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

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.