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Equiatomic TaNbHfZr and TaNbHfZrTi high-entropy alloys (HEAs) have beeninvestigated for their phase stability, short-range clustering (SRC) typechemical ordering, other structural phenomena and and the effects onstrengthening. Ab-initio calculations have been done to determine thethermodynamic stability parameters. The as-cast structures do not contain anylocal chemical order, but the high-resolution transmission electron microscopy(HRTEM) studies reveal their locally distorted lattice. The local latticedistortion of the average structure in their as-cast and annealed state isquantified by refining atomic displacement parameters (ADPs) fromsingle-crystal X-ray diffraction (XRD) and powder neutron diffraction (ND)studies. The static component of the ADPs appears to dominate over the thermalADP, suggesting a high degree of local lattice distortion in the HEAs. Thelattice distortions cause solid-solution like strengthening, which increasesthe 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 tothe <1 0 0> directions. The SRCs mainly are co-clustering of Zr and Hf. TheSRCs have a higher atomic volume causing local tetragonal lattice distortions,which in terms produce diffuse streak-like intensities in the XRD datasets andin the fast Fourier transform (FFT) of the HRTEM lattice images. The presenceof SRCs locally reduces the lattice potential energy of the structure and putsadditional energy barrier to the movement of dislocations. This increases thehardness causing around 50% of additional strengthening in the annealed HEAs.Thermodynamic analysis shows that the relaxed structure containing the SRCslower the enthalpy and the configurational entropy.

Phase stability, local chemical disorder and its effect on the mechanical properties of refractory high-entropy alloys