臺灣博碩士論文加值系統

本研究結合了摩擦攪拌製程之熱加工以及鋁與氧化鈰之放熱反應製造原位鋁基複合材料。鋁與氧化鈰之壓錠是由傳統擠壓及燒結所製成，然後經多道次摩擦攪拌之燒結壓錠,其微觀組織藉由使用X光繞射、掃描式電子顯微鏡及穿透式電子顯微鏡觀察描繪其特徵。強化相已被鑑定為Al11Ce3和 δ*Al2O3。氧化鋁均勻分布於鋁基地，其平均顆粒大小約10 nm。鋁基材晶粒平均大小 390-550 nm。穿透式電子顯微鏡觀察顯示氧化鋁與鋁基材具特定晶向關係；其關係為 (223)δ*-Al2O3平行(111)Al 和 [1-10]δ*-Al2O3 大約平行於 [1-10]Al。氧化鋁與鋁之晶向關係明確顯示氧化鋁是經固態析出所產生。推測反應過程應是，摩擦攪拌製程產生過飽合氧於鋁基材中，然後均勻析出奈米級氧化鋁顆粒。
研究結果也顯示，燒結溫度及摩擦攪拌製程之工具走速會明顯影響複合材料微結構及機械性質。此複合材料展現在常溫與高溫均具有高強度；在燒結溫度833 K及走速30 mm/min條件下，楊氏係數為109 GPa，抗拉強度488 MPa及延伸率約3%。造成此複合材料高強度之主要貢獻有二，包括鋁基材次微米晶粒結構以及奈米級氧化鋁顆粒造成之Orowan強化。另外，由於氧化鋁之高溫穩定性，此複合材料在723K下仍有高強度。

In this study, a novel technique was used to produce aluminum based in situ composites from powder mixtures of Al and CeO2. This technique has combined hot working nature of friction stir processing (FSP) and exothermic reaction between Al and oxide. Billet of powder mixtures was prepared by the use of conventional pressing and sintering route. The sintered billet was then subjected to multiple passages of friction stir processing (FSP). The microstructure was characterized by the use of TEM, SEM and XRD. The reinforcing phases were identified as Al11Ce3 and δ*-Al2O3. The Al2O3 particles with an average size of ~10 nm are uniformly distributed in the aluminum matrix, which has an average grain size about 390-500 nm. The analysis of TEM indicated that these Al2O3 particles exhibit crystallographic orientation relationship with the aluminum matrix, i.e., (223)δ*-Al2O3//(111)Al and [1-10]δ*-Al2O3 roughly parallel to [1-10]Al. The precipitates of Al2O3 exhibiting crystallographic orientation relationship with the aluminum clearly indicates that they were formed from solid state precipitation. Apparently, significant supersaturation of oxygen in aluminum had been created in FSP, and nanometric Al2O3 particles were then precipitated uniformly in the aluminum matrix.
This study shows that both sintering temperature and the tool traversing speed used in FSP have significant influence on the microstructure and mechanical properties of the composite. The composites produced exhibit high strength both at ambient and elevated temperatures. For example, the composite produced by 833K sintering followed by FSP with tool traversing speed of 30 mm/min possesses enhanced modulus (E = 109 GPa) and strength (UTS = 488 MPa) as well as a tensile ductility of ~3%. The major contributions to the high strength of the composite are the submicrometer grain structure of aluminum matrix and the Orowan strengthening caused by the fine dispersion of nanometer size Al2O3 particles inside aluminum grains. In addition, the composite also exhibits high strength at elevated temperatures up to 773 K. The good thermal stability and high temperature strength of the composite may be attributed to the uniform dispersion of nanometric Al2O3 particles, which are very stable at elevated temperatures.

Table of Contents…………............................................................................................... I
List of Tables…………………………………………………………………………… III
List of Figures………………………………………………………………………….. IV
Abstract…………………………………………………………………………………. X

Chapter 1 Introduction...................................................................................................... 1
Chapter 2 Literature Review ........................................................................................... 4
2.1 Metal matrix composites (MMCs)........................................................................... 4


2.1.1 Strengthening mechanisms of particulate reinforced MMCs. ……………. 5
2.1.2 Fabrication of in-situ MMCs……………………………………….. 8
2.1.3 Mechanical properties of in situ aluminum matrix composites…………… 12
2.2 Friction stir welding (FSW) …………………………………………………….. 13
2.2.1 Metal flow in FSW…………………………………………………………14
2.2.2 Temperature distribution in FSW…………………………………………..16
2.2.3 Microstructure evolution in FSW…………………………………………. 16
2.3 Friction stir processing (FSP)……………………………………………….. 18
2.4 In situ MMCs produced by FSP…………………………………………….. 19
Chapter 3 Experimental Procedures …………………………………………………. 22
3.1 Materials………………………………………………………………………… 22
3.2 Fabrication of billets for friction stir processing………………………………… 22
3.2.1 Powder mixing…………………………………………………………….. 22
3.2.2 Powder ball-milling……………………………………………………..… 22
3.2.3 Cold compaction…………………………………………………………... 22
3.2.4 Sintering …………………………………………………………………... 23
3.3 Friction stir processing ………………………………………………………….. 23
3.4 Isothermal heat treatment (IHT)……………………...…………………………. 24
3.5 Microstructure analysis………………………………………………………….. 24
3.5.1 Microscopic observation…………………………………………………... 24
3.5.2 X-ray diffraction (XRD)…………………………………………………... 24
3.5.3 Transmission electron microscopy (TEM)………………………………… 25
3.6 Mechanical properties…………………………………………………………… 25
3.6.1 Microhardness measurement……………………………………………… 25
3.6.2 Ambient temperature tensile and compressive tests………………………. 25
3.6.3 Elevated temperature compressive tests………………………………...… 26
Chapter 4 Results………………………………………………………………………. 27
4.1 Microstructure evolution in pre-FSP processing………………………………... 27
4.1.1 Microstructure of cold compaction ……………………………………….. 27
4.1.2 Microstructure after sintering……………………………………………... 27
4.2 Microstructure and mechanical properties of specimens produced by FSP…...... 29
4.2.1 Effect of FSP parameters………………………………………………….. 29
4.2.2 Influence of sintering temperature ………………………………………... 32
4.2.3 Effect of CeO2 content…………………………………………………….. 35
4.2.4 Mechanical properties at elevated temperatures…………………………... 36
4.3 Formation of nanometric Al2O3 particles in composites produced by FSP…. 37

Chapter 5 Discussion ……………………...……………………………… 41
5.1 In-situ reaction and microstructure evolution in FSP…………………………… 41
5.2 Tensile-compressive strength difference ……………………………………… 42
5.3 Strengthening mechanisms……………………………………………………… 43
Chapter 6 Conclusion …………………………………………………………………. 45
Chapter 7 References ……………………………………………………………...…. 113

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