UC Santa Barbara

Advanced characterization techniques in materials science permit a greater understanding of how a material's processing history affects the microstructure of metallic alloys. With the goal of understanding how different processing and loading conditions impact the relationship between microstructure, damage, and defect evolution, an investigation on dislocation-void interactions, what defects exist in as-built materials, and how grain-level properties such as grain size and orientation affect the development of defect structures in a variety of material systems. This investigation was performed by using 3D spatially resolved orientation information obtained via TriBeam Tomography to calculate 3D spatial mappings of Geometrically Necessary Dislocations (GNDs), which are dislocations that satisfy elastic-plastic compatibility when grains within polycrystals develop lattice orientation gradients. A general approach to calculating GND densities is provided, emphasizing the ability to calculate GND densities for any crystal structure, provided proper motivation for the inclusion of specific slip systems or modes. Implementing this theory and extending it from FCC slip systems to BCC and HCP, GND density maps for IN718, AlNiCo, AM pure Ta, spalled wrought Ta, spalled Additively Manufactured (AM) Ta, and Ti7Al are investigated. AM IN718, pure Ta, and AlNiCo exhibit GND densities present in the as-built microstructures, with various subboundaries clearly present in the IN718 and pure Ta microstructures but not apparent in the AlNiCo material. GND density formation is also shown to be linked to void growth in response to shock loading for the spalled wrought Ta and spalled AM Ta samples, with grain boundary nucleation of voids preferred over nucleation of voids near high GND density subboundaries in the AM spalled Ta specimen. GND densities were also shown to accumulate in Ti7Al in response to plastic deformation, and the disparity in calculated GND densities between the differently strained samples supported the idea that microstructures experiencing greater mechanical strain have a greater amount of stored dislocations, and are important to include in forward modeling approaches. The 3D microstructures used for these GND calculations clearly provide the 3D information crucial for calculating a full Nye tensor and are necessary for full characterization of subboundaries and other subgranular features in a variety of pure metals and alloys.