**3. Results**

After SMGT, a severely deformed gradient structure formed on the surface, as shown in the optical microscopy image in Figure 1a. NG structures (as labeled by a dotted line) were observed on the topmost region of the gradient layer (based on TEM studies shown later). The Vickers hardness indents and the corresponding hardness along the depth direction were labeled, indicating the formation of gradient microstructures after SMGT. After annealing at 700 ◦C, a sharp interface formed between the topmost NG region and the

deeper region of the sample, as revealed by the optical microscopy images in Figure 1b,c. It is worth mentioning that the thickness of the NG region varies with positions due to the inhomogeneous penetration depth of the gradient structure after SMGT, with a maximum depth of 30 μm.

**Figure 1.** Optical microscopy images of (**a**) as-processed (AP) and annealed gradient IN718 alloy at 700 ◦C for (**b**)5h and (**c**) 24 h. The topmost nanograined (NG) regions are labeled by dotted lines. Vickers indentation and corresponding hardness values (in GPa) are labeled.

The TEM image of the as-processed sample in Figure 2a reveals that NG structures formed near the surface of IN718 alloy after SMGT, as confirmed by the inserted selected area diffraction (SAD) pattern. It is worth mentioning that fine and coarse nanograin layers were observed in the NG region (referred to as FNG and CNG hereafter, respectively), as labeled by the dotted lines in Figure 2a. The TEM image in Figure 2b shows the alternately distributed FNG and CNG layers in the topmost NG region (at a depth range of 2–5 μm from surface). The corresponding grain size distribution profile reveals that the average grain size is 14 and 28 nm in the alternating FNG and CNG layers, respectively. The average grain size of the CNG layers increases gradually from 25 nm at a depth of 100 nm from the surface to over 32 nm at the depth of 2200 nm from the surface (as shown in Figure 2b), further confirming the formation of complex gradient microstructures. The corresponding scanning transmission electron microscopy (STEM) image and EDS maps of the NG region in Supplementary Figure S1 (see supplementary materials) reveal that the chemical composition remains uniform. The ASTAR inverse pole figure map in Figure 2c shows the nanograins with various orientations in both FNG and CNG layers. The grain boundary map in Figure 2d shows that a majority of the grain boundaries in both the FNG and CNG layers are high-angle grain boundaries (HAGBs, indicated by blue lines). The fractions of low-angle grain boundaries (LAGBs) and coincidence site lattice (CSL) boundaries (indicated by red and yellow lines, respectively) are low.

**Figure 2.** (**a**) TEM image showing the formation of an NG structure in the topmost region of the IN718 specimen after surface mechanical grinding treatment (SMGT). Fine nanograin (FNG) layers were sandwiched by coarse nanograin (CNG) layers. (**b**) TEM image of NG region and the corresponding grain size vs. position profile showing the grain size evolution of both FNG and CNG layers at various depth. The corresponding (**c**) ASTAR crystal orientation analyses and (**d**) grain boundary map showing the high-angle grain boundaries (blue lines), low-angle grain boundaries (red lines) and twin boundaries (yellow lines).

Upon annealing (700 ◦C/24 h), the nanograins of the topmost NG region retained, whereas recrystallization and grain coarsening occurred in the rest of the gradient layers. Figure 3a shows the microstructure of the distinct interface (as denoted by a dashed line) formed between the thermally stable topmost NG area and the grain coarsened areas. Furthermore, the alternatively distributed FNG/CNG structures were sustained after annealing (as labeled by dotted lines in Figure 3a). The statistic distributions of grain size in Figure 3b show that the average grain size of FNG and CNG layers in the thermally stable area is 18 and 37 nm, respectively, whereas the grain size of the adjacent area coarsened to 90 nm. The STEM images and corresponding EDS maps of both thermally stable and grain coarsened area in supplementary Figure S2a,b show that large Ni- and Nb-rich δ phases and Al-, Ni- and Nb-rich η phases formed after annealing. The difference between these two areas is that nanoscale α-Cr phases have higher density and smaller size in the thermally stable area than the grain coarsened area.

**Figure 3.** TEM images showing the sharp interface formed between the thermally stable area and the grain coarsened area of NG IN718 specimens after annealing at 700 ◦C for (**a**) 24 h and (**c**) 100 h. (**b**,**d**) The corresponding statistic distributions revealing the average grain sizes of FNG (DF) and CNG (DC) layers in the thermally stable and grain coarsened area.

Increasing the annealing time further to 100 h coarsened the grains in both thermally stable and grain coarsened areas of the NG region. However, the sharp interface between those two areas sustained, as labeled by the dashed line in Figure 3c. The alternatively distributed FNG/CNG structures were also observed, whereas the grain size difference between these two layers is much larger than the specimen annealed for 24 h (in Figure 3a). The statistical analyses reveal that the grain size of the FNG and CNG layers increased to 62 and 219 nm, respectively (Figure 3d). In comparison, in the grain coarsened area, the grains coarsened further to 295 nm.
