Martensitic transition in Fe via Bain path at finite temperatures: A comprehensive first-principles study

Due to the magnetic nature of Fe, various phenomena during structural transitions in Fe-based alloys, including martensitic transition (MT), cannot be accurately interpreted even by the state-of-the-art first-principles methods based on density functional theory (DFT), which is mostly limited to zero Kelvin. In the present work, thermodynamics and kinetics of Bain transition in pure Fe, i.e. the simplest model for fcc/bcc transition, are studied by analyzing the minimum energy path (MEP) at finite temperatures. Energies of various lattices and magnetic configurations at ground state are calculated by the standard DFT methods, which are further fitted by the Birch-Murnaghan equation of state (EOS) to obtain the ground state properties. By combing the quasi-harmonic Debye-Grüneisen model with the magnetic partition function approach (PFA), the Helmholtz energies for the body-centered tetragonal lattices with fixed c/a ratio and volume (V) are calculated, where the PFA accounts for the fluctuations of the magnetic configurations. Using free energy surface in the {c/a, V} space, the MEP is searched and a correlation between driving force and energy barrier for the fcc/bcc transition is observed. Further combined with previous heterogeneous nucleation models for MT, the correlation shown in the present work is found to be ubiquitous of MTs, and thus governing the formation of martensite.