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High pressure torsion (HPT) is one of the ways to form nanostructuredmaterials with high strength properties. However, HPT hardening mechanisms varyfrom material to material and are poorly understood for some BCC metals,particularly niobium and vanadium. This work aims to identify strain hardeningstages for Nb and V metals during HPT. Two approaches have been used toidentify the deformation stages during high pressure torsion. The approachesare based on the application of a "piecewise" model, taking into account thedifferent deformation mechanisms that determine the type of the formingstructure, and on the analysis of the hardness vs. true strain dependenceaccording to the $H$$-$${e}^{0.5}$ law. We compared the identified stages withthe results of the electron microscopic study of the structure. Both modelsdescribe well the structural changes observed microscopically in HPT-deformedniobium. However, we have shown that only the piecewise model gives an adequatedescription of the stages of structure development in vanadium. We haveprovided an explanation for the observed difference in the behavior of niobiumand vanadium upon HPT.

Strain-Hardening Stages and Structure Evolution in Pure Niobium and Vanadium upon High Pressure Torsion