中文提要

本研究藉由粉末冶金方式混合鋁-氧化鋁、鋁-鋅-氧化鋁、鋁-鋅三種不同系統，
接著利用摩擦攪拌製程製作晶粒範圍300 nm 到3μm的不同晶粒大小範圍的鋁合金複材，針對這些複合材料的微觀結構的觀察利用掃瞄式電子顯微鏡（SEM）與穿透式電子顯微鏡（TEM），而使用拉伸試驗來幫助瞭解其機械性質，此外也針對三種不同的複材的複材的變形機制的演化有深入的探討。
在鋁-氧化鋁系統中，利用摩擦攪拌製程添加奈米氧化鋁粉均勻的散佈在鋁基材中可以有效的增加變形時差排的累積進而增加晶粒內其加工硬化率，而此增加對於次晶粒增加均勻延展性而言非常的關鍵。
在鋁-鋅-氧化鋁系統中，可以利用摩擦攪拌製程添加奈米氧化鋁粉、固溶原子與微細小的G-P zone均勻的分散在鋁基材中，可以同時增加其均勻延展性與強度，其獲得的均勻延展性比一般未添加鋅的系統來得更好。此外因為固溶鋅之緣故造成晶界的擴散係數大為提升增加差排的相消速率，因而晶界上觀察到應力鬆弛與乾淨差排的現象，而這些現象並不會因為氧化鋁粉的參與而消失。在機性的表現上因上述的結果使Hall-Petch 中 值隨鋅含量增加而降低。
在鋁-鋅系統中，實驗清楚的發現鋅含量從0到15wt%可以大大增加其總伸長量但其均勻伸長量並未明顯的貢獻，那是因為常溫變形時微小滑移帶的均勻散佈與晶界滑移的啟動雙重貢獻使加工硬化率與延展性隨著鋅含量的增加而增加。此外因為摩擦攪拌製程形成的高比例的高角度晶界與因為添加鋅固溶造成快速的晶界擴散使晶界差排快速相消導致晶界乾淨與晶界鬆弛，此結合效果促使晶界在常溫變形過程中發生晶界滑移，此發現更可解釋鋁-鋅-氧化鋁系統中 值降低的現象。

Abstract

Friction stir processing (FSP) is modified to produce various grain sizes of aluminum matrix composites (Al-Al2O3, Al-Zn-Al2O3 and Al-Zn) ranging from 300 nm to 3μm. The microstructures of the composites were characterized using SEM and TEM. Tensile tests were performed to evaluate the mechanical properties of these composites.
In the Al-Al2O3 system, it was found that the nano-scale alumina made it very effective to accumulate dislocations within grains during deformation, and resulted in increasing working hardening rate which is very critical to extend uniform elongation for materials with submicron grain sizes.
In the Al-Zn-Al2O3 system, addition of Zn which dissolved into Al matrix to form solid solution and subsequently uniformly distributed G-P zones can improve strength and uniform ductility to some extent, comparing to those without addition of Zn. In addition, the relaxed and dislocation-free boundaries were observed regardless the existence of Al2O3 particles on boundaries. As a result, ko derived from Hall-Petch equation from various strain region decrease as Zn increases.
In the Al-Zn system, experimental evidence suggests that increasing Zn content from 0 to 15wt% can enhance the total elongation but not uniform elongation as a result of the uniform spreading of the fine slip bands all over the gauge length and the contribution of grain boundary sliding (GBS) at RT. The relaxed and dislocation-free boundaries and GBS are attributed to the combination of high fraction of high-angle GBs and high GB diffusion to help fast dislocations annihilation at boundaries.

Table of Contents

Table of Contents I
List of Table III
List of Figures IV
Abstract X
中文提要 XII
Acknowledge XIII

Chapter 1 Introduction 1
1.1. Background 1
1.2. Scope of Thesis and Objectives 3
1.3. Overview of Thesis 5
Chapter 2 Literature Review 7
2.1 Mechanical Properties of ufg and/or nc Metals and Alloys 7
2.1.1 Ultra-High Strength 7
2.1.2 Ductility Limitation 9
2.2 The Approaches to Improve the Tensile Ductility of High-Strength ufg and nc Metals 11
2.2.1 Grain Size Distribution (Bimodal ultrafine-grained nanostructured metal) 11
2.2.2 Stacking Fault Energy 12
2.2.3 Second-Phase Particles 13
2.2.4 Twinning 14
2.2.5 Strain Rate Controlled 15
2.3 Deformation Mechanisms of ufg and nc Materials 17
2.4 Structure and Phase Transformation in Al-Zn Alloys 20
Chapter 3 Experimental Procedure 21
3.1 Initial Powder 21
3.2 Specimen Preparation 21
3.2.1 Powder mixing 21
3.2.2 Cold compaction 21
3.2.3 Friction stir processing (FSP) 21
3.3 Microscopy 23
3.3.1 Scanning electron microscopy 23
3.3.2 Transmission electron microscopy 23
3.4 Tensile test 24
Chapter 4 Results 25
4.1 Al-Al2O3 system 25
4.1.1 The Observation and Statistic Distribution of Grain Size 25
4.1.2 Mechanical Properties 26
4.1.3 Microstructures 27
4.2 Al-Zn-Al2O3 system 28
4.2.1 The Statistic Distribution of Grain Size 28
4.2.2 Mechanical Properties 29
4.2.3 Microstructure 31
4.3 Al-Zn system 33
4.3.1 The Statistic Distribution of Grain Size 33
4.3.2 Mechanical Properties 33
4.3.3 Microstructure 35
Chapter 5 Discussions 40
5.1 Uniform and Total Elongation 40
5.2 Grain Size Dependence of Yield Stress and Flow Stress 42
5.3 The Correlations between Deformation Mechanism and Grain Boundary Diffusion 45
5.4 Strain Rate Sensitivity 51
Chapter 6 Summary 53
Reference 55

List of Table

Table 2-1 Partial list of papers in inverse Hall-Petch relationship [only first author named] 67
Table 3-1 The Al grain size obtained with specific the content of Zn and Al2O3 by the pre-determined preparing parameters of FSP 70
Table 4-1 Tensile properties obtained at room temperature for the A-l, A-2, A-3, A-4, A-5 and A-6 samples from RT at σy is the yield stress and σu is the ultimate tensile stress. 72
Table 4-2 Tensile properties obtained at room temperature for the A5Z-1, A5Z-2, A5Z5-3, A5Z-4, A5Z-5, A5Z-6, A15Z-1, A15Z-2, A15Z-3, A15Z-4, A15Z-5, and A15Z-6 samples from RT at σy is the yield stress and σu is the ultimate tensile stress. 73
Table 4-3 Tensile properties obtained at room temperature for the Z5-1, Z5-2, Z5-3, Z5-4, Z5-5, Z5-6, Z5-7, Z15-1, Z15-2, Z15-3, Z15-4, Z15-5, Z15-6 and Z15-7 samples from RT at σy is the yield stress and σu is the ultimate tensile stress. 74
















List of Figures

Figure 2-1. Variation in Vicker microhardness with for nanocrystalline Cu and Pd produced by IGC method, showing the negative slope of H-P relation [39]. 75
Figure 2-2. Yield stress dependence on both grain and cell size for pure Ni [68]. 76
Figure 2-3. Variation in Vicker microhardness with for Al-3%Mg produced by torsion straining, showing possible division into distinct regions [42]. 77
Figure 2-4. Complied yield stress versus grain size plot for Cu from various sources ranging from coarse to nanograin size. The plots show different trend as the grain size falls below a critical size. 78
Figure 2-5. True stress-strain curves of the 1100-Al with various grain sizes [72]. 79
Figure 2-6. The influence of the volume fraction of ultra-fine grains on the tensile yield, UTS (a) and elongation (b) of AA5754 [87]. 80
Figure 2-7. Room-temperature tensile engineering stress–strain curves for Cu with different microstructures: curve A, the ECAP-Cu; curve B, conventional coarse-grained Cu; curve C, bimodal nanostructured Cu; curve D, the ECAP-Cu cold rolled at liquid nitrogen temperature and annealed at 180 oC for 3 min. The strain rate for all the tests in the figure is 1×10-4 s-1 [85]. 81
Figure 2-8. (a) Tensile engineering and true stress-strain curves of the UFG Cu and bronze. The open squares mark the uniform elongations and the open circles mark .(b) and the inset show the normalized work hardening rate against the true strain and true stress, respectively. The true stress-strain curves and the curves are calculated from the engineering stress-strain curves by assuming a uniform deformation [19]. 82
Figure 2-9. Effects of alloying on (a) true-stress-strain curve and (b) strain hardening rate of nanocrystalline nickel [97]. 83
Figure 2-10. Tensile engineering stress-strain curves of the CG, NS, and NS+P samples [98]. 84
Figure 2-11. Effect of nanotwin density on the mechanical response of copper [108]. 85
Figure 2-12. True stress vs. true strain for ufg Al with 5 ARB passes and for cold-rolled Al tested at different strain rate varying form to . All experiments were performed at room-temperature [111]. 86
Figure 2-13. The measured strain-to-fracture as a function of strain rate for the nc Cu sample and coarse-grained Cu specimen [112]. 87
Figure 2-14. Nominal engineering stress-strain curves of ufg Cu at different strain rates [113]. 88
Figure 2-15. Schematic of the variation of flow stress as a function of grain size in mc, ufg, and nc metal and alloys [115]. 89
Figure 2-16. Volume fractions of the crystalline, the grain boundary, the triple line and quadruple node as a function of grain size. The grain-boundary thickness is assumed to be 1 nm [116]. 90
Figure 2-17. (a) Coble creep and (b) twinning believed to accommodate deformation for grain sizes lower than 100 nm [117]. 91
Figure 2-18. Phase diagram of the Al-Zn system [140]. 92
Figure 3-1. Schematic illustration of the set-up used for cold compact of powders. 93
Figure 3-2. Schematic illustration of the processing procedures. 94
Figure 3-3. The geometry of the tensile specimen. 95
Figure 3-4. Configuration of polished tensile test specimen showing the marker line and marker positions of strain. The width of marker line is 70 nm. 96
Figure 3-5. Flow chart of the experimental procedures in this research. 97
Figure 4-1. (a)(c)(e)(g)(i)(k): TEM micrographs for the A-l, A-2, A-3, A-4, A-5 and A-6 samples, and (b)(d)(f)(h)(j)(l): grain size distribution charts of the A-l, A-2, A-3, A-4, A-5 and A-6 specimens. 99
Figure 4-2. (a) Tensile engineering stress-strain curves of the A-l, A-2, A-3, A-4, A-5 and A-6 samples from RT at (b) True stress-strain curves converted from the curves in (a) using standard formula to demonstrate strain hardening. The curves are truncated because the conversion requires assumption invalid for strains beyond that corresponding to the maximum engineering stress in (a). 100
Figure 4-3. Plot of log true stress vs. log true strain of the A-l, A-2, A-3, A-4, A-5 and A-6 samples. 101
Figure 4-4. Bright-field TEM image from the gauge sections of the A-1 sample: (a) before and (b) after about =0.05. 102
Figure 4-5. (a)(c)(e)(g)(i)(k): TEM micrographs for the A5Z-l, A5Z-2, A5Z-3, A5Z-4, A5Z-5 and A5Z-6 samples, and (b)(d)(f)(h)(j)(l): grain size distribution charts of the A5Z-l, A5Z-2, A5Z-3, A5Z-4, A5Z-5 and A5Z-6 samples. 104
Figure 4-6. (a)(c)(e)(g)(i)(k): TEM micrographs for the A15Z-l, A15Z-2, A15Z-3, A15Z-4, A15Z-5 and A15Z-6 samples, and (b)(d)(f)(h)(j)(l): grain size distribution charts of the A15Z-l, A15Z-2, A15Z-3, A15Z-4, A15Z-5 and A15Z-6 samples. 106
Figure 4-7. (a) Tensile engineering stress-strain curves for the A5Z-l, A5Z-2, A5Z-3, A5Z-4, A5Z-5, and A5Z-6 samples tested at RT and (b) True stress-strain curves converted from the curves in (a) using standard formula to demonstrate strain hardening. The curves are truncated because the conversion requires assumption invalid for strains beyond that corresponding to the maximum engineering stress in (a). 107
Figure 4-8. (a) Tensile engineering stress-strain curves for the A15Z-l, A15Z-2, A15Z-3, A15Z-4, A15Z-5, and A15Z-6 samples tested at RT and (b) True stress-strain curves converted from the curves in (a) using standard formula to demonstrate strain hardening. The curves are truncated because the conversion requires assumption invalid for strains beyond that corresponding to the maximum engineering stress in (a). 108
Figure 4-9. Plot of log true stress vs. log true strain of the A5Z-l, A5Z-2, A5Z-3, A5Z-4, A5Z-5 and A5Z-6 samples. 109
Figure 4-10. Plot of log true stress vs. log true strain of the A15Z-l, A15Z-2, A15Z-3, A15Z-4, A15Z-5 and A15Z-6 samples. 110
Figure 4-11. The SEM surface topography of the broken A-1 sample tested at a strain rate of 1x10-3 s-1 with different strains (a) (b) (c) and (d) , showing the offsets of surface marker line and evolution. The tensile axis is horizontal for all images. 111
Figure 4-12. The SEM surface topography of the broken A5Z-1 sample tested at a strain rate of 1x10-3 s-1 with different strains (a) ; (b) ; (c) and (d) , showing the offsets of surface marker line (marked by white arrows) and porosities induced by grain boundary sliding. The tensile axis is horizontal for all images. 112
Figure 4-13. The SEM surface topography of the broken A15Z-1 sample tested at a strain rate of 1x10-3 s-1 with different strains (a) ; (b) ; (c) and (d) , showing the offsets of surface marker line (marked by white arrows) and porosities induced by grain boundary sliding. The tensile axis is horizontal for all images. 113
Figure 4-14. Bright-field TEM image from the gauge sections of the A15Z-1 sample: (a) before and (b) after about =0.15 tensile tests. The white arrows mark the nano-scale alumina. 114
Figure 4-15. HREM images of grain boundary of A15Z-1 sample in as FSPed before tensile test. The area marked with a small white square on the inset of HREM images above (a) and (c). (b) and (d) The Fourier-filtered images obtained from Fourier Transformation of the area marked with a big white square on the HREM image above (a) and (c). 115
Figure 4-16. HREM images of grain boundary of A15Z-1 sample in as FSPed after about =0.05. The area marked with a small white square on the inset of HREM images above (a) and (c). (b) and (d) The Fourier-filtered images obtained from Fourier Transformation of the area marked with a big white square on the HREM image above (a) and (c). 116
Figure 4-17. (a)(c)(e)(g)(i)(k)(m): SEM micrographs for the Z5-1, Z5-2, Z5-3, Z5-4, Z5-5, Z5-6, and Z5-7 samples, and (b)(d)(f)(h)(j)(l)(n): grain size distribution charts of the Z5-1, Z5-2, Z5-3, Z5-4, Z5-5, Z5-6, and Z5-7 samples. 119
Figure 4-18. (a)(c)(e)(g)(i)(k)(m): SEM micrographs for the Z15-1, Z15-2, Z15-3, Z15-4, Z15-5, Z15-6, and Z15-7 samples, and (b)(d)(f)(h)(j)(l)(n): grain size distribution charts of the Z15-1, Z15-2, Z15-3, Z15-4, Z15-5, Z15-6, and Z15-7 samples. 122
Figure 4-19. (a) Tensile engineering stress-strain curves of the Z5-l, Z5-2, Z5-3, Z5-4, Z5-5, Z5-6 and Z5-7 samples tested at RT and (b) True stress-strain curves converted from the curves in (a) using standard formula to demonstrate strain hardening. The curves are truncated because the conversion requires assumption invalid for strains beyond that corresponding to the maximum engineering stress in (a). 123
Figure 4-20. (a) Tensile engineering stress-strain curves for the Z15-l, Z15-2, Z15-3, Z15-4, Z15-5, Z15-6 and Z15-7 samples tested at RT (b) True stress-strain curves converted from the curves in (a) using standard formula to demonstrate strain hardening. The curves are truncated because the conversion requires assumption invalid for strains beyond that corresponding to the maximum engineering stress in (a). 124
Figure 4-21. Plot of log true stress vs. log true strain of the Z5-l, Z5-2, Z5-3, Z5-4, Z5-5, Z5-6 and Z5-7 samples. 125
Figure 4-22. Plot of log true stress vs. log true strain of the Z15-l, Z15-2, Z15-3, Z15-4, Z15-5, Z15-6 and Z15-7 samples. 126
Figure 4-23. The SEM surface topography of the broken Z15-1 sample tested at a strain rate of 1x10-3 s-1 with different strains (a) ; (b) ; (c) and (d) , showing the offsets of surface marker line (marked by white arrows) and porosities induced by grain boundary sliding. The tensile axis is horizontal for all images. 127
Figure 4-24. The SEM surface topography of the broken Z15-5 sample tested at a strain rate of 1x10-3 s-1 with different strains (a) ; (b) ; (c) and (d) , showing the offsets of surface marker line (marked by white arrows) and porosities induced by grain boundary sliding. The tensile axis is horizontal for all images. 128
Figure 4-25. The SEM surface topography of the broken Z15-6 sample tested at a strain rate of 1x10-3 s-1 with different strains (a) ; (b) ; (c) and (d) , showing the offsets of surface marker line (marked by white arrows) and porosities induced by grain boundary sliding. The tensile axis is horizontal for all images. 129
Figure 4-26. The SEM surface topography of the broken Z5-1 sample tested at a strain rate of 1x10-3 s-1 with different strains (a) ; (b) ; (c) and (d) , showing the offsets of surface marker line (marked by white arrows) and porosities induced by grain boundary sliding. The tensile axis is horizontal for all images. 130
Figure 4-27. Bright-field TEM image from the gauge sections of the Z5-1 sample: (a) before and (b) after tensile test at RT ( ). 131
Figure 4-28. Bright-field TEM image from the gauge sections of the Z15-1 sample: (a) before and (b) after tensile test at RT ( ). 132
Figure 4-29. (a) HREM image of a grain boundary for Z15-1 sample before deformed at . Grain 1 is aligned along the zone axis. The angle between the (002) planes in grain 1 and grain 2 is 60o. (b) Fourier-filtered image. Grain-boundary relaxation can be observed at the grain boundary. Residual dislocations (encircled) in as- FSPed are observed at the grain boundary. 133
Figure 4-30. (a) HREM image of a grain boundary for Z15-1 sample after deformed at . Grain 1 is aligned along the zone axis. The angle between the (002) planes in grain 1 and grain 2 is 60o. (b) Fourier-filtered image. Grain-boundary relaxation can be observed at the grain boundary. 134
Figure 4-31. An HREM image along a matrix zone axis of the gauge section of tensile sample of Z15-1 sample showing regions of dark contrast (spherical G-P zones) after deformation at a strain rate of 1×10-3. 135
Figure 5-1. Grain size dependence ( ) of (a) uniform and (b) total elongation at RT for pure Al, and A (A-1~A-6) sample. 136
Figure 5-2. Grain size dependence ( ) of (a) uniform and (b) total elongation at RT for pure Al, A5Z, and A15Z samples. 137
Figure 5-3. Grain size dependence ( ) of (a) uniform and (b) total elongation at RT for pure Al, Z5, and Z15 samples. 138
Figure 5-4. Comparison of the normalized work hardening rates, Θ, against the true
strain for pure Al, A-1, A5Z-1, and A15Z-1 samples. The black circle
indicate the strain positions which is work hardening rate equal to zero for
A-1, A5Z-1 and A15Z-1. 139
Figure 5-5. Hall-Petch for the effect of grain size on the yield and flow stress of
Al-15Al2O3 composite from RT at ε =1×10−3s−1 140
Figure 5-6. Work hardening behavior of two fine (A-1 sample) and coarse-grained
(A-5 sample) conditions. 141
Figure 5-7. Hall-Petch for the effect of grain size on the yield and flow stress of
Al-5Zn-15Al2O3 (a) and Al-15Zn-15Al2O3 (b) samples from RT
atε =1×10−3s−1 142
Figure 5-8. Dependence of friction stress (a) e σ and (b) k at various strains for
Al-15Al2O3, Al-5Zn-15Al2O3 and Al-15Zn-15Al2O3 samples. 143
Figure 5-9. Hall-Petch for the effect of grain size on the yield and flow stress of
Al-5Zn (a) and Al-15Zn (b) samples from RT atε =1×10−3s−1 144
Figure 5-10. The statistic boundary misorientation angles distribution of Z15-2 samples. 145
Figure 5-11. Tensile true stress-strain curves of the (a) A-1, (b) A5Z-1 and (c) A15Z-l
samples from RT atε =1×10−3s−1 and ε =1×10−4s−1
146
Figure 5-12. Tensile true stress-strain curves of the (a) Z5-1 and (c) Z15-l samples
from RT atε =1×10−3s−1 and ε =1×10−4s−1 147
Figure 5-13. Strain rate sensitivity of Al as a function of grain size. The reference
(including of Z5-1 and Z15-1) and its first author corresponding to each
data point are also given in the plot. 148

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