Journal of the Japan Institute of Metals and Materials
Online ISSN : 1880-6880
Print ISSN : 0021-4876
ISSN-L : 0021-4876
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Volume 75, Issue 2
Displaying 1-8 of 8 articles from this issue
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Regular Articles
  • Redistribution of Solute during Cellular Solidification of Single Phase Alloys
    Toshiro Owadano
    Article type: Regular Article
    2011 Volume 75 Issue 2 Pages 83-90
    Published: 2011
    Released on J-STAGE: February 01, 2011
    DOIhttps://doi.org/10.2320/jinstmet.75.83
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      Computer simulation of the upward cellular solidification of Al-Cu alloy was carried out considering the solidification to be a reaction controlled by diffusion. In the simulation, direct solving method of differential equation was applied to the diffusion in solid and liquid in contact with each other during cooling. The results of simulation showed that the solute distribution in liquid groove ahead of the solid-liquid interface is parabola whose bottom zenith lies at cell boundary. These results, together with the negligible diffusion in solid, showed that Scheil's equation is approximately applicable to the solute distribution in the solid cell after the solidification. Numerical analysis of the simulated results yielded a general equation which expresses the parabolic relationship between cooling rate and the cell size. The validity of equation thus induced was verified by the experimental results obtained by other researchers. The transition of planar/cellular interface in solidification was also investigated from the standpoint of the diffusion controlled solidification.
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  • Effect of Tool Shape on Friction Stir Welded Joint Strength of Fine-Grained Mg-Al-Zn Magnesium Alloy
    Naotsugu Yamamoto, Jinsun Liao, Kazuhiro Nakata
    Article type: Regular Article
    2011 Volume 75 Issue 2 Pages 91-96
    Published: 2011
    Released on J-STAGE: February 01, 2011
    DOIhttps://doi.org/10.2320/jinstmet.75.91
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      Friction stir welding was applied to join a fine-grained high tensile strength AZ31 alloy and a commercially extruded AZ31 alloy, and the effect of tool shape on the joint strength of the two magnesium alloys was investigated. For the commercially extruded AZ31 alloy with mean grain size of ca. 11 μm, the tool shape has almost no influence on the joint strength; for the fine-grained AZ31 alloy with mean grain size of ca. 1.9 μm, however, the joint strength was strongly influenced by the tool shape. The joint strength of the fine-grained AZ31 alloy increases with the ratio of shoulder diameter to probe diameter. It is found that the texture in the stir zone changes with the tool shape, which is one of the predominant factors controlling the joint strength of the fine-grained AZ31 alloy.
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  • Mechanical Properties and Microstructures of Silicon Doped Chromium Oxynitride Thin Films
    Jun Shirahata, Aoi Sato, Kazuma Suzuki, Tetsutaro Ohori, Hiroki Asami, ...
    Article type: Regular Article
    2011 Volume 75 Issue 2 Pages 97-103
    Published: 2011
    Released on J-STAGE: February 01, 2011
    DOIhttps://doi.org/10.2320/jinstmet.75.97
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      Cr-Si-N-O thin films were prepared by radio frequency reactive unbalanced magnetron sputtering. Compositional analysis of the thin films was carried out by Rutherford backscattering spectrometry and electron energy loss spectroscopy. It was found that these thin films contained approximately up to 11 mol% silicon. Film thicknesses were measured by using a scanning electron microscope; specimens had varying thicknesses from 200 nm to 2.5 μm arising from the changes in the locations of the samples and sputtering rates of targets. Phases in the samples were identified by X-ray diffraction. All samples showed only broad peaks based on NaCl-type-CrN. Diffraction peaks attributed to Cr2N, chromium oxides, and the silicon compounds were not observed. From the analysis of Fourier transform infrared spectroscopy, peaks attributed to the Cr-N bond of CrN, peaks attributed to the Si-O bond of SiO2 and peaks attributed to Si-N bond of Si3N4 were observed. There were no peaks attributed to chromium oxides. Microstructures were observed by transmission electron microscopy; grain size decreased with increasing silicon content. A nano-indentation method was used to measure indentation hardness and elastic modulus of the thin films. In accordance with a change of silicon content, the hardness increased up to a maximum value of 40 GPa and the elastic modulus was also increased from 185 to 215 GPa.
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  • Microscopic Phase-Field Modeling of Edge and Screw Dislocation Core Structures and Peierls Stresses of BCC Iron
    Hideki Mori, Hajime Kimizuka, Shigenobu Ogata
    Article type: Regular Article
    2011 Volume 75 Issue 2 Pages 104-109
    Published: 2011
    Released on J-STAGE: February 01, 2011
    DOIhttps://doi.org/10.2320/jinstmet.75.104
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      We investigate edge and screw dislocation core structures and Peierls stresses of BCC iron using microscopic phase-field (MPF) modeling. Parameters needed for the MPF modeling, such as the generalized-stacking-fault (GSF) energy, are determined based on first principles density functional theory (DFT) calculations. Screw dislocation core has six-fold symmetric structure and 0.05 nm width. The edge dislocation core is three times wider than the screw dislocation core. The Peierls stresses of edge and screw dislocations are estimated as 0.07 GPa and 2.8 GPa, respectively.
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  • Application of Theoretical Design to Selection of TiNi-X Alloys with Both High Strength and Oxidation Resistance
    Kazuhiro Matsugi, Yong-Bum Choi, Gen Sasaki
    Article type: Regular Article
    2011 Volume 75 Issue 2 Pages 110-114
    Published: 2011
    Released on J-STAGE: February 01, 2011
    DOIhttps://doi.org/10.2320/jinstmet.75.110
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      Optimization in both alloy-composition and manufacture-process has been required to obtain the TiNi-X compounds with high-strength and -oxidation resistance. Both Ti-49 mol%Ni-1 mol%Cr and Ti-45Ni-5Cr were designed for the achievement of tensile strength of 500 MPa and 800 MPa, respectively, and high oxidation resistance, by the d-electrons concept. The levitation melted castings showed the limited contamination amount and homogeneous microstructure. The values of tensile strength for Ti-49Ni-1Cr and Ti-45Ni-5Cr castings without solidification defects were 585 MPa and 840 MPa, which meant the achievement of the target values. The oxidation amount of Ti-45Al-5Cr was 0.28 times, compared with Ti-50Ni. It is concluded that TiNi-X intermetallic compounds having the objective constructed phase, tensile strength and oxidation resistance, are obtained successfully by the theoretical alloy design, without doing any trial-and-error experiments.
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  • Thermodynamic Property Change in Li-N-H Hydrogen Storage System by Melting Lithium Amide
    Toshihisa Izuhara, Hiroyuki T. Takeshita, Hidekazu Miyake
    Article type: Regular Article
    2011 Volume 75 Issue 2 Pages 115-121
    Published: 2011
    Released on J-STAGE: February 01, 2011
    DOIhttps://doi.org/10.2320/jinstmet.75.115
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      In this study, we used the Sieverts' method to measure the hydrogen pressure-composition isotherms of LiH and LiNH2 mixed in a molar ratio of 2:1 in order to obtain the standard reaction enthalpy and entropy of the hydrogenation reaction. The amounts of hydrogen absorbed and desorbed by the mixture were equal to the theoretical value at temperatures ranging from 653 K to 873 K (which were higher than the melting temperature of the mixture), where the standard reaction enthalpy and entropy were −32±3 kJ•(mol H2)−1 and −68±4 J•K−1•(mol H2)−1, respectively. On the other hand, the amounts were less than the theoretical value at 553 K and 623 K (which were lower than the melting temperature) due to the incomplete reaction, indicating that these isotherms would not afford reliable estimates of the standard reaction enthalpy and entropy. Hence, we instead calculated the standard enthalpy and entropy below the melting temperature using the values above the melting temperature and the standard enthalpy and entropy of fusion of the mixture. The values thus obtained, −49±3 kJ•(mol H2)−1 and −95±5 J•K−1•(mol H2)−1, respectively, are significantly different from those previously reported. Here, the differences are discussed from the standpoint of the difficulty in measuring the hydrogen pressure-composition isotherms at lower temperatures as well as the validity of the method.
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  • Production of Ni-Cr-P-B Alloy-Coated Bipolar Plates for PEMFC by HVOF Spray-Coating and Their Surface Analysis by XPS
    Sung-Chul Kim, Shin-ichi Yamaura, Takanori Igarashi, Yuta Shimizu, Koj ...
    Article type: Regular Article
    2011 Volume 75 Issue 2 Pages 122-130
    Published: 2011
    Released on J-STAGE: February 01, 2011
    DOIhttps://doi.org/10.2320/jinstmet.75.122
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      In this study, the newly designed bipolar plates for proton exchange membrane fuel cells (PEMFC) were produced by the HVOF spray-coating the Ni65Cr15P16B4, Ni60Cr20P16B4 and Ni55Cr25P16B4 alloys on Al plates having a flow field. As a result of XRD observations, it was found that the HVOF spray-coated Ni65Cr15P16B4 and Ni60Cr20P16B4 alloy films deposited on Al plates showed mainly a broad hallo peak coming from the glassy matrix and a small peaks from the crystalline phases. The spray-coated Ni55Cr25P16B4 alloy showed sharp distinct peaks coming from crystalline phases. So, it was difficult to prepare the coating layer with single glassy phase by the HVOF spray-coating in this study.
       The corrosion resistance of these Ni-Cr-P-B alloy films deposited by the HVOF-spray-coating was studied under simulated PEMFC environments. As a result, it was found that the corrosion current density of these films was smaller than that of the high-corrosion-resistant stainless steel SUS316L.
       Then the I-V performance of a single fuel cell with these bipolar plates produced in this work was studied and we found that the single fuel cells with the alloy-coated bipolar plates showed high I-V performance as well as the cell with the carbon bipolar plates. Among them, the Ni60Cr20P16B4 alloy showed the highest I-V performance, showing the largest current density at 0.3 V. After that, the 24 h durability tests were conducted at the constant current density of 200 mA•cm−2. As a result, the cell voltage of a single fuel cell with the alloy-coated bipolar plates did not show significant voltage drop during the tests. It can be concluded that the most suitable alloy for bipolar plate prepared in this study is Ni60Cr20P16B4 alloy.
       XPS analysis of the surface layer of the Ni-Cr-P-B alloy-coated bipolar plates was conducted after the 24 h durability tests. As a result, the Cr2O3 passive film was found in all the Ni-Cr-P-B alloy surface films and also the P2O5 in the Ni65Cr15P16B4 and Ni55Cr25P16B4 alloy surface films and Ni(OH)2 in the Ni65Cr15P16B4 alloy surface film. This observation indicates that the surface passive film of Ni60Cr20P16B4 alloy contains the Cr2O3 of the highest concentration among the three alloy surface films, resulting in the highest I-V performance and good durability.
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  • Corrosion Evaluation of Various Stainless Steels in Environments that Simulate Automotive Salt Damage
    Shunji Kajikawa, Tomoko Ito, Yasuaki Isobe, Masazumi Okido
    Article type: Regular Article
    2011 Volume 75 Issue 2 Pages 131-140
    Published: 2011
    Released on J-STAGE: February 01, 2011
    DOIhttps://doi.org/10.2320/jinstmet.75.131
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      Corrosion properties of stainless steels were evaluated by the corrosion test developed to simulate automotive salt damage environment in our previous work. In addition, corrosion resistance of the stainless steels in the field was estimated from comparison of test results and measurement of wet/dry condition of the engine compartment in typical salt damage regions. As a result of the corrosion test, the stainless steels with large pitting index had small corrosion depth both inside and outside of crevice. In case where the pitting index was equal between ferritic and austenitic stainless steels, corrosion depth of ferritic stainless steels tended to be deeper than that of austenitic stainless steels especially in low-humidity test condition. Electrochemical measurements under the same condition as the corrosion test indicated that the pitting potential of ferritic stainless steels was easier to lower with increase of chloride concentration in solution than that of austenitic stainless steels. Thus, the reason for the difference of corrosion depth in low-humidity test condition was thought to be the increase of chloride concentration in the surface water film. Comparing these results, the relative corrosion resistance of stainless steels in automotive salt damage environment was estimated on the basis of SUS304. The corrosion resistance of SUS316L was 1.5 to 1.8 times that of SUS304, and that of SUS444 was 1.0 to 1.3 times that of SUS304.
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