First-principles and classical modeling study of the phase stability and phase transformation mechanism of CrMnFeCoNi high-entropy alloy

• Main Authors: Kang-Tien Hsieh, 謝岡典
• Other Authors: 郭錦龍
• Format: Others
• Language: en_US
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/4u9r84

碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 106 === Density functional theory (DFT) and modified embedded atom method (MEAM) are applied in this thesis with aim of investigating the fundamental reasons of phase stability and phase transition process of CrMnFeCoNi quinary high-entropy alloy (HEA). These two atomistic approaches are used in different aspects of researches due to their accuracy and computational demands. In the first part of the thesis, the phase stability of the quinary CrMnFeCoNi, quaternary CrFeCoNi and ternary FeCoCr alloy systems are investigated by DFT static calculations and ab initio molecular dynamics (AIMD). A new idea of reverse Monte Carlo (RMC) method is presented to systematically construct different structures with different local chemical ordering. Quinary CrMnFeCoNi alloy is initially considered as a random solid solution, but phase separation phenomenon is observed in recent studies. Our results show that the experimentally observed phase separation is an enthalpy driven process and the entropy in high-entropy alloys may not be that “high”. We further suggest that the quaternary CrFeCoNi alloy is more stable than its quinary parent and that Mn plays a crucial role in the relative phase stability of the quinary alloy. Futhermore, the local chemistry ordering may greatly affect the stacking fault energies of the system. The lacking of proper atomistic potential model can greatly prohibit the outgrowth of material studies. In the second part of the thesis, a set of MEAM parameters is developed and validated. When comparing with the results of existing parameters, our results show better agreement with ab initio calculations and experimental values. The FCC-to-HCP phase transformation during high-pressure compression is investigated by large scale molecular dynamics (MD). The results suggest that the locally anisotropic pressure can activate FCC-to-HCP phase transformation while hydrostatic pressure cannot. Among <001>, <011> and <111> directions, the stress applied on <001> is the most effective in turning FCC into HCP, reaching a 66% transformation. An extra mechanism is found to be responsible for this stacking fault mediated phase transformation process. Moreover, by biaxial compression with and without free surface, we suggest that the homogeneous nucleation of dislocations plays a more important role than heterogeneous nucleation in FCC-to-HCP phase transformation for Cantor alloy.