Formation Mechanism of Ultrafine Grained Microstructures: Various Possibilities for Fabricating Bulk Nanostructured Metals and Alloys

  • Tsuji Nobuhiro
    Department of Materials Science and Engineering, Kyoto University Elements Strategy Initiative for Structural Materials (ESISM), Kyoto University
  • Gholizadeh Reza
    Department of Materials Science and Engineering, Kyoto University
  • Ueji Rintaro
    National Institute for Materials Science (NIMS)
  • Kamikawa Naoya
    Department of Mechanical Science and Engineering, Hirosaki University
  • Zhao Lijia
    Department of Materials Science and Engineering, Kyoto University ArcelorMittal Global Research and Development
  • Tian Yanzhong
    Department of Materials Science and Engineering, Kyoto University Elements Strategy Initiative for Structural Materials (ESISM), Kyoto University Key Laboratory for Anisotropy and Texture of Materials, School of Materials Science and Engineering, Northeastern University
  • Bai Yu
    Department of Materials Science and Engineering, Kyoto University Elements Strategy Initiative for Structural Materials (ESISM), Kyoto University
  • Shibata Akinobu
    Department of Materials Science and Engineering, Kyoto University Elements Strategy Initiative for Structural Materials (ESISM), Kyoto University

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Abstract

<p>Severe plastic deformation (SPD) have had a revolutionary impact in fabricating ultrafine grained (UFG) or nanostructured metallic materials with bulky dimensions. It is, however, difficult to understand from a viewpoint of conventional metallurgy why UFG microstructures form in the as-deformed (as-SPD-processed) state without annealing process. The mechanism of UFG evolution has not yet been perfectly recognized and even some confusions have been induced. Experimental results of systematic observations on the evolution of microstructures during SPD were introduced in the present manuscript, and it was concluded from the results that the formation of UFG microstructures during SPD could be understood in terms of the grain subdivision mechanism. Consequently, the UFG microstructures formed by SPD have the deformed (strain-hardened) characteristics, which influences their mechanical properties. Besides the breakthrough in grain refinement, some features of SPD processes have become barriers for practical applications of UFG materials and scaling-up of the processes. Several new processes combining phase transformation with plastic deformation were introduced for fabricating UFG microstructures without huge plastic strains, and UFG microstructures successfully obtained by the new processes were shown. Finally, recent finding of fully recrystallized UFG microstructures in some alloys having average grain sizes of several hundred nano-meters was exhibited. The fully recrystallized UFG metals all showed both high strength and large ductility, because of the capability of strain hardening in the structures. These recent results are expected to open great possibilities of UFG or nanostructured metals as advanced structural materials.</p>

Journal

  • MATERIALS TRANSACTIONS

    MATERIALS TRANSACTIONS 60 (8), 1518-1532, 2019-08-01

    The Japan Institute of Metals and Materials

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