  High pressure torsion (HPT) is one of the ways to form nanostructured
materials with high strength properties. However, HPT hardening mechanisms vary
from material to material and are poorly understood for some BCC metals,
particularly niobium and vanadium. This work aims to identify strain hardening
stages for Nb and V metals during HPT. Two approaches have been used to
identify the deformation stages during high pressure torsion. The approaches
are based on the application of a "piecewise" model, taking into account the
different deformation mechanisms that determine the type of the forming
structure, and on the analysis of the hardness vs. true strain dependence
according to the $H$$-$${e}^{0.5}$ law. We compared the identified stages with
the results of the electron microscopic study of the structure. Both models
describe well the structural changes observed microscopically in HPT-deformed
niobium. However, we have shown that only the piecewise model gives an adequate
description of the stages of structure development in vanadium. We have
provided an explanation for the observed difference in the behavior of niobium
and vanadium upon HPT.
