主催: The Iron and Steel Institute of Japan
p. 96-102
Martensite is an important phase in steel, but evaluating its free energy is challenging both experimentally and theoretically. In this study, we explored a method to assess the free energy of martensite using cluster expansion and variational methods (CE-CVM), with the additional application of intermittent lattice deformation. Our analysis of the formation energy of dilute solid solution Fe64C showed that in regions where the c/a ratio exceeds 1.00, carbon dissolution in the c-site is energetically favorable. The dependence of the energy difference on the c/a ratio showed that as c/a increases, the occupancy probability of the c-site approaches 1, while that of the ab-site approaches 0. In previous studies using cluster expansion, energy calculations were performed without considering local strain relaxation due to issues with carbon position deviations affecting precision. This study addresses this by incorporating local strain relaxation into the calculations. Specifically, the initial structure was adjusted to increase the distance between the nearest Fe and C atoms by approximately 0.5 Å, followed by relaxation calculations using first-principles methods. Free energy calculations at T = 400 K indicated that higher axial ratios become more stable with increasing carbon concentration. Furthermore, while previous research overestimated changes in the axial ratio, this study's consideration of local atomic position relaxation improved alignment with experimental data.
