Computer simulation of phase decomposition in Fe-Cu-Mn-Ni quaternary alloy based on the phase-field method

Recently, the phase-field method is becoming a powerful tool to simulate and predict complex microstructure evolutions in interdisciplinary fields of materials science. In this study, the phase-field simulations are demonstrated on the phase decomposition in the alpha (bcc) phase during isothermal aging in Fe-Cu-Mn-Ni quaternary system, which is a base alloy system of the light-water reactor pressure vessel. Since the CALPHAD method based on a thermodynamic database of equilibrium phase diagrams is used for the evaluation of a chemical free energy in this simulation, the calculated microstructure changes are directly linked to the phase diagram of the Fe-Cu-Mn-Ni system. At the early stage of phase decomposition, the Cu-rich zone with bec structure begins to nucleate, and the component X (= Mn, Ni) is partitioned to the Cu-rich phase. When the Cu composition in the precipitate reaches almost the equilibrium value. the component X inside the precipitates moves to the interface region between the precipitate and the matrix. Finally, there appears the shell structure that the Cu precipitates are surrounded by the thin layer with high concentration of component X. This microstructure change is reasonably explained by considering the local equilibrium at the compositionally diffused interface region of Cu-rich nano-particles surface.