with focused-ion-beam, were almost independent of the fiber diameter and notch depth. The distribution of the fracture toughness was described by the three-parameter Weibull distribution function. The distributions of fiber diameter and original fracture strength were described by a normal distribution function and fiber diameter-incorporated three-parameter Weibull distribution function, respectively. These distribution functions and a Monte Carlo method were used to simulate the change in fracture strength distribution with notch depth. With this simulation, the experimental results were well described. Also the statistical features in notch size-dependence of distribution of fracture strength, arising from the decrease in fraction of intrinsic-defect fractured fiber specimens to all specimens and hence the increase in the fraction of notch-fractured specimens with increasing notch size, were elucidated.
  • Chemical Thermodynamic Processes at Metal-Mold Interface

    pp. 1925-1929

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    DOI:10.2320/matertrans.M2013190

    At the contact surface between the liquid metal and the mold wall a number of chemical and thermodynamical reactions were taken. The products of those reactions are spread out on both the sides, i.e., in the solidified metal or into the mold wall. The processes which took place in the casting mold — here made from sand mix, during the pouring of liquid metal, solidification and cooling down to room temperature, are pretty complex.
    The thermodynamic reactions at the contact surface of liquid metal/sand mold in the process of castings forming are discussed in this paper. Experiments were performed in molds made by CO2 sand casting process. As a pouring metal we used steels from an ordinary production program of a steel foundry, exactly it was the steel GS 50CrMo4, assigned by DIN.
    Some results obtained in our investigations during studying those processes are shown here.
  • Solidification Structure and Casting Defects in High-Speed Twin-Roll Cast Al–2 mass% Si Alloy Strip

    pp. 1930-1937

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    DOI:10.2320/matertrans.L-M2013824

    Al–2 mass% Si alloy, which has wide freezing temperature range, was cast using a laboratory-scale vertical-type high-speed twin-roll caster. The solidification structure and several kinds of casting defects were examined using OM, SEM and SEM-EDS. The results showed that the unstable melt pool condition could cause the surface defects such as the ripple mark, “un-shiny” zone and the inverse segregation. In the present nozzle type, constructing a stable high melt pool level was seemed to be essential to obtain a sound strip with no defect on the strip surface. A large-scale internal cracking along the casting direction was also observed in the high-speed twin-roll casting of aluminum alloy. The present results revealed that the cracking was related to distribution of the residual liquid in the central band region, which could be controlled by the roll separating force near the roll nip. It is considered that the roll separating force caused the shear localization in the central band region and promoted a formation of continuous liquid film in the shear localized region. The liquid film was a cause of the internal cracking when the strip passed through the roll nip.
  • Surface Hardening of Ferritic Flake Graphite Cast Iron by Friction Stir Processing

    pp. 1938-1943

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    DOI:10.2320/matertrans.F-M2013818

    Ferritic flake graphite cast iron, which is less commonly used due to its relatively low strength, was friction stir processed (FSP) for surface hardening in this study. We clarified that a Vickers hardness of above 700 HV can be obtained even for the ferritic matrix due to the formation of the fine martensite structure which can be seen in an ordinary quenched cast iron. Additionally, the effects of the graphite shape and microstructure of the matrix on the FSP conditions were investigated.
  • Effects of Solidification Behavior during Filling on Surface Defects of Aluminum Alloy Die Casting

    pp. 1944-1950

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    DOI:10.2320/matertrans.F-M2013819

    Die castings are prone to contain considerable porosities due to the entrapment of air or gases in the molten metal during mold filling. Reducing the die filling velocity is effective for reducing the entrapment, but it increases surface defects, such as surface folds and cold shut on die castings.
    In this research, the solidification behavior of molten metal during mold filling was investigated by developing a highly sensitive thermosensor with a response time of 0.015 s that can correctly measure the temperature of flowing molten metal. The criterion for the formation of surface defects was further examined based on the solidification behavior of molten metal during mold filling.
    It was found that the type of surface defects varies with the solidification manner of aluminum alloys. Surface fold defects occur on die castings made of JIS AD12.1 alloy with skin-formation type solidification. The occurrence of surface folds can be predicted by the thickness of the solidified layer of the molten metal from the surface of cavity. The critical thickness for the formation of surface folds increases with increasing casting pressure. On the other hand, cold shut defects occur on die castings made of JIS AC4C alloy with mushy-formation type solidification. The molten metal temperature drops toward the tip of the molten metal flow. The occurrence of cold shut defects can be estimated by the temperature of this molten metal flow tip at the time this flow converges with other flows.
  • Development of Load Reduction Technology by Swing-Type Forging and Lubrication for Large-Deformation Forging by High-Speed Large-Reduction Forging — Production Technology for Fine Grained Steel by Large Deformation Forging II —

    pp. 1951-1956

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    DOI:10.2320/matertrans.P-M2013823

    To reduce the forging load of the proposed large deformation forging technology, a new type of forging system is proposed. In this forging system, the contact length between forging dies and a work can be reduced by adjusting the swing motion of the dies during forging. This swing motion results in load reduction because contact length is a dominant factor of the load for this type of forging. Also, this forging technology is relatively stable when using lubrication because of its basic up-and-down motion. In this study, load reduction effect with the newly proposed forging system and lubrication are examined by laboratory-scale experiments and FE analysis. It appears that a nearly 30% load reduction can be achieved by the swing-type forging compared with the flying-type forging under the condition of a large thickness reduction. Width spread is also reduced by the swing type forging because contact length has a large influence on a material’s lateral flow. Moreover, another 20 to 30% load reduction is confirmed and seizure on the forging dies was prevented by lubrication.
  • Epitaxial Growth of Chromium Oxynitride Thin Films on Magnesium Oxide (100) Substrates and Their Oxidation Behavior

    pp. 1957-1961

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    DOI:10.2320/matertrans.MAW201310

    Epitaxially grown Cr(N,O) thin films were prepared on MgO substrates, with a misfit of −1.7% with respect to CrN, using pulsed laser deposition. X-ray diffraction patterns showed the peak for the (200) reflection of Cr(N,O) around the peak for the (200) reflection of MgO. The X-ray diffraction pattern of the ϕ scan for the (111) reflection of Cr(N,O) showed a narrow peak appearing every 90 degrees. From microstructural observations, grain boundaries in the thin films could not be confirmed. In order to evaluate the oxidation behavior of Cr(N,O) thin films, oxidation tests in air were carried out. After the oxidation tests, a Cr2O3 phase was formed at 873 K and the B1 (NaCl-type) phase disappeared at 1173 K. In the thin film oxidized at 1073 K, a Cr2O3 layer on the surface of the thin film as well as a compositional gradient of oxygen were observed. This indicates that Cr(N,O) hard coatings can form oxidation barrier layers to extend the lifetime of cutting tools.
  • Comparison Studies of the Flow Characteristics of the Newtonian and Thixotropic Fluids

    pp. 1962-1967

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    DOI:10.2320/matertrans.M2013157

    Comparison studies of the effects that the viscosity and the injection speed of the fluids have on the die filling behaviors have been performed via an approach that involves both experiments and simulations. Two different fluids, i.e., thixotropic fluid (paint) and Newtonian fluid (water), were selected as the model fluids to monitor the differences in the rheological behaviors and the associated flow patterns during die filling. According to high-speed photography recorded for the two fluids, in comparison to the Newtonian fluid, the thixotropic fluid flow with higher viscosity proceeded into the die cavity in a more controllable manner while maintaining its free surface. Such experimental observations were then compared with the analytical results obtained from computer simulations.
  • Surface Tensions of Fe–(30–40 mol%) Si–C Alloys at 1523–1723 K

    pp. 1968-1974

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    DOI:10.2320/matertrans.M2013211

    To analyze the Marangoni effect on fluid flow in an Fe–Si solution during solution growth of SiC, knowledge of the temperature coefficient of surface tension of the solution is essential. In this investigation, the surface tensions of Fe–(30–40 mol%) Si alloys and alloys with added carbon were measured at 1523–1723 K by the maximum bubble pressure method. Surface tensions and temperature coefficients were precisely determined. The measured surface tensions were also compared with estimations based on the modified Butler’s model, and the effect of carbon on the surface tension of Fe–Si–C alloys was assessed.
  • Effect of Surface Modification by Aqueous NaOH Solution on Bond Strength of Solid-State Bonded Interface of Al

    pp. 1975-1980

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    DOI:10.2320/matertrans.M2013101

    Aluminum alloys have high strength and are easily recyclable owing to their low melting point. Therefore, aluminum is widely used for manufacturing cars and electronic devices. Currently, the most common method for bonding aluminum surfaces is brazing. However, brazing requires positional accuracy and results in the formation of voids by the flux residue; therefore, to avoid these problems, solid-state bonding methods are considered as a possible alternative. However, solid-state bonding also suffers from some problems that need to be overcome. One of these problems is the presence of an oxide film on aluminum surfaces, necessitating the need to remove or destroy the oxide film without applying high temperature and high load. Hence, in this study, a bonding surface was treated with NaOH (aq) for removing the oxide film; moreover, the effectiveness of this treatment was determined by observing the bonding interfaces and fractured surfaces of specimens. For solid-state bonding, the specimens were first subjected to surface modification by boiling the specimens in NaOH (aq) for 20 s. Solid-state bonding was then carried out at bonding temperatures ranging from 693 to 813 K in a vacuum chamber with the bonding pressure and bonding time fixed at 6 MPa and 1.8 ks, respectively. After solid-state bonding, the specimens were subjected to the tensile test for evaluating their strength. The results revealed that the tensile strength of a joint increased with the bonding temperature with or without surface modification. Moreover, surface modification resulted in high-strength joints with less deformation at low bonding temperatures. In addition, the bonding temperature and degree of deformation needed to achieve joints with strength higher than the 0.2% proof stress of aluminum were 100 K and 25% lesser, respectively, in the case of surface modification. On the basis of the experimental results, it was established that surface modification is effective for removing oxide films and processing layers on an aluminum surface.
  • Improvement in Thermoelectric Properties of Non-Stoichiometric Titanium Dioxide by Reduction Treatment

    pp. 1981-1985

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    DOI:10.2320/matertrans.E-M2013820

    Compacts of non-stoichiometric TiO2−x were fabricated by SPS and the following reduction treatment in carbon powder. XRD, SEM and EPMA were used to characterize the compacts. The results showed that they were composed of non-stoichiometric TiO2−x inside and TiO as the surface layer. The thickness of the surface layer was increased with the increase of the holding time and exceeded 100 µm when it came to 20 h. Carbon did not penetrate into the compacts during the reduction treatment. To examine the carrier density and the non-stoichiometric number, 2 − x, thermogravimetry was carried out. The study showed that the value of 2 − x was decreased and the carrier density was increased as the holding time increased. The thermoelectric properties including electrical resistivity, Seebeck coefficient and power factor was measured. The electric resistivity was considerably reduced by the reduction treatment. The values of the Seebeck coefficient was also decreased by the reduction treatment but maintained at a large absolute value of 250 µV·K−1 at 573 K. Power factor reached 2 × 10−4 W·m−1·K−2 at 573 K. The value was much higher than that of the samples by SPS only. Therefore, the thermoelectric performance was improved by the reduction treatment. The influence of the TiO layer on the thermoelectric performance was also examined. It was believed that the influence should result from its comprehensive effect on Seebeck coefficient and electrical resistivity.
  • Structural and Thermoelectric Properties of Rare-Earth-Substituted Sr3Fe2O7

    pp. 1986-1992

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    DOI:10.2320/matertrans.E-M2013821

    The structural and thermoelectric properties of rare-earth (Sm)-substituted Ruddlesden–Popper-type Sr3Fe2O7, Sr3−xSmxFe2O7 (SSFO, x = 0, 0.1, 0.15), were investigated by measuring the temperature dependence of its crystallographic parameters, electrical conductivity, and Seebeck coefficient (S). A sharp increase in electrical conductivity (σ) up to 60 S/cm (log σ = 1.78 S/cm) at 400–500°C occurred as a result of oxygen reduction in the x = 0 sample. S values changed from negative to positive with increasing temperature, presumably due to the increase in the number of oxygen vacancies, resulting in Fe ion hopping conduction. The introduction of Sm increased both S and σ, and the power factor of the oxide prepared with x = 0.1 was approximately fivefold higher than that of the oxide with x = 0.
  • Effects of Electrolyte Species and Their Combination on Film Structures and Dielectric Properties of Crystalline Anodic Alumina Films Formed by Two-Step Anodization

    pp. 1993-1999

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    DOI:10.2320/matertrans.L-M2013826

    The effects of electrolyte species and their combination during the two-step anodization of anodic oxide films formed in various ammonium salts solutions on aluminum in terms of the crystallinity and dielectric properties for use as an electrolytic capacitor were investigated. The aluminum substrates were pretreated by dipping in boiling water to form a hydrated layer. Anodic films were annealed at 500°C after the first anodization. The CV (capacitance C multiplied by withstand voltage V) of the films formed with a second anodization in ammonium borate was higher than that of the films formed using the same electrolyte for the first and second anodizations. The increase in CV due to the improved crystallinity when using borate for the second anodization and the following electrolytes for the first anodization increased in the order salicylate < succinate < tartrate < phosphate < citrate < adipate < borate. When phosphate or organic electrolytes were used in the second anodization, considerable spike noise was observed. However, the spike noise was significantly suppressed when borate was used in the second anodization.
  • Improvement of Tensile and Fatigue Properties of β-Titanium Alloy while Maintaining Low Young’s Modulus through Grain Refinement and Oxygen Addition

    pp. 2000-2006

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    DOI:10.2320/matertrans.M2013151

    To investigate methods of improving the tensile and fatigue properties of a solutionized Ti–29Nb–13Ta–4.6Zr (TNTZ) alloy without increasing its Young’ modulus, two types of TNTZ alloys having oxygen contents of 0.06 and 0.14 mass% (TNTZ–0.06O and TNTZ–0.14O), respectively, were subjected to cold swaging and a subsequent heat-treatment. The effects of the grain refinement caused by the cold swaging and the subsequent heat-treatment as well as those of oxygen addition on the microstructures, Young’s moduli and tensile and fatigue properties of the two alloys were investigated.
    The grain diameters of the TNTZ–0.06O and TNTZ–0.14O decrease from 27 µm (as-received) to 1.7 µm and from 33 µm (as-received) to 1.0 µm, respectively, after subjected to cold swaging and the subsequent heat-treatment. These results suggest that cold swaging, followed by heat treatment, is effective in refining the grains of TNTZ alloys. However, a β (110) texture develops in the alloys as a result of the cold swaging.
    Young’s moduli of as-cold swaged and heat-treated TNTZ–0.06O and TNTZ–0.14O are within the range of 61–68 GPa and as low as those of solutionized TNTZ–0.06O and TNTZ–0.14O.
    The tensile strengths and elongations of the as-cold swaged, heat-treated and solutionized TNTZ–0.14O are approximately 30% higher and 20% lower, respectively, than those of the corresponding TNTZ–0.06O. Moreover, the 0.2% proof stresses of the heat-treated TNTZ–0.14O are approximately 110% higher than that of the corresponding TNTZ–0.06O. On the other hand, the values of the Hall-Petch constant (k) for the TNTZ–0.06O (kTNTZ–0.06O = 0.02) and TNTZ–0.14O (kTNTZ–0.14O = 0.005) are much smaller than those for pure Ti and another β-type Ti alloy (Ti–15.2Mo). These results indicate that the addition of oxygen can improve the tensile properties of TNTZ alloys. However, the grain refinement caused by cold swaging and a subsequent heat-treatment does not have a significant effect on the tensile properties of TNTZ.
    The fatigue limit of the heat-treated TNTZ–0.14O (540 MPa) is much higher than those of the heat-treated and solutionized TNTZ–0.06O (290 and 230 MPa) and solutionized TNTZ–0.14O (330 MPa). These results indicated that it is possible to improve the fatigue properties of solutionized TNTZ using grain refinement, which can be induced by cold swaging and a subsequent heat-treatment and by the addition of oxygen.
  • Influence of Heat Treatment on the Microstructure and Magnetic Properties of Mn–Sn–Co–N Alloys

    pp. 2007-2010

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    DOI:10.2320/matertrans.MAW201312

    This paper describes the influence of heat treatment on the microstructure and magnetic properties of Mn–Sn–Co–N alloys. After homogenization at 950°C for 20 h in Ar, Mn82.5Sn10Co7.5 alloys were annealed at 900°C for 1–20 h in N2 and quenched to room temperature (first-annealing). The alloys were subsequently annealed at 400–700°C for 5 h in N2 and quenched to room temperature (second-annealing). The coercivity of the alloys changed drastically by the second annealing. The samples subjected to second annealing at 500°C showed maximum coercivity of 1270 kAm−1 and exhibited a fine two-phase microstructure. Wavelength Dispersive X-ray Spectrometer (WDS) and X-ray Diffraction (XRD) analyses revealed that the fine two-phase microstructure consists of β-Mn and perovskite-type (Mn4N-like) phases, as well as that the two phases emerged from the ζ′ phase during the second-annealing at 500°C.
  • Wear of Cemented Carbide Dies for Steel Cord Wire Drawing

    pp. 2011-2017

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    DOI:10.2320/matertrans.M2013185

    Cemented carbide dies with five categories of composition, namely, WC–Co straight alloys, TaNbC-, Cr3C2- or VC-containing WC–Co alloys and WC–Ni alloys, were fabricated by sintering and HIP treatment. The wear behavior of these cemented carbide dies for drawing steel cord wire is examined, and material properties such as hardness, transverse-rupture strength and corrosion resistance are evaluated. It is found that the TaNbC-containing alloy shows the longest lifetime as a steel cord drawing die, followed by the Cr3C2-containing alloy, WC–Co, VC-containing alloy and WC–Ni alloy. The strength and corrosion resistance show no relation with the drawing die life. The hardness has some influence on the die life; for example, within the same alloy category, there is a tendency that the longer die life is obtained for the material with the higher hardness. However, it is not possible to explain simply from the viewpoint of hardness even why the TaNbC-containing alloy has the longest life though the VC-containing alloy has the finest grain size and highest hardness. The wear mechanism of cemented carbide dies for steel cord wire drawing was discussed, focusing on WC/Co interface adhesion.
  • Microstructure Characterization of Oxide Dispersion Strengthened Steels Containing Metallic Chromium Inclusions after High-Temperature Thermal Aging

    pp. 2018-2026

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    DOI:10.2320/matertrans.M2013217

    Microstructure characterizations of 9Cr-oxide dispersion strengthened (ODS) steels were carried out after high-temperature thermal aging to reproduce the anomalous microstructure change that occurred in the BOR-60 irradiation test — formation of abnormally coarse and irregular precipitates a few tens of micrometers in size. The 750°C thermal aging for 8,000 h produced the precipitate growth in normalized-and-tempered (NTed) 9Cr-ODS steels, however, the largest precipitates were only a few micrometers. The furnace-cooling (FC) heat treatment produced the matrix structure change from tempered martensite to α-ferrite, but the precipitate size after the thermal aging was in the same level as that in the NTed 9Cr-ODS steel. In the case of NTed defective 9Cr-ODS steel containing metallic Cr inclusions, coarse and irregular precipitates were formed nearby metallic Cr inclusions after the 750°C thermal aging for 8,000 h. In the case of FCed defective 9Cr-ODS steel containing metallic Cr inclusions, the 750°C thermal aging for 8,000 h produced the pronounced growth of coarse and irregular precipitates, which was equivalent to the anomalous microstructure change in the BOR-60 irradiation test. Based on the analyses using energy dispersive X-ray spectrometry (EDX) and electron backscattered pattern (EBSP), coarse and irregular precipitates were identified as M23C6.
  • Mn(II)-Oxidizing Activity of Pseudomonas sp. Strain MM1 is Involved in the Formation of Massive Mn Sediments around Sambe Hot Springs in Japan

    pp. 2027-2031

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    DOI:10.2320/matertrans.M-M2013825

    There has been an increasing interest in the application of Mn(II) oxidizing microbes for Mn(II) containing waste water treatment, as well as biogenic Mn oxides for remediation of a number of toxic metals. Sambe hot spring in Shimane Prefecture, Japan is known for its Mn-rich spring water, and Mn oxide sediments are widely distributed over an area of 60 m × 80 m of this region. With an aim to find out Mn(II) oxidizing bacteria involved in this massive Mn(II) oxide deposit formation, culture enrichment was undertaken. Pseudomonas sp. strain MM1 was isolated as dominant Mn(II) oxidizing bacterium from the site. Biogenic Mn oxide formed by strain MM1 was shown to be poorly-crystalline birnessite, having the large specific surface area of 90 m2·g−1. Mn(II) was readily oxidized by strain MM1 during the stationary phase and initial Mn(II) concentrations of up to 1.0 mmol·dm−3 (55 mg·dm−3) went well below the effluent standard of dissolved manganese (10 mg·dm−3; set by Japanese Ministry of Environment). Strain MM1 grew heterotrophically and Mn(II) did not serve as the sole electron donor. Nano-sized (30–50 nm) Mn oxide particles were produced by strain MM1, which later chained or aggregated to form larger Mn oxide minerals. Cells were observed eventually encrusted inside the Mn oxides. This study showed the important role of strain MM1 in the production of massive Mn oxide deposits at Sambe hot spring. The Mn(II) oxidizing ability of this strain is potentially applied for bioremediation of toxic metals.
  • Investigation of Microstructure and Mechanical Properties on Surface-Modified Inconel 718 Alloy

    pp. 2032-2036

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    DOI:10.2320/matertrans.M2013096

    This study was carried out to evaluate the development of the microstructure and mechanical properties of a surface-modified post-heat-treated Inconel 718 alloy. The friction stir process (FSP), as the surface-modification method, was applied to an overlap-welded Inconel 718 alloy. The FSP was conducted at a tool rotation speed of 200 rpm and a tool downforce of 39.2 kN; post-heat treatment was carried out in two steps: (i) at 720°C for 8 h and (ii) 620°C for 6 h in vacuum. FSP was effective for developing grain refinement, accompanied by dynamic recrystallization, thus improving the mechanical properties compared to the original overlap-welded material. Furthermore, the post-heat treatment after FSP accelerated the formation of precipitates such as gamma prime (γ′) and metal–carbon (MC) carbides, which led to a significant improvement in mechanical properties. In this study, the development of the microstructure and the improvement of the mechanical properties through the application of FSP and post-heat treatment are systematically discussed.
  • High-Cycle Fatigue Properties of Automobile Cold-Rolled Steel Sheet with Stress Variation

    pp. 2037-2043

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    DOI:10.2320/matertrans.M2013208

    Since many components in the automotive body and chassis are produced by various manufacturing processes, have different chemical compositions, and are subjected to complex loading cycles, it is important to understand their loading mechanisms and susceptibility to damage. This research examined the mechanical properties of cold-rolled steel sheets and evaluated the effects of stress variations on fatigue behavior. Specifically, a series of load-controlled high-cycle fatigue tests were conducted by varying the stress levels of SPCC and SPRC340 sheet materials. The results showed that fatigue life and the fatigue limit increased with higher tensile and yield strengths. In addition, testing results indicated that the fatigue limit was higher than the monotonic yield strength due to cyclic hardening with plastic deformation during fatigue cycling. Regarding tensile properties upon pre-deformation, the yield strength increased with a higher amount of pre-deformation and was greater than the fatigue limit after deformation. Based on these experimental results, two types of fracture modes were observed under the applied stress range. General fatigue fracture mode, which denotes failure by crack initiation, propagation and final rupture at low stress amplitude, was observed with fatigue lives larger than 4 × 105 cycles. On the other hand, constrained fracture mode occurred at stress levels higher than 0.89 times the tensile strength and exhibited a fracture surface without fatigue crack initiation or propagation.
  • Two- and Three-Dimensional Grain Growth in the Cu–Al–Mn Shape Memory Alloy

    pp. 2044-2048

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    DOI:10.2320/matertrans.M2013167

    The normal grain growth behaviors at 750, 800 and 900°C in block and sheet samples of Cu–Al–Mn shape memory alloy with the bcc single-phase structure were investigated. The grain growth exponents, n, evaluated in the two- and three-dimensional grain growths were found to be larger than those in most other metals and were located between 10 and 18. Furthermore, the two-dimensional growth in thin sheet samples was much slower than the three-dimensional growth in block samples due to the grooves formed along grain boundaries on the specimen surface, these grooves constituting a hindrance to the movement.
  • Characterization of the MC Carbides and Mechanical Properties in a TiC Particles-Strengthened Cobalt-Based Alloy through HIP, Solid-Solution and Aging Treatments

    pp. 2049-2054

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    DOI:10.2320/matertrans.M2013210

    In previous studies, we mixed and added different amounts of TiC powders (0, 10, 15 and 20 mass%) to the cobalt-based alloy powders. The results showed that adding 15 mass% TiC powder to the cobalt-based alloy and sintering at 1280°C for 1 h was the optimal process. The aim of this paper is continue exploring a series of HIPing processes and various heat treatments for T0 and T15 composite materials, and examining the effects on the microstructure and mechanical properties of TiC particles-strengthened cobalt-based alloy. The experimental results showed that the highest TRS value of 1566.7 MPa was obtained by the 15 mass% TiC additive powders, which was sintered at 1280°C for 1 h, followed by HIP plus solid-solution heat treatment. The experiment proved that the closed pores can be effectively eliminated (1.34% → 0.69%) by HIP treatment. In addition, the more uniform precipitation of M6C carbides appeared in the grain-boundary and matrix after aging treatment. Moreover, the added amounts of TiC powder play an important role in improving the mechanical properties of cobalt-based alloy.
  • The Electrical and Optical Properties of Fe–O–N Thin Films Deposited by RF Magnetron Sputtering

    pp. 2055-2058

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    DOI:10.2320/matertrans.M2013170

    The electrical and optical properties of Fe–O–N films were investigated in order to find their possibilities for solar cell application. Fe–O–N thin films were deposited on glass substrates by RF magnetron sputtering using an Ar–N2–O2 reactive gas. Under optimum flow rates of nitrogen and oxygen, the Fe–O–N films showed equivalent electrical properties to amorphous Si that has been conventionally used for thin film solar cells. Bandgap narrowing was also observed from 2.0 to 1.9 eV. The observed results were considered to be due to the formation of hematite-magnetite mixed phase, and the introduction of oxygen vacancies and/or nitrogen interstitials.
  • A Novel Repair Method of Carbon Fiber Reinforced Plastics with Reinforcing Fibers Intact

    pp. 2059-2063

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    DOI:10.2320/matertrans.M2013148

    A novel repair method of carbon fiber reinforced plastics (CFRPs) has been developed by means of our novel technology based upon the thermal activation of oxide semiconductors (TASC). TASC means the appearance of significant catalytic effects when the semiconductor is heated at about 350–500°C. This technology allows us to remove the polymer matrix alone in damaged areas of CFRP in the presence of thermally activated semiconductors while retaining the embedded fibers intact. The polymer-eliminated area is then refilled with an epoxy resin to complete the repair. In parallel, characterization of the carbon fiber has also been carried out by optical microscope, scanning electron microscope (SEM), X-ray diffraction (XRD), as well as thermogravimetric analysis and differential thermal analysis (TGA/DTA). The analysis revealed that no noticeable deterioration of the carbon fiber is recognized, but the sizing agent for the fiber is found to be eliminated by TASC.
  • Photovoltaic Characteristics of a Dye-Sensitized Solar Cell (DSSC) Fabricated by a Nano-Particle Deposition System (NPDS)

    pp. 2064-2068

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    DOI:10.2320/matertrans.M2013220

    Nano particle deposition system (NPDS) was evaluated for the potential photoelectrode fabrication method for dye-sensitized solar cell (DSSC). TiO2 powders as a photoelectrode for DSSC, were fabricated via NPDS and screen printing methods to compare its photovoltaic characteristics. The NPDS is a novel system for depositing ceramic and metallic powders on substrates at room temperature without using solvents by accelerating particles to supersonic velocity. The conventional method for preparing working electrodes uses TiO2 paste where poor adhesion to the substrate and numerous steps to properly disperse the powders are the main problems. To overcome these issues, TiO2 powders were directly sprayed onto ITO glass using the NPDS. The nano-sized TiO2 particles were densely deposited, no adhesion problems were noted and the solar-cell properties were improved. The photovoltaic performance of a cell having TiO2 layers first deposited by the NPDS method and then deposited by the paste method was compared to a cell fabricated only using the NPDS and only using the paste. A light conversion efficiency of 3.5% was obtained for the former cell, which indicates synergy effect of using these two methods. The screen-printing step right after NPDS deposition served to densify the TiO2 film, which decreased the series resistance. Therefore, NPDS was found to be a promising approach for improving DSSC properties and efficiencies.
  • Properties of Nanostructured TiCN and TiCN–TiAl Hard Materials Sintered by the High-Frequency Induction-Heating

    pp. 2069-2074

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    DOI:10.2320/matertrans.M2013200

    In the case of cemented TiCN, Ni or Co is added as a binder for the formation of composite structures. However, the high cost of Ni or Co and the low corrosion resistance of the TiCN–Ni cermet have generated interest in recent years for alternative binder phases. In this study, TiAl was used as a binder and consolidated by the high-frequency induction heated sintering (HFIHS) method. Highly dense TiCN–TiAl with a relative density of up to 100% was obtained within 2 min by HFIHS under a pressure of 80 MPa. The method was found to enable not only the rapid densification but also the inhibition of grain growth preserving the nano-scale microstructure. The average grain sizes of the sintered TiCN and TiCN–TiAl were lower than 100 nm. The addition of TiAl to TiCN enhanced the hardness and toughness due to increase of relative density.

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