2NN MEAM 古典力場模型之建立及 CoCrFeMnNi 相分離與 CoCrNi、 CoCrFeMnNi 機械性質之原子尺度研究

Abstract

本論文主要探討 CoCrFeMnNi 高熵合金與 CoCrNi 中熵合金之原子尺度結構穩定性、相分離機制與機械性質，研究內容可分為三個部分。首先，本研究建立適用於高熵合金系統之 2NN MEAM 古典力場模型參數組，並以第一原理計算結果作為參考，針對純元素、二元系統及多元局域結構之凝聚能、晶格常數、彈性常數、空缺形成能、結構穩定性與疊差能進行系統性驗證。結果顯示，本研究所建立之參數組不僅可合理描述五元高熵合金中局域化學有序之能量特徵，亦能捕捉 FeCoCr 子系統中 FCC 與 BCC 結構之相對穩定性，並保留中熵合金系統原有之機械性質描述能力，證明本參數組具備良好之物理合理性與延伸性。 其次，本研究利用混合式蒙地卡羅/分子動力學模擬探討高熵合金於退火過程中之相分離行為。模擬結果顯示，高熵合金之分解機制是由 NiMn 之優先聚集與析出所觸發。當 Ni 以 NiMn 相形式析出後，殘餘 FeCoCr 區域失去足夠之 FCC 穩定作用，進一步發生 spinodal decomposition，形成 Cr BCC 與 CoFe B2 結構。隨溫度升高，NiMn L10 與 CoFe B2 有序相逐漸崩解，系統由三相分離逐步轉為 Cr BCC 與 FCC CoNiFeMn 共存之狀態，並最終於高溫下恢復為近似 SQS 排列之單相 FCC 固溶體。此結果說明 Ni 對於維持高熵合金FCC 固溶體穩定性具有關鍵作用，而 NiMn 析出則為啟動相分離之主要步驟。 最後，本研究進一步比較 高熵合金與中熵合金之拉伸機械行為。結果顯示，中熵合金具有較高之強度與較佳之延展性，而高熵合金雖具有較低之疊差能，卻未表現出較佳之延展性。變形機制分析指出，中熵合金在拉伸過程中較易形成變形雙晶，而 高熵合金則主要形成 extrinsic stacking faults。進一步由廣義疊差能分析可知，影響材料雙晶形成與延展性之關鍵因素為 slip barrier，而非疊差能本身。當 slip barrier 降低時，雙晶較易形成，材料延展性亦隨之提升；反之則會抑制雙晶生成並降低延展性。 綜合而言，本論文建立了一套可用高熵合金與中熵合金不同系統的原子尺度研究架構，成功串聯古典力場模型建立、相分離機制分析與機械性質比較，並釐清 CoCrFeMnNi 高熵合金於退火過程中之相演化行為及其熱力學驅動因素。本研究成果可作為未來高熵合金材料設計與性質優化之理論基礎。

This thesis investigates the atomic-scale structural stability, phase separation behavior, and mechanical properties of the CoCrFeMnNi high-entropy alloy and the CoCrNi medium-entropy alloy. First, a 2NN MEAM interatomic potential was developed for the CoCrFeMnNi alloy system and systematically validated against first-principles calculations. The results show that the developed potential can reasonably reproduce cohesive energies, lattice constants, elastic constants, vacancy formation energies, structural stability, and stacking fault energetics for unary, binary, and multicomponent structures. It also captures the energetic characteristics of local chemical ordering in quinary CoCrFeMnNi and the relative stability of FCC and BCC structures in the FeCoCr subsystem. Second, the phase separation mechanism of CoCrFeMnNi during annealing was investigated using hybrid Monte Carlo/molecular dynamics simulations. The results show that phase separation is primarily triggered by the preferential segregation and precipitation of NiMn. Once Ni is consumed through NiMn formation, the remaining FeCoCr region loses sufficient FCC stabilization and undergoes spinodal decomposition into Cr BCC and CoFe B2 structures. With increasing temperature, the ordered phases gradually collapse, and the alloy evolves from a three-phase-separated structure into a mixed state of Cr BCC and FCC CoNiFeMn, before eventually returning to a nearly single-phase FCC solid solution at sufficiently high temperature. These results indicate that Ni plays a critical role in maintaining the FCC stability of CoCrFeMnNi. Finally, the tensile behaviors of CoCrNi and CoCrFeMnNi were compared through nanowire tensile simulations. The results show that CoCrNi exhibits both higher strength and better ductility than CoCrFeMnNi. During tensile deformation, CoCrNi readily forms deformation twins, whereas CoCrFeMnNi mainly develops extrinsic stacking faults. Although CoCrFeMnNi has a lower stacking fault energy, it does not exhibit better ductility. Further generalized stacking fault energy analysis shows that the slip barrier plays a more important role than stacking fault energy in governing twinning behavior and tensile ductility. Overall, this thesis establishes an atomistic framework for potential development, phase-separation analysis, and mechanical-property investigation in multi-principal-element alloys, and provides a theoretical basis for the future design and optimization of medium- and high-entropy alloys.