In this study, we develop the multiscale model of phase transformation in Fe-Cr alloy applicable for the simulation of \(\alpha^{\prime}\) -phase formation in the grain bulk as well as segregation at the grain boundaries (GBs). In this multiscale model, we use molecular statics for the calculation of the interaction parameters in the solid solution, including concentration dependence of mixing enthalpy. The obtained interaction parameters are applied for the calculation of the phase diagram for the homogeneous Fe-Cr alloy and GB Σ5(210). The diffusion coefficient of chromium is derived using ab initio (DFT) calculations. The dynamics of the phase transformation and GB segregation are calculated within phase-field modeling, accounting for the strong anisotropy of the GB transport of chromium in iron. The developed approach allows us to simulate nucleation and growth of Cr-rich precipitates in the Fe-Xat%Cr alloy (X = 16–20 at%) in the grain bulk up to ~ 71 years of thermal aging at 600 K. Also, the simulation predicts very fast heterogeneous enrichment in Cr of GB Σ5(210) for X ≥ 18 at% with the formation of needle-like Cr-rich particles located in the GB region. At lower concentrations (X < 16 at%), the segregation of Cr is expected to be almost homogeneous over the GB surface area at the temperature of 600 K.
