高熵合金是一種具有高溫應用潛力的新興金屬材料，具有高強度、耐磨耗、抗腐蝕及抗氧化等特性。但傳統的高熵合金大多是基於鐵、鈷、鎳、鉻、錳等有較高密度的過渡元素，也因此限制其發展性。

在本研究中，我們採用新型的中密度TiAlV高熵合金作為實驗材料，TiAlV 高熵合金的密度大約是4.4 g/cm3，與傳統鐵基高熵合金密度大約是8 g/cm3相比有顯著的下降。而我們將深入探討TiAlV高熵合金的顯微結構及機械性質，除了常溫的性質外，我們也會利用奈米壓痕技術去探討在高溫環境下的機械性質變化及潛變特性。並將結果與傳統FeCoNiCrMn及FeCoNiCrMn-Al高密度高熵合金，以及商用Ti-6Al-4V合金比較，觀察其中的差異。

實驗結果顯示，TiAlV高熵合金在室溫環境下擁有相當高的硬度，維氏硬度達到543±14 Hv，利用奈米壓痕所測得的硬度為7.2±0.1 GPa。潛變實驗方面，在溫度400-600oC範圍內透過相同應力水平（σ/ E）為1.3x10-2下進行分析，可以發現在400-450oC區間應力指數約為4.7，在500-600oC區間應力指數則約為3.5，即應變速率敏感係數分別為0.21及0.29，這表示潛變的機制主要由差排進行主導。另一方面，雖然TiAlV高熵合金在常溫下擁有較高強度及硬度，但在高溫的潛變實驗中，TiAlV的潛變活化能約為208±7 kJ/mol，相較於FeCoNiCrMn及FeCoNiCrMn-Al的259±10和260±8 kJ/mol都有明顯下降，與 Ti-6Al-4V約280±50 kJ/mol相比也較低。在活化體積方面，TiAlV也是較FeCoNiCrMn、FeCoNiCrMn-Al及Ti-6Al-4V為低。

The high entropy alloys (HEAs) is a new emerging class of metallic alloys which are frequently considered as potential structural materials for high-temperature applications. Nowadays, several refractory HEAs compositions based on transition element have demonstrated remarkable compressive strength at elevated temperatures. But their high density over 8.0 g/cm3 would restrict the application range and the development foreground of HEAs mostly.

In this study, we use novel medium-density TiAlV HEA as material. The theoretical density of TiAlV equiatomic alloy is about 4.44 g/cm3 which is much lower than that of conventional Fe based HEAs. The TiAlV alloy is firstly investigated by characterizing its microstructure and mechanical properties. In addition to the room temperature (RT) properties, the high temperature (HT) performance is studied by using nanoindenter to extract creep behavior. The results were compared with the traditional FeCoNiCrMn and FeCoNiCrMn-Al high density high entropy alloys, as well as with commercial Ti-6Al-4V alloys.

The experimental results reveal that TiAlV possess high hardness in room temperature. The Vickers hardness of TiAlV is about 543±14 Hv and the nano-scaled hardness is about 7.2±0.1 GPa. On the other hand, the results of creep testing show that at the temperature regime from 400 to 600oC, under a normalized stress level (σ/E) of 1.3x10-2, the stress exponents for TiAlV are found to be 4.7 from 400 to 450οC and 3.5 from 500 to 600oC, or the strain rate sensitivities are about 0.21 and 0.29 separately. The results indicate the dislocation climb power law creep as the dominant creep mechanism. In addition, although the TiAlV high entropy alloy has higher strength and hardness at room temperature, the creep activation energy of TiAlV (208±7 kJ/mol) is lower than those of FeCoNiCrMn (259±10 kJ/mol), FeCoNiCrMn-Al (260±8 kJ/mol) and Ti-6Al-4V (280 kJ/mol) in high temperature creep experiments. In terms of activation volume, TiAlV also has lower activation volume than those of FeCoNiCrMn, FeCoNiCrMn-Al and Ti-6Al-4V.

論文審定書 i
誌謝 ii
中文摘要 iii
Abstract v
Content vii
List of Figures x
List of Tables xiii
Chapter 1 Introduction 1
Chapter 2 Background and literature review 3
2-1 The development of alloys 3
2-2 High entropy alloys (HEAs) 3
2-3 Four core-effects of HEAs 5
2-3-1 High entropy effect 6
2-3-2 Severe lattice distortion effect 7
2-3-3 Sluggish diffusion effect 8
2-3-4 Cocktail effect 8
2-4 The solid solution of high entropy alloys 9
2-5 The properties of high entropy alloys 10
2-6 HEA classification in terms of density and temperature in use 11
2-7 Lighter weight HEA development 13
2-8 Creep 14
2-8-1 Introduction of creep 15
2-8-2 The creep curve 15
2-8-3 Mechanisms of creep deformations 16
2-8-3-1 Dislocation glide 16
2-8-3-2 Dislocation creep 16
2-8-3-3 Diffusion creep 18
2-8-3-4 Grain boundary sliding 19
2-8-4 The indentation creep 20
Chapter 3 Experimental procedures 22
3-1 Raw material 22
3-2 Mechanical polishing and electropolishing process 22
3-3 Materials characterization 23
3-3-1 X-ray diffraction (XRD) 23
3-3-2 Optical microscopy (OM) 23
3-3-3 SEM and EDS analysis 23
3-3-4 Electron back scattered diffraction (EBSD) 24
3-3-5 Density 24
3-3-6 Vickers hardness (Hv) 24
3-4 Dual beam focused-ion-beam (FIB) 25
3-5 Nanoindentation 25
3-5-1 Hardness and modulus 25
3-5-2 Creep testing 25
Chapter 4 Results and discussions 27
4-1 X-ray diffraction analysis 27
4-2 EDS and SEM analysis 27
4-3 Density of TiAlV alloy 28
4-4 EBSD analysis 29
4-5 Hardness and modulus measurements 29
4-6 Micro-scaled micro-pillar mechanical properties of TiAlV alloy at room temperature 30
4-7 Nanoindentation creep testing 31
4-7-1 Creep behavior of TiAlV 31
4-7-2 Discussions for the creep testing results 36
Chapter 5 Conclusions 37
References 39
Figures 45
Tables 92

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