Segregation--Assisted Spinodal and Transient Spinodal Phase Separation at Grain Boundaries

Segregation to grain boundaries affects their cohesion, corrosion and embrittlement and plays a critical role in heterogeneous nucleation. In order to quantitatively study segregation and phase separation at grain boundaries, we derive a density-based phase-field model. In this model, we describe the grain boundary free energy based on available bulk thermodynamic data while an atomic grain boundary density is obtained using atomistic simulations. To benchmark the performance of the model, we study Mn grain boundary segregation in the Fe--Mn system. 3D simulation results are compared against atom probe tomography measurements. We show that a continuous increase in the alloy composition results in a discontinuous jump in the Mn grain boundary segregation. This jump corresponds to an interfacial spinodal phase separation. For alloy compositions above the interfacial spinodal, we found a transient spinodal phase separation phenomenon which opens opportunities for knowledge-based microstructure design through the chemical manipulation of grain boundaries. The proposed density-based model provides a powerful tool to study thermodynamics and kinetics of segregation and phase separation at grain boundaries.