Studies on Microstructure of Annealing Twin and Deformation Twin in α-brass

雙晶是指材料中兩個晶體其方位具有鏡面對稱關係。此論文將針對以下兩種型態的雙晶加以討論：退火雙晶發生在退火時再結晶成長過程中，偶發的堆疊錯誤造成，而機械雙晶則是當基地受到應力應變，為了減少系統總能而產生。較早期的文獻認為機械雙晶僅僅發生在體心立方晶體內，但近期的實驗證明面心立方晶體中不僅有退火雙晶，也可能產生機械雙晶；因此，此篇論文中將介紹較廣為接受的理論及模型來解釋退火雙晶以及機械雙晶的生成原因。 由於雙晶晶界是由Shockley 部分差排以及疊差生成，在某些情況下可以阻礙差排的前進，其往往被視為增強材料機械性質的要素之一；但有時候在雙晶介面附近會有差排滑移現象，因此雙晶材料的延展性可比其他加工硬化材料來得佳。若雙晶晶界附近產生滑移，差排分解成部分差排，則僅有特定方向的部分差排可以繼續往前滑動，晶界會阻礙剩下的差排，因此晶界處會有階梯狀特徵。實驗中我們將以光學顯微鏡以及穿透式電子顯微鏡來觀察退火雙晶、機械雙晶、階梯狀特徵、滑移現象。 此外，本實驗也以電子背向散射繞射儀進行退火雙晶結晶面方位分析，同時以矩陣來計算晶體之間的misorientation；最後利用電子背向散射繞射的結果，施打不同方位的結晶面硬度，以及不同結晶面之間的雙晶晶界硬度。The term of twin in materials represents two crystals with a mirror symmetry relationship. In this thesis, two types of twins will be discussed: one is the annealing twin, and the other is the deformation twin. The formation of annealing twin, as a result of annealing treatment, can be traced back to growth accidents or stacking faults during recrystallization. The deformation twin, on the other hand, is the accommodation to the deformation in matrix owing to the energy minimums. Additionally, some previous studies show that not only in BCC materials but also in FCC materials do deformation twins exist. Some conceivable models and mechanisms will be presented for the formation of annealing twins and deformation twins. The twin boundary, formed by Shockley partial dislocations and stacking faults, is believed to enhance the mechanical property of materials because it can obstruct the movement of dislocations; therefore, the interaction of them and the incoming dislocations are noticeable. However, not every twin boundary will hinder the dislocation; unlike other strengthening method, the existence of twin boundary will improve the ductility. Some of dislocations may cross-slip at the twin boundary, and thus the ductility is maintained. One of the trace of cross-slipping is the ledge of twin. When dislocation dissociates into Shockley partials, only part of incoherent twin boundary can glide continuously, while the other part stops and forms a “step” on boundary. In this thesis, the morphology of annealing twin, deformation twin, the ledge, cross-slipping will be shown by optical microscopy and transmission electron microscopy. Besides, the orientation of annealing twins will be manifested by EBSD, and the identification of misorientation will be identified by orientation matrices. The result of EBSD will be used in the hardness test, which considers the effect of twin boundary in different orientation