Fig. 1. Inverse pole figure (IPF) and corresponding phase maps of (a) FG and (b) CG alloys before the mechanical tests. The alloys were rolled along the vertical axis. The red, black, and blue lines in the phase maps indicate low-angle grain boundaries (LAGBs, 2° < θ < 15°), high-angle grain boundaries (HAGBs, 15° < θ < 58°), and Σ3 boundaries (58° < θ < 62°), respectively; the fractions of these boundaries are presented in each map. The white and gray areas in the phase maps indicate γ and ε phases, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2. Back-scattered electron images of (a) FG and (b) CG before mechanical loading. The higher magnification of the yellow rectangular in (a) is shown in (c). The EDS elemental maps are from the yellow rectangular in (c). Red arrows in (c) show the location of carbide particles. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3. Uniaxial tensile behavior of the investigated alloys: (a) engineering, (b) true stress-strain and (c) strain-hardening curves. The previous data [13] were also included for discussion purposes.

Fig. 4. XRD line profiles of FG and CG HEAs before and after the tensile test.

Fig. 5. Deformed microstructures of (a) FG and (b-d) CG alloys subjected to uniaxial tension along the vertical axis. The black-lined region in Fig. 2(b) was magnified in the form of (c) a phase map and (d) corresponding IPF map of the γ phase. The black, blue, and green lines in the phase maps indicate HAGBs (15° < θ < 58°), Σ3 boundaries (58° < θ < 62°), and the Shoji-Nishiyama orientation relationship [42], respectively. The white and gray areas indicate γ and ε phases, respectively. The yellow and red arrows in Fig. 4(c) indicate the mechanical twinning in and bidirectional transformation of the alloys, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 6. Fatigue behaviors of FG and CG alloys, and the reported CoCrFeMnNi-0.7C alloys with different grain sizes [13]: (a) Coffin–Manson plot, (b) $\frac{\text{ }\!\!\Delta\!\!\text{ }\sigma }{2}$ plot, and (c) $\frac{\text{ }\!\!\Delta\!\!\text{ }{{\varepsilon }_{\text{t}}}}{2}$ plot with respect to the terms of 2Nf.

Fig. 7. Maximum stress at various numbers of cycles during the LCF test at different values of $\frac{\text{ }\!\!\Delta\!\!\text{ }\sigma }{2}$.

Fig. 8. EBSD phase maps and corresponding image quality maps of the CG HEA at (a) 0, (b) 20, (c) 200, (d) 1000, and (e) 3000 cycles at $\frac{\text{ }\!\!\Delta\!\!\text{ }\sigma }{2}$ = 0.7%. The alloys were subjected to cyclic loading along the vertical axis. The black, blue, and green lines in the phase maps indicate HAGBs (15° < θ < 58°), Σ3 boundaries (58° < θ < 62°), and the Shoji–Nishiyama orientation relationship [42], respectively. The white and gray areas indicate γ and ε phases, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 9. XRD line profiles of CG HEA for various numbers of loading cycles at $\frac{\text{ }\!\!\Delta\!\!\text{ }\sigma }{2}$ = 0.7%. The inset is a magnified line profile for the ($10\bar{1}1$)ε peak at 2θ between 46.0° and 47.6°

Fig. 10. TEM micrographs of the CG alloy subjected to cyclic loading at $\frac{\text{ }\!\!\Delta\!\!\text{ }\sigma }{2}$ = 0.7%: (a–d) 1000 cycles, (e) 4000 cycles, and (f) fractured. (b) is the selected area diffraction pattern for the yellow-lined region in (a). (c) and (d) are the dark-field images corresponding to the spots designated in (b). (e) is the magnified image of the region in Fig. 9(d). The yellow arrows indicate fragmented SFs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 11. The ECCI micrographs (a, c, d) and EBSD phase map (b) of CG after fatigue fracture at $\frac{\text{ }\!\!\Delta\!\!\text{ }\sigma }{2}$ = 0.85%. The ECCI micrograph in Fig. 11(a) and EBSD phase maps in Fig. 11(b) are from the same location. The higher magnification of the region indicated by yellow rectangular in Fig. 11(a) is presented in Fig. 11(c). Fig. 11(d) is from other grain of this specimen. The black, blue, and green lines in the phase map indicate HAGBs (15° < θ < 58°), Σ3 boundaries (58° < θ < 62°), and the Shoji–Nishiyama orientation relationship [42], respectively. The white and gray areas indicate γ and ε phases, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 12. ECCI micrographs (a, c, d) and EBSD phase map of FG after fatigue fracture at $\frac{\text{ }\!\!\Delta\!\!\text{ }\sigma }{2}$= 0.85%. The ECCI micrograph in Fig. 12(a) and EBSD phase map in Fig. 12(b) are from the same location. The higher magnification of the regions indicated by yellow rectangular in Fig. 12(a) are presented in Fig. 12(c) and (d). The black, blue, and green lines in the phase maps indicate HAGBs (15° < θ < 58°), Σ3 boundaries (58° < θ < 62°), and the Shoji–Nishiyama orientation relationship [42], respectively. The white and gray areas indicate γ and ε phases, respectively.

Fig. 13. EBSD phase maps of the CG alloy after fatigue fracture at $\frac{\text{ }\!\!\Delta\!\!\text{ }\sigma }{2}$ of (a) 0.4%, (b) 0.55%, (c) 0.7%, and (d) 0.85%. The alloys were subjected to cyclic loading along the vertical axis. The red, black, blue, and green lines in the phase maps indicate LAGBs (2° < θ < 15°), HAGBs (15° < θ < 58°), Σ3 boundaries (58° < θ < 62°), and the Shoji–Nishiyama orientation relationship [42], respectively. The white and gray areas indicate γ and ε phases, respectively, of which the area fractions were provided. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 14. XRD analysis of the investigated HEAs after fatigue fracture at $\frac{\text{ }\!\!\Delta\!\!\text{ }\sigma }{2}$ values from 0.4% to 0.85%: (a) XRD line profiles for the FG alloys, (b) those for the CG alloys, and (c) ε-martensitic fractions calculated from the XRD results. The insets in Fig. 14(a) and (b) are the magnified line profiles for the ($10\bar{1}1$)ε peak. The line profile of the initial alloys (“As”) were also provided for comparison purposes.