Kinetics of crystallization in undercooled phase-separated molten Fe₈₀C₂₀ alloys.

• Other Authors: Yip, Kai Hou.
• Format: Others
• Language: English; Chinese
• Published: 2012
• Subjects: Crystallization; Interfaces (Physical sciences); Chemical kinetics; Alloys > Structure
• Online Access: http://library.cuhk.edu.hk/record=b5549135; http://repository.lib.cuhk.edu.hk/en/item/cuhk-328594

研究證實，助焊劑能將溶融合冷卻至其溶點之下，仍保持溶液態，亦即達到過冷態。這些溶液因而能夠進液態調幅分。它們凝固之後，會有種互相接的相。在這篇文，這種結構的合簡稱為網絡合。近期研究顯示，白鑄鐵Fe₈₀C₂₀ 亦可憑上述方法，冶成網絡合。其網絡合由種相構成。它們分別是αFe 子網絡及Fe₃C 子網絡，前者柔韌後者堅硬，因此這種網絡合有很優越的機械性能。 === 久之前，有報告研究Fe₇₉.₅B₆.₅C₁₄ 及Fe₈₄B₁₆ 網絡合的結晶過程。這篇文中，研究會集中在過冷Fe₈₀C₂₀ 的結晶過程。其微觀結構分為三區：區由無序網絡構成，十分細小。區之外遍佈高碳的Fe₃(C,B)，由於成份上與Fe₇₉.₅B₆.₅C₁₄的C區相，因此Fe₈₀C₂₀A區外的區稱作C₁ 區及C₂ 區，以相對照。C₁ 區的種子網絡成棒。C₂ 區與Fe₇₉.₅B₆.₅C₁₄ 的C 區一樣，網絡有明顯方向性，且長成樹幹圖案，其Fe₃(C,B) 子網絡屬多晶結構。 === 我們認為合的碳含是引致Fe₈₄B₁₆，Fe₇₉.₅B₆.₅C₁₄，Fe₈₀C₂₀ 三種網絡合，於微觀結構上有差別的原因。高碳的Fe₃(C,B)比低碳的Fe₃(C,B)難生長。以此，我們解釋上述三個合的結晶過程。在Fe₈₀C₂₀ 系統，大碳原子堆積於生長中的固體/液體界面前，這引致細小的A區､C₁ 區的高碳Fe₃(C,B)枝晶出現。 === By employing a fluxing technique, molten alloys can be undercooledsubstantially below its liquidus. The melts carry out phase separation by liquid state spinodal decomposition. After crystallization, solids with interconnected phases are obtained. They are called network alloy in this work. Recently, it is reported that a Fe₈₀C₂₀ eutectic ingot can be cast into a network alloy. The network alloy has two constituent phases. One of which is a ductile αFe subnetwork and the other one is a strong Fe₃C subnetwork. Therefore the network alloy has attractive mechanical properties. === The kinetics of crystallization in undercooled Fe₇₉.₅B₆.₅C₁₄ and Fe₈₄B₁₆ are latelyreported. In this thesis, the kinetics of crystallization in undercooled Fe₈₀C₂₀ alloy was studied. The microstructure can be classified into three zones. Zone A is a small random network. Outside zone A, the microstructure contains high-carbon Fe₃(C,B). In terms of the composition of Fe₃(C,B), they are analogous to the zone C inFe₇₉.₅B₆.₅C₁₄ system. Therefore the two zones outside zone A are named zone C₁ and C₂. Zone C₁ contains dendrites of the two subnetworks. Zone C₂ is the same as thezone C in Fe₇₉.₅B₆.₅C₁₄ systems, which is an aligned network structure showing patterns. The structure of Fe₃(C,B) subnetwork is polycrystalline. === The difference in microstructures between Fe₈₄B₁₆, Fe₇₉.₅B₆.₅C₁₄ and Fe₈₀C₂₀ isattributed to the carbon concentration. The formation of high carbon Fe₃(C,B) is less favoured than low carbon Fe₃(C,B). By this, the kinetics of crystallization in the 3 systems is explained. In Fe₈₀C₂₀, a high concentration of carbon atoms is established in front of the growing solid/liquid interface. This results in the presence of a small zone A and high carbon Fe₃(C,B) dendrites (zone C₁). === Detailed summary in vernacular field only. === Detailed summary in vernacular field only. === Detailed summary in vernacular field only. === Yip, Kai Hou = 因過冷引致相分離的網絡結構Fe₈₀C₂₀合金的結晶動力學 / 葉繼豪. === Thesis (M.Phil.)--Chinese University of Hong Kong, 2012. === Includes bibliographical references. === Abstracts also in Chinese. === Yip, Kai Hou = Yin guo leng yin zhi xiang fen li de wang luo jie gou Fe₈₀C₂₀ he jin de jie jing dong li xue / Ye Jihao. === Abstract --- p.i === Acknowledgements --- p.v === List of Figures --- p.ix === Chapter Chapter 1: --- Introduction --- p.1 === Chapter 1.1 --- Introduction --- p.1 === Chapter 1.2 --- Composites --- p.1 === Chapter 1.2.1 --- Different Types of Composites --- p.1 === Chapter 1.2.2 --- Fabrication of Composites --- p.3 === Chapter 1.3 --- Phase Transformation --- p.3 === Chapter 1.4 --- Nucleation --- p.5 === Chapter 1.4.1 --- Homogeneous Nucleation --- p.5 === Chapter 1.4.2 --- Heterogeneous Nucleation --- p.6 === Chapter 1.5 --- Growth --- p.7 === Chapter 1.5.1 --- Solidification in Pure Metals --- p.8 === Chapter 1.5.2 --- Solidification in Alloys --- p.9 === Chapter 1.5.2.1 --- Growth of Single-Phase Alloys --- p.9 === Chapter 1.5.2.2 --- Solidification in Eutectic Binary Alloys --- p.11 === Chapter 1.6 --- Phase Separation by Spinodal Decomposition --- p.12 === Chapter 1.6.1 --- Spontaneous Phase Separation --- p.12 === Chapter 1.6.2 --- Uphill Diffusion --- p.13 === Chapter 1.6.3 --- Modified Diffusion Equation --- p.14 === Chapter 1.6.4 --- Solution to the Equation --- p.16 === Chapter 1.6.5 --- Morphology Resulted from Spinodal Decomposition --- p.17 === Chapter 1.7 --- Aim of This Project --- p.18 === Figures --- p.20 === References --- p.28 === Chapter Chapter 2: --- Experiment --- p.30 === Chapter 2.1 --- Introduction --- p.30 === Chapter 2.2 --- Preparation of Fused Silica Tube --- p.30 === Chapter 2.3 --- Sample Preparation --- p.31 === Chapter 2.3.1 --- Preparation of Eutectic Fe₈₀C₂₀ ingots --- p.31 === Chapter 2.3.2 --- Fluxing with Dehydrated B₂O₃ --- p.32 === Chapter 2.4 --- Optical Microscopy Analysis --- p.33 === Chapter 2.5 --- Scanning Electron Microscopy (SEM) Analysis --- p.34 === Chapter 2.6 --- Transmission Electron Microscopy (TEM) Analysis --- p.34 === Chapter 2.6.1 --- TEM Specimen Preparation --- p.34 === Chapter 2.6.1.1 --- Polishing --- p.35 === Chapter 2.6.1.2 --- Ion Milling --- p.35 === Chapter 2.6.2 --- TEM Characterization: Indexing Diffraction Patterns --- p.36 === Chapter 2.6.3 --- TEM Characterization: Electron Energy Loss Spectrum (EELS) --- p.37 === Figures --- p.39 === References --- p.43 === Chapter Chapter 3: --- Kinetics of crystallization in undercooled phase-separated molten Fe₈₀C₂₀ alloys --- p.44 === Chapter 3.1 --- Introduction --- p.45 === Chapter 3.2 --- Experimental --- p.45 === Chapter 3.3 --- Results --- p.46 === Chapter 3.3.1 --- SEM studies --- p.47 === Chapter 3.3.2 --- TEM studies --- p.49 === Chapter 3.4 --- Discussion --- p.55 === Chapter 3.5 --- Conclusion --- p.61 === Figures --- p.62 === References --- p.92 === Bibliography --- p.93