Grain refinement of pure Ti during plastic deformation

The mechanism of grain refinement was studied in titanium subjected to high-rate surface mechanical attrition treatment. By cross-sectional transmission electron microscopy, the deformation structures were systematically characterized in a nanocrystalline (NC) layer of the treated surface which exhibits a grain size gradient. The microstructural evolution with increasing strain involves (1) the formation of slip bands with dislocation-free cells in their interiors, (2) nucleation and migration of high-angle grain boundaries at cell boundaries consisting of high-density tangled dislocations, leading to the formation of submicrosized grains inside the slip bands, followed by (3) the formation of subgrains and dislocation cells inside the submicrosized grains with increasing misorientations, which finally become NC grains. The mechanism of grain refinement was interpreted in terms of the classical migration dynamic recrystallization (DRX) and continuous rotation DRX, respectively, leading to submicrosized and NC grains with increasing strains at high strain rates. The cooperative grain boundary sliding is active at high strain rate, by the presence of arrays of coplanar grain boundaries.