AbstractIn an AA 8090 alloy, the microstructural changes that occur as a result of variation in the extrusion processing parameters have been investigated. Light microscopy revealed that under all processing conditions the surface layers of the extrudates are fully recrystallised, whereas the core is essentially unrecrystallised. The depth of the recrystallised surface layer, and recrystallisation at the core, are affected by changes in the processing conditions: recrystallisation is most sensitive to extrusion ratio and nucleates at prior grain boundaries. Transmission electron microscopy (TEM) revealed that a well recovered subgrain structure is produced under all deformation conditions and variation of both extrusion temperature and ratio affect the subgrain morphology. Examination of extrudates using TEM together with results from hot torsion and post-extrusion annealing, provide evidence of the recrystallisation mechanisms: at lower deformation rates recrystallisation occurs by the classical grain boundary bulging mechanism and at higher deformation rates this process is supplemented by rotation recrystallisation.MST/1072