  Equiatomic TaNbHfZr and TaNbHfZrTi high-entropy alloys (HEAs) have been
investigated for their phase stability, short-range clustering (SRC) type
chemical ordering, other structural phenomena and and the effects on
strengthening. Ab-initio calculations have been done to determine the
thermodynamic stability parameters. The as-cast structures do not contain any
local chemical order, but the high-resolution transmission electron microscopy
(HRTEM) studies reveal their locally distorted lattice. The local lattice
distortion of the average structure in their as-cast and annealed state is
quantified by refining atomic displacement parameters (ADPs) from
single-crystal X-ray diffraction (XRD) and powder neutron diffraction (ND)
studies. The static component of the ADPs appears to dominate over the thermal
ADP, suggesting a high degree of local lattice distortion in the HEAs. The
lattice distortions cause solid-solution like strengthening, which increases
the hardness of the HEAs by a factor of 2-3 compared to the rule of mixture.
The SRCs that form in the annealed HEAs have a planar profile perpendicular to
the <1 0 0> directions. The SRCs mainly are co-clustering of Zr and Hf. The
SRCs have a higher atomic volume causing local tetragonal lattice distortions,
which in terms produce diffuse streak-like intensities in the XRD datasets and
in the fast Fourier transform (FFT) of the HRTEM lattice images. The presence
of SRCs locally reduces the lattice potential energy of the structure and puts
additional energy barrier to the movement of dislocations. This increases the
hardness causing around 50% of additional strengthening in the annealed HEAs.
Thermodynamic analysis shows that the relaxed structure containing the SRCs
lower the enthalpy and the configurational entropy.
