EBSD-Derived Misorientation Analysis of Stage-Dependent Grain Refinement in High-Pressure-Torsion-Processed AA1050 Aluminium

The grain refinement of high-stacking-fault-energy metals during high-pressure torsion (HPT) is governed by dislocation activity and boundary evolution. However, how local misorientation evolves during different stages of grain refinement remains insufficiently clarified. In this study, AA1050 aluminium was processed by HPT to selected equivalent strains of 0, 0.90, 2.69, 10.76 and 53.78, and electron backscatter diffraction (EBSD) was used to analyse grain morphology, boundary fractions and EBSD-derived misorientation parameters. The results reveal strongly stage-dependent grain refinement during HPT. The average grain size decreases rapidly from 12.63 μm in the initial state to 3.29 μm at ε = 0.90 and 2.19 μm at ε = 2.69, remains nearly unchanged at ε = 10.76, and finally decreases to 0.58 μm at ε = 53.78. The fraction of low-angle grain boundaries increases markedly at ε = 0.90, indicating intensive formation of dislocation substructures, whereas high-angle grain boundaries become dominant at high strain. Grain orientation spread (GOS) and grain reference orientation deviation (GROD) exhibit non-monotonic evolution, whereas the geometrically necessary dislocation (GND) density increases markedly at the early deformation stage and subsequently decreases with further deformation. These results indicate that the stage-dependent grain refinement of AA1050 during HPT is closely associated with dislocation-mediated grain subdivision involving alternating grain elongation and fragmentation.