Fig. 1. Chemical characterizations of ordered oxygen complexes (OOCs) in the (TiZrHfNb)98O2 HEA. (a) The HAADF-STEM image for the [011]bcc crystal axis with a differently adjusted contrast to reveal the existence of chemical short-range ordering. (b) The atomic number contrast analysis of the HAADF-STEM image reveals the OOCs. Red squares represent the Zr/Ti-rich regions and yellow squares indicate the Hf/Nb-rich regions. (c) The aberration-corrected STEM-annular bright field image. The inset in (c) is an enlarged view of the OOCs, with the white arrows indicating the positions of the oxygen atom columns. (d) APT reconstruction of the O-doped HEA [16].

Fig. 2. Mechanical behaviors of O/N doped TiZrHfNb HEAs. (a) Room temperature tensile stress-strain curves for the as-cast TiZrHfNb, (TiZrHfNb)98O2 (O-2) and (TiZrHfNb)98N2 (N-2) HEA; (b) Dislocation configuration of O-2 HEA at three strain stages: 2.5 %, 8%, and fracture [16].

Fig. 3. Schematic illustration of the interplay between SROs and different deformation carriers in a representative FCC HEA.

Fig. 4. Typical experimental methods utilized for characterizing SROs in HEAs. (a) ATP measurements indicate existence of network-like Zr/Hf enriched SROs in an annealed TaNbHfZr HEA [68], (b) energy-filter diffraction pattern technique reveals the clear streaking feature originated from the diffuse scattering signals of SRO domains in the CoCrNi MEA [14], (c) EXAFS analysis suggests that the preferred Ni/Co-Cr SROs are present in the CoCrNi MEA [12], and (d) the local concentration distribution of individual elements for the same region in the FCC CrPdFeCoNi HEA by the STEM-HAADF technique [17].