  Equiatomic TaNbHfZr refractory high-entropy alloys (HEAs) were synthesized by
arc-melting. The HEAs were annealed at 1800{\deg}C for different times, at
maximum up to 8 days. Their on average body-centered cubic (bcc),
solid-solution structure was confirmed by X-ray (XRD) and neutron (ND)
diffraction, respectively. The HEAs are characterized by high average values of
the static atomic displacements from the ideal lattice positions and the local
internal strain. The short-range clustering (SRC) of a subset of atoms,
enriched in Hf and Zr takes place perpendicular to the <1 0 0> directions.
Furthermore, it becomes increasingly interconnected as a function of the
annealing time. This is revealed by the evaluation of diffuse XRD intensities,
high-resolution transmission electron microscopy (HRTEM) images, and atom probe
tomography (APT). The local structural disorder and distortions at the SRC were
modeled by molecular dynamics (MD) relaxations. The hardness and compressive
yield strength of the as-cast HEA is found to be many times of what can be
expected from the rule of mixture. The yield strength further increases by 76 %
after 1 day of annealing, which can be explained by a strengthening mechanism
resulting from the SRC. With further annealing to 4 days, a minor phase with
hexagonal close packed (hcp) structure and rich in Hf and Zr nucleates at the
larger connecting nodes of the SRCs.
