Integration of the system free energy concept and the Phase-field method for the risk-based materials design

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The main purpose of this research is to predict the microstructural evolution of metallic materials showing a complex microstructure on the basis of the integration method of the system free energy concept and the Phase-field method. The system free energy is represented by the chemical free energy, the interfacial energy and the elastic strain energy, and it can predict the stable microstructure of the materials at a given time. On the other hand, the Phase-field method is based on the field variables as a function of the field indicating the state of microstructure, and the microstructural evolution is represented by the time evolution of the varialbles. In this study, high Cr ferritic steels were treated, because they show the most complex microstructure among metallic materials and hence they are suitable for the purpose of this study. For the ferritic steels, the TTP(Time-Temperature-Precipitation) diagram was constructed for Fe-C-Cr-Co-W-Mo system, and the phase boundary between the coherent and incoherent Laves phases was determined for Fe-C-Cr-W system. Also, The phase-field simulation was carried out for the recrystallization of iron. Through these results, it was found that microstructural evolution can be estimated when the change in the system free energy was evaluated quantitatively with a sequence of time. From this point of view, the system free energy was evaluated quantitatively for the martensite structure. As a result, the contribution of the interfacial energies such as grain boundary energy and packet boundary energy were found to be about one tenth of the elastic strain energy in the martensite phase. Furthermore, it was found that about 80% of the elastic strain energy was released by the tempering at 1053K for 1h. In conclusion, the most important thing for the risk based materials design is to evaluate the system free energy change quantitatively and it makes easy for the phase-filed simulation.