Microstructure evolution and nanograin formation during shear localization in cold-rolled titanium

The microstructure evolution within the shear localization areas formed in commercial titanium subjected to cold rolling is systematically investigated. Sheared micro-regions are first initiated followed by the formation of distinct microscopic shear bands, which gradually grow and coalesce to form a macroscopic shear band. The latter contains thin lath structures in the boundary regions, fine elongated subgrains in the outer areas and roughly equiaxed (sub)grains with a mean size of 70 nm in the centre region. The early stage of shear localization involves the formation of twin/matrix lamellae aligned along the shear direction. The lamellae subsequently undergo longitudinal splitting into thin laths, which are in turn subjected to transverse breakdown, giving rise to fine elongated subgrains. The continuing thermally assisted lath breakdown, in conjunction with lateral sliding and lattice rotations, ultimately leads to the formation of roughly equiaxed, nanosized (sub)grains in the macroscopic shear band centre at large strains.