Abstract
In this study, ultrafine grain structure evolution during high pressure torsion (HPT) of commercial aluminium alloy AA6082 at an elevated temperature is presented. Two different initial structural states of the alloy were prepared by thermal treatment. The dependence of the progress of microstructure refinement on shear strain was investigated by TEM observation of thin foils. The impact of various amounts of strain (ε_ef) was analysed with respect to the increased temperature of deformation. Microhardness data measured across the deformed discs show scatter. Observation of microstructure revealed that an ultrafine grain (UFG) structure formed in the deformed disc as early as the end of the first turn, regardless of the initial structure of alloy resulting from the prior thermal treatment. The heterogeneity of UFG deformed structure in the deformed discs is consistent with the scatter in microhardness values. By increasing the strain level through adding turns (N = 2, 4, 6), the UFG structure was homogenized in the deformed discs. The effect of the increase in deformation temperature became more evident and dynamic recrystallization modified the UFG structure locally. In the specimens prepared by two-stage thermal treatment (quenching and ageing) prior to torsion deformation, the growth of new grains was inhibited and the UFG microstructure was more stable. Tensile strength values suggest that strengthening was partially relaxed by local recrystallization. Torque vs. time plots reveal that the torque required to deform the sample was increasing until the completion of the first turn and then remained stable or even decreased slightly.

Key words
6082 Al alloy, thermal treatment, structure modification, HPT severe plastic deformation, ultrafine grain structure, mechanical properties, torque

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