Nanostructuring Nb by various techniques of severe plastic deformation

Evolution of structure of polycrystalline Nb at severe plastic deformation (SPD) by equal-channel angular pressing (ECAP), high-pressure torsion (HPT) and combination of these techniques has been studied by transmission and scanning electron microscopy and EBSD (electron back-scatter diffraction) analysis. ECAP results in obtaining grain–subgrain structure which refines to submicrocrystalline sizes only at the highest strain of 18.6 (16 passes). HPT by 5 revolutions of anvils (the strain at the radius middle is 90) enables to obtain submicro- and nanocrystalline structure, but its refinement is limited by an achievement of saturation stage. Microhardness after HPT is considerably higher than that after ECAP. The ECAP combination with further HPT does not result in an additional microhardness increasing which demonstrates that the saturation at HPT is achieved independently on the initial state (single crystalline, polycrystalline or after the ECAP).