在鎳基超合金中，元素在γ與γ´相之間的分佈行為(partition behavior)對於合金的機械性質有顯著的影響，因此元素的分佈行為在設計一款鎳基超合金時，扮演十分重要的角色。元素的分佈行為已有被廣泛的研究，然而，研究指出鎢元素在含有強烈γ´相形成元素 (strong γ´ former)鉭的系統中，鎢原子將會受鉭原子的影響，分佈行為將會從原先的γ´相轉變成γ相，鎢原子展現出相反分佈行為(reverse partition)。近年來矽元素的微量添加至鎳基超合金將有助於改善高溫抗氧化性質與高溫疲勞性質，在含矽的TMS-138A與KC系列鎳基超合金中矽原子表現出不同的分佈行為，在TMS-138A中，矽原子將會分佈至γ´相，而在KC系列合金中，矽原子有著分佈至γ相的傾向。矽元素的分佈行為會受到強烈形成γ´相元素鈦原子的影響，藉由場發射電子微探儀、CALPHAD模擬與第一原理的模擬，對矽原子的分佈行為與鈦原子對矽原子分佈行為的影響進行探討。場發射電子微探儀結果顯示矽原子將會分佈至γ´相，且隨著鈦的添加，矽原子將會更加傾向分佈至γ相，CALPHAD模擬結果指出矽原子將會分佈至γ相且矽將受鈦添加的影響而更加分佈至γ相。根據第一原理模擬結果，鈦原子將會取代矽在γ´晶格中的矽原子，被取代的矽原子將會傾向分佈至γ晶格中。矽原子亦將隨著時效時間的上升而更加分佈至γ相，然而在真實實驗狀況，矽原子將會受系統達熱平衡狀態與否與點缺陷的影響而被留在γ´晶格中，使得矽原子CALPHAD模擬結果與實際實驗量測中展現出不同的分佈行為。矽的分佈行為將會與強烈γ´相形成元素鈦的存在、系統點缺陷濃度與系統達熱平衡與否有所影響。

The elemental partitioning behavior between gamma and gamma prime phase play an important role for designing a new Ni-based superalloys. The partition behavior of Si would be affected by the strong gamma prime former Ti. EPMA analysis, CALPHAD and first principle simulation are performed to study the effect of Ti on the partition behavior of Si. EPMA result shows that Si would partition to gamma prime phase, while CALPHAD simulation shows that Si have the partition tendency toward gamma phase. Also, both EPMA and CALPHAD simulation shows that Si would partition to gamma matrix more strongly as the content of Ti increase. Based on first principle simulation, Ti atoms would replace Si atoms in gamma prime lattice and reject Si atoms into gamma lattice rather than occupy the Ni-sites in gamma prime lattice. The elemental partition ratio will also change gradually as the aging time increases. Long-term aging and Strain aging process also was conducted to calculate the partition ratio at the system which is closed to the thermodynamic equilibrium state. The existence of point defects would trap Si atom in the gamma prime lattice, so Si would shows the partition tendency toward gamma prime phase in the real experimental cases. The partition behavior of Si between gamma and gamma prime phase is sensitive to the addition of Ti and the system reach the thermodynamic equilibrium or not.

Abstract I
摘要 II
Acknowledgements III
List of Figures VIII
List of Tables XI
I. Introduction 1
II. Literature review 3
2.1. Alloying effect 3
2.2. The point defects in the Ni-based superalloys 4
2.1.1. Point defects in γ matrix 4
2.1.2. Point defects in γ´ precipitates 7
2.3. Lattice misfit between γ and γ´ phase 9
2.4. CALPHAD simulation 11
2.5. First principle simulation 13
2.6. Site preference in the Ni3Al lattice 15
2.7. The formation enthalpy of intermetallic compound 20
2.8. Elemental partitioning behavior 23
2.9. Addition of Si 27
III. Experimental method 29
3.1. Experimental procedure 29
3.2. Alloy design 30
3.3. Arc melting process 30
3.4. Solution heat treatment (SHT) 31
3.5. Normal aging process 31
3.6. Long-term aging process 31
3.7. Strain aging process 32
3.8. Optical metallography (OM) 32
3.9. Field Emission Scanning Electron Microscope (FE-SEM) 32
3.10. Electron Probe Microanalyzer (EPMA) 33
3.11. CALPHAD simulation 33
3.12. First principle simulation 34
IV. Results and analysis 35
4.1. Microstructure evolution 35
4.1.1. As-cast microstructures 35
4.1.2. Microstructure after full heat treatment 39
4.2. Elemental partitioning behavior 46
4.3. CALPHAD simulation results 48
4.4. First principle simulation 50
4.4.1. Site preference 50
4.4.2. Formation enthalpy of γ and γ´ lattice 51
4.5. Long-term aging process 55
4.6. Strain aging process 56
4.7. Formation enthalpy of point defects in γ´ lattice 62
V. Discussion 64
VI. Conclusion 67
Reference 68

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