Formation Mechanism of Spheroidal Graphite in Nickel Alloy Cooled Ultra-Rapidly

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  • Wasai Kyoko
    Department of Materials Science and Engineering, Faculty of Engineering, Kyushu Institute of Technology
  • Yanagi Yousuke
    Graduate Student, Kyushu Institute of Technology
  • Yamaoka Hajime
    Graduate Student, Kyushu Institute of Technology
  • Shinozaki Nobuya
    Department of Materials Science and Engineering, Faculty of Engineering, Kyushu Institute of Technology
  • Era Hidenori
    Department of Materials Science and Engineering, Faculty of Engineering, Kyushu Institute of Technology

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Other Title
  • 超急冷したニッケル合金中の球状黒鉛の生成機構
  • チョウキュウレイ シタ ニッケル ゴウキン チュウ ノ キュウジョウ コクエン ノ セイセイ キコウ

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Description

The formation mechanism of spheroidal graphite (S.G.) in high purity Ni–C and Ni–C–Mg alloys cooled ultra-rapidly from the liquid state at four different rates (Exp. 1–Exp. 4) was investigated. The cooling rate increased from 2×104 to 5×105 (K/s) in the order of the experimental number. Contrary to the previous result on S.G. in Ni–C alloy with impurities, no core of the different phase was found at each center of S.G. (1–3 μm in dia.) in Ni–C alloy in Exp. 1 and a number of the S.G. appeared to have collapsed or burst during the process of growing. In Ni–C–Mg alloy samples of Exp. 1 and Exp. 2, the S.G. with various sizes (1–10 μm in dia.) were observed; some had hallow in each center and some appeared to have collapsed or burst. In Ni–C samples of Exp. 2, Exp. 3 and Exp. 4, and Ni–C–Mg samples of Exp. 3 and Exp. 4, only a few S.G. was found.<br>The results could provide two growing mechanisms of the S.G., namely, a heterogeneous nucleation of graphite on a core of the impurity phase formed during cooling, and a nucleation of graphite on the interface between a gas bubble and supercooled liquid nickel. The gas bubble came from vaporized Mg or Ar atmosphere; the Ar gas had been contained in porous alloy sample made of powder of Ni and graphite by pressing. At high cooling rate, the increase or decrease in gas pressure in the bubble would cause the collapse and burst, thus results in a few S.G.

Journal

  • Tetsu-to-Hagane

    Tetsu-to-Hagane 96 (8), 482-491, 2010

    The Iron and Steel Institute of Japan

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