||Develop the manufacturing process to improve the properties of medical Ni-Ti memory alloy wire
||Department of BioMedical Engineering
phase transformation temperature
鎳鈦 (Ni-Ti) 合金是形狀記憶合金 (SMA) 的一種，屬於獨特的智能和功能性之類別，且具有形狀記憶效應和超彈性兩種獨特的特性。此外，鎳鈦合金還具有較高的機械強度、耐腐蝕性佳、良好的可加工性和生物相容性，由於在具備這些優異性質的條件下，近年來被廣泛用於各個領域。鎳鈦合金還能透過多種表面改質的技術在其表面被覆含有些許或是無鎳的TiO2 之氧化層，進一步提高其表面之抗腐蝕能力、親水性以及生物相容性，使鎳鈦合金在醫療設備中的應用日新月異；且已有許多公開的文獻表明，鎳鈦合金的特性可以通過冷加工，熱處理或兩者的結合來處理，以用於所需的醫療器械，包括心血管支架、矯正線和導線。
在這次研究中，設計了一系列的製程對鎳鈦合金線材 (Ni含量為54.1 at%) 進行冷拔和退火處理。我們使用數個鑽石眼模進行冷拔，以多階段方式增加冷加工的百分比，受到減面率的作用鎳鈦線材達到預期的線徑尺寸並獲得冷加工效果；接著有別於一般研究常見之箱型爐，本研究使用連續爐對鎳鈦線進行退火以調整真直度與恢復其性質，且探討其效果是否適用於未來生產上。除了探討直徑0.44 mm之鎳鈦線材(冷加工百分比50％)在530℃下進行退火10秒之最終樣品的性質分析外，透過將直徑0.62 mm之鎳鈦線材(冷加工百分比30％)進行不同的時間之800°C退火，來研究不同退火時間的影響與數據趨勢。最後，在線材經製程處理完成後，對待測樣品進行表面形貌、氧化層厚度與線徑分析、拉伸性質測試、硬度測試以及相轉變溫度的量測，來判斷冷拔與退火所帶來的影響。
Nickel- Titanium(Ni-Ti) alloy is one of shape memory alloys (SMA) which are a distinct class of smart and functional material and have two unique properties, shape memory and superelasticity. Besides, Ni-Ti alloy also possesses high mechanical strength, good corrosion resistance, good workability and biocompatibility, so it has been used for a wide range of applications in various fields in recent years. It can form a Ni-free TiO2 layer on the surface of the Ni-Ti alloy through a variety of surface modifications, further improving its corrosion resistance, surface hydrophilicity and biocompatibility to make the applications of the Ni-Ti alloy in medical devices more suitable. Some literatures indicate that the properties of Ni-Ti could be manipulated through cold work, heat treatment or a combination of both for applicable medical devices including vascular stents, orthodontic wires, and guide wires.
In this study, the Ni-Ti wire (54.1%) was subjected to a series of cold drawing process and annealing heat treatment to evaluate their influence in mechanical alternation and shape memory transformation temperature. A series of diamond dies were used to draw Ni-Ti wire, in gradual reduction in cross sections to obtain desirable dimension and the effect of cold working. A continuous furnace, which was different from the box furnace in common research, was used to adjust the straightness and recover the properties of the wires by annealing. Diameter at 0.62 mm Ni-Ti wire were annealed at 800 ℃ for different times to find the relation between the annealing time and the properties of Ni-Ti wire. All the treatment in Ni-Ti wires were analyzed by surface morphology, oxide layer thickness, tensile properties, hardness value, and phase transformation temperature.
The results showed that the cold drawing process possessed excellent dimensional control, and the wires were not twisted after multiple stages drawing. The ductility of annealed Ni-Ti wires successfully returned so that next cold drawing process could be continued. After the drawing process, the strength and hardness of Ni-Ti wires were greatly enhanced. It could be observed that annealed wires had good straightness. As the annealing time increases, the coverage and thickness of the oxide layer that can suppress Ni ion release also increased. The relationship between the annealing time and the properties of Ni-Ti wires was evaluated from the results, so it provided the guidance to control the desired properties of Ni-Ti wires as well as the proper phase transformation temperature for various applications.
Table of contents
Table of contents VI
List of tables VIII
List of figures X
Chapter 1 Introduction 1
1.1 Nickel-titanium alloy (Ni-Ti alloy) 1
1.2 Cold working and heat treatment 4
1.2.1 Cold drawing 4
1.2.2 Annealing 6
1.3 Shape Memory Alloy (SMA) 8
1.4 Motivations and aims 12
Chapter 2 Materials and methods 13
2.1 Materials 13
2.2 Instrument 13
2.3 Experiment flow chart 15
2.4 Drawing Ni-Ti wire 16
2.5 Annealing 17
2.6 Surface morphology 18
2.7 Oxide layer thickness analysis 19
2.8 Tensile properties 20
2.9 Hardness test 21
2.10 Phase transform temperature 22
Chapter 3 Results 24
3.1 Surface morphology of Ni-Ti wires with cold drawing and annealing 24
3.1.1 Macroscopic surface morphology of nickel-titanium wires with cold drawing and annealing 24
3.1.2 Micro-surface morphology of nickel-titanium wire after cold drawing and annealing 27
3.2 Oxide layer thickness analyzed 33
3.2.1 The variation of thickness of oxide layer 33
3.2.2 The dimension of Ni-Ti wires with the cold drawing process 38
3.3 Ni-Ti wires with cold drawing and annealing for tensile test 40
3.3.1 Ultimate tensile strength 40
3.3.2 Elongation 43
3.4 Ni-Ti wires with cold drawing and annealing for hardness test 46
3.5 Phase transition temperature of Ni-Ti wires with cold drawing and annealing 49
Chapter 4 Discussion 51
Chapter 5 Conclusion 57
List of tables
Table. 1. Chemical composition of Ni-Ti alloy wire (at%) 13
Table. 2. Record the drawing times and the percentages of cold working for each stage. 16
Table. 3. The annealing condition of Part 1 and Part 2. 17
Table. 4. The designation of all experiment groups. 23
Table. 5. The surface morphology of the NiTi wire with cold drawing under the macroscopic condition. 25
Table. 6. The surface morphology of the NiTi wire with annealing under the macroscopic condition. 26
Table. 7. The surface morphology of the NiTi wire with cold drawing for stage 1 under the microscopic condition. 28
Table. 8. The surface morphology of the NiTi wire with cold drawing for stage 2 under the microscopic condition. 29
Table. 9. The surface morphology of the NiTi wire with cold drawing for stage 3 under the microscopic condition. 30
Table. 10. The surface morphology of the NiTi wire with annealing under the microscopic condition. 31
Table. 11. EDS chemical composition of cold-drawn and annealed Ni-Ti wires analysis (at%) from the edge to the inside. 35
Table. 12. EDS chemical composition of cold-drawn and annealed Ni-Ti wires analysis (at%) near the edge. 37
Table. 13. The expected dimension and practical dimension of Ni-Ti wires after the cold drawing process 39
Table. 14. Characteristic transformation temperatures (℃) of Ni-Ti wires with annealing for different time. 49
Table. 15. The straightness that tested Ni-Ti wire reveal after heating. 50
List of figures
Fig. 1. Light reflected on the oxide layer. 3
Fig. 2. The die structure and deformation during drawing process. 4
Fig. 3. Experimental phase diagram of the Ni-Ti system. 7
Fig. 4. Phase transition process of Ni-Ti alloy by heating and cooling. 9
Fig. 5. Stress-strain-temperature data exhibiting the shape memory effect and superelasticity for a typical Ni-Ti SMA. 10
Fig. 6. One-way memory effect 11
Fig. 7. Two-way memory effect 11
Fig. 8. Flow chart of this study 15
Fig. 9. The cold-drawn and annealed Ni-Ti wires were (a.) judged the quality by naked-eye and (b.) fixed on the carrier to be observed and analyzed by SEM. 18
Fig. 10. (a.) Fixed the Ni-Ti wires in the epoxy carrier; (b) SEM image and the chemical composition of the cross section by EDS. 19
Fig. 11. Ni-Ti wire with the tensile test by universal testing machine. 20
Fig. 12. The pattern on the surface of Ni-Ti wires during the hardness test. (a.) The projection of the indentation on the surface of the material; (b.) The side view of the material with hardness test. 21
Fig. 13. Schematic diagram of phase transform temperature testing. 22
Fig. 14. The surface content of Ni-Ti wires with annealing for different time. 32
Fig. 15. SEM images of cold-drawn and annealed Ni-Ti wires morphology from the edge to the inside. (a) 0.62 mm Ni-Ti wire, (b) 0.44 mm Ni-Ti wire, (c) 0.62 a_1800s, and (d) 0.44 fa. 34
Fig. 16. SEM images of cold-drawn and annealed Ni-Ti wires morphology near the edge. (a) 0.62 mm Ni-Ti wire, (b) 0.44 mm Ni-Ti wire, (c) 0.62 a_1800s, and (d) 0.44 fa. 36
Fig. 17. The ultimate stress for the tensile testing of the Ni-Ti wires with the various stages of the drawing by universal testing machine. The data presented as the mean ±SEM (n=3), *p<0.05. 41
Fig. 18. The ultimate stress of the Ni-Ti wires of diameter 0.62 mm after annealing at different time by universal testing machine. The data presented as the mean ±SEM (n=3), *p<0.05. 42
Fig. 19. The elongation for the tensile testing of the Ni-Ti wires with the various stages of the drawing by universal testing machine. The data presented as the mean ±SEM (n=3), *p<0.05. 44
Fig. 20. The elongation of the Ni-Ti wires of diameter 0.62 mm after annealing at different time by universal testing machine. The data presented as the mean ±SEM (n=3), *p<0.05. 45
Figure 21. The hardness of the Ni-Ti wires with the various stages of the drawing by hardness tester. The data presented as the mean ±SEM (n=3), *p<0.05. 47
Figure 22. The hardness of the Ni-Ti wires after annealing at different time by hardness tester. The data presented as the mean ±SEM (n=3), *p<0.05. 48
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