κ-carbides phase transformation in FeMnAlCSi steels: A Monte Carlo simulation

Austenite-based lightweight steels are designed to develop local chemical inhomogeneities—such as chemically ordered κ -carbide formation—to achieve superior mechanical performance. To model the structural transformation of these complex steels, we develop a flexible pairwise energy model coupled with lattice Monte Carlo (MC) simulations. Extensive MC simulations are conducted to investigate the thermodynamic atomic structures of FeMnAlCSi alloys as a function of composition and temperature. Our simulation results are in good quantitative agreement with reported experimental data, and provide new insights into phase decomposition, species segregation and site preferences, and chemical order–disorder transitions. The simulations further offer microscopic views of atomic segregation phenomena, including the incorporation of interstitial carbon atoms at grain boundaries (GBs). We identify the primary mechanisms underlying Si-accelerated κ -carbide formation as: (i) repulsive Si–C and Si–Si interactions that globally exclude Si from the ordered κ -carbide phase, and (ii) thermodynamic preference and segregation of carbon to GBs, which locally supersaturate the boundary region and promote GB precipitation. The present approach—combining DFT-parameterized pair energies with large-scale MC simulations—is computationally efficient, captures the essential thermodynamics of multi-component systems with interstitials, and can be readily extended to a wide range of multi-principal element alloy systems with constant lattice structures.