Thermodynamic analysis of the formation process of metastable carbides in iron–carbon martensite

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Abstract

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.

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Enoki, M., Osawa, Y., & Ohtani, H. (2020). Thermodynamic analysis of the formation process of metastable carbides in iron–carbon martensite. Tetsu-To-Hagane/Journal of the Iron and Steel Institute of Japan, 106(6), 52–61. https://doi.org/10.2355/tetsutohagane.TETSU-2019-098

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