  We have carried out plastic deformation experiments on single-phase samples
of the body centered ZrNbTiVHf high-entropy alloy between room temperature and
1150 K. The experiments were carried out on polycrystalline samples with two
different grain sizes, in compression at a true constant strain rate of 10-4
1/s. Incremental tests such as stress-relaxation tests, strain-rate changes and
temperature changes were carried out in order to determine thermodynamic
activation parameters of the plastic deformation process. The material displays
a yield-stress anomaly in the temperature range between about 500 and 800 K,
where the yield stress increases with increasing temperature. In the same
temperature range, we find an extremely low strain-rate dependence of the flow
stress, which is reflected in almost constant flow-stress values in
stress-relaxations and strain-rate changes. At temperatures below 400 and above
800 K we find regular plastic deformation behavior with a linear temperature
dependence of the yield stress of about -1.1 MPa/K, an activation volume of
around 1.8 nm3 and 0.2 nm3 at low and high stresses, respectively, and an
activation enthalpy of 2.7 eV in the high-temperature range.
