本文研究中，我們使用超音波珠擊和奈米壓痕設備作為研究工具，進行高熵合金的表面晶粒細化和潛變性能之研究，相關結果將在論文中分別進行描述和討論。
在表面細晶化處理之研究中，在兩個常用高熵合金，單相結構(面心FCC)Fe20Co20Ni20Cr20Mn20，與雙相結構(面心FCC+體心BCC) Fe18Co18Ni20Cr18Mn18Al9，其表面上進行超音波珠擊表面處理(SMAT)，以提高其在室溫下的表面性質。在適當的處理下，晶粒尺寸可以從約50 μm減小到約0.1-1 μm，硬度從約2.5-5.0 GPa增加到約5.0-8.5 GPa，並且拉伸強度和伸長率可以增強接近一倍。由上述可以知道漸進的細化層和表面的強化層提升了高熵合金的機械性質。本研究並建立這些強化機制和疊加規則，且與實驗結果進行比較。
另一方面，利用了奈米壓痕系統在單相FCC結構Fe20Co20Ni20Cr20Mn20與FCC+BCC雙相結構Fe18Co18Ni20Cr18Mn18Al9的高熵合金，分別在溫度300-600oC範圍內且針對FCC(111)方向上的晶粒進行潛變實驗。透過相同應力水平(σ/ E)為2.5x10-3下進行分析，可以發現兩種合金的應力指數約為4，即應變速率敏感係數為0.25。這表示潛變的機制主要由差排進行主導。在固定的相同應力水平下，兩種合金的活化能分別為259±10和260±8 kJ/mol。然而，由於兩種合金在400-600oC會有析出反應，所以在固定結構條件下的實際活化能應大於260 kJ/mol，推測大約為280-300 kJ/mol。另外，兩種高熵合金較純鎳或鎳基超合金會有較大的活性體積，例如：在600oC時約250 Å3，比純鎳或鎳基超合金(~150 Å3或更小)要大，相關的潛變數據並會與鎳基超合金的潛變數據進行比較和討論。

In this research, we used the ultrasonic mechanical surface attrition treatment and nanoindentation as research tools, conducting the surface grain refinement and creep properties of high-entropy alloys. The relevant results will be separately described and discussed in the manuscripts.
For the first part, most high entropy alloys (HEAs) are cast to form single phase solid solution. Their hardness and strength at room temperature under the as-cast condition are typically lower than expectation. In the research, the ultrasonic surface mechanical attrition treatment (SMAT) is conducted on the surface of two HEAs, face-centered cubic (FCC) single-phased Fe20Co20Ni20Cr20Mn20 and the face-centered plus body-centered cubic (FCC+BCC) dual-phased Fe18Co18Ni20Cr18Mn18Al9, to upgrade their room temperature surface characteristics. By proper SMAT multiple paths, the grain size can be reduced from ∼50 μm down to ∼0.1-1 μm, the hardness increased from ~2.5-5.0 GPa up to ~5.0-8.5 GPa, and the tensile strength and elongation can be nearly doubled. The gradient refined and strengthened surface layers are demonstrated to appreciably upgrade the HEA performance. The strengthening mechanisms and superposition rules are established and are compared well with the experimental measurements.
Additionally, the creep responses under nanoindentation for the FCC single-phased Fe20Co20Ni20Cr20Mn20 and the FCC+BCC dual-phased Fe18Co18Ni20Cr18Mn18Al9 HEAs are examined on the FCC (111) grains over the temperature regime from 300 to 600oC, under a normalized stress level (σ/E) of 2.5x10-3. The stress exponents for both alloys are found to be about 4, or the strain rate sensitivity is about 0.25, indicating the similar dislocation climb power law creep as the controlling dominant creep mechanism. The extracted activation energy for these two under the “constant normalized stress” is 259±10 and 260±8 kJ/mol, respectively. However, since there is precipitation effect in both alloys over ~400-600oC, the actual activation energy under the “constant structure condition” should be greater than 260 kJ/mol, presumably about 280-300 kJ/mol. The current two HEAs possess relatively large volume, for example, about 250 Å3 at 600oC, larger than those for pure Ni or typical Ni based superalloys (~150 Å3 or less). The current creep response is compared and discussed with that of the Ni based superalloys.

論文審定書 i
誌謝 ii
摘要 iv
Abstract vi
Content viii
List of Tables xii
List of Figures xiv
Chapter 1 Introduction 1
1-1 Introduction and application of high entropy alloys 1
1-2 Motivation 2
Chapter 2 Background and Literature Review 7
2-1 The History of alloys 7
2-2 Definition of high entropy alloys 7
2-3 Four core-effects of HEAs 10
2-3-1 High entropy effect 10
2-3-2 Severe lattice distortion effect 11
2-3-3 Sluggish diffusion effect 12
2-3-4 Cocktail effect 13
2-4 The solid solution of high entropy alloys 14
2-5 The behavior of high entropy alloys 15
2-5-1 Mechanical behavior 16
2-5-1-1 The mechanical behavior at room temperatures 16
2-5-1-2 The mechanical behavior at high temperatures 17
2-5-2 Chemical behavior 18
2-5-3 Physical behavior 19
2-6 Surface mechanical attrition treatment (SMAT) 20
2-6-1 Introduction of surface mechanical attrition treatment 20
2-6-2 Grain refinements 21
2-6-3 Applications of surface mechanical attrition treatment 23
2-7 Creep 24
2-7-1 Introduction of creep 25
2-7-2 The creep curve 25
2-7-3 Mechanisms of creep deformation 26
2-7-3-1 Dislocation glide 26
2-7-3-2 Dislocation creep 26
2-7-3-3 Diffusion creep 28
2-7-3-4 Grain boundary sliding 29
2-7-4 The indentation creep 30
Chapter 3 Experimental Procedures 33
3-1 Sample preparation 33
3-1-1 Raw material 33
3-1-2 Mechanical polishing and electropolishing process 33
3-1-3 Surface mechanical attrition treatment (SMAT) 34
3-1-4 Dual beam focused-ion-beam (FIB) 34
3-2 Property measurements and analyses 35
3-2-1 X-ray diffraction (XRD) 35
3-2-2 Vickers hardness (Hv) 36
3-2-3 Scanning electron microscopy (SEM) 36
3-2-4 Transmission electron microscopy (TEM) 37
3-2-5 Nanoindentation 37
3-2-6 Universal testing machine (UTM) 38
3-2-7 Differential scanning calorimeter (DSC) 38
Chapter 4 Results and Discussions 40
4-1 SMAT response 40
4-1-1 The experiment results for SMAT 40
4-1-1-1 XRD analysis 40
4-1-1-2 EDS and SEM analyses 41
4-1-1-3 EBSD analysis 42
4-1-1-4 TEM analysis 43
4-1-1-5 Hardness measurements 43
4-1-1-6 Tensile results 44
4-1-2 Discussions for SMAT 45
4-1-2-1 Strengthening analyses by rule of mixture 45
4-1-2-2 Strengthening analyses in terms of grain size and work hardening strengthening 48
4-2 Creep behavior 51
4-2-1 The experiment results and analyses for nanoindentation creep 51
4-2-1-1 XRD analysis 51
4-2-1-2 EDS analysis 52
4-2-1-3 SEM and EBSD analyses 52
4-2-1-4 Room and elevated temperature nanoindentation testing 52
4-2-2 Discussions for creep testing 57
4-2-2-1 In situ hardening during creep at 300 to 600oC 57
4-2-2-2 Correction for the activation energy for “constant structure condition” 58
4-2-2-3 Creep deformation mechanisms 60
Chapter 5 Conclusions 62
References 65
Tables 81
Figures 91

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