**5. Microscopic characterization on the plasma nitrided AISI316**

Through the macroscopic and mesoscopic analysis, the plasma nitrided AISI316 is characterized by the refined and two-phase microstructure, by the plastically strained microstructure, and by the modified microstructure below NFE. STEM was employed to analyze the microstructure to microscopically describe the grain-size refining, the two-phase structuring, the plastic straining, and the microstructure modification below NFE during the plasma nitriding of AISI316 at 673 K for 14.4 ks.

The Cs-corrected STEM (Spherical Aberration Corrected Scanning Transmission Electron Microscope; JEM-ARM200F; JOEL, Tokyo, Japan) was employed for this microscopic characterization on the nitrided layer. This system has a cold FEG (Field Emission Gun) with a capacity of 200 kV and a resolution of 0.08 nm by using a Cs-corrector. Dual SDD (Solid State Detector)-EDS detector was utilized for local element mapping with Cs-STEM. Argon ion milling was used to make a thin slice of the plasma nitrided AISI316 specimen for Cs-STEM analysis.

**Figure 15a** depicts the STEM image in the vicinity of nitrided layer surface. This surface region is composed of two zones; e.g., a smooth zone and a rough zone. The electric diffraction at the former zone is shown in **Figure 15b**. Since a single spot is only detected, this former zone has a single-crystal like microstructure. On the other hand, the electric diffraction at the latter zone consists of two or three spots as shown in **Figure 15c**. That is, this zone has a polycrystalline microstructure.

**Figure 15** proves that the nitrided layer with high nitrogen solute content is composed of single-crystal and poly-crystal grains. After [28], a single crystal grain with its size less than 10 nm has no dislocations or no defects in its lattice structure. Those defects in the fine single crystals have high free energy enough to be pushed out of the inside of crystals to fine-grain boundaries. Hence, this ultra-fine singlecrystal zone also has no nitrogen solute to occupy the octahedral vacancy sites in the single crystal. That is, these fine single crystals are generated by the refining process

### **Figure 15.**

*TEM image on the surface of the nitrided AISI316 at 673 K for 14.4 ks. a) Cross-sectional TEM image with two zones, b) single-crystal like zone, and c) poly-crystal like zone.*

through the cascading reduction of zone size, co-working with the intense plastic straining in the synergic relationship.

In later, the microstructure of this single crystal zone is precisely analyzed by HAADF (High-Angle Annular Dark-Field)-imaging, ABF (annular bright-field) imaging, and LAADF (Low-angle annular dark-field)-imaging, respectively. Most of the microstructure in **Figure 15a** consists of the poly-crystal zones. Remember that the nitrided layer consists of the fine two-phase structures in the mesoscopic characterization. This poly-crystal zone is expected to be formed by two neighboring crystals with different lattice structures. In later, HAADF, ABF, and LAADF imaging methods are also utilized to describe this correlation.

In the STEM analysis, HAADF-imaging produces an annular dark-field image formed by very high angle, incoherently scattered electrons (Rutherford scattered from the nucleus of the atoms). ABF provides a robust technique for simultaneous imaging of light and heavy elements since its contrast has a low scaling rate with the atomic number. LAADF receives the diffracted or inelastically scattered electrons at low to medium angles (25 to 60 mrad) using an ADF (annular dark-field) detector.

**Figure 16** shows the HAADF, ABF, and LADDF-images at the single-crystal zone in **Figure 14b**. ABF-imaging explains that every constituent atom is aligned in (111) direction. From LAADF imaging, this single crystal with (111) and (200) crystallographic orientations. The above nano-structural analysis reveals that this single-crystal has γ-phase, the crystallographic orientation of which coincides with the easiest slipping plane orientation of (111). Remember that the mesoscopic analysis on the nitrided layer. The γ-phase zones coexist with α-phase zones to form the fine two-phase nanostructures and the nitrogen unsaturated zones are plastically strained to compensate for the strain incompatibility between the nitrogen saturated and unsaturated zones. ABF-imaging in the above proves that a refined

**Figure 16.**

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