Quasi-four-dimensional characterization of dislocation interactions in FCC and HCP systems

Transmission electron microscopy (TEM);Dislocations;Electron tomography;Mechanical deformation

Kacher, Joshua

关键词: Transmission electron microscopy (TEM);    Dislocations;    Electron tomography;    Mechanical deformation;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/42379/Joshua_Kacher.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Dislocation/grain boundary and dislocation/dislocation interactions have been investigated in austenitic stainless steel and α-Ti systems using in situ deformation in a transmission electron microscope, diffraction contrast electron tomography for three-dimensional analysis, and electron backscatter diffraction for orientation determination. In situ deformation experiments were conducted at room temperature and elevated temperature, as well as combined experiments where the temperature was varied over the course of an experiment. A novel technique to combine in situ deformation experiments with periodic three-dimensional snapshots is introduced and its application to understanding dislocation interactions is demonstrated. This technique removes the requirement inherent to diffraction contrast electron tomography of maintaining consistent diffraction conditions during image acquisition by using manual digital processing of the micrographs making up a tilt series prior to image alignment and tomographic reconstruction.In the study of stainless steel, it was found that previously determined criteria for slip transfer across grain boundaries hold true at elevated temperature as well as room temperature. Details of the interactions observed at elevated temperatures, however, varied in that raising the temperature resulted in the interactions reaching higher levels of complexity earlier as well as a reduction in the barrier strength of grain boundaries. The slip transfer criteria were extended to include the emission of partial dislocations at grain boundaries. It was found that only the lead partial dislocation need be considered initially. If the trailing partial dislocation significantly increases the strain energy in the boundary, the primary system shuts down and a differentiiisystem activates. Thin film effects were not found to play a significant role in determining dislocation interactions at grain boundaries in stainless steel.Dislocation/grain boundary interactions in α-Ti deformed at room temperature were found to adhere to previously determined slip transfer criteria during in situ deformation. In all interactions characterized, minimization of the strain energy at the boundary was found to dictate the evolution of the interaction. Thin film effects were found to facilitate dislocation glide on planes not usually active at room temperature, increasing the number of available slip systems and potentially simplifying the interactions. Dislocation/grain boundary interactions initiated during deformation of bulk samples were found to be much more complex, emitting multiple dislocation systems simultaneously. The majority of these interactions were found to reduce the grain boundary strain energy, though not in all cases.Dislocation interactions previously observed only post mortem including double cross-slip and dislocation/dislocation interactions were observed in situ. It was found that the development of jogs and cross-slip events led to dislocation generation. Sequential double cross-slip events were also seen to occur, resulting in the emission of multiple loops and half-loops from a single gliding dislocation.