Fig. 1. Cross-sectional SEM images of as-deposited GS specimens I, II, and III, respectively.

Fig. 2. TEM/SEM images at normalized distances x of (a) 0.08, (b) 0.25, (c) 0.40, (d) 0.53, (e) 0.67 and (f) 0.90 in GS specimen I. The top-right insets in (a-d) are the corresponding selected area electron diffraction (SAED) patterns, and the bottom-right insets in (a-f) are the distributions of grain size.

Fig. 3. (a) Hardness and (b) grain-size-distribution profiles of as-deposited GS specimens, where the grain sizes are calculated from the hardness profiles using the Hall-Petch relationship. The grain sizes in specimen I were also measured by TEM and SEM (red spheres).

Fig. 4. Hardness profiles of GS specimens after annealing at 503 K for 3 h, where the critical hardness values marked by the stars are identical for all three GS specimens.

Fig. 5. (a) Typical cross-sectional SEM image of the specimen I after annealing at 503 K for 3 h. (b) The magnified SEM image showing the original microstructural morphology at the specific location (i.e., the region c marked in (a) in the as-deposited specimen. (c-e) The enlarged SEM images at the regions marked in (a).

Fig. 6. TEM images at the critical position (x = 0.44 for GS specimen I) in the (a) as-deposited and (b) annealed specimens. The top-right and bottom-right insets in (a) and (b) show the corresponding SAED patterns and grain size distributions, respectively.

Fig. 7. Relationship between the critical size of grain growth dc and the grain coarsening temperature TGC in Ni. Data for ED Ni and severe plastic deformed (SPD) Ni [8,[10], [11], [12], [13], [14], [15], [16],[24], [25], [26], [27], [28], [29]] are also provided for comparison.

Fig. 8. (a) The logarithmic dc as a function of logarithmic t, and (b) as a function of 1000/TGC in our GS Ni.

Fig. 9. The softening degree ΔH as a function of initial hardness H0 in our GS Ni.

Fig. 10. (a) XRD patterns of GS specimen I at various positions along deposition direction, where the initial grain sizes are 31, 37, 45, 61, and 73 nm, respectively. (b) Grain boundary energy as a function of the initial grain size, and data for ED Ni [9,20,35,43,44] are also included for comparison.