運用第一原理計算探討含碳鈷鉻鐵鎳高熵合金系統疊差能和雙晶生成度影響與鈷鉻鐵錳鎳古典力場模型發展

本篇論文第一部份研究為利用第一原理搭配密度泛函理論計算方式研究含碳之CoCrFeNi高熵合金系統當中疊差能與雙晶生成容易度影響。第二部份的主要研究在於修正與改善我們團隊現有的CoCrFeMnNi MEAM古典力場模型參數組。於論文的第一部分我們運用第一原理計算搭配我們所發展的逆蒙地卡羅方式來探究四元CoCrFeNi高熵合金系統於添加碳元素前後疊差能的變化行為。我們藉由逆蒙地卡羅方法來建構多組均勻混和元素排列之結構，隨後將此結構分別產生疊差缺陷結構與置入碳元素再搭配後續的結構能量計算來探究碳元素對於疊差能的影響。我們發現碳元素與疊差結構的相對距離會顯著影響系統之疊差能變化行為。當碳元素所在之原子層與疊差結構間隔一定距離的原子層後，系統的疊差能出現低於未添加碳元素之前的情況。此外，藉由進一步的純元素系統之計算，我們探討多種純元素對於加壓過程中FCC與HCP結構之相對相穩定度的變化行為與含碳前後疊差能變化情況，結果顯示不同系統添加碳元素後的疊差能下降與否主要取決於各種元素系統對於加壓過後FCC與HCP之相對相穩定度變化的敏感程度。最後，我們進行電子結構特性的分析來研究添加碳元素前後電子結構的變化情況，搭配前述的相關計算，我們以系統性地理論計算方式解釋了碳元素對於疊差能與雙晶生成容易程度影響之主要物理因素。在第二部分的研究中，我們修正且改善我們團隊原有的CoNiCrFeMn MEAM力場模型參數，並驗證這些參數純元素與二元對的結構、機械性質，以及異相結構之間的相對能量趨勢。隨後，我們針對不同組成數目高熵合金系統進行相轉變溫度的預測，一方面用於驗證此次精進後參數組的可靠性；另一方面，我們藉由大尺度古典力場模型的預測方式，提供了另一種相較於第一原理計算更大的尺度來進行未來材料開發設計上另一種可靠的設計工具。

關鍵字

高熵合金；第一原理計算；古典力場模型

並列摘要

In this thesis, there are two major topics will be systematically studied. The first is the carbon effect on the stacking fault energy (SFE) change and twinnability in CoCrFeNi alloys, the other part is the development of MEAM classical force field.. In the first part of this study, an algorithm by which we can generate multiple random CoCrFeNi structures is applied and is combined with first-principles static calculations to study the SFE change in the carbon-doped CoCrFeNi alloys. We find out that addition of carbon can alter the SFE in CoCrFeNi systems and the distance between carbon and stacking fault also affects the SFE change behavior. With the comprehensive studies of pure elements, we discovered the carbon-induced SFE change behavior is strongly related to the FCC-HCP relative phase stability change during the (111) applied strain. Furthermore, we perform the electronic properties calculations and the unstable stacking fault energy barrier change in carbon-doped systems. Combining the above mentioned works, we systematically study the carbon-doped CoCrFeNi alloys and its derived systems. More importantly, we provide the solid scientific explanation about the carbon effect on SFE and twinnability of the CoCrFeNi system. The second part of this thesis will focus on the development and refinement of our CoCrFeMnNi MEAM force field and relative validation tests. The unary and binary properties of this MEAM model are refined by the parameters optimization, while the properties of unary, quaternary, and quinary are improved by the fine tune of the screening function in the MEAM force field model. After the development and validated test, we utilize our MEAM model to perform the energetic and phase transition temperature prediction in high-entropy alloys. By this kind of prediction, we not only demonstrate the stability of our parameters, but also provide another efficient materials design tool for the future materials development process.

並列關鍵字

High-entropy alloys ； First-principle calculations ； Classical force field

參考文獻

[1] Yeh, J. W., Chen, S. K., Lin, S. J., Gan, J. Y., Chin, T. S., Shun, T. T., ... Chang, S. Y. (2004). Nanostructured high‐entropy alloys with multiple principal elements: novel alloy design concepts and outcomes. Advanced Engineering Materials, 6(5), 299-303.

Google Scholar

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