ABSTRACTCHARACTERIZATION OF DEFORMATION NEAR GRAIN BOUNDARIES IN POLYCRYSTALLINE METALSByJames Robert SealUnderstanding and describing plastic deformation in polycrystalline materials is fundamentally challenging due to the complex atomic rearrangements that must occur at grain boundaries. These atomic rearrangements can have long-range and substantial impacts on a material's bulk behavior and material properties. Thus, there is a significant need to develop new techniques to study, correlate, and describe deformation accommodation at grain boundaries. Understanding how grain boundaries accommodate plastic deformation at the microscale will provide new insight into the evolution of heterogeneous deformation, stress concentration, and damage nucleation. A series of comprehensive experiments have been conducted in order to develop a quantitative and crystallographically based understanding of the relationships between deformation behavior, material microstructure, and slip transfer mechanisms across grain boundaries in polycrystalline materials.Slip transfer events in polycrystalline metals were investigated using novel analysis techniques in scanning electron microscopy (SEM). The objective of these experiments was to correlate observations of slip transfer with a geometric parameter m', which can be used to identify and predict crystallographic arrangements that are better suited for slip transfer. An emphasis was placed on understanding how the parameter m' can be correlated with heterogeneities in local lattice orientations and local stresses near grain boundaries. A large population of slip transfer reactions across α/β phase boundaries in Ti-5Al-2.5Sn were imaged by SEM and slip system activity was characterized using electron backscattered diffraction (EBSD) and slip trace analysis. Statistical correlations identified that slip transfer across the α/β phase boundary was strongly influenced by slip plane alignment across the interface. Slip direction alignment was not strongly correlated to observations of slip transfer and the parameter m' was not useful for correlating slip transfer across the phase boundary. A brittle interfacial phase was observed between the α/β phase boundary and prevented deformation accommodation through slip transfer. Microcantilever beams in Ti-5Al-2.5Sn were fabricated and deformed in order to isolate slip transfer events at α/β phase boundaries and study stresses that accompanied slip transfer events. Microbeam bending experiments were ineffective for repeatedly activating slip transfer across the α/β phase boundary and it was difficult to relate measured loads with specific instances of slip transfer, deformation response, and local stresses. A new approach, using electron channeling contrast imaging (ECCI) combined with EBSD, was developed to further study local lattice misorientations and stresses near grain boundaries with evidence of slip transfer and correlate these measurements with the parameter m'. The distribution of dislocations near grain boundaries was measured using ECCI and successfully fit to a stress model that followed a 1/√x relationship. Local lattice misorientations matched modeled stress distributions. Boundaries that showed little evidence of slip transfer and were correlated with low values of m' had discontinuous stress distributions across the grain boundary interface and large local lattice misorientations.This work has led to a better understanding of the relationship between heterogeneous deformation, slip transfer, and the evolution of local stresses at grain boundaries. Future studies following these approaches may lead to a greater ability to identify and anticipate slip transfer events, which can be helpful for predicting and mitigating damage nucleation and failure evolution.
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