Yanming He

The background of deep peak-shaving poses demanding requirements for the performance of metal components in thermal power plants. The cracking of the connecting fins on the side wall of the flue seriously endangers the safe operation of the system. Herein, welding simulations were carried out using the finite element method to understand the cracking process of the connecting fins. By changing the welding process and fin size, their effect on stress and deformation was explored. The results showed that increasing the welding rate would decrease the flexural deformation of the flue-side wall. Additionally, the yield range of the connecting fin depended more on width than thickness, and increased with increasing width. As for the relationship between fin size and flexural deformation, the maximum deformation decreased with the increase in thickness, while it first decreased and then increased with increasing width. Overall, the post-welding stress and deformation of the boiler flue side wall exhibited more sensitivity to the fin width compared with the fin thickness and welding rate. This article clarifies the stress distribution status of the connecting fins in the flue side wall under different welding conditions, providing a basis for analyzing its cracking phenomenon, and further providing theoretical guidance for optimizing the structural parameter design of the side wall.

Micro-arc oxidation (MAO) is performed on Ti-6Al-4V electron-beam-welded joints, and the microstructure, phase composition, and corrosion resistance of the joint and surface coating are investigated systematically by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), as well as electrochemical and stress corrosion analyses. SEM analyses revealed that the joint undergoes a phase transformation. The coating morphology of the joint and base materials is similar, but the joint coating is denser and thicker. XRD analyses recognize the rutile and anatase phases in the coating. Polarization and electrochemical impedance spectroscopy (EIS) corrosion tests reveal that the MAO treatment results in a decrease of two orders of magnitude in the corrosion current density of the welded joint and an increment of corrosion resistance. Stress corrosion evaluation reveals that a dense layer is exposed to protect the joint after long-term exposure to a high-stress corrosion environment. No stress corrosion-induced cracking or defects are observed in the joints, indicating the corrosion resistance of the joint has significantly improved.

Ti₂AlC is a typical MAX (M: early transition metal, A: main group element, and X: carbon and/or nitrogen) phase with ceramic and metallic properties due to its unique nano-layered structure. In order to investigate the interaction behavior between Ag and Ti₂AlC, a sessile drop experiment was conducted at 1080 °C for 5 min. The atomic rearrangement occurred at the Ag–Ti₂AlC interface was revealed using high-angle annular dark-field scanning transmission electron microscopy coupled with high-resolution transmission electron microscopy analysis. The results show that Ag nanoclusters generally appeared in most of the Ag–Ti₂AlC interaction regions thermally processed at 1080 °C. In addition, Ag can also substitute for Al and Ti atoms in the Ti₂AlC, promoting local structural decomposition of the Ti₂AlC and producing 4H–Ag with a hexagonal close-packed (hcp) structure. Additionally, Al atoms released from the Ti₂AlC lattices can dissolve locally into the liquid Ag, particularly at the grain boundaries. When the loss concentration of Al exceeded the critical level, the Ti₂AlC started to decompose and the residual Ti₆C octahedrons and Al atoms recombined, giving rise to the production of anti-perovskite Ti₃AlC with a cubic structure. Lastly, the discrepancy in substitution behavior of Ag in the Ti₂AlC was compared when thermally processed at different temperatures (1030 °C and 1080 °C). This work contributes to the understanding of the intrinsic stability of Ti₂AlC MAX ceramics under high-temperature treatment.

The present work introduced first-principles calculation to explore the substitution behavior of Ag atoms for Al or Ti atoms in the Ti₂AlC MAX phase ceramic. The effect of Ag substitution on supercell parameter, bonding characteristic, and stability of the Ti₂AlC was investigated. The results show that for the substitution of Ag for Al, the Al-Ti bond was replaced by a weaker Ti-Ag bond, decreasing the stability of the Ti₂AlC. However, the electrical conductivity of the Ti₂AlC was enhanced after the substitution because of the contribution of Ag 4d orbital electrons toward the density of states (DOS) at the Fermi level coupled with the filling of Ti d orbital electrons. For the substitution of Ag for Ti, new bonds, such as Ag-Al bond, Ag-C bond, Al-Al bond, Ti-Ti anti-bond, and C-C anti-bond were generated in the Ti₂AlC. The Ti-Ti anti-bond was strengthened as well as the number of C-C anti-bond was increased with increasing the substitution ratio of Ag for Ti. Similar to the substitution of Ag for Al, the stability of the Ti₂AlC also decreased because the original Al-Ti bond became weaker as well as the Ti-Ti and C-C anti-bonds were generated during the substitution of Ag for Ti. Comparing with the loss of Ti d orbital electrons, Ag 4d orbits contributed more electrons to the DOS at the Fermi level, improving the electrical conductivity of the Ti₂AlC after substitution. Based on the calculation, the substitution limit of Ag for Al or Ti was determined. At last, the substitution behavior of Ag for Al or Ti was compared to discriminate that Ag atoms would tend to preferentially substitute for Ti atoms in Ti₂AlC. The current work provides a new perspective to understand intrinsic structural characteristic and lattice stability of the Ti₂AlC MAX phase ceramic.