本研究採用分子動力學模擬研究AlCoCrFeNi 高熵合金(HEA)在奈米刮刻和奈米壓印下的機械性質。結果表明，表面磨損特性和刮痕所引起的表面破壞明顯取決於雙晶界間的結晶取向、間隔和傾斜角度等影響因素。對於晶體取向的變化，最大壓痕力會隨著晶體取向[111]、[001]和[101]的順序而增加。在晶體取向[001]時有最大的摩擦係數，這表明壓頭在基板中的移動受到的限制最大。微結構演變顯示了Lomer-Cottrell 和 Hirth 差排在各個滑移系統中不同角度之間的形成。Hall-Petch關係和inverse Hall-Petch關係可以透過不同的雙晶界間距所觀察到，並由於差排和雙晶界之間的各種相互作用，會在彼此傾斜角度為0°時達到最大壓痕力。顯然，變形和微觀結構演變以及原子流向很大程度上取決於雙晶界彼此間的間隔和傾斜角度，其中雙晶界遷移是重要因素。工件的表面形貌會因為表面的彈性恢復、差排的成核與滑移等，存在著顯著差異，這意味著材料的結構會決定磨損量的多寡。此外，在奈米壓印過程中仔細分析了晶體結構、合金成分、晶粒尺寸和雙晶界距離對力學性能的影響。壓印結果表明，最高負載力依序為多晶、奈米雙晶多晶和單晶。合金比例的變化表明，壓印力會隨著合金中鋁比例的增加而增加。由於差排和晶粒/雙晶界之間的相互作用，可以在多晶結構中觀察到reverse Hall-Petch關係，而在多晶奈米雙晶中發現Hall-Petch和reverse Hall-Petch關係。變形行為表明，剪切應變和局部應力不僅集中在壓頭周圍，且還集中在晶界處與晶界周圍。晶界的滑移和扭曲在多晶結構的變形機制中起了重要作用。在奈米雙晶多晶的奈米壓印過程中觀察到雙晶界遷移。此外，合金成分和晶粒尺寸的改變對材料的彈性恢復影響不大，且隨著雙晶邊界距離的減小，圖案的可成形性會更高。

The mechanical properties of AlCoCrFeNi high-entropy alloys (HEAs) are brought to light through nano-scratching and nano-imprinting processes using molecular dynamics (MD). The results show that the surface wear characteristics and the scratching-caused surface damage depend significantly on influencing factors such as crystallographic orientation, spacing and inclination angle of the twin boundary. For the variation of crystallographic orientation, the greatest indentation force increases with the order of crystallographic orientation as [111], [001] and [101]. The largest friction coefficient belongs to the crystallographic orientation [001], indicating that the movement of the indenter in this substrate is most restricted. The microstructure evolution reveals the formation of Lomer-Cottrell and Hirth dislocation locks because of the distinctness of angle between various slip systems. Both the Hall–Petch and inverse Hall–Petch relationships are observed for the difference of twin boundary spacing, and the maximum indentation force achieves with a tilt angle of 0° resulting from the various interactions among the dislocations and twin boundaries. Obviously, the deformation and microstructure evolution and the atomic flow are greatly dependent on the spacing and the inclination angle of twin boundary, where the twin boundary migration being the significant factor. The surface morphology is significantly different between workpieces due to the elastic recovery at the surface, nucleation and slipping of dislocation, which implies that the wear volume depends on the structure of the material. Furthermore, the influences of crystal structure, alloy composition, grain size, and twin boundary distance on the mechanical properties are carefully analyzed under the nano-imprinting process. The imprinting load indicates that the highest loading force is in ascending order with polycrystalline, nano-twinned polycrystalline, and monocrystalline. The change in alloy composition suggests that the imprinting force increases as the Al content in the alloy increases. The inverse Hall–Petch relation found for the polycrystalline structure, while the Hall–Petch and inverse Hall–Petch relations are discovered in the nano-twinned polycrystalline, which is due to the interactions between the dislocations and grain/twin boundaries. The deformation behavior shows that shear strain and local stress are concentrated not only around the punch but also on grain boundaries and adjacent to grain boundaries. The slide and twist of the grain boundaries play a major in controlling the deformation mechanism of polycrystalline structure. The twin boundary migrations are detected during the nanoimprinting of the nano-twinned polycrystalline. Furthermore, the elastic recovery of material is insensitive to changes in alloy composition and grain size, and the formability of the pattern is higher with a decrease in twin boundary distance.

摘要 i
ABSTRACT iv
Acknowledgments vi
Contents viii
List of abbreviations and symbols x
List of table captions xii
List of figure captions xiii
Chapter 1 INTRODUCTION 1
1.1 Overview 1
1.2 Thesis objectives 4
1.3 Organization of the dissertation 5
Chapter 2 LITERATURE REVIEW 7
2.1 High-entropy alloy 7
2.2. Nano-scratching process 11
2.3 Nano-imprinting process 13
Chapter 3 SIMULATION METHODS 15
3.1 Molecular dynamics simulation 15
3.2 Physical model for nano-scratching process 19
3.3 Physical model for nano-imprinting process 23
3.4 Interaction potential 28
3.5 Deformation distribution and local structure identification 29
Chapter 4 RESULTS AND DISCUSSION 32
4.1 Influences of crystallographic orientation on nanotribological characteristics and subsurface damage of AlCoCrFeNi high-entropy alloy subjected to scratching process 32
4.2 Influences of twin boundary on nanotribological characteristics and subsurface damage of AlCoCrFeNi high-entropy alloy subjected to scratching process 48
4.2.1 Effect of twin boundary spacing 48
4.2.2 Effects of twin boundary inclination 59
4.3 Microstructure and composition dependence of mechanical characteristics of nanoimprinted AlCoCrFeNi high-entropy alloys 73
4.3.1. Influence of crystal structure 73
4.3.2 Influence of alloy composition 85
4.3.3. Influence of grain size 91
4.3.4 Influence of twin boundary distance 97
4.3.5 Evaluations of elastic recovery ratio 103
4.4 Summary and comparison 105
Chapter 5 CONCLUSIONS AND FUTURE WORK 108
5.1 Conclusion 108
5.2 Future work 111
References 113
Curriculum vitae 128
Publication list 129

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