Experiments were conducted on an ultrafine-grained Al-3wt.%Mg-0.2wt.%Sc alloy to characterize the microstructure changes occurring during large tensile deformation at elevated temperature within a transition between superplasticity and creep. The coarse-grained material was subjected to equal-channel angular pressing at room temperature using a die which had a 90° angle between the channels. The billets were rotated by 90° in the same sense between each pass in the processing route Bc. The

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... ts were subsequently pressed by 2 and 8 passes. Microstructure was investigated by scanning electron microscope equipped with an electron back scatter diffraction unit. Microstructure analysis showed that the movement of dislocations and the growth of grains were restricted by Al3Sc coherent precipitates formed during stabilizing annealing at 623 K for 1h. It was found that deformation behaviour of investigated Al alloy was influenced by inhomogeneity of microstructure and deformation-induced coarsening of grains. The fracture behaviour was affected by cavity formation due to high local concentration of plastic deformation at the triple points and inefficient accommodation processes of grain boundary sliding. The results showed that total tensile deformation is a consequence of a various mechanisms occurring at different stages of tensile deformation. The superplastic tensile deformation was predominantly realized by grain boundary sliding and formation of mesoscopic shear bands. The role played by grain boundary sliding is discussed.