Dislocation Density Effects on Annealing Twin Nucleation and Recrystallization Mechanism Activation

Grain boundary engineered microstructures consisting of highly interconnected twin-related boundary networks have been developed through empirical refinement of processing parameters. These microstructures show improved performance in terms of mechanical properties, intergranular corrosion, and segregation of solutes. Two distinct processing regimes have been identified, consisting of low deformation followed by intermediate temperature, long anneals and intermediate deformation followed by intermediate temperature short anneals. The underlying mechanisms by which grain boundary microstructures form during these processes are not well understood. Improving the understanding of the underlying twin formation and recrystallization mechanisms will allow grain boundary engineering processes to be refined. In order to gain insight into these mechanisms, a set of analysis tools were developed and then applied to the study of recrystallization in copper and Cu90Zn10 specimens at a range of processing conditions. Specifically, a method was developed to estimate geometrically necessary dislocation densities using spatially resolved orientation data acquired in both scanning electron microscopes and transmission electron microscopes. A similar method of assessing disclination densities was explored but determined not to be viable. Additionally a method of determining the crystallographic orientation of grain boundary planes in transmission electron microscopy specimens was developed. Twin nucleation and recrystallization mechanisms in copper and Cu90Zn10 were studied using in situ annealing in a scanning electron microscope in conjunction with electron backscatter diffraction for the characterization of crystallographic orientations. The geometrically necessary dislocation density analysis method was used to measure stored energy and track recrystallization. Specific grain boundaries that formed during recrystallization were prepared for transmission electron microscopy using a focused ion beam in order to gain insight into nucleation mechanisms. It was determined that there is a transition between long-range strain induced boundary migration and primary nucleation and growth as the dominant mechanism of recrystallization as deformation is increased. This transition was found to be at higher levels of deformation in Cu90Zn10 than copper due to the inhibition of grain boundary motion by solutes. Annealing twin nucleation densities were found to be dependent on prior deformation and independent of recrystallization rate, which differs from prior observations. Based on this observation a modification to the growth accident theory of twinning was proposed. Almost all twin formation that was observed likely occurred via growth accidents. A few possible cases of dissociation of random high angle grain boundaries were observed but no direct evidence of this mechanism was acquired. Dissociation of [sigma]9 grain boundaries to form grains bounded by two [sigma]3 boundaries frequently was observed and confirmed as an additional nucleation beyond growth accident twinning.Ph.D., Materials Science and Engineering -- Drexel University, 201