  Severe plastic deformations under high pressure are used to produce
nanostructured materials but were studied ex-situ. We introduce rough diamond
anvils to reach maximum friction equal to yield strength in shear and perform
the first in-situ study of the evolution of the pressure-dependent yield
strength and nanostructural parameters for severely pre-deformed Zr.
{\omega}-Zr behaves like perfectly plastic, isotropic, and
strain-path-independent. This is related to reaching steady values of the
crystallite size and dislocation density, which are pressure-, strain- and
strain-path-independent. However, steady states for {\alpha}-Zr obtained with
smooth and rough anvils are different, which causes major challenge in
plasticity theory.
