高熵合金獨特的設計理念使合金設計的自由度大增，根據成分設計可以得到各式各樣的組織結構，如單相FCC、BCC或是生成各式各樣的介金屬化合物，可以在低溫擁有極佳的破壞韌性或是在高溫擁有優異的壓縮強度，但在高溫應用的探討較少，固本研究主要探索高熵合金在高溫應用的可能性，
本研究一開始著重於一款擁有高硬度的FCC高熵合金Co1.5CrFeNi1.5Ti0.5 (莫耳比)，將此合金鑄造成單晶研究其鑄造偏析行為，並用多晶樣品研究不同時效溫度下析出強化效果及相穩定性；結果顯示Co、Cr、Fe等元素在凝固時會先行成樹枝相，Ni和Ti則是被排除到未凝固液體中，因此在樹枝間相富含Ni和Ti進而生成相(DO24 hexagonal phase)；使用Scheil equation 分析EPMA(電子微探儀)所量測的元素從樹枝中心(最先凝固)到樹枝間(最後凝固)的分佈曲線，可以得到各元素的分配係數(partition coefficient)，結果顯示此高熵合金的各元素分配係數與傳統Ni基超合金十分相似，代表其凝固行為與傳統合金相似。此合金經過時效熱處理後可以析出奈米級顆粒狀的相(L12 FCC結構)，此析出相為超合金可以擁有高高溫強度的強化機制，室溫硬度的結果顯示此析出物可以提供有效的析出強化效果，然而此相在高溫不穩定(> 800 oC)會轉變成片狀的相，相雖然硬度高但析出強化效果低反而使合金硬度大幅下降。
因此，我們進一步研究如何改善此高熵合金的高溫相穩定性；根據熱力學模擬軟體ThermoCalc (CALPHAD method)加上Ni基的資料庫(TCNi5)進行不同成分的相圖模擬，結果顯示藉由添加Al取代部份的Ti可以有效改善相的高溫穩定性，但是過多的Al (高Al/Ti比)會使析出相不穩定進而生成相(B2 結構)。一系列的AlxCo1.5CrFeNi1.5Tiy (x, y 值為莫耳比， x + y = 0.5)高熵合金利真空電弧熔煉並經過均質化及時效熱處理，實際實驗結果顯示在高溫(>900 oC)的相組成與預測的相圖一致，但在低溫(<900 oC)的時效熱處理，所有合金都有析出奈米級相，因為實際的相轉變必須考量到動力學擴散的時間因素，我們使用的時效時間並不足夠使相轉變為其他的金屬化合物。室溫硬度結果顯示Ti含量較的合金擁有較高的硬度，因為其相的體積百分比較高且Ti可以增加相的反相邊界能(antiphase boundary energy)提高該相的強度。高溫硬度結果顯示這些相強化的合金可以擁有不錯的高溫硬度表現，尤其是Ti含量較高的兩款高熵合金甚至超越商用Ni基超合金IN718。高溫氧化實驗顯示此系列可以生成連續緻密的CrO2保護層，擁有良好的高溫抗氧化能力。
前一階段的研究顯示此高熵系統控在一定的Al/Ti比可以同時擁有不錯的高溫強度、相穩定性、抗氧化能力，其相組成與緞造型超合金相似。因此我們選用其中一組成分(Al02Ti03)並進一步改善其成分，使其擁有更高的相體積百分率並維持其相穩定，並進行熱機處理(Thermo-mechanical process)；結果顯示經過熱機處理後可以比未熱機處理之樣品擁有更高的拉伸強度及更好的延展性，尤其在650 oC仍然保持良好的延性，原因為熱機處理使其晶粒細化得到細晶強化的效果，而晶粒等軸化使應變時更均勻，因此可以進一步優化其延性表現。然而，在750 oC時會有延性降低的現象，此為在晶界上的柱狀析出物(cellular precipitate)所導致，在更高溫時此柱狀析出物會回溶回基底延性得以回復，此晶界柱狀析出物為時效後產生，TEM擇區繞射顯示其為相，根據文獻此種析出現相為一種不連續析出的現象(Discontinuous precipitation)，主要之趨動力來自不穩定的飽和基底相並伴隨晶界的移動，因此藉由降地退火後的冷卻速度或提高相回溶溫度可以促進相在退火後冷卻過程中生成，冷卻生成時相可以消耗反應驅動力及阻礙時效時晶界的移動進而抑制晶界柱狀物生成。綜觀所有結果，本研究可以做為設計析出強化鍛造型高熵合金於高溫應用的指標。

In this study, we try to understand the potential of high entropy alloys (HEAs) for high temperature application. There are many essentials need to be met before the application. We started from studying the solidification behavior of a single crystal Co1.5CrFeNi1.5Ti0.5 HEA and its high temperature microstructural stability. This material is shown to possess mainly FCC structure;  phase is present at the interdendritic region in the as-cast condition and it is stable between 1073K and 1273K (800C and 1000 C) after homogenization and aging;  particles are found throughout the microstructures below 1073K (800 C). Segregation analysis was conducted on a single crystal sample fabricated by a directional solidification process with a single crystal seed. Results show that Co, Cr, Fe partitions toward the dendritic region, while Ni and Ti partition toward the interdendritic areas. Scheil analysis indicates that solid-liquid partitioning ratio of each element are very similar to those in typical Ni-based superalloys. The results show that HEAs can be strengthened through the precipitation of  phase, which is also a crucial strengthening mechanism in superalloys. However, the  phase in Co1.5CrFeNi1.5Ti0.5 is not stable and would transform to brittle η phase.
Thus, we further studied the effects of Al and Ti contents on the phase stability of  phase and high temperature properties of AlxCo1.5CrFeNi1.5Tiy (x, y values in molar ratio, x + y = 0.5) HEAs. With an increase in Al content and a decrease in Ti concentrations, precipitated phases in the FCC high entropy  matrix can evolve from  to  to  phases. Some of these alloys can possess high hardness values at elevated temperatures due to  precipitations, and continuous Cr2O3 can form on the surfaces of these alloys for protection against oxidation. The results showed the HEAs with proper Al/Ti ratio can have a stable γ+γ׳ structure.
Based on the study of phase stability, a precipitation strengthened high entropy alloy with stable γ+γ׳ structure was designed. This HEA was subjected to thermo-mechanical process, which is standard process for wrought superalloy to homogenize the grain microstructure. Tensile tests from room temperature to 1000 oC were conducted; microstructures were observed by scanning electron microscope and transmission electron microscope. Good tensile properties can be obtained at temperature up to 650 oC. Formation of cellular precipitate along grain boundaries was observed and could be related to a hot ductility drop at 750 oC (1023 K). Experimental analysis has indicated that driving force for the formation of cellular precipitates could be resulted from the chemical instability of supersaturation after annealing and migration of grain boundaries, and this phenomenon could be suppressed either through alloy design to increase γ׳ solvus, and to hinder the migration of grain boundaries. This study serves as a guideline to design composition and thermo-mechanical process for precipitation strengthened wrought high entropy alloys for high temperature application.

摘要 I
Abstract III
Acknowledgement V
Table of content VI
List of figures VIII
List of tables XII
1. Introduction 1
2. Literature review 4
2.1 Superalloys for advance turbine engine 4
2.2 High entropy alloy 9
2.3 Solid-solution phases in HEAs 11
2.4 Intermetallic phases in HEAs 13
2.5 Tensile properties of solid-solution HEAs 15
2.6 Tensile properties of HEAs with intermetallic compounds 17
3. Research scope 23
4. On the Solidification and Phase Stability of a Co-Cr-Fe-Ni-Ti High-entropy Alloy 24
Abstract 24
4.1 Research aim 24
4.2 Experimental procedures 25
4.3 Results and Analysis 28
4.4 Discussions 34
4.5 Conclusions 36
5. The phase stability and high temperature properties of AlxCo1.5CrFeNi1.5Tiy high entropy alloys 37
Abstract 37
5.1 Research aim 37
5.2 Experimental methods 38
5.3 Results and discussions 41
5.4 Conclusions 52
6. Development of a wrought precipitation-strengthened high entropy alloy 53
Abstract 53
6.1 Research aim 53
6.2 Experimental procedure 54
6.3 Results and discussion 56
6.4 Conclusion 72
7. Conclusions 74
8. Future work 76
9. Major contributions of this PhD thesis 78
10. Publication list 79
11. Reference 80

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