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標題: | 運用第一原理計算與與古典力場模型探討鉻錳鐵鈷鎳高熵合金之相穩定度和相轉變機制 First-principles and classical modeling study of the phase stability and phase transformation mechanism of CrMnFeCoNi high-entropy alloy |
作者: | Kang-Tien Hsieh 謝岡典 |
指導教授: | 郭錦龍 |
關鍵字: | 高熵合金,第一原理計算,古典力場,蒙地卡羅,相穩定度, high-entropy alloy,first-principles calculation,atomistic simulation,Monte Carlo, |
出版年 : | 2018 |
學位: | 碩士 |
摘要: | 本論文之研究目標為通過第一原理配合密度泛函理論計算以及古典力場模型兩種方式來探討CrMnFeCoNi五元高熵合金之相穩定度與相轉變機制,並探究其根本之原因。這兩種方法擁有不同的準確度以及計算效能而因此用來進行不同方向之探討。
在第一部分的研究中,我們運用第一原理計算與分子動力學模擬來進行針對五元CrMnFeCoNi,四元CrFeCoNi以及三元FeCoCr合金系統進行相穩定度之探討。首先,我們開發了一個新的逆蒙地卡羅法來進行系統性的結構建置,而這些結構會擁有不同的局域化學排序。最初CrMnFeCoNi合金被視為隨機固溶體,但在最近的研究中觀察到相分離現象。而我們的結果顯示,實驗觀察到的相分離是一個「焓驅動過程」,而高熵合金中的熵可能不是那麼「高」。我們進一步建議,四元CrFeCoNi合金比五元CrMnFeCoNi更穩定,且Mn在五元合金的相對相穩定度中起到關鍵之作用。此外,局部化學排序對於系統的疊差能可能會有相當大的影響。 適當的古典力場模型對於材料研究的發展扮演相當重要的角色。在論文的第二部分,我們開發並且驗證了一組MEAM參數。當與現有參數的結果進行比較時,我們的參數顯示出與第一原理計算有更好的一致性。通過大尺度分子動力學模擬,我們能夠研究並探討高壓壓縮時的面心立方(FCC)至六方最密(HCP)之相轉變過程。而我們的結果顯示,局域各向異性之壓力可以觸發FCC-HCP相轉變而等向壓力則沒有辦法觸發。在<001>、<011>和<111>方向的單軸壓縮模擬中,我們發現,施加在<001>上的應力能夠最有效的將FCC相轉變為HCP相,最高之轉變率可達到66%。同時我們也發現一個額外的機制會主導這個由疊差導致的相轉變過程。此外,通過有無自由表面的雙軸壓縮模擬,我們認為在五元CrMnFeCoNi中,差排的均質成核比異質成核在相轉變過程中更為重要。 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. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/70725 |
DOI: | 10.6342/NTU201802482 |
全文授權: | 有償授權 |
顯示於系所單位: | 材料科學與工程學系 |
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