Fig. 1. (a) FE-SEM morphology of gas-atomized CoCrFeMnNi powder, (b) and (c) high magnification image of particle, (d) EDS elemental maps of ball-milled HEA/TiN powder, (e) FE-SEM image of ball milled HEA/TiN powder, (f) a higher magnification of (e) and (g) schematic representation of HEA/ nano-TiN powder bed for SLM. Ruler bars: (a) 50 μm, (b) 10 μm, (c) 2 μm, (d) 10 μm, (e) 50 μm, and (f) 10 μm [45].

Fig. 2. FE-SEM images of TaNbHfZrTi powders after (a) dehydrogenation and (b) plasma treatment [41].

Fig. 3. EBSD images of the CrMnFeCoNi HEA bulk sample after different straining levels at 77 K. (a) Phase map; (b) Euler map; (c) and (d) enlarged Euler map of the area circled by the red rectangle in (b) [69].

Fig. 4. (a-d) Tensile behavior of samples fabricated by different laser powers in the scanning (S) and deposition (D) direction [71]. (e) Optical microscope (OM) images of the as built HEA in the YZ plane, (f) OM images of the rectangular areas in (e), (g) and (h) are corresponding images of the rectangular areas in (b) showing equiaxed grains and dendrite columnar grains. The white, black and blue dashed lines represent the layer boundaries, laser movement direction and melt pool boundaries, respectively [72].

Fig. 5. Potentiodynamic polarization curves of alloys in (a) 3.5 wt % NaCl and (b) 1.0 M H2SO4 solutions, (c) engineering tensile curves of the as-built CoCrFeNiMo0.2 high entropy alloys at 77 K [76].

Fig. 6. The backscattered electron (BSE) images and corresponding EDS elemental maps of the as-built (a) CoCrFeNiNb0.1 and (b) CoCrFeNiNb0.2 HEAs [77].

Fig. 7. BF-STEM graphs along the 〈101〉 zone axis showing dislocations in the as built (a, b) and multiple slip systems in deformed (engineering strain, ε = 58 %) AM CrCoNi alloy with lower strain rate (LSR) of 2 × 10-5 s-1 (c). Selected area diffraction (SAD) pattern (d) taken at the red circle in (c) showing marked twin spots. High-resolution HAADF-STEM images along the < 101 > zone axis of the AM CrCoNi sample (e, f) deformed ( ε = 58 %) with LSR (2 × 10-5 s-1), (g) Fast Fourier transformation (FFT) of the TEM image of (f, h) deformed (ε = 47 %) with HSR (2 × 10-3 s-1). (i) True stress-strain curves and stacking fault energies (SFE, mJ/m2) as a function of true strain [81].

Fig. 8. X-ray diffraction patterns of AlCrFeCoNi coatings synthesized under different laser-processing conditions [91].

Fig. 9. (a) A schematic representation of compositionally graded AlxCoCrFeNi HEA. BSE graphs, high magnification EBSD inverse pole figures, and the corresponding phase maps of region I (b), transition region (c) and region II (d) of graded AlxCoCrFeNi HEA. Hardness profile across the longitudinal section (e) and dynamic true stress-true strain curves of different samples (f) [110].

Fig. 10. (a) Photo of the fabricated specimen, (b) BSE image of the longitudinal section of specimen. The white dashed line at the bottom shows the substrate surface. The EDX elemental profile is the average of measured data along the green rectangles shown at the top. (c) Elemental (colored lines) and hardness (black symbols) profiles of the DLD specimen from the top (left) to the bottom (right) of the sample (d) EBSD phase maps (top row) and grain orientation maps, bcc unit cell, (bottom row) of the outlined areas as A, B, C in (b). Arrows indicate areas with small and large grain sizes in region A [123].

Fig. 11. (a) SEM image showing the cross sectional morphology of the coating. (b), (c) and (d) are magnified images of outlined rectangular regions B, C and D in (a), respectively [130].

Fig. 12. (a) Bright-field TEM micrograph indicating two {111} glide traces intersecting at 70.5°, the inset is SAED of the red circled region at [011] zone-axis; (b) Magnified TEM image at the [001] zone axis of B2 phase, the inset shows the SAED of the blue circled region; (c) HRTEM micrograph at the [001] zone axis of B2 phase. (d) HRTEM image of a B2 lamella at the [111] zone axis; (e) Magnified HRTEM image of the yellow dotted region in (d), the inset is the corresponding FFT pattern; (f) Magnified HRTEM image of the white outlined region in (d), inset is the FFT pattern of the yellow dotted box [151].

Fig. 13. compressive stress-strain curves of SEBM and casting samples at room temperature [32].

Fig. 14. The phase maps and IPF maps of the cast and the SEBM specimens acquired at the cross-section perpendicular to the build direction at the top and bottom sections [34].

Fig. 15. (a) EBSD showing the polycrystalline nature of HEA powder and (b) XRD patterns of the powder and the as built specimen. OM images obtained from (c) XZ and (d) XY planes indicating the epitaxial growth and cellular structure of grains in the SEBM-built part [35].

Fig. 16. (a) OM images of the as-built Fe49.5Mn30Co10Cr10C0.5 HEA. (b) SE image of dotted region in (a). The inset displays the equiaxed cellular structures. (c) STEM micrograph showing the equiaxed cellular structures. The inset shows the distance across five {111} planes in HRTEM image. (d) (a1-a4) EBSD phase and GBs maps revealing microstructural evolution at different strain levels. TEM images of deformed structure for 4 % strain (e, f) and 12 % strain (g). The insets show the SAED pattern [160].

Fig. 17. (a) Tensile behavior of the SLM-built (CoCrFeMnNi)99C1 HEA along the x and y directions. (b) The yield strength versus elongation values for (CoCrFeMnNi)99C1 and some alloys referenced in their work [162].

Fig. 18. (a) A 2D diffractogram of mixed oxide prior (left) and after (right) the TGA measurement in H2 disclosing the complete transition of blended oxide to a fcc CoCrFeNi HEA. Scale bar is 20 nm-1. (b) SEM of the blended oxide powder prior to TGA, indicating agglomerates (<10 μm) of sub-micron particles. Scale bars are 5 μm and 1 μm (inset). (c) Micro-lattice with 200 μm diameter struts before and after sintering. Scale bars are 3 mm. In situ XRD 2D-phase evolution plot (temperature vs. the scattering vector q, with diffraction peak intensity as color map) illustrating oxides reductions and the complex route to the synthesis of CoCrFeNi HEA during heating in H2 atmosphere [176].

Fig. 19. (a) SEM image showing microstructural features of the as-built specimen. Inverse pole figures of the as-printed sample (b1), and the samples annealed for 2 h at 1173 K (b2), 1373 K (b3), and (b4) 1573 K. Twinning distribution of the as-printed sample (c1), and the samples annealed for 2 h at 1173 K (c2), 1373 K (c3), and (c4) 1573 K. TEM images of the deformed as-built (d1, d2) and samples annealed at 1573 K (d3, d4). Dislocation wall inside the dislocation network (d1), interactions between dislocations and the dislocation network (d2), and massive t wins (d3, d4) are indicated [178].

Fig. 20. TEM graphs showing distinct microstructures evolved along the depth direction after 5-impact of LSP. Near the upper surface (a), and at depths of 25 μm (b), 150 μm (c), and 250 μm (d). Schematic representations of the evolved microstructure along the depth direction (e) and transition in tensile to compression residual stress (f1-f3) [184].

Fig. 21. OM images of SLM-built CoCrFeNi with (a) chessboard (denoted as C) and (b) stripe (denoted as S) scanning strategies. EBSD IPF color maps parallel to the build direction in the outlined areas for C sample (c) and S sample (d). Lack-of-fusion cracks and spherical pores are encircled in (a) and (b), respectively. Enlarged EBSD band contrast images and IPF color maps showing intergranular cracks within C (e) and S (f) samples. Schematics of the chessboard (g) and stripe (h) scanning strategies for SLM process. (i) A schematic representation of hot cracking mechanism in AM process. (j) Hot tearing inclination as a function of grain size and typical depression pressures for some SLM-built alloys [187].

Fig. 22. Relative densities of SLM-printed CoCrFeMnNi and AlCoCrFeMnNi HEA systems as a function of VED. Solid and open symbols represent the density of CoCrFeMnNi and AlCoCrFeMnNi HEA, respectively. The VED ranges corresponding to balling effect, keyhole porosity, and sound printing are labeled. The transparent colors indicate transition regions.

Fig. 23. Microhardness of AM-built HEAs [[65], [66], [67], [68],73,84,87,[94], [95], [96], [97],[99], [100], [101],109,110,113,119,120,[122], [123], [124], [125],132,136,137,139,144,146,[148], [149], [150],166,[190], [191], [192]]. The data for conventional HEAs are taken from elsewhere [193]. Each column represents the corresponding HEA system with all its derivatives.

Fig. 24. Summary of ultimate tensile and yield strengths versus elongation for various HEAs fabricated by different AM techniques. The references are taken from Table 3. The UTS of conventional HEAs are adopted from [63]. AlCoCrFeNi2.1-DPHL (Dual-phase heterogeneous lamella), which indicates a high strength state of the art HEA prepared by arc melting followed by severe cold rolling is included for comparison [194].