Evolution of ultrafine microstructures in commercial purity aluminum heavily deformed by torsion

Commercial purity aluminum (1100Al) bars were severely plastic deformed by torsion deformation at room temperature. The specimens were deformed to ultrahigh equivalent strain of 5.85 in maximum. Microstructure evolution during the torsion deformation was characterized using electron back scatter diffraction analysis on two different sections: the longitudinal section parallel to the torsion axis and transverse section perpendicular to the torsion axis. The grain size decreased and the fraction of high angle grain boundary increased with increasing equivalent strain. Elongated ultrafine grained structure was obtained after an equivalent strain of 3.27. We have found that the microstructure evolution in the specimen deformed by torsion exhibited similar behavior to those in the same material heavily deformed by accumulative roll bonding. The average grain size of 0.32 μm with the high angle boundary fraction of 0.76 was achieved in the specimen deformed to an equivalent strain of 5.27. Though the microstructure and hardness on the transverse section varied depending on the radial positions, they could be arranged as a simple function of equivalent strain. The present work confirmed that the torsion deformation worked as a kind of severe plastic deformation.