The evolution of fretting wear behavior and damage mechanism in Alloy 690TT with cycle number was investigated via laser scanning confocal microscopy (LSCM), scanning electron microscopy (SEM), focus ion beam (FIB), and transmission electron microscopy (TEM). The results showed that the fretting running status underwent a transition from partial slip and mixed stick-slip to final gross slip with the transformation of Ft–D curves from the ellipse to the parallelogram. The coefficient of friction (COF) experienced three drops throughout the fretting process, which indicated the transformation from high-friction wear to low-friction wear. The first drop was due to the transition from two-body to three-body contact. The second and third drops were mainly related to the evolution of the glaze layer from a localized distribution to completely covering the whole contact surface. The competition between fretting induced fatigue cracking (FIF) and fretting induced wear (FIW) ran through the entire fretting wear process. Before the 1.2 × 10⁴th cycle, the fatigue crack growth was faster than wear, and FIF won the competition. As the fretting cycle continued to increase, the wear velocity was obviously faster than that of FIF, which indicated that FIW defeated FIF. The tribologically transformed structure (TTS) participated in the competition between FIF and FIW. The gain boundaries and dislocations in the TTS were a suitable pathway for crack initiation and propagation and oxygen permeation.

All light or heavy water reactors fabricated with carbon steels suffer from flow-accelerated corrosion (FAC). The FAC degradation of SA106B with different flow velocities was investigated in terms of microstructure. As flow velocity increased, the major corrosion type changed from general corrosion to localized corrosion. Severe localized corrosion occurred in the pearlite zone, which can be the prior location for generating pits. After normalizing, the improvement in microstructure homogeneity reduced the oxidation kinetics and lowered cracking sensitivity, causing a decrease in FAC rates of 33.28%, 22.47%, 22.15%, and 17.53% at flow velocity of 0 m/s, 1.63 m/s, 2.99 m/s, and 4.34 m/s, respectively. Additionally, localized corrosion tendency was decreased by reducing the micro-galvanic effect and tensile stresses in oxide film. The maximum localized corrosion rate decreased by 21.7%, 13.5%, 13.8%, and 25.4% at flow velocity of 0 m/s, 1.63 m/s, 2.99 m/s, and 4.34 m/s, respectively.

Microstructure and deformation properties of both unaged and aged cladding material were studied at 400 °C for 10,000 h. The results indicated that carbide formation occurred in the cladding material, while thermal aging treatment resulted in spinodal decomposition and G-phase formation in the aged ferrite phase. Furthermore, intensive straight slip bands formed in both unaged and aged austenite phases. Continual straight slip bands formed in the unaged ferrite phase, while curvilinear slip bands formed in the aged ferrite phase during the plastic deformation process. Microcracks preferred to nucleate at the points of interaction between phase boundaries and carbides, while the aged ferrite phase experienced lowered microcrack formation along the carbide/ferrite phase boundary. Microcracks propagated along the straight slip bands in the unaged ferrite phases and curvilinear slip bands in the aged ferrite phases.

Gradient-nanostructured Alloy 600MA was fabricated via surface mechanical attrition treatment (SMAT) for 0–180 min. Pre-corrosion XRD confirms that SMAT with severe plastic deformation does not induce phase transformation of the γ-FCC Ni-Cr-Fe matrix, while progressive peak broadening verifies grain refinement and accumulated lattice strain, consistent with TEM-observed gradient nanolayers. Quantitative grain-boundary misorientation and mechanical-twinning information cannot be obtained solely from TEM-SAED. Pre-corrosion SEM-EDX reveals no obvious elemental segregation after SMAT. Corrosion behavior was evaluated in oxygen-saturated deionized water at 290 °C and 8.7 MPa for a single duration of 720 h. Raman spectroscopy detects crystalline corrosion products of NiCr₂O₄, though amorphous or disordered nanocrystalline oxides cannot be excluded. Under 720 h exposure, all SMAT-treated samples exhibit higher mass loss than the pristine alloy; the 120 min-SMAT sample shows moderate mass-loss among treated groups. Since defect-driven electrochemical activity and Cr diffusion were not directly measured, the competitive mechanism between defect-accelerated corrosion and Cr-enriched passivation is proposed based on experimental results and published literature. This work provides fundamental data for gradient-nanostructured nickel-based alloys under high-temperature pressurized-water conditions.