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

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.