Comparison between superplastic deformation mechanisms at primary and steady stages of the fine grain AA7475 aluminium alloy

Evolution of the deformation behaviour, surface and bulk structures at superplastic flow of the АА7475 aluminium-based alloy were studied by scanning and transmission electron microscopes and a focused ion beam technique. Differences between deformation behaviour at a primary stage with strains below 0.69 and a steady stage with strains above 0.69 were discussed. The research showed the grain growth and grain elongation to the stress direction at a primary stage of deformation. Stabilisation of both mean grain size and grain aspect ratio was found at a steady stage of deformation. Grain neighbour switching, grain rotation, dispersoid free zones and some insignificant intragranular strain were observed at both stages. Appearing and disappearing of the grains on the sample surface, with increased dislocation activity occurred at the steady stage of deformation. The current results highlighted the importance of diffusion creep as a dominant mechanism at the beginning of superplastic deformation and as an accommodation mechanism of the grain boundary sliding at the steady stage of deformation. The dislocation creep as an additional accommodation mechanism of the grain boundary sliding at the steady stage of superplastic deformation is suggested.