Pattern initiation in plastically deformed metals

The emergence of crystalline defects known as dislocations and their subsequent self-organisation into complex patterns is crucial for understanding processes such as the strengthening of a metal via deformation, called work-hardening. Existing techniques are inadequate for studying this process in embedded crystals in thick samples, making it challenging to derive theoretical models that accurately explain microstructure evolution from first principles.
This thesis explores the utilization of Dark-Field X-ray Microscopy (DFXM) to investigate the early stages of microstructural evolution during tensile deformation in pure single crystal aluminum. The main goal is to fill gaps left by traditional techniques like Transmission Electron Microscopy (TEM), with the main hypothesis being that the initial structural changes in aluminum microstructure occur much earlier than previously documented.
The study is divided into two primary experiments. The first is an ex-situ analysis of aluminum sheet were incrementally deformed. Findings revealed that the formation of Geometrically Necessary Boundaries (GNBs) initiates at as low as 0.6% strain, significantly earlier than reported. This was followed by detailed characterization at 1.7% and 3.5% strain, where intersecting planes in CoM maps confirmed the presence of GNBs, aligning with known data for more substantial deformations. Quantitative analysis via autocorrelation enhanced the depth of these findings.
The second experiment conducted an in-situ investigation into cell formation during tensile deformation. Using a pure aluminum single crystal oriented along the (111) tensile axis, the study mapped cell boundaries and their evolution through mosaicity maps and KAM masks at varying deformation levels. Observations indicated cell formation and refinement, with cell size distribution fitting a log-normal model, thus suggesting a Markovian growth-fragmentation process. Peak broadening, which is a proxy of dislocation density, allowed detailed insights into dislocation density at cell boundaries. This part of the study also contributed a novel data analysis pipeline, simplifying data extraction from DFXM measurements.
These findings establish DFXM as a uniquely capable technique for capturing early microstructural evolution in deformed metals, bridging gaps left by traditional methods and enabling more accurate theoretical models. The evidence shows that the early formation of dislocations and GNBs at minimal strain levels provides new insights into work-hardening mechanisms, leading to the development of stronger and more resilient metal alloys. In summary, this research enhances our understanding of microstructural evolution in metals and highlights the transformative potential of DFXM in advancing both material science and engineering.