First-Principles Study of Interface between bcc Fe and Transition Metal Carbides

• Main Authors: Yi-Ting Chen, 陳奕廷
• Other Authors: Chin-Lung Kuo
• Format: Others
• Language: zh-TW
• Published: 2017
• Online Access: http://ndltd.ncl.edu.tw/handle/26505215326285026654

碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 105 === The aim of this thesis is to reveal the role of Mo addition to the Ti-based steel using first-principles calculations based on density functional theory. In the first part of this thesis, we calculated the chemical interface energy and strain interface energy (chemical interface energy including elastic strain) to investigate the effect of the addition of different composition Mo,W and Nb to the Ti-based interface system Fe/Ti1-xMxC. We built the interface system following Baker-Nutting orientation relationship. Our results of interface enrgy calculation shows that Mo and W addition can significantly decrease interface energy. The Fe/(Ti0.5Mo0.5)C interface system, which has Mo present at interface, has been found to be having the lowest chemical and strain interface energy in all the complex interface system. Furthermore, we found that interface energy isn’t a constant but decreases as particle size increases. We then employed density of state calculation to the different interface systems, the results shows that the interaction between segregated transition metal and its first nearest neighbor Fe has increasedfor the interface system that has the lowest interface energy. Moreover, from the analysis of charge difference calculation, the Mo addition to the interface can cause more ssignificant electron redistribution when the interface formed, and eventually induced stronger interface dipole to help stabilizing the interface. In the second part of the thesis, we investigated the formation energy of the Fe/(Ti1-xMx)C interface system in the viewpoints of thermodynamics. For the dependence of atomic configuration of the TMCs, various stacking sequences have been considered at different composition. We discovered that the complex carbide of (Ti1-xMx)C which has M present at the interface were the most stable. For the Fe/(Ti1-xMox)C precipitation, our results show that there is a turnig point of formation energy at x=0.352 in Fe/Ti1-xMoxC system and x=0.292 for Fe/Ti1-xWxC ststem. Our results suggest that the substitution composition between x=0.352 to x=0.5 can possibly lead to ultimate precipitation hardening enhancement because the formation energy of the interface system in this composition range is low enough for TMC to nucleate,meanwhile, the formation energy increases as precipitation size increases.The results will lead to large amount of extreamly fine precipitations .For the Fe/(Ti1-xWx)C precipitation, our results shows that the substitution composition between x=0.292 to x=0.5 will have same effect.