Features of severe plastic deformation as compared to conventional deformation modes

Different stress-strain characteristics of a material deformed by different conventional deformation modes get identical if one considers the texture evolution being specific of the deformation mode. Densities of deformation induced lattice defects, and also the average sizes of cells/subgrains are identical, too, although the shape of the latter is highly anisotropic, with ultrafine grain (UFG) size in one dimension only. While the qualitative sequence of work hardening stages II, III and IV is the same, Severe Plastic Deformation (SPD) achieves much more isotropic grain shapes, with UFG sizes in all dimensions. This effect is due to the presence of enhanced hydrostatic pressure at SPD techniques which not only allows for much larger strains to be reached but also for much higher lattice defect densities, yielding a natural explanation for the higher density of grain boundaries. The presence of higher defect densities especially that of deformation induced vacancies suggests the enhanced hydrostatic pressure to restrict the dynamic defect annihilation, via restriction of lattice diffusion.