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

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.