高熵超合金是結合高熵合金及超合金的合金設計概念，以高熵FCC γ相為基底，結合L12 γ’析出物做為強化相的新型態合金。有別於傳統超合金，高熵超合金的成分設計可含有更多的鐵及鈦元素，使其擁有低成本及輕量化的優勢，而整體性價比表現更可超越部分鑄造型超合金。本研究提出單晶鑄造型高熵超合金的層次結構，可在高熵超合金的低成本及輕量化優勢下進一步提升高溫強度，使其具有極佳的高溫應用潛力。主要的研究內容可分為兩部分，一是研究此層次結構在高熵超合金中的形成及演變機制，二是探討此層次結構的強化效應。
在第一部分中，本研究利用製程控制介穩態的相變化過程，在γ’析出相中形成奈米級的γ顆粒，創造出含有γ基底相- γ’析出相- γ顆粒的層次結構。根據熱力學模擬，奈米級γ顆粒是由過飽和的高固溶γ’析出相於熱處理的動態相平衡變化過程產生。此層次結構處於一介穩定狀態，若持續延長熱處理，γ顆粒的組成元素會傾向藉由擴散穿越γ’析出相而溶入γ基底相中，以降低整體系統的能量。理論上，整個系統最終會變回穩定γ基地相加上穩定γ’析出相的平衡態。所幸大部分的商用工程合金並非以最終平衡態做為實際應用，以合金設計搭配CALPHAD模擬的製程改良，可望延長整體層次結構在高溫環境下的應用時間。
在第二部份中，本研究以高溫拉伸測試搭配理論模型計算來檢驗層次結構的強化效果。擁有層次結構的高熵超合金在室溫及高溫的拉伸降伏強度可超越商用單晶超合金逾120 MPa，同時保有延展性達20 %以上。根據TEM分析發現，其高強度可歸因於奈米級γ顆粒對γ’析出相中的差排移動造成牽制，提供額外的強化效果。若考量到層次結構高熵超合金整體的強度、密度及成本優勢，其性價比更可為商用單晶超合金的8倍。本研究展現層次結構之高熵超合金作為更強、更輕、更便宜、延展性更好的新型高溫合金系統的潛力。

This study presents a hierarchical microstructure strengthened single crystal high entropy superalloy (HESA) with superior cost-performance for elevated temperature applications.
Design of HESA has adopted the template of FCC-structured γ matrix and coherent L12-structured γ’ precipitates resembled that of conventional superalloys. Compositions of HESA are distinctively different from those of cast superalloys with higher contents of Fe and Ti, making HESA cheaper and lighter. In addition, HESA has medium entropy γ’ enriched with solutes with higher intrinsic strength, and the high entropy γ matrix attributed to a good combination of strength and ductility. HESA have exhibited superior cost-performance than that of cast superalloys, while this study further introduces a novel hierarchical microstructure that can provide additional strengthening in HESA without altering the alloy cost and density. Formation and evolution of the hierarchical microstructure and the strengthening mechanisms are investigated.
In the first part, processing control HESA phase transformation pathway through metastability can create a hierarchical microstructure containing a dispersion of nano-size disordered FCC γ particles inside ordered L12 γ’ precipitates that are within the FCC γ matrix. The formation mechanism and microstructure evolution have been studied by experimental observation and CALPHAD calculation. According to thermodynamic simulations, composition complexity of HESA is an important factor to the formation of FCC γ particles inside γ’ phase. However, this hierarchical microstructure is in a metastable state. After prolonged ageing, the γ particle formers tended to diffuse across the γ’ phase into the γ matrix to eliminate the addition surface energy created, and the whole system could evolve toward the equilibrium state. Although thermodynamic process is inevitable during thermal exposure, engineering alloys are rarely designed to be used in thermodynamic equilibrium state. Thermal stability of hierarchical microstructure may be improved by alloy design to prolong its service life for high temperature application.
In the second part, room to high temperature tensile tests and theoretical models are utilized to examine the strengthening contribution of the hierarchical microstructure. The average tensile yield strength of hierarchical HESA from room temperature to 750 ℃ could be 120 MPa higher than that of advanced single crystal superalloy, while HESA could still exhibit an elongation greater than 20 %. According to TEM analysis, γ particles in γ’ phase could serve as additional dislocation barrier that contributed to the higher yield strength in hierarchical HESA. Composition complexity in HESA was found to improve the intrinsic ductility of γ’ phase, and lead to the excellent elongation in tension. Considering all the advantages in cost, density and strength, the cost specific yield strength of HESA can be 8 times that of some advanced superalloys. A template for lighter, stronger, cheaper, and more ductile high temperature alloy is proposed.

摘要 I
Abstract II
致謝 IV
Table of content VI
List of figures VIII
List of tables XIII
1. Background and Introduction 1
1.1. High temperature materials 1
2. Literature review 4
2.1. Ni-based superalloys 4
2.2. High Entropy Alloys (HEAs) 10
2.3. High temperature performance of FCC-type HEAs 13
2.4. L12 phase strengthened FCC-type HEAs 15
2.5. High Entropy Superalloy (HESA) 20
2.6. Hierarchical microstructure strengthening 27
3. Research scope 29
4. Experimental design 30
4.1. Materials and methods 30
5. Results and Discussion 34
5.1. Formation and evolution of hierarchical microstructure in HESA 34
5.2. Hierarchical microstructure strengthening in HESA 53
6. Conclusions 68
7. Future Work 69
8. Publication list 70
9. References 71

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