無間隙原子鋼之連續冷卻相變態組織分析及差排密度量測

Abstract

本文探討極低碳無間隙原子鋼(0.001-0.002 wt% C)的連續冷卻相變態產物。設計中空圓柱式樣進行熱膨脹儀量測以獲得到極高冷速，利用熱膨脹曲線進行分析並獲得Massive Ferrite的相變態溫度及臨界冷速。利用光學顯微鏡(optical microscopy, OM)，電子背向散射繞射技術(electron backscattered diffraction, EBSD)、穿透式電子顯微鏡技術(transmission electron microscope, TEM)來了解massive ferrite的組織型態。massive ferrite中的次結構隨著冷速上升而增加，且擁有較高的差排密度。 第二部分針對材料的非彈性平均自由徑(inelastic mean free path)量測進行探討。能量散失能譜儀(electron energy-loss spectrometry)為一厚度量測之利器。本實驗利用能量散失能譜儀，在已知厚度的情況下，利用TEM與STEM模式量測非彈性散射平均自由徑之值。探討不同電子顯微鏡操作條件下，平均自由徑與各項參數之間的趨勢關係。平均自由徑在TEM與STEM模式下，皆會隨著收斂半角的上升而下降。而在STEM中匯聚角所產生的影響較為顯著，並隨著匯聚角上升，非彈性平均自由徑會隨之下降。同時，差排密度以及材料內非晶質層對平均自由徑所造成的影響也在研究中被探討。

Ultra-low carbon IF steels (0.001-0.002 wt% C) were studied in the present research. Hollow cylinder specimens were used to obtain rapid cooling by the high resolution dilatometer. The transformation mechanism was analyzed by dilatometric measurement. The transformation temperature of massive ferrite and the critical cooling rate in corresponding steel were obtained. Optical microscopy (OM), electron backscattered diffraction (EBSD), and transmission electron microscope (TEM) were used to realize the effects of cooling rate and characterize grain morphologies of massive ferrite. The fraction of massive ferrite, containing lots amount of substructure, increases with the cooling rate. Dislocation density of massive ferrite was measured and is higher than that in polygonal ferrite. In the second part, the research focuses on the inelastic mean free path measurement by using electron energy-loss spectrometry (EELS). Log-ratio method is used widely in the thickness measurement. The inelastic mean free path was measured under both TEM and STEM mode. Different experimental conditions were operated. The mean free path decreases with the increasing of semi-collection angle in both TEM and STEM mode. The convergent angle effect is more significant under STEM mode, and the results show that lager convergent angle make the mean free path small. The effects of dislocation and amorphous layer have also been revealed.