*Nitrogen Supersaturation of AISI316 Base Stainless Steels at 673 K and 623 K… DOI: http://dx.doi.org/10.5772/intechopen.102387*

γ-phase zone with its average size of 5 nm is formed by plastically straining the nitrogen unsaturated γ-phase region.

In the mesoscopic characterization, the network of zone boundaries plays an essential role to transport the nitrogen solute from the surface to the NFE of the nitrided layer and to drive the homogeneous nitrogen supersaturation. **Figure 16** also proves that zone boundaries in this single-crystal should work as a fine network of nitrogen diffusion paths and propel the nitrogen supersaturation to the unnitrided regions. The grain size refinement co-works with the homogeneous nitrogen supersaturation and terminates itself at this formation of ultrafine γ-phase single crystals without nitrogen solutes.

In **Figure 15**, this refined γ-phase single-crystal is neighboring to the poly-crystal zones; the other phase in the refined two-phase microstructure corresponds to this poly-crystal. Let us make STEM analysis on this point.

**Figure 17** shows the HAADF, ABF, and LAADF-images on the poly-crystal zone at the vicinity of nitrided AISI316 specimen surface at 673 K for 14.4 ks. These imaging methods in low magnification prove that two zones with different crystallographic structures are aligned in series and in parallel in the inside of the polycrystal. After STEM analysis in high resolution and dual SDD-EDX detection, either of these two zones mainly consists of nitrogen-enriched chromium or Cr (N) rich lattices. On the other hand, another zone consists of nitrogen-poor iron and nickel or Fe/Ni (N). HAADF and KAADF images also prove that two neighboring zones have different nitrogen content. In correspondence to the mesoscopic analysis on the two-phase structure, this nitrogen-rich Cr (N) has a γN-phase structure while the nitrogen-poor Fe/Ni(N) has αN-phase structure.

To be described later, the microstructure of AISI316 below NFE has no separation among iron, nickel, and chromium contents. This local segregation of chromium from iron and nickel is induced by the difference in chemical compatibility to

**Figure 17.**

*HAADF, ABF, and LAADF analysis on the poly-crystal like zone at the surface of nitrided AISI316 at 673 K for 14.4 ks.*

nitrogen among the three elements. In the austenitic stainless steels, chromium is a substitutional element to occupy the iron site without significant change of the original lattice constant for iron [7]. Nickel works to stabilize the fcc-structure [8]. In the absence of nitrogen solute in AISI316 substrate, its crystalline system has γphase while this local system has a fine mixture of γN- and αN-substructures by their chemical compatibility to nitrogen solutes. This local separation of a constituent element in AISI316 among chromium, nickel, and iron, drives the nitrogen supersaturation and zone-boundary diffusion to advance into the original matrix. The zone boundaries between Cr (N) and Fe/Ni (N) work as a local network of nitrogen diffusion paths to modify the original, homogeneous AISI316 matrix to a fine twophase crystalline structure of Cr (N) and Fe/Ni (N). This formation of nitrogen supersaturated γ- and α-phase zones reveals that nitrogen solute diffuses through the zone boundary network to the inside of each nano-crystalline zone.

**Figure 17** also proves that the chemical compatibility of AISI316 constituent elements to nitrogen solute separates the homogeneous γ-phase crystalline structure into two-phase zone structure. Hence, the interface between two nano-crystalline structure distorts by itself to form an irregular zone boundary.

Let us summarize the microscopic view on the nitrided layer at the vicinity of the surface. Its microstructure consists of the γ-phase single-crystal structure and the γN/αN, two-phase polycrystalline structure. The former structure is sparsely formed in the nitrided layer by plastic straining with nitrogen supersaturation. The fine grain boundaries with co-orientation of (111) are also formed along (111) slipping planes so that no nitrogen solute is present in its inside and it diffuses to the depth through these fine and larger grain boundaries. The latter structure is common to the nitrogen supersaturated zones. Each zone is composed of nitrogen-rich, γ-phase crystal and nitrogen-poor, α-phase one. This formation of the two-phase nanostructure is induced by different chemical compatibility of nitrogen solute to Cr and {Fe, Ni} in local. The local disturbance of nitrogen solute content triggers the local phase separation; this continuously propagates into the depth of the nitrided layer to form the fine two-phase structure.

Remember that the nitrogen supersaturated zones are advancing into the A- and B-grains in **Figure 9**. The highly strained zones are present in neighboring to the fine, two-phase zones. This local area in **Figure 9** is just resembling the microstructure at the surface of nitrided AISI316 in **Figure 15**. The γ-phase single-crystals are yielded by highly plastic straining in the similar manner to the fine distribution of high KAM zones in **Figure 9b**. Each refined g-phase single crystal in **Figures 9** and **15** is formed by enclosure of slipping lines with the orientation (111). The fine twostructured poly-crystals are also formed just near these high KAM zones in **Figure 9c**. Under high nitrogen enrichment through the single crystal zones, a twophase nanostructure is formed by nitrogen supersaturation with its different chemical compatibility.

STEM analysis was utilized to describe the microstructure below NFE. In the mesoscopic analysis by EBSD, the α-phase zones, the modified grains, and the plastic strains are observed even below NFE in **Figure 10**. STEM analysis provides proof of microstructure modification by plastic straining. **Figure 18** shows the HAADF, ABF, and LAADF images below NFE. In the low-resolution imaging, the original AISI316 grain is divided into several subgrains in correspondence to the IPF profiles in **Figure 10c**. To be noticed in the HAADF image, this subgrain boundary has almost (111) plane; these subgrains are formed by the plastic straining in the easiest slipping lines. In addition, the inside of the subgrain also has slip-lines in (111) direction as seen in both ABF and LAADF images. This cross-slipping in (111) direction is just corresponding to the formation of γ-phase single-crystals in **Figure 16**. When the plastic straining co-works with the nitrogen supersaturation

*Nitrogen Supersaturation of AISI316 Base Stainless Steels at 673 K and 623 K… DOI: http://dx.doi.org/10.5772/intechopen.102387*

**Figure 18.**

*HAADF, ABF, and LAADF analysis on the microstructure of the nitrided AISI316 just below NFE.*

and boundary-diffusion processes, the microstructure in **Figure 18** is further modified and refined by the cross-slipping in plastic straining to be a single-crystal with the zone boundaries in (111) directions. Since the nitrogen content is nearly zero below NFE, the synergic relationship among the nitrogen supersaturation, the boundary diffusion, the plastic training, and the microstructure refinement stops intermediately to leave the plastically strained granular structure below NFE.
