Fig. 1. (a) Schematic diagram of the FSP process, (b) photograph of the overview of the FSPed HEA. WD, TD, and ND indicate the welding, transverse and normal directions of the FSW process.

Fig. 2. (a) Schematic diagram of the FSPed HEA showing the sectioned areas for the electrochemical tests, (b) schematic diagram of the electrochemical test set up. CE, RE, and WE are counter, reference, and working electrodes, respectively. (DAQ: data acquisition).

Fig. 3. (a) Schematic of the computational domain showing the dimensions, governing equation as well as the boundary conditions. The red line represents the anodic surface (i.e. the active region during pitting corrosion), while the blue line represents the cathodic surface (or the non-corroding surface). (b) Schematic of the mesh system along with the boundary conditions used for the ALE method in COMSOL Multiphysics. The reference frame has fixed (X,Y) coordinates while the spatial frame has (x,y) coordinates moving with time.

Fig. 4. Microstructural features of HEA BM: (a) IPF-ND, (b) IQ (overlaid with grain boundaries), (c) KAM, (d) GOS maps, (e) IQ (overlaid with twin boundaries) (f) misorientation-angle distribution (the blue line represents Mackenzie random distribution and (g) {111} pole figure.

Fig. 5. (a) Optical micrograph of the cross-section of the FSPed HEA after etching showing the shoulder width, plunge depth as well as the various zones resulted from the FSP. The green dashed lines indicate the boundary between the SZ and TMAZ and the numbered-red square shapes also mark the locations for the EBSD and EDS analyses. The result of the EDS analysis is presented in Table 1. (b) Calculated equilibrium phase mole fraction versus temperature for the CrMnFeCoNi HEA using ThermoCalc. The region marked with a gray rectangle within the FCC region indicates the estimated peak temperature range (～1016 °C to ～1265 °C) during the FSP of the HEA. (c) XRD patterns of the FCC phases in regions 1, 5, and 2 in Fig. 5(a), corresponding to the BM, RS-TMAZ, and Top SZ of the FSPed HEA, respectively.

Fig. 6. EBSD IPF-ND maps of (a) region 2 (as Top SZ), (b) region 3 (as Middle SZ), and (c) region 4 marked by red squares in Fig. 5(a). (d) Shear processed zone (SPZ) and (e) region below the SPZ marked by black squares in region 4, which are shown at higher magnifications.

Fig. 7. Microstructural features of the (a1?e1) Top SZ, (a2?e2) Middle SZ, (a3?e3) SPZ, and (a4?e4) region below the SPZ: (a1?4) KAM, (b1?4) GOS, (c1?4) IQ (overlaid with grain boundaries) maps, (d1?4) misorientation-angle distribution (the blue line represents Mackenzie random distribution), and (e1?4) {111} pole figure (red triangle represents the dominant γ-fiber component of the shear texture).

Fig. 8. Microstructural features of (a1-f1) RS-TMAZ and (a2?f2) AS-TMAZ: (a1?2) IPF-ND, (b1?2) KAM, (c1?2) GOS, (d1?2) IQ (overlaid with grain boundaries) maps, (e1?2) misorientation-angle distribution (the blue line represents Mackenzie random distribution), and (f1?2) {111} pole figure (red triangle represents the dominant γ-fiber component of the shear texture).

Fig. 9. (a) Optical micrograph of the FSPed HEA cross-section showing blue, green, and pink dashed lines that represent indentations at uniform intervals both horizontally and vertically. The centerline perpendicular to the cross-section is represented by a yellow dotted line. The hardness in the region below the SZ is measured separately along the black dotted line. (b) Hardness profile of the dashed lines in Fig. 9(a) across the cross-section. The orange and violet dashed lines indicate regions corresponding to the tool pin and shoulder in the SZ, respectively. The brown dashed line indicates the boundary between the TMAZs and the BM at the RS and AS based on Row_1.

Fig. 10. (a) OCP versus time curves and (b) potentiodynamic polarization curves of the specimens in aerated 0.5 M Na2SO4 electrolyte solution at room temperature (Ecorr is corrosion potential; Ep is the pitting potential). The measured Ecorr and icorr values are presented in Table 2. INSET: The transpassive region of the polarization curves shown at a higher magnification. (c) SEM image showing a passive layer formed on the surface of the FSPed region (FSPed_front) identified as Cr2O3 using EDS point scans and listed in Table 4. (d) SEM image showing the measured lengths of some pits formed on the surface of the FSPed_front specimen.

Fig. 11. (a) Potential distribution (color) and absolute potential gradient (black arrows) in the electrolyte after 5 h. The color in the figure legend corresponds to the changes in the electrolyte potential in nanovolts (nV), with a maximum potential of 875.2376763 mV located at the undamaged film. The varying sizes of the black arrows represent the intensity of the absolute potential gradient, where the highest intensity is located close to the interface and decreases upwards in the electrolyte. (b) Variations in the change in the electrolyte potential after 5 h along with the passive film of the front surface of the FSPed region. (c) Current density distribution as contour lines in the electrolyte after 5 h. The color in the figure legend corresponds to the electrolyte current density. The varying sizes of the arrows represent the intensity of the current density distribution, where the highest intensity is located close to the interface and decreases upwards in the electrolyte. (d) Variations in the local current density along with the passive film of the front surface of the FSPed region after 5 h.

Fig. 12. (a) Potential distribution (color) and absolute potential gradient (black arrows) in the electrolyte after (a) 5 h, (b) 2, (c) 5, and (d) 10 months along with the passive film of the front surface of the FSPed region with potentials around ～875, ～872, ～868, and ～862 mV, respectively. The varying sizes of the black arrows represent the intensity of the absolute potential gradient, where the highest intensity is located close to the interface and decreases upwards in the electrolyte.