Ni-Ti記憶合金亦稱做Nitinol合金，主要用於醫療產品上的應用，如牙齒矯正線、心導管支架等。首先本研究以金屬中心所提供不同製程下之Nitinol合金及線材，分析其各階段製程下之微結構，以此為基礎並探討微結構與對機械性質之影響，如：成型性影響與裂紋生成原因，並針對分析結果配合不同文獻比較與探討其中之關聯，其次則是注重其NiTi2與Ni2Ti4O氧化物形成機制與關連性。
首先於製程微結構分析中結果顯示：第一真空感應熔煉(VIM)與真空電弧熔煉(VAM)之Nitinol鑄錠內部產生NiTi2之偏析物並延晶界分布，且將因鑄錠內部之內應力產生延晶界裂痕，第二此NiTi2偏析物於熱機加工製程中產生裂痕並造成鑄錠之破裂，第三於終端產品中可發現此偏析物之均勻分布並存在著微小裂痕，因此此NiTi2脆硬相之存在將造成Nitinol製程良率與產品機械性質性質之下降。所以如何控制並了解其形成機制與對機械特性之影響是本研究重點之一。
其次本研究利用1273K分別固溶處理24、36與48小時，由實驗結果可發現延晶界分布之脆硬相能有效的被降低；然而其中卻維持大約1%左右之Ni2Ti4O之氧化物存在，於TEM分析中可以發現此Ni2Ti4O之氧化物，其電子繞射分析之lattice constant與NiTi2差異並不明顯，因此推斷Ni2Ti4O氧化物之生成與NiTi2之偏析物將有密切的關聯性，所以本研究利用Material Studio 7.0 軟體針對其中之NiTi2偏析相與Ni2Ti4O氧化物，進行第一原理計算與探討其形成能量與兩相之間的關聯性，發現NiTi2偏析物之生成焓為隨著氧的參雜而降低，並最終形成穩定之Ni2Ti4O氧化物，因此也可證明隨著氧的參雜將造成驅動力使NiTi2偏析物相變成更穩定之Ni2Ti4O。

Ni-Ti shape memory alloy also called Nitinol is majorly used in applications of medical products such as orthodontic wire, stent, etc. In the present study, the microstructure analysis of the Nitinol ingot and samples from different mechanical process supplied by Metal Industries Research & Development Centre was conduct. Based on the results of analysis and the paper review, the investigation on the connection between the microstructure and mechanical properties e.g., the formability of Nitinol and the origin of cracks were done. And the correlation between the NiTi2 segregation and Ni2Ti4Ox oxides was probed then to realize the mechanism of formation.
The microstructures analysis of Nitinol ingots from vacuum induction melting (VIM) and vacuum arc melting (VAM) shows that NiTi2 segregation formed along the grain boundary. And the internal stress in VIM ingots will cause crack along the distribution of NiTi2 segregation. The NiTi2 segregation in VAM ingots would also induce crack during the thermomechanical process and may cause the rupture in final. In the products, the NiTi2 segregation distributed uniformly and the small crack may exist in it. Therefore the existence of this brittle phase will decrease the yield rate of Nitinol and may cause the mechanical properties go down due to the small cracks in the product.
Then the solution heat treatment at 1273K for 24, 36 and 48 h was conducted on VIM samples and shows that the amount of NiTi2 segregation can be decreased effectively. However, there still existed about 1% NiTi2 even though sustained for a longer time. Moreover, the NiTi2 segregation left behind may solute oxygen and become the Ni2Ti4Ox. Therefore the Material Studio 7.0 software was used to investigate the correlation between the NiTi2 and Ni2Ti4Ox by calculating the formation enthalpy of both phases based on first principle simulation. The result shows that the formation enthalpy decreases as increasing the amount of oxygen mixed in NiTi2 phase. It means that the oxygen mixed in NiTi2 phase will cause the phase transform into Ni2Ti4Ox which is more stable than former.

Abstract................................................I
摘要 ..................................................II
List of Figures .......................................VI
List of Tables ......................................VIII
I. Introduction ........................................1
II. Literature review ..................................3
2.1. The shape memory properties of Nitinol ............3
2.2 .The medical application of Nitinol ................5
2.3 .The problem of fabrication process ................6
2.4 .The influence on the fatigue life of medical products ...............................................7
2.5. The correlation between the surface treatment and biocompatibility of medical product ...................10
2.6. First principle simulation .......................12
2.7. The formation enthalpy of intermetallic compound..14
III. Experimental method ..............................15
3.1. Experimental procedure ...........................15
3.2. The samples melted by arc melting ................16
3.3. The samples fabricated by thermomechanical process .......................................................16
3.4. Solution heat treatment ..........................17
3.5. The analysis by optical metallography ............17
3.6. The analysis by X-ray diffractometer .............17
3.7. The analysis by scanning electron microscope .....17
3.8. The analysis by electron probe microanalyzer .....18
3.9. The analysis by transmission electron microscope .18
3.10. First principle simulation ......................19
IV. Results and discussion ............................20
4.1. The microstructure analysis of VIM ingot and the samples after solution heat treatment .................20
4.2. The first principle simulation ...................35
4.3. The microstructure analysis of VAM ingot and the samples after thermomechanical process ................43
V. Conclusion .........................................51
Reference .............................................53

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