Observation of the formation and sharpening of geometrically necessary boundaries

We present in-situ visualisation of incidental dislocation boundaries (IDBs) and geometrically necessary boundaries (GNBs) in single-crystal aluminium under tensile load along [111]. Below 5.10% strain, plastic deformation manifests through the stochastic formation of IDBs. GNBs emerge abruptly between 5.15 and 5.20% strain, forming along pre-existing dislocation structures and aligning with slip traces, as confirmed by autocorrelation analysis. This transition occurs across tens of micrometers. Orientation and dislocation density mapping reveal that strain accommodation occurs through boundary sharpening, dislocation migration, and elastic energy release. These findings provide insights into dislocation patterning mechanisms and establish a foundation for refining physics-based plasticity models.