Thermodynamic Analysis of the Formation Process of Metastable Carbides in Iron–Carbon Martensite

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  • Enoki Masanori
    Institute of Multidisciplinary Research for Advanced Materials, Tohoku University
  • Osawa Yohei
    DMG Mori Co.,Ltd.
  • Ohtani Hiroshi
    Institute of Multidisciplinary Research for Advanced Materials, Tohoku University

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Other Title
  • 鉄-炭素系マルテンサイトにおける準安定炭化物の生成過程の熱力学的検討
  • テツ-タンソケイ マルテンサイト ニ オケル ジュンアンテイ タンカブツ ノ セイセイ カテイ ノ ネツリキガクテキ ケントウ

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Abstract

<p>In this study, the free energy of iron-carbon binary BCT martensite was calculated using the first-principles calculation and the cluster expansion and variational method. The free-energy curves of BCT martensite show the possibility of promoting the clustering of carbon atoms in the tempering process, because there is two-phase separation associated with the formation of metastable BCT-Fe2C ordered structure. This BCT-Fe2C structure was found to have many crystallographic similarities to η-carbide (Fe2C). Then, the energy barrier required for the transition from the BCT-Fe2C ordered structure to η-carbide was calculated by means of the G-SSNEB method. The obtained activation energy was sufficiently small, suggesting that η-carbide may be formed through the BCT-Fe2C ordered structure. According to these findings, it was suggested that η-carbide in the low-temperature annealing process of BCT martensite may precipitate through a two-step process in which BCT-Fe2C ordered structure is formed by the two-phase separating tendency in BCT martensite, and the ordered structure transitions to η-carbide over the energy barrier.</p>

Journal

  • Tetsu-to-Hagane

    Tetsu-to-Hagane 106 (6), 342-351, 2020

    The Iron and Steel Institute of Japan

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