Fig. 1. First row shows photographs of the as-eroded appearance of the (a) FG, (b) MG and (c) CG cathodes. Second and third rows show SE-SEM plane-view and BSE-SEM cross-sectional images of the corresponding cathodes, respectively. The inserts in (i) show details (BSE) of the modified layer on top of respective Al and Cr grains.

Fig. 2. X-ray diffractograms obtained from the surface of FG (upper), MG (middle) and CG (lower) cathodes.

Fig. 3. BSE-SEM images demonstrating the microstructure of the modified layer on the eroded FG cathode in the first row (a, b), MG cathode in the second row (c, d), and CG cathode in the third row: (e, f) on top of an Al grain and (g, h) on top of a Cr grain. The blue rectangles mark the sites from where the TEM lamellae were lifted out. Red dotted lines highlight boundaries of the stacking sublayers.

Fig. 4. (a) TEM BF micrograph of the Cr-rich cells surrounded by the Al-rich matrix observed in the modified layer of the MG cathode, (b) HRTEM images and corresponding FFT of the Cr-rich cell and the Al-rich matrix, and (c) simulated SAED patterns of r-Al8Cr5 and c-Al8Cr5 along the [012] zone axis.

Fig. 5. 3D reconstructions of an atom probe tip prepared from the modified layer of the MG cathode: upper-left Al 50 at.% iso-surface showing the Al-rich matrix (c-Al8Cr5 phase); lower-left Cr 50 at.% iso-surface demonstrating the Cr-rich cells, (Cr) phase. Right-hand side diagram shows elemental distribution along the z-axis of the cylinder shown in the 3D reconstructions (scanning direction: top to bottom).

Fig. 6. First row shows the TEM lamella used for TEM studies in Fig. 4: (a) PQ map where the dotted-line square indicates the region studied by TEM, (b) phase mapping results, (c) orientation mapping results, IPF-Z. The second row shows (d) BSE SEM image of the region with partial intermixing and PQ map from the corresponding TEM lamella, (e) phase map of the lamella together with an enlarged dendrite, and (f) IPF-Z map of the lamella and the dendrite.

Fig. 7. (a) TEM BF micrograph of the Cr-rich cells and separating Al-rich matrix in the modified layer of the FG cathode, (b) HRTEM image of cell-matrix interface along the [001] zone axis and (c) filtered HRTEM image and corresponding FFT of the regions marked as 1, 2 and 3 in (b), all along the [001] zone axis.

Fig. 8. 3D reconstructions of an atom probe tip prepared from the modified layer of the FG cathode(a) Al 50 at.% iso-surface showing the Al-rich matrix (c-Al8Cr5); (b) Cr 50 at.% iso-surface demonstrating the Cr-rich cells, (Cr) phase. (c) shows elemental distribution along the z-axis of the Ø 20 nm cylinder shown in the 3D reconstructions (scanning direction: top to bottom).

Fig. 9. (a) PQ map of the lamellae prepared from the modified layer of the FG cathode, marked by the rectangle in Fig. 3(b); the boundary of two stacking sublayers is marked by the dashed line in the upper lamella, (b) phase mapping results, (c) orientation mapping results, IPF-Z. The insets show higher resolution maps obtained with reducing the scanning step size to 3 nm.

Fig. 10. (a) PQ map of the lamella prepared on top of a Cr grain (CG cathode) shown in Fig. 3h together with HRTEM and SAED results, and (b) TKD phase map and (c) IPF-Z map of the lamella.

Fig. 11. (a) 3D reconstructions of an atom probe tip prepared from the homogeneous part of the modified layer on top of a Cr grain shown in Fig. 3(h). (b) shows elemental distribution along the z-axis of the Ø 40 nm cylinder shown in the 3D reconstructions (scanning direction: top to bottom).

Fig. 12. (a) STEM BF micrograph of the lamella prepared from the upper sublayer of the modified layer on top of the Al grain (CG cathode), the upper rectangle shown in Fig. 3(f), (b) SAED patterns obtained from regions 1, 2 and 3 as well as a HAADF-STEM image of region 4.

Fig. 13. 3D reconstructions of an atom probe tip prepared from the upper sublayer of the modified layer on top of the Al grain shown in Fig. 3(f). (a) Al 76 at.% iso-surface showing the supposedly QC phase; (b) Cr 22 at.% iso-surface demonstrating the c-Al8Cr5 phase. (c) shows elemental distribution along the z-axis of the Ø 15 nm cylinder shown in the 3D reconstructions (scanning direction: left to right).

Fig. 14. (a) PQ map from the lamella investigated by TEM in Fig. 12 and (b) corresponding phase mapping.

Fig. 15. (a) STEM BF micrograph of the lamella prepared from the lower sublayer of the modified layer on top of the Al grain (CG cathode), the lower rectangle marked in Fig. 3(f), (b) HRTEM image obtained from region 1 with corresponding phase analysis and (c) SAED patterns obtained from areas marked with 2 and 3.

Fig. 16. 3D reconstructions of an atom probe tip prepared from the lower sublayer of the modified layer on top of the Al grain shown in Fig. 3f. (a) Al 88 at.% iso-surface showing the Al and/or (Al) phase(s); (b) Cr 17 at.% iso-surface demonstrating the i-QC phase. (c) Right-hand side diagram shows the elemental distribution along the z-axis of the cuboid (X:7 nm/Y:30 nm/Z:65 nm) shown in the 3D reconstructions (scanning direction: top to bottom).

Fig. 17. (a) pattern quality map from the lamella investigated by TEM in Fig. 15, (b) corresponding phase map, (c) Kikuchi patterns obtained from the area marked by the arrow showing 5-, 3-, 2-fold rotation axes of an icosahedral quasicrystalline phase.

Fig. 18. (a) BSE SEM image of an area of the modified layer of the MG cathode showing variations in dendrite size and (b) TEM BF image of a fully grown dendrite obtained from an area with medium size microstructure and intensity profile used to determine the secondary dendrite arm spacing (SDAS). The SDAS was estimated to be 150-200 nm.

Fig. 19. Dendrite simulations as a function of cooling rate $\dot{T}$, temperature gradient G, and initial thermal undercooling conditions $\mathbf{\Delta }T$of 0 and 100 K applied for the dendrite phase (Cr). The highlighted boxes show the cooling conditions that yielded a dendritic microstructure where the secondary dendrite arm spacing (SDAS) could be determined.

Fig. 20. Schematic illustration of the droplets’ angular emission from imaginary cathode spots on top of Al and Cr grains of the CG cathode and subsequent droplet redeposition on the cathode. Droplets are largely ejected at low angles with respect to the cathode plane [97].

Fig. S1.  3D reconstructions of an atom probe tip prepared from the modified layer of the FG cathode: (a) Al 50 at.% iso-surface showing the Al-rich matrix (c-Al8Cr5); (b) Cr 50 at.% iso-surface demonstrating the Cr-rich cells, (Cr) phase. (c) shows elemental distribution along the z-axis of the Ø 35 nm cylinder shown in the 3D reconstructions (scanning direction: top to bottom).