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

本研究的主要目的是基于系统自由能概念和相场方法的集成方法来预测具有复杂微观结构的金属材料的微观结构演化。系统自由能由化学自由能、界面能和弹性应变能表示，可以预测材料在给定时刻的稳定微观结构。而相场法则是将场变量作为表征微观结构状态的场的函数，用场变量的时间演化来表征微观结构的演化。在这项研究中，高铬铁素体钢进行了处理，因为他们表现出最复杂的显微组织中的金属材料，因此，他们是适合本研究的目的。对于铁素体钢，绘制了Fe-C-Cr-Co-W-Mo系的TTP（Time-Temperature-Precipitation）图，确定了Fe-C-Cr-W系共格和非共格Laves相的相界。并对铁的再结晶过程进行了相场法模拟。通过这些结果，发现当系统自由能随时间序列的变化被定量评估时，可以估计微观结构的演变。从这个角度出发，定量地计算了马氏体组织的系统自由能。结果表明，晶界能和包边界能等界面能的贡献约为马氏体相弹性应变能的十分之一。在1053 K回火1h后，约有80%的弹性应变能被释放。因此，基于风险的材料设计最重要的是定量地评估系统自由能的变化，这使得相场模拟变得容易。

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.